Restoration / Repair
1962 VOX AC30
It's one of those stories that makes you think: "This isn't really happening!"
Yes it does. A few months ago we were able to buy an old VOX AC30 "Copper Panel" that was (get this) found in a dumpster in southern France. Only the amp chassis survived. The combo housing looked "like after a splatter movie".
We probably don't have to explain that this is an absolute Holy Grail amp with extremely high collector's value. The VOX AC30 has written music history and, in addition to "Fender Clean" and "Marshall Distortion", characterizes one of the 3 sound pillars of classic electric guitar sounds.
We are honored to bring new life and glory to this pitiful amp, which had a near-death experience in a french dumpster.
History of VOXES
let's call it "charming chaos"
Every guitarist has heard of VOX and the legendary AC30. This amp has written music history and, alongside Fender and Marshall, has shaped one of the sound pillars of the electric guitar. The Beatles, Queen, Tom Petty, Foo Fighters... you name it! The list of legendary bands swearing on the AC30 can be continued almost indefinitely.
Let's take a quick trip back to the late 1950s:
Founded in the early 1950s as the "Jennings Organ Company", the company founder Tom Jennings caught the unstoppable trend towards electric guitars at the end of the 1950s and renamed his company JMI (Jennings Musical Industries) in order to also market guitar amplifiers under the VOX brand name.
The first "big hit" from chief designer Dick Denney was the VOX AC15, which was introduced in 1958 and was also offered in a 30 watt version, the VOX AC30/4, on from 1959. What we know today as the AC30 saw the light of day in 1960, being called the VOX AC30/6. It included a revised preamp circuit without the mechanically sensitive EF86 tube and had 3 independent channels with 6 input jacks: normal, brilliant and vib-trem.
This is where the "golden era" of VOX began, which we consider today to be the period from 1960-1967 when all products were manufactured with the label "a JMI product".
A look behind the nostalgically curtain shows that this was a very successful time for VOX, but also a rather chaotic time that was very much influenced by constant product revisions and production bottlenecks. This can still be recognized today by the fact that dating the AC30's from this time is a science in itself because there were only few constants. Within VOXES from this era, you sometimes have the feeling that whatever was at hand and available was used.
This left its mark as early as 1964 when Tom Jennings sold his company to Royston and culminated in himself being fired as managing director in 1967. By this time, chief designer Dick Denney had already left the company. Things got worse rapidly, and in 1968 Royston had to file for bankruptcy. Subsequently VOX was taken over by former company-executives under the label of "VOX Sound Equipment Ltd.". From this point on, the (now legendary) "a JMI product" label disappeared from the amplifiers and was replaced by "a VOX product" label.
Over the years, VOX has had a wide variety of owners, often manufacturing at different sites from England to Italy to the USA at the same time. Despite this, the VOX brand has survived to this day, and even today you can still buy new AC30's. In a significantly different version, but still with the tonal ingredients from the "Golden Era".
A detailed (though not complete) overview can be found in the VOX showroom. If you are particularly interested in the "Golden Era" there, you want to click here. And also voxac30.org.uk is a great nerd-source for detailed information on old VOX amps.
what is left of the amp?
Honestly, it doesn't look that bad!
Ok, that part comes from a French dumpster... the housing was completely destroyed. BUT: The amp chassis itself maa good impression.
Not massively bent... no handicraft ruins... 98% of the amp chassis look like they left VOX back then. Very good! To be honest, it's rare to find old AC30s in one so original condition.
The circuit itself looks completely untouched. Top!
All original Woden transformers are still there and the windings show realistic DC resistances. Top!
Ok, the electrolytic capacitor mounted on the chassis no longer has a retaining clip and is "up in the air"...
The fuse holder makes a strange impression and half is missing. There was already someone going on...
A plastic nut on the input sockets is missing. We forget that...
There are no power tubes anymore... but we have an old GZ34 and a set of old Brimar ECC83 and ECC82. Top!
The finder of the amp was clever enough to unscrew everything useful on the destroyed housing and take it with him. Hence we have additionally:
The original type plate including original fastening nails
Many of the old screws and decorative washers
The original brackets of the EL84 tubes
So let's say:
Cool, that could have been a lot worse!
when did you see the light of day?
Ok, admittedly... it's also a bit of a hobby of ours.
But honestly, with a Holy Grail amp like a VOX AC30 Copper Panel, you want to know exactly when it's from.
As written above, dating old VOX amps is a science in itself, because back then VOX changed specifications, suppliers and manufacturing locations like others change their underpants. For example, transformers from different manufacturers were often used in parallel in different factories. And oh yes... there was of course no really unique serial number system either. It's clear, isn't it?
Our VOX is an AC30/6 without Top Boost in the "Normal" version (there was also a "Bass" version). This can be recognized by the embossed "N" behind the serial number on the type plate. This limits the period to 1960-1964, because from 1964 the so-called "Treble" version was introduced and the optional top-boost circuit was the "standard" from 1965 at the latest. Here, www.voxshowroom.com was very helpful.
While Westrex Co. Ltd. already produced VOX amps from 1961, Burndept Electronis in Erith (Kent) was acquired as a production facility from mid-1962 (due to the high demand). Our AC30 clearly comes from the Burndept production. This can be seen from the fact that a chassis serial number is stamped next to the output transformer. Has probably only been made at Burndept. This is supported by the use of Woden transformers, Welwyn load resistors and Hunts electrolytic capacitors. That was typical of Burndept.
So we narrow down the manufacturing period to mid 1962-1964. Here, we have been enlightened by www.voxac100.org.uk.
Date codes on components:
We can also make it a little easier for ourselves!
Date codes on components have proven to be the safest way of dating old amps. Ok, you don't know how long e.g. a capacitor has been floating around in storage before it was installed, but in most cases you can assume that components were not in storage forever but were installed fairly quickly. At least at VOX where everything was "urgent". Component codes give us a clear indication of when the "earliest production date" was. A huge thank you to www.voxac30.org.uk which helped us decipher the old component codes.
We find in our amp:
Woden transformers with the date code "LT" = Nov. 1962
Welwyn resistors with the date code "LB" and "LG" = Feb. / July 1962
Egen Potis with the date code "JJ" = Oct. 1962
Hunt's capacitors with the date code "ISH" = Week 35 1962
Bingo! We're pretty sure our AC30/6 "Normal" dates late 1962 and is therefore almost exactly 60 years old...
This is how we do it
we need a plan...
In a 1962 VOX AC30 you don't just start soldering around! The amp is far too precious and rare for that. You should think carefully in advance what you want to do with such a piece of rock history and what will become of it.
We don't want to disfigure the amp. No wild drilling in the chassis or high-gain mods or something like that...
But we are also not slavishly "all original". If something no longer works or is in a questionable condition, it must be replaced in order for the amp to function properly again.
We'll probably introduce a few tweaks along the way to the restored amp as the original AC30 is far from perfect. We're not talking about modifications to the AC30's sound itself. We're talking about things like improved grounding, power supply filtering, etc. ... things that VOX could have done "better" back then to minimize background noise.
We would like the amp to live in a housing that does justice to a VOX AC30 from the early 60s as much as possible. No Frankenstein amp such as a 1962 AC30 in a black sprayed plywood box elapsing its future.
So the bottom line is that it's about bringing the amp back to its original condition as much as possible. Both sonically and visually. However, it shouldn't become a museum piece, we want to play it. Therefore, we won't slavishly hold on to "originality" in places where it doesn't make sense. The goal is to bring an early '60s AC30 back to life with a sound that can be described as "AC30 at its best!" goes through A high goal that we have set ourselves!
If we were to restore a collector's amp, we would have to be more uncompromising in terms of originality and "vintage correct". But the amp is OURS and should be the sonic blueprint of an optimal VOX AC30 in the Tube WorkShop... and hopefully we will get it there.
Where can we get a housing that is as original as possible?
Check tubes: Can the old "Brimars" still do anything good, or ready for the dustbin?
Overhaul power supply
Overhaul power stage
Start up and optimize the amp with power amplifier and phase inverter.
Commission and optimize the Bright Channel.
Commission and optimize the normal channel.
Put the Vib/Trem Channel into operation and optimize it.
Assembling the finished amp and final test.
This is the approximate plan for our AC30. Let's see what we encounter along the way...
scrubbing instead of soldering
This time we don't actually start soldering directly, but first devote ourselves to a thorough cleaning of the chassis and the control panel.
On the one hand, this is due to the wisdom "you don't wanna cook in a dirty kitchen", on the other hand, it is also partly due to the mechanical concept of the AC30. In the AC30, the control panel is on the top of the combo, as are the vents above the power amp section. With regard to the operation and convection cooling of the 4 EL84 tubes, this is also a very good idea. In terms of what gravity does to dirt, dust, spilled drinks, etc., it's pretty much non-ideal. The dirt deposits on the control panel and migrates through the ventilation slots to the power amplifier section and the transformers where they bake wonderfully due to the heat of the tubes.
It looks the same here in our AC30... although in other specimens we have found worse. Much worse!
So get ready for the cleaning cloths and start scrubbing. It can't be that complicated, right? In principle it isn't, but cleaning an old amp can do more harm than good if you're using the wrong cleaning agents or cleaning utensils.
As already written before, glass cleaner and lint-free cotton cloths are the weapon of choice for most control panels. Old kitchen towels, old T-Shirts, if necessary even old underwear (please wash them beforehand). Glass cleaner is relatively mild, but still removes greasy dirt very well. With the vast majority of amplifiers you can get the panel wonderfully clean again without running the risk of rubbing off the lettering. If in doubt, please do not "rub" too hard, but rather run several cleaning cycles with little force.
The old Woden transformers in particular are extremely sensitive. To be more precise, the printing on the transformers. I have no idea how to get such poorly adhering printing. Actually, fact is that you might have rubbed it off faster as you will expect. Even with mild cleaners. The finder of the amp probably noticed this too when he started to clean the mains transformer and removed half the printing at the same time. Thank God he noticed it in time and stopped before you could read any more of the printing.
So here the rule is: Spray on glass cleaner and dab carefully. Yes: Dab...don't rub!
With this, you can get the printed transformer caps clean again investing a lot of patience. Outside of the printing-area, you are welcome to rub and scrub... and if the dirt is too baked on, you are also welcome to use cleaning spirits such as Naphta. We like to use Zippo lighter fluid here because it is very pure.
This is where all the dirt and liquids (that get inside through the ventilation slots) end up. Often quite scary what you get to see there. And since the power amp section of the AC30 chassis is made of steel, it is also susceptible to rust film.
In the first step, we flood the top of the chassis generously with isopropanol and scrub away all "loose" dirt with a brush (an old toothbrush is ideal). Here you can invest all your energy and you are welcome to even use your fingernails or a guitar-pick if the brush isn't enough in some places. You will be really amazed what you find in your cleaning cloth after wiping. Yummy!
Next we turn our attention to the tube sockets, flooding them with isopropanol as well and scrubbing the surface with our toothbrush and the contacts with interdental brushes. Our dentist would be very proud of us. Then tighten the contact pins and blow them out with compressed air. Clean! The remaining isopropanol evaporates without leaving any residue within a very short time. No worries.
Now comes an important step before we turn our attention to the rust film on the chassis: Please mask off all tube sockets with masking tape!
We use good ole WD40 for the wellness treatment of the steel chassis. It has the wonderful property of dissolving both dirt and rust, penetrating the metal surface and sealing it a bit, thus slowing down the formation of further rust. BUT: We don't want WD40 on the contact pins of our tube sockets. Here we want bare metal and no WD40 anti-rust film. So: Mask it! Important...
Now we can generously rub the chassis with WD40 and leave it for about 30 min. to unfold its magic. Then brush the metal surfaces intensively and rub dry with a cotton cloth. And we mean "dry". No visible smear should remain. Otherwise, it will quickly attract dust like a magnet. We really only want to keep the WD40 here that has penetrated the metal surface and protects it in the future.
From a purely visual point of view, you may not notice a big before/after difference. Where there were rust spots, you can also see them afterwards. But: Run your finger over the surface... smooth as a baby's bottom! It felt pretty rough before. This means that we actually managed to remove the rust film with WD40 and can assume that the rust spots have now received a good protective film.
Our WorkShop now smells wonderfully of glass cleaner, isopropanol and WD40. That's the scent of clean amp chassis. We love it! ;-)
sure, we always wanted to do that...
In the short video clip you have already seen the sparkling clean control panel without potentiometer knobs. But these have us in the meantimebut really brought a few beads of sweat to the forehead.
First of all: It's great that all the original knobs are still there!
The downside was that one of the chickenhead knobs had half of the mounting screw broken off. Somehow this is the classic with old brass screws and then you have nen old knob that you can not unscrew. At least we in the TWS haven't found a working method yetn. If you have a "brilliant move" for something like this... please let us know!
Sometimes you are lucky and the screw is not particularly tight. Then you can often pry the potentiometer down with the help of WD40 and the "2 teaspoon method". Not so in this case: the old, brittle potentiometer button broke down into its components when it was levered down.
Of course we could just use a new chickenhead button as a replacement. But we want to keep as much of the original amp as possible. So you sit patiently with the potentiometer puzzle andSuperglue there and tries to put everything back together without getting the devil's stuff on your fingers and sticking a potentiometer knob to your hand permanently.
Mission accomplished! We puzzled an intact knob...
Obviously others had already tried something similar on some of the other poti-knobs. On one pot knob we could probably even take the fingerprints of the hobbyist that did his work here. "Don't get any superglue on your fingers" obviously didn't work so well here.
Whatever, finally we have 6 intact knobs from 1962 that we can use again. We're just wondering how to fix the fact that a piece of 3 of these potentiometer knobs has chipped off on the underside. Either someone has already tried to pry it down with brute force... or a Frisbee has been smashed into it. We will never know...
A happy PSU
Ok, we cleaned and glued like the world champions... now it's about getting the amp back into a safe condition where you can put it into operation bit by bit.
The first thing on the agenda here is the power supply.
We can't repeat it often enough: Please don't just plug an old amp in an unknown condition into the socket to see if it's still working! Electrolytic capacitors that have aged over the years can sometimes cause short circuits that can kill a valuable, original mains transformer, since there are no other safety precautions in an AC30 apart from the mains fuse. And oh yes... the mains fuse holder is only half there and not usable.
1. First we need a new fuse holder
2. The power pack electrolytic capacitors are 60 years old and some don't even have a mount anymore. Not good at all!
3. The power cord was of course (thank God) also snapped off. So we need a new one.
Ok, the original mains fuse holder seems to have quit service at some point. Can happen. But folks, let's be honest... you really have to drill out the control panel and chassis with a rough milling machine (of course without deburring it) and then NENtoo big Fuse holder (which you happened to still have in the craft box)?
In any case, after removing the fuse holder, a horror film awaits us with a roughly enlarged hole into which nothing common fits and a charred inside of the chassis where we ask ourselves: "Was the fuse holder removed with the cutting torch?". Guys, really... what were you doing there?
Aaawell.... calm down again...
We still have original Belling fuse holders as they were built into the AC30's back then. But they don't fit in the extended hole. So washers are needed in exactly the format that covers the grossly enlarged hole and holds the original fuse holder. We have had success here with shims in the format 16x22x0.5mm. It's not really pretty, but with this we get an original fuse holder back in the place where it belongs.
Power pack electrolytic capacitors:
Without much discussion: 60-year-old electrolytic capacitors have to be exchanged. Point! Yes, we know all the techniques with "re-forming" etc. In any case, the fact is that electrolytic capacitors are designed for a lifespan of 10-20 years. Of course you can also try to drive a 60-year-old classic car with the original tires. I wouldn't do that for safety reasons though...
What do we need? And what do we want?
A 1962 AC30 has a standing, radial 2x16uF capacitor and a radial 2x8uF capacitor for filtering the "Normal" and "Brillant" channels and also an axial 32uF capacitor for filtering the "Tremolo" channel. Replacements with suitable mechanical dimensions can be found at JJ, F&T and TADTubetownandTube Amp Doctor.
You can of course stay "historically correct" with the values, for a collector we would definitely do that. However, it must be said that the early AC30s were notoriously "underfiltered". Ie the filtering of the power pack was measured more than just to save money. Since we are restoring this AC30 for ourselves, we take the liberty of installing the standing e-cap on 2x32uF/500V from TAD to upgrade and use a 2x15uF/450V version from F&T instead of the 2x8uF electrolytic cap. The axial 32uF Elko was renewed with a F&T 32uF/500V. The upgrades aren't dramatic, but should give us a little more stability and less hum. We would like to have both. If the amp turns out to be a bit too "stiff", we can "downgrade" it later.
Unfortunately, the grounding of the AC30's in their 2-part housing is not particularly well thought out ex works. We will encounter this topic a few more times and has a good share in the fact that old VÖXE have the reputation of "always buzzing a little".
In the power pack, the screen of the mains transformer and the center tap of the high voltage are grounded on a soldering strip right next to the mains transformer. Good thing, these are the "dirtiest" ground connections in terms of background noise and should be as far away from the rest as possible! The first filter cap with 2x32uF shares a ground connection with the cathodes of the output stage and the axial 32uF filter cap intended for the vibrato channel. Here the opinions differ somewhat. Some prefer to ground the first filter capacitor together with the center tap of the high voltage and have an exclusive ground point for the cathodes of the output stage. Others prefer the first filter cap (where the anode voltage of the amplifier tubes is also taken) to be grounded at the same point as the cathodes of the output stage, because this is supposed to give the best common-mode rejection. We prefer the latter method and have had good experiences with it. But there are always several roads that lead to Rome.
The fact that the 32uF filter capacitor for the vibrato channel should ideally be grounded in the area of the preamp section is relatively undisputed. At least in theory. For the time being we left this on the ground point of the power amp and will see later (when we take care of the vibrato channel) whether we can get away with it or not.
One more request:
Make sure that all grounding points make really good contact with the chassis! Over the years, the Original Equipment brass terminal lugs often develop layers of oxide. So: Remove old connections... clean... reinstall... and tighten guuuut! The ground connections should be as low resistance as possible. This job is not fun, but unfortunately it has to be.
There is nothing spectacular to report here. New power cord...wire fuse holder installed...make sure the ground has the longest wire...phase goes first through the fuse, then through the power switch...neutral goes straight to the power switch. That's how you know it and that's how it should be.
Introducing the amp to mains voltage:
Now the big moment has come when we introduce the AC30 to mains voltage again for the first time. But gently. We put only the rectifier tube in the amp and the voltage is gradually ramped up with an isolating transformer. We want to see whether the amp draws an unnatural amount of current in this state (or rather THAT is exactly what we don't want to see) and we want to know whether there is voltage at all relevant points in the amp.
Everything good! The mains transformer doesn't seem to have a problem seeing mains voltage, and the wiring in the amp also seems to be in order and supplying voltage wherever we want it to be.
In this state (without tubes), the voltage in the electrolytic capacitors remains stored even after switching off. In addition to the valuable technician's advice to always discharge electrolytic capacitors before working on the amp, we have got used to temporarily soldering a 100k/5W resistor from the first filter capacitor to ground on any amp "under work". This ensures that the electrolytic capacitors are discharged to a harmless level within approx. 15s and that you are also "safe" if you accidentally forget to discharge the electrolytic capacitors.
Some TLC for the Power Amp!
an AC30 needs that...
The VOX AC30 has a reputation for being the amp that eats EL84s for breakfast.
That's true to a certain extent. With the legendary 50 ohm cathode resistor, the 4x EL84 in an AC30 are driven with approx. 130% plate dissipation and the 100 Ohm screen grid resistors also don't exactly help that the power tubes are in a "comfort zone". Many say that this is exactly what makes the sound of an AC30. Hmmm, let's see... anyways, that's why you'll find a charred ring around the power tube sockets in many AC30s due to the heat.
We do find a lot of melted wax in our 1962 AC30, but only a few "burn marks". Interesting!
As so often: First we have to clean! Again...
As already mentioned, a thick layer of wax awaited us between the tube sockets and even ON the tube sockets. Unfortunately we don't have a really good photo of it.
Honestly, we have no idea how the wax got there!
It is not uncommon in AC30s for wax to melt out of the transformers and this can be found under the transformers on the slider board. Since this no longer exists, we cannot say whether it was the same here. Of course, the wax can also run a little along the cables from the transformer, but for this to collect as a puddle in the middle of the chassis between the tube sockets, the amp would actually have to be upside down.
Most interestingly, you can also see traces of wax on the top of the chassis between the tube sockets. This probably didn't drip from above, but seems to have flowed through the tube sockets from below. And that actually only works when the amp is upside down! Unless in the southern France, gravity is reversed.
You poor little AC30... what have those barbarians done to you? Upside down SM-Torture-Games in dark French recording studio only to be disposed of in the dumpster at the end? We'd better not know what this amp has already experienced...
Ok, we're professionals... we can deal with it:
Soldering on waxed tube sockets is not a particularly good idea. So how do we get the wax out of there? Very simple: The resistors have to raise their legs up in the air, then you melt the wax with the hot air gun and soak it up with paper towels. Q-Tips work great in corners and on the tube sockets. The last wax film can then be easily removed with isopropanol and at the end a thin layer of WD40 seals the galvanized metal chassis.
Sounds easy, but it's a huge mess. In the end, however, we have a sparkling clean chassis and can solder the power amplifier back together again.
Cathode Bias resistor:
We are very surprised to find an 82 ohm cathode resistor in the power amp. At first we thought it had been replaced later... but the solder joints were "untouched" and the date code on the resistor also shows the year 1962. So our AC30 came "ex works" like this. After intensive internet research, we are now certain that VOX equipped the AC30 with a cathode resistor of 82 ohms up to around mid-1963, which is almost "optimal". Only then did VOX reduce it to 50 ohms to get a little more power out of their amps. After all, they were in competition with the "Almighty Fender Bassman" that came with 45W. Incidentally, this is also where the VOX race began after more convection cooling in the amps, as the power amplifier was simply running too hot.
With the Brimar and Mullard tubes of the time, one could probably get away with this. Many thanks at this point, by the way, to Lyle Caldwell from Psionic Audio for his advice. Lyle is probably one of THE AC30 experts on the planet.
Long story short:
The original Welwyn 82Ohm/5W resistor is still in top form and should also give us optimal BIAS conditions in the future. Better than the later "standard" of 50 ohms which brought the AC30s to the brink of a meltdown...
And let's not waste many words about the cathode bypass capacitor: You know my opinion on 60-year-old electrolytic capacitors. So let's get it out replace it with a 250uF/100V type from TAD.
Screen grid resistors:
Yes, that's such a controversial topic in AC30s!
Originally, these are equipped with 100 ohms, which put a lot of strain on the power tubes. In our AC30 we decided to replace the screen grid resistors of the power amp tubes with heat-resistant 1KOhm/2W metal oxide resistors. Some also choose 470 ohms here. However, we would not recommend a higher value, as the output of the power tubes would be slightly limited above approx. 1.5KOhm.
Normally we like to use 5W types as screen grid resistors. But we only do this in amps that have an HT fuse! Why? Dying power tubes often tend to draw an ever-increasing screen grid current up to a short circuit. In such a case, the HT fuse blows. That's its job. Until then, 5W types are coping best with the stress they are exposed to.
However, an AC30 does not have an HT fuse. That's why we deliberately use weaker 2W types, since these are the fuse themselves in the event of a fault. This means that the screen grid resistance must burn out before the current that is too high can damage the filter choke or the mains transformer. If you want to play it really safe, use only 1W types here.
Yes, you could also stay with the historically correct 100Ohm/1W resistors and subsequently install an HT fuse for them. There are different worldviews. But hey: Our amp, our decision! And we go to 1k/2W. Amen.
These have drifted away in many AC30s because they are exposed to a lot of heat directly above the boiling EL84's, and are therefore often automatically swapped out. In our amp, the 4 Erie's were at the upper end of the tolerance band (where they may have been right from the factory), but have very even values around 1.8kOhm and are otherwise in good condition. So the Erie's are allowed to stay.
Short check on the Bias of the EL84's:
Before we even start thinking about the preamp and phase inverter, let's do a quick check of the power amp's operating point. So a matched quad of new JJ's move into the amp and we slowly turn up the variac. We pay attention to whether we hear crackling or ticking noises. This could indicate arcing in the primary winding of the output transformer, since this is now confronted with high voltage and current again for the first time. With us, thank God, everything stays still and the voltage drop across the primary winding is stable. Happy days!
Measure the voltage at the cathode... approx. 12.5V... calculate the current per tube... subtract 5% for the screen grid current... multiply by the anode/cathode voltage...
Baaaam: We end up with pretty much exactly 12W per tube which is pretty much exactly 100% plate dissipation. Almost perfect! But later we will measure this again more precisely when all tubes are in the amp and all voltages have their final values.
To everyone out there accusing VOX of bad amp design because AC30's running so hot:
EAT THIS! Old fox Dick Denney knew exactly what he was doing when he designed the 82 Ohm cathode resistor in the 1960's AC30.
Well, until he was convinced by the sales guys in mid-1963 that it was ok if this box was glowing, and persuaded himself to use the well-known 50 ohm cathode resistor, which squeezed out a little more power.
Phase Inverter & Grounding
an AC30 does not have to be a grumpy bear...
Now that the power amp of the AC30 is up to pace again, the phase inverter is next on the agenda.
The questions that move the world are here:
1. Are the beautiful Wima Durolits still ok, or are they leaky?
2. Are the old Erie resistors still ok or have they drifted into nirvana and making noise?
3. Do we have the typical AC30 50Hz mains hum or not?
It's a pity. The old Wima Durolits are "right on the edge" and let 0.7V - 1V DC through. That's not much and you could still use them in this condition... but it doesn't get any better either. To be more precise, the DC leakage gets worse the longer the amp is used. So we swap the coupling capacitors for 2x 150nF Vishay 1813 with 630V rating. These are sonically suitable and qualitatively one of the best you can currently get.
Really interesting. The old Erie resistors are all still in good condition and do not show any microphonics or excessive background noise. However, almost all of them have drifted to almost 120% of their intended value. We had already noticed that within the grid stoppers of the power amplifier and it continues here. As already mentioned, either all Erie's have drifted upwards very evenly, or they were already in this range (+/- 20% tolerance) ex works. Or both. Anyway, the Erie's stay in for now.
After the phase inverter is ready for use again, we find in our 1962 AC30 exactly what is found in almost all AC30's: They hum. Like a grumpy bear at 50Hz. For many years we also dismissed this as "that's how it is with an AC30"... Until Lyle Caldwell from Psionic Audio drew our attention to a design flaw from VOX that has a large share in the buzzing Voxes...
VOX simply screwed up the grounding concept of the AC30!
They have made every effort with twisted heating conductorsments of the tubes. Since the heating voltage winding of the mains transformer has no center tap, they did what was common at the time and was considered "good practice". They grounded one of the heater voltage lines.
With this you lose the effect that the 50Hz hum is extinguished by the twisted heating cables because we have two 3.15V cables in opposite phase. If one side is grounded, we only have 6.3V with a nice electromagnetic field at 50Hz around it.
Next, VOX grounded the heating cable at the most unfavorable point of the entire amp: At the grounding point of the sensitive preamp and the phase inverter! This is an extremely sensitive spot for background noise and at the same time the heater voltage is one of the "dirtiest" companions in an amp. That can't go well. And neither does it.
So we removed the inconspicuous connection at the preamp ground point to the heater lines and created a ground reference through a "virtual center tap" at a remote location. With a "Virtual Center Tap" the two heating lines are each grounded with approx. 100 ohms. This ensures that the heating voltage has a defined potential and that both lines move symmetrically around the outside at approx. 3.15V, thereby eliminating the electromagnetic fields. Traditionally, the "Virtual Center Tap" is grounded, but a good tweak is from Lyle Caldwell to use a slightly increased DC voltage instead of ground and thereby create a certain decoupling.
Why does this work? It works because an AC voltage (like the 50Hz heaters) interprets any DC voltage as "ground". So, for example, the bias voltage of the power tubes is a good point. Here we incorporate our "Virtual Center Tap" in which both sides of the heating voltage with 100 Ohms are applied to the cathodes of the output stage. We use heat-proved 2W metal oxide resistors here. If you want to add a little more security, you only use 0.5W resistors that burn out very quickly if there is a defect in the tube heating. Here the resistor also has the function of a fuse at the same time.
The effect is really amazing! As soon as you have freed the preamp and phase inverter from the "dirty" heating lines, the basic 50Hz hum is reduced massively. The humming of the AC30's is not God-given but is due to poor grounding. We recommend this modification for all old AC30's. In any case, our 1962 is no longer a "grumpy bear" but extremely quiet. Thanks to Lyle for his suggestions!
Normal & Brilliant Channel
we make the amp playable..."Hero" or "Zero""?
Power amp is running fine. Also the Phase Inverter. Background noise behavior: Top!
We have also fed a sine wave to the phase inverter with a signal generator to see whether the power amplifier is amplifying correctly and a resilient signal is coming out of the speaker out. It does. So all good up to here.
So we have a perfect starting point to put the preamp back into operation so that we can finally connect a guitar and hear how the amp sounds. A sinusoidal signal that is fed into the phase inverter is only "musical" to a limited extent and is not really fun to listen to.
Since our AC30 still comes without a top boost circuit, we decided to put the "Normal" channel and the "Brilliant" channel back into operation together, since both only share a single 12AX7 and only a handful of components are used.
Ok, we have to be clear that VOX didn't specifically design a pure guitar amp back then. You should also be able to operate other instruments such as a bass or an organ on it. The normal channel was intended for this. Very linear frequency response... deep bass... no treble-bleed capacitor for more clarity in the sound. There are definitely instruments that sound good with it. It's definitely not a guitar. So "Needs Work"...
VOX's "weapon of choice" to amplify instruments that should be a little clearer and more assertive. Like a guitar! However, VOX decided at the time that this sound was not achieved by letting more treble through, but by cutting off the bass. And not subtle, but rather violent. Yes, somehow you get a sound that is "brilliant"... but only because the very small 500pF coupling capacitor cuts off everything in the low-frequency range, which could somehow be fun. "Needs also work"...
"Let the games begin!":
So let's start working on the preamp. We start with the input sockets and find that they are still in good condition and, above all, the switched ground contacts are still working. These ensure that an input is short-circuited to ground when nothing is connected, and thus no background noise is caused by an "open input". The Erie resistors on the jack sockets are also still in good condition, as are the shielded cables leading to the preamp tube. Happy days!
Next we check the value of the anode resistors on the shared 12AX7 of the two channels and background noise, as well as the coupling capacitors to see whether they let DC voltage through and thus mess up the bias of the subsequent phase inverter.
The old Erie resistors have also drifted up by about 20% at this point (typical in this amp), but they still work well and don't make much more background noise than new types. The coupling capacitors also do not leak DC-Voltage and are absolutely OK. Again: Happy days!
We could have left the Erie anode resistors in the circuit with no problem. Nevertheless, we decided to replace them with new Carbon-Comp resistors to tickle a little bit less background noise out of the amp
So everything is fine and we just put another tube in and have "AC30 at its best"? Unfortunately, no! The 60-year-old bias electrolytic capacitor definitely needs to be replaced on the cathode, and we also have a problem with crosstalk in the normal and brilliant channels in almost all old AC30s.
"Crosstalk" is a phenomenon where 2 channels of an amp influence each other, i.e. where the signal from channel A can also be heard in channel B and vice versa. I know, it's all very technical and exhausting. Don't need to understand it in detail. In practice, this simply gives us an "unclean" sound, because signals from 2 channels are superimposed, and that's not always beneficial.
The hunt for crosstalk:
Allright, let's start with the "basics".
An extremely sensitive point in an AC30 is the high-impedance mixer where the normal and brilliant channels are combined with 2x 220k mixing resistors and fed to the phase inverter input. Even if we are only talking about cable routes in the 10cm range, it is very noticeable how sensitive this area reacts to interference. This can be significantly improved by combining the mixing resistors at the volume potentiometers and making the connection to the phase inverter with a single, shielded cable. The difference is amazing!
Even if this area is now significantly less sensitive to interference, we still have a massive problem that both channels influence each other. In practice this means: I plug the guitar into the brilliant channel, have the brilliant volume set to "0" and turn up the normal channel volume. I clearly hear a signal. That shouldn't really be the case. It is the same the other way around.
Maybe that's "normal" with old AC30's... and you can also say: "Well, just turn the volume potentiometer of the channel you're using". But at the end of the day I find it annoying because you always have to pay attention to how the volume pots are set and whether something here influences each other.
After we experimented with different tubes (and they had no effect), the cause was clearly the "shared cathode" of the 12AX7, which supplies both the normal and the brilliant channel. The VOX design provides that each input channel has its own triode system, but both channels share the cathode circuitry.
We have decided to take a "radical" step here:
We remove the "Shared Cathode" circuit and give each triode system of the 12AX7 input tube its own 1.5k bias resistor in combination with its own 25uF cathode electrolytic.
Yes, the "vintage correct" fraction will tear each other apart and we will be accused of "blasphemy". But: Hey guys, face it... the amp just works better with it! And with that, we have almost completely eliminated crosstalk between the normal and brilliant channels. And that's exactly what we want!
So now we've blasphemed, but the result is a Normal and a Brilliant channel that actually work independently and have no crosstalk.
Nevertheless, we still have the problem that VOX refused to feature assertive highs at the time. The keyword here is "treble bleed capacitors". VOX decided to cut off massive bass in the Brilliant channel.
First of all, let's bring the Brilliant Channel into a form where it doesn't sound "anaemic", but has an appealing depth foundation. For this purpose, the much too small coupling capacitor of the Brilliant Channel with 500pF was exchanged for a 2.2nF mustard cap, which has the correct sound and is historically correct.
In addition, the Normal channel received a 150pF treble-bleed capacitor in the form of a Vishay "Class 2" ceramic capacitor which adds some liveliness and a little "grind" without making the channel overly "bright". We wanted to keep the basic character of the channel but make it "usable". In the Brilliant Channel we decided on a 250pF Cornell Dubilier Silver Mica which sounds very "beautiful" and "open" without being "exhausting".
The BIG moment... Playing Test:
So the big moment has come...
The power amplifier is running. The phase inverter is running. And we put the normal and brilliant channels into operation in prepress. So we can finally connect a guitar.
But stop: expectation management!
We know some AC30's. Especially late 60's and early 70's models. Also current models.
All of these amps sounded "Fairly Good" but none had the "magic" of an excellent AC30 as we envision it. All these amps sounded somewhat like AC30, but always seemed a bit strained and pressed. Our idea of a "perfect" AC30 is an amp that effortlessly masters the transition from clean to distorted. It asserts itself perfectly without being shrill and has the perfect "chime" that makes all the notes shine.
Guitar plugged in...amp turned up...a big "Open-D"...
YESSSSSSS! That's the sound!
Not quite perfect yet, but our AC30 already shows everything we could hope for from an excellent AC30. Is it "vintage voodoo"? No idea. In any case, it is clear that this amp has that "certain something" that our previous AC30's lacked. direct hit!
Home Sweet Home - Pt. I
the VOX needs a home...
The time has come to think about where the AC30 should live in the future, since unfortunately we only have the "bare" chassis.
We thought of it quite simply:
"Okay, well, we'll just have to buy a replica of an old AC30 combo case. It won't be cheap, but it will be a perfect fit."
Dang it... there are NO replicas of old AC30 cases! That was unexpected...
About 10 years ago there was the company JMI in England for a short time, which offered excellent VOX replicas and housings. But they've been "out of business" for quite some time. Another source we knew was "North Coast Music". Here we found out that they closed their "Cabinet Shop" in 2019 and that all leftovers have been sold since 2021. Damn!
In fact, there is no supplier of replicas of the VOX cabinets anywhere in the world. There are various theories as to why this is so:
1. These things are very complex to build and the market is very small. Nobody wants to do that.
2. Korg (as the owner of VOX) keeps a very close eye on not selling copies and stops them from doing so. That's why no one dares to do so
You can choose for yourself what your preferred conspiracy theory is. ;-)
The fact is: We are stuck here with all of our talent and have an original 1962 AC30 but we don't have access to a housing.
Irony of fate: We "recycle" ourselves a housing
After a few sleepless nights it occurred to us that we still have a Frankenstein case from an early 1960s VOX 2x12 box in the basement. This came to us many years ago along with an old AC15. Since it probably didn't have a case anymore, someone roughly built a case for the amp with a jigsaw and a few boards from a VOX 2x12 box. Why is it that old VOX amps keep losing their housings?
So we go down to the basement with the meter rule... blowing of some dust... and BINGO. Identical dimensions and identical structure. At that time VOX seems to have used the same basic housing for AC30's and 2x12 boxes, and depending on the orders it became either an amp or a box.
It's almost ironic that we've wanted to place the old, battered housing in the dumpster a few times, but for some reason we never did. And now it gets its "grand appearance" as a home for a '62 VOX AC30 that comes from a dumpster. Kind of as if the housing was waiting for the amp.
Positioning: What have we got?
We have an early '60s VOX 2x12 cabinet. The dimensions are identical to the AC30. However, the box is "closed-back". In addition, someone used a jigsaw to cut openings for an AC15 chassis and ventilation grilles. Rails for a slider board were also nailed in, but they don't fit for an AC30. The attached handles are something out of the craft box. And the old tolex and the covering are in a pitiful state too. And of course the multiplex wood has already started to delaminate in a few places.
But ok... "easy" is something everyone can do. We love challenges!
In our case, it quickly became clear that the housing had to be stripped down to the base and then rebuilt.
Let's see the positives: This gives us the opportunity to get as close as possible to the original housing.
The search for the "Original Spec."
So, we were highly motivated to research the original housing spec. made. Wow, not that easy! You can find a lot of information on the Internet, but it is often not very clear to assign an exact time stamp because the transitions at VOX were often "fluent" and what was "at hand" was being used. So you will always find specimens that do not fit into a certain scheme.
However, the following things have emerged:
From 1960 to early 1962 virtually all AC30's were covered with "Fawn" Tolex.
From 1962 there was also a version with "Smooth Black Tolex" because it was a better match for the VOX Continental organ. Customers loved this look! This option was offered in parallel with the "Fawn Variant" until 1963. These amps had no protective corners, 3 narrow brass ventilation slots, brown speaker fabric and brown leather handles.
In the course of 1963, VOX gradually went from offering only black AC30s with a coarser grained Tolex, black speaker fabric, 8 protective corners, black ventilation slots made of cast metal and the well-known "dogbone handles" made of plastic. This is the VOX look we know today.
So, to summarize, there are 2 options for our late 1962 AC30:
Fawn Tolex, Brown Grill-Cloth, no protective corners, 3x narrow brass ventilation slots, brown leather handles
Smooth Black Tolex, Brown Grill-Cloth, no protective corners, 3x narrow brass ventilation slots, brown leather handles
In the end we decided on a housing in Fawn Tolex.
Why? We just think "Fawn" is kinda cool!
So, after we decided how our housing should finally look like, we still had to find the appropriate parts for it. There are really a few tough nuts to crack here.
Let's start with the simple things:
- Fawn Tolex: Yep, you can get a decent replica at either TAD or Tube Town
- Brown Diamond Grillcloth: Yep, you can also get it at TAD or Tube Town
- Narrow brass air vents: Yep, see above
- White piping: Yep, see above
- Wide gold stripe between grill cloth and case: TAD is your friend!
Now it gets more difficult:
Also available from TAD. Unfortunately, their article is very far from the original and very dark. Almost like a "stealth look". A variant from a supplier from Hungary was lighter, but not really "Golden". Looked more like a caramel candy. Ultimately, the hit was the gold piping from "North Coast Music" to "Original Spec". Unfortunately, it's not exactly cheap to have the stuff sent from California either.
Brown Leather Handles:
To make it short: The original, brown leather grips with their typical end caps are no longer a standard part. After a long search we have found with ampaholics a small manufacturer in England who offers handmade original replicas. No a cheap offer... but very good quality and alternatives.
So we have everything together:
A battered VOX 2x12 speaker cabinet from the early 1960s... Tolex... fabric... white piping... gold piping... gold stripe... brown leather handles.
As we are not focused on woodwork at Tube WorkShop, we will entrust the case restoration to an expert we know well.
We will "update" the status of the cabinet from time to time to see the progress. o, there will probably be a "Pt. II" of the cabinet restoration...
To be continued
More amp nerd stuff to come...