Restoration / Repair
1956 Siemens V72 mic preamp
Sometimes you just have to be lucky in life!
This is roughly how our customer feels after this V72 has been given to him as a present app. 20 years ago. It has since then spent its life unused as a dust collector. In the meantime, the old V72 preamps have become collectors items and one of THE Holy Grails for high-quality studio recordings. In other words: "The really hot shit!".
The prices for a well-preserved and perfectly functioning V72 are now around €2,000.
More than reason enough to bring the present V72 out of retirement and put it back into service.
History
a studio legend
We are dealing here with some of the most important institutions in German studio and broadcast history.
In 1928, the companies AEG and Siemens joined forces to develop the first professional preamplifier for the broadcast industry, which was still young at the time. The joint venture was called "Telefunken" and the first product was the V1 preamp. After several development steps, this resulted in the V42 based on an EF12 tube, which was to remain the absolute standard until after World War II.
With the spread of FM technology at the end of the 1940s (which allowed higher audio quality), studio technology was also forced to be upgraded and higher-quality microphone preamps used. In a cooperation between the NWDR (North West German Radio) and the IRT (Institute for Broadcasting Technology) the V72 was developed between 1949 and 1952 as the new studio standard in terms of preamplifiers. Prof. Nestler, the former head of development at AEG/Telefunken, was in charge here.
From the perspective of todays fast-moving world, one looks back wistfully to the time when several years were invested in the development of a new preamp. We should please not fall back into the usual whining of "everything was better then"... but we can certainly state that devices, in which a lot of care and effort was put into the development, are obviously still relevant, sought-after and valued 70 years later are. Why?
Because they are simply GOOD!
In any case, the result was the V72 preamplifier. The core elements were the then new EF40 tubes in a glass housing (later replaced by the higher-quality EF804 and then EF804s) and very high-quality, multi-layered transformers.
In total, more than 25.000 V72s were made. Most of them simply ended up in the scrap-bin at the end of the last century when broadcasting technology was renewed across Germany. Thank God, however, not a few went into the private possession of employees of the radio stations, who then gave them away. As in our case.
I can recommend this site to anyone who would like to deal with the history of the V72 in detail: www.tab-funkenwerk.org. Warmly recommend.
Mechanical construction
let's disassemble this box
It is always fascinating how well thought-out, complex and yet service-friendly devices were designed in Germany in the 50s and 60s that are made to last forever. This is where the legendary reputation for quality "Made in Germany" arose, from which we still draw today.
With the V72, the engineers at that time around Prof. Nestler managed to create a mains transformer, a symmetrical input transformer, a multi-nested output transformer, a full 230V power supply and a complex amplifier circuit with 2 EF804s that makes 34dB gain in a compact "Danner Cassette" with the dimensions 47 x 134 x 265mm to accommodate. Respect!
Although the extremely tightly packed structure is reminiscent of 3D Tetris, the V72 module is designed to be extremely service-friendly. The complete cassette can be opened with only 2 (removable by hand) knurled screws on the back. Only 4 more screws hold the front panel (which you don't necessarily have to remove). Inside you can easily disassemble the EF804s, and the complete power supply can also be removed as a module after removing 3 screws and unsoldering 5 cables.
We bow to the great job that the designers of that time did. This also puts the (from today's point of view) quite long development time in a different light. It wasn't about developing a product as quickly as possible.. it was about developing a product that was as perfect as possible.
However, this also means that 66 years later we have to cope with the limited space available when restoring the V72 and we have to fit modern components with other form factors into the given space conditions.
"Challenge Accepted!"
Dating
not talking 'bout Tinder...
Even if it is not really important for a restoration, driven by our own curiosity, we always try to find out when the device was built on our workbench. Sometimes it's simply printed on the type plate... but often it's not. As with our V72. The nameplate tells us that it came from the inventory of the IRT and had the inventory number LV72 No.411 there... but nothing more.
So Sherlock Holmes mode:
1. The device was built by Siemens. This can be seen on the front panel by the Siemens symbol and the components in the V72, all of which are labeled for Siemens. In 1954, Siemens and TAB were commissioned as subcontractors to build the V72... ie our device dates from the period 1954-1963, when the V72 was then replaced by the transistor version V72t.
2. Codes on the components. At that time, Siemens used an in-house dating system in which the movement, the year of manufacture and the month of manufacture were encoded. On the power supply capacitors of our V72 we find the number: 44L12.
Deciphered this means: Plant No. 44 / 1955 / December. You can find detailed information about the Siemens codeshere.
Conclusion:
Our V72 dates from the "Siemens era" and was most likely made in the year of 1956. The earliest possible production date would be Dec. 1955 if the electrolytic capacitors were installed in the V72 immediately after manufacture.
Game plan
How do we approach this?
Ok, so we have a Siemens V72 from 1956. It was in use at IRT until it was retired here. We can assume that it was well cared for in the IRT and was retired in working order. After that it was not used for about 20-30 years.
Many would say: Then let's just connect the part and see if it still works. Very bad idea!
Yes ok, the probability that the V72 is still up and running is quite high. But: If, for example, the power supply electrolytic capacitors have dried out completely during long storage, this is playing with fire. The last thing we want is for the power supply to consume power and eventually kill the power transformer. There is simply no adequate replacement for this in such a special and tightly packed device and the V72 would only have scrap value. We do not want!
So the following procedure:
1. Check all transmitters and transformers for their DC resistance. This gives a good picture of whether they are still OK or have short circuits or interruptions somewhere. If a problem occurs here, it may not make sense to continue the restoration.
2. Review and revision of the power supply. Exchange of all electrolytic capacitors and check of the selenium rectifier
3. Check the function, operating point and characteristics of the EF804s tubes with our eTracer Curve Tracer
4. Checking and, if necessary, replacing the coupling capacitors
5. Check of all other components for function and values
6. Make a connection adapter for the multipin connector of the V72.
Transformers-City
we have them in abundance.
One appeal of the V72 is that it is based on very high-quality transformers on both the input and output sides. In addition, of course, there is the mains transformer. Somehow the art of producing high-quality transformers has gotten lost these days.
It is all the more important to check whether all the transformers and transmitters in our V72 are still OK.
A quick but powerful test is to check the DC resistance of all windings.
In principle, transformers and transmitters can produce 2 faults:
1. Short circuit: DC resistance is "hard" 0 ohms
2nd break: DC resistance is "infinity"
In our case, we were still able to measure realistic DC resistances for all windings. Thus, the transformers and transformers are still OK with a very high probability. Very good!
A defective transformer or transformer would put the V72 directly in the "scrap" department, since nowadays there are simply no longer any replacement types that fit into the very tightly packed design of the V72.
The power supply
hardest thing first...
Why the hardest?
We just want to exchange a few 66-year-old electrolytic capacitors...
First of all: Why do these electrolytic capacitors have to be exchanged?
Well, electrolytic capacitors have a lifespan of around 15-20 years. Depending on use and temperature, they are "done" afterwards and tend to no longer filter sufficiently and become "leaky". By "leaky" here I don't mean that they actually start spitting out their innards (they only do that when they're really completely"broken"). By "leaky" I mean that the electrolytic capacitors start to let through DC voltage.
Although the electrolytic capacitors in our V72 appear to be OK both optically and with regard to capacitance and ESR measured values, it can be seen that the DC resistance has already dropped to a value of approx. 50MOhm (which should actually be "infinite"). By the way, you can't measure that with a common multimeter... 4-wire measurement is the keyword here.
Long story short, the electrolytic capacitors still make a good impression but are definitely on the way to becoming a problem.
I know there are different opinions about when components should be replaced and when not. Some swear that you can bring old electrolytic capacitors back to life with "reforming"... Personally, I don't dig it. Not at all!
Power supply capacitors are safety-critical components in an amplifier. If they die, they can take other parts, such as the mains transformer, with them to their deaths. These are virtually "not replaceable" in a V72. We don't want to take that risk.
Let's compare it to a vintage car:
We have a 66-year-old classic car and we say: "The original tires are still good. They still have enough tread and aren't losing any air!". And then you drive over the first pothole and wonder why the tire bursts. Well, because the rubber has simply become brittle after 66 years...
But I digress...
I actually wanted to explain why replacing the power supply capacitors in a V72 is not an "easy job". Well, the V72 in the "Danner Cassette" is a device that is extremely tightly packed internally. The designers have put a lot of thought into service-friendly disassembly, but at the end of the day everything is based on having the original components with the appropriate dimensions. Just like 3D Tetris...
Rebuilding the PSU - I
we have challenges
So, now we have the little heap of misery in front of us.
Multiple Challenges:
1. The old electrolytic capacitors have a special mechanical design in which the plus connection is inserted and twisted through a slot in a Pertinax plate. This is also the mechanical attachment. Electrolytic capacitors of this design no longer exist today. Really not! Of course you could simply use modern radial capacitors... put the legs through the holes... twist them together somehow... and generously try to secure them halfway mechanically with silicone. But it won't be pretty... and mechanically it's also a more than shaky affair.
2. The positions of the electrolytic capacitors (especially the one next to the long, black selenium rectifier) are designed in such a way that they exactly correlate with the dimensions of other components during assembly and fit together perfectly in our 3D Tetris. If you don't pay attention to this, you will be in for a nasty surprise when assembling.
3. The Selenium rectifier. Back then it was the "really hot shit" (semiconductor diodes didn't exist yet). A huge part... optically quite distinctive... and was then used as a "mechanical guide" for the installation of the power supply module. And did I mention that Selenium is classified as "Toxic" these days?
Rebuilding the PSU - II
let's do it...
Ok, to shorten it a bit at this point:
There are exactly 2 ways to rebuild the old power supply of the V72 with modern components so that it fits mechanically. With both we take advantage of the fact that after removing the old electrolytic capacitors, the existing holes in the Pertinax plate fit perfectly to mechanically and reliably secure new elements with M3 screws.
1. We cut a perforated board that is securely connected to the Pertinax plate with the existing holes and spacer bolts and equip it with modern radial electrolytic capacitors. Technically a great solution... but optically rather unattractive, since the mix of old Pertinax plate, modern perforated board and modern radial electrolytic capacitors looks a bit like Frankenstein.
2. We screw several standing Pertinax soldering strips to the base plate and use axial electrolytic capacitors. Technically also a very good solution and optically much closer to the style of the 50s. The latter was ultimately the reason why we decided to do this. Nevertheless, it must be said: If the remaining 24.999 V72s also come to us for service, we would probably design a circuit board which is equipped with modern, radial electrolytic capacitors and can be installed in the V72s power supply module as a drop-in replacement.
And then we still have:
1. The dropping resistors between the Elko filter stages.
Even after 66 years, these are still absolutely "spot-on" and in great shape. Presumably these were not made by Siemens, but only labeled for Siemens. As a manufacturer, I suspect Rosenthal, and even by today's standards, Rosenthal resistors are still among the best you can get. So they can continue to live in the V72.
2. The Selenium rectifier.
It's a question of conscience again: leave it in or not? Maximum originality or safe function? Many swap out selenium elements in old amps principally for health reasons. I don't always agree with that. In the present case, the selenium rectifier is installed in a closed metal cassette and the probability that it will leak and then someone will sniff or lick it is negligible.
Although the selenium rectifier in our V72 still works perfectly, we will decommission it (but leave it in the housing due to its mechanical function) and replace it with a modern bridge rectifier. Why? Firstly, this is only in the power supply and has no sonic function. Secondly, a modern rectifier bridge gives you an approx. 10V higher internal voltage, which has a positive effect on the headroom. And thirdly, I just don't trust a 66-year-old selenium rectifier... ;-)
Mains fuse
You little bastard you!
Ok, the power supply has been restored... so purely as a matter of routine, we briefly measure whether the mains transformer is also connected to the power supply. And lo and behold: No! Fuse checked... is OK... what the f...?!?!
The fuse holder (and only this one) has been shown to have built up the thickest oxide layer imaginable over the years. An inserted fuse simply didn't make contact anymore, or only very shaky contact... and only in certain positions. And no, even almighty WD40 only wrung a weary smile from the oxide layer of our fuse holder.
Loose contact with fuses is not good!
Just imagine: "Joe Bonamassa is playing the guitar solo of his life and in the middle of the recording the V72 through which the signal from the microphone of his guitar amp goes goes out. The sound engineer then also says: "Strange, the preamp just went out ... he was only recently overtaken by the Tube WorkShop." Very bad!
So what do you do in such a case?
You get a coffee, wrap fine sandpaper around the end of a suitable screwdriver... and then, with the patience of an angel, you grind around the fuse holder until all the contacts are blank again. We don't want an oxide layer to be between us and Joe Bonamassa... ;-)
Tubes
how are you?
66 year old tubes? They have to be exchanged!
Nope...
I hate it when old components are replaced by suspicion. Especially with tubes, many old treasures end up in the trash-bin just because someone suspects that they must be bad. For this reason, we purchased a complex tube testing system some time ago, which not only measures tubes at a specific operating point, but can also determine the complete family of characteristics.
And what should I say:
The old Telefunken EF804s in our V72 are still in top condition after 66 years and can continue to live in the module. I'm always impressed by the quality of the tubes that Telefunken, Valvo and others produced back then.
And oh yes, if any of you know "Bea02"... please pass on contact details to me... ;-)
Tube sockets
getting clean
While we're already dealing with the tubes, then the tube sockets are also next. Thorough cleaning is definitely good for them after such a long time, because over time some deposits will always settle on the contacts and oxidation is also an issue.
No, we do not use WD40!
Some claim with WD40 and Gaffa you can solve 90% of the world's problems. I don't even want to disagree. Dirty tube sockets definitely belong to the remaining 10%. The stuff is great for any corroded metal surface to refurbish. However, it leaves a lubricating film on contacts that attracts new dirt like a magnet.
And no, we don't use DeoxIT D5 or F5 either.
Both are great in small doses for pots and contacts of switches. However, DeoxIT also leaves a conductive lubricating film which we do not want in tube sockets.
So what do we use for tube sockets?
Pure isopropanol and interdental brushes are your friends! Place a paper towel under the tube sockets and then generously flood them with isopropanol from above. After a short exposure time, the contacts for the tube pins are then scrubbed with interdental brushes.
You'd be surprised how much dirt and film you can bring out with it in a V72 (where the tubes are in a closed metal cassette) and then find it on the paper towel. But hey, after 66 years it's time for a professional dental cleaning. Isn't it?
Then blow it out with compressed air and let the remaining isopropanol evaporate (which it does without leaving any residue). We are CLEAN!
Coupling capacitors
Hero or Zero?
Once again we are faced with the question of conscience: "Maximum authenticity or maximum functionality"
The V72 doesn't make it easy for us here. The 4 coupling capacitors that can be found in the device are mechanically "strong charcters". Their mechanical size (partly mounted in special clamps) also shapes the optics of the device to a certain extent. From a purely metrological point of view, it can also be attested that they still have good capacitance and ESR values.
If only it weren't for the DC resistance, which has already dropped to a measurable, double-digit MegOhm range. The capacitors would still work fine with this, but it's just a clear indication that they've already started their journey of becoming a resistor and allowing DC voltage to pass.
Since we not only want to make the V72 functional again for a few years, but actually want to bring it back into a technically "as new" condition, we decided with a heavy heart to replace the existing coupling capacitors.
2 of our TWS Cream Caps (polyester in oil) were used here, which have a very even and even sound and are made on the original Philips production machines from the 60s. So we're pretty much period correct here. High-quality MKP types from Solen Fast in France were used for the larger coupling capacitors in the uF range. These are among the best you can buy today.
We would have loved to just take the old capacitors out of the circuit but leave them in the device for aesthetic reasons. Unfortunately, the narrow form factor does not allow this, especially since the mechanical position of the capacitors is partly decisive for whether the V72 runs stable or tends to oscillation. So a sure instinct was required here.
Many engineers underestimate the importance of layout and component placement in old tube amps in order to tame background noise and oscillation. Old devices are often patched back together somehow and if something vibrates, snubber caps are soldered in until everything is stable, even if you sacrifice a lot of sound.
In many cases this is not necessary with the correct positioning of the components and the correct routing of the cables. Unfortunately, this also means a lot of "trial and error" and takes time. But it's well worth it in the end!
Check of remaining components
easy job
Transformers checked... PSU overhauled... Tubes checked... Coupling capacitors replaced.
Only a handful of resistances remain to be checked. As already written, I strongly suspect that these are resistors from Rosenthal of the highest quality, labeled for Siemens. And as already suspected, measuring all the resistances has shown that they are still in top form. Nice one!
Connection adapter
let's get connected
In a guitar amplifier, we would have already made this after overhauling the power supply unit in order to power up the amp for the first time without tubes. However, since the V72 has such a nested design, we first carried out all the necessary work here and are only now thinking about the first commissioning.
We are very fortunate that with our V72 the right multipin plug was connected directly (because the cables were simply clipped off and the plug remained on the module). We were therefore able to produce a corresponding cable harness with manageable effort.
Otherwise we would have had to search hard for the appropriate plug or a (high-quality but expensive) wiring harness from accent audio and have to charge our customer for it.
In general one can say that a big factor for the popularity of the V72 is that it is operated directly with common 230V (integrated power supply) and the pin assignment of the connector plug allows it either for symmetrical signals (microphone preamplifier) as well as for unbalanced signals (line amplifier) to configure.
We opted for the symmetrical variant as a microphone preamp and attached XLR input and output jacks accordingly. The different connection variants would go beyond the scope here, but I will post a verified circuit diagram from which you can see which other options are possible.
We just want to point out one thing:
Since the V72s were operated in a professional environment, it was assumed that the technicians installing the modules knew exactly what your application required and configured the connections on the multi-pin connector accordingly.
For example, the metal case ground and the circuit ground are separate. This can be an advantage to prevent ground loops with the following device. In this case, however, it is imperative that the following device connects a defined ground potential to the circuit ground of the V72 (in our case via the ground pin of the XLR output on the cable harness).
If this is not the case, the circuit in the V72 has no defined ground potential (floating) and tends to start oscillating when the output is not assigned. To prevent this, we have grounded the circuit ground on the multipin connector (as is actually usual). If this leads to ground loops in combination with the following device, this ground bridge can be removed. However, you must then ensure that the V72 receives its ground potential from the following device. Rule of thumb: If you don't understand what I'm talking about here, then you shouldn't wire the V72 yourself... ;-)
Installation
V72 comes to live
Well, finally the time has come to put the V72 back into operation.
Of course, this is not simply plugged hard into the socket, but slowly ramped up at the control isolating transformer.
This is where "problem areas" appear directly:
Over a mains voltage of 180V, the V72 tends to fall into a massive 32KHz oscillation at the output. There are 2 reasons for this:
1. We have an open housing that is not grounded because it is connected to the isolating transformer...
2. The position of the coupling capacitors in the circuit massively influences the stability of the circuit
After some "back and forth" regarding the placement of the coupling capacitors and the general layout, we found the optimum, which is even more electrically stable than the original design with the coupling capacitors in the retaining clips provided.
We spare you the "deep dive" for this...
Readings
and stress tests
Yay, we're finally at the point where the V72 is running reliably and stably again and we can start measuring what it's capable of.
What would we give for a professional audio test system like an Audio Precision. Unfortunately, these are extremely expensive, but they are very high on our "must have" list. If anyone has one lying around and wants to get rid of it, please contact us via eMail.
Until then, we'll do it old-school with a signal generator, scope and Excel list.
We feed a -20dBV sinusoidal signal (100mVpp) into the input of the V72 and have an oscilloscope at the output with a dummy load of 10kOhm. And then we patiently sweep by hand from 20Hz to 20KHz and note the values.
Result:
-
In the frequency range from 50Hz - 20,000KHz we move within a tolerance of +/-1dB. That's very good for a 66 year old device!
-
The data sheet says the V72 has a fixed gain of 34dB. The measured gain of our V72 over the frequency range is 34.4 dB on average. Spot on!
Better living
WD40 and glass cleaner are our friends
Now that the inside of the V72 is tip-top again, we reassemble the "Danner cassette". Over time, a little rust has formed on the outside. Here finally WD40 has its grand entrance! When treating old metal surfaces, the stuff works wonders in terms of cleaning and also leaves behind a thin film that protects against further corrosion.
Cleaning the front panel with glass cleaner shows that a decent layer of deposits has formed here over the years, which can only be removed with patience and several cleaning cycles. Our guess: In the 60s, 70s and 80s it was quite common to smoke at work, and back then there was a lot of smoking... probably also in the IRT. In any case, the deposits (which we are cleaning up little by little) look very much like a decent layer of nicotine.
Yes, there are also more aggressive cleaning agents with which something like this might go faster. But if you don't want to explain to your customer that the new miracle cleaning agent has unfortunately also removed the paintwork on your device, you're better off sticking with very mild cleaning agents such as Windex or similar.
Sound of the V72
some thoughts
Over the years, the V72 has become an icon that is said to have a particularly desirable sound.
Why actually? The preamp was actually designed to be as neutral and linear as possible. Actually, it shouldn't have a "unique" sound.
You have to keep in mind that the V72 has a very high fixed gain of 34dB. With this you can very quickly reach the point where the V72 is no longer linear with close miking with common microphones, but starts to clip asymmetrically.
Slight asymmetrical clipping (especially with tubes) is perceived by many as an extremely good-sounding tone coloration.
Of course, at the input of the following device, the high output level of the V72 is reduced again, but by then it has already irretrievably left its mark on the audio signal. Whether linear or not, it seems to sound very cool!
But that's just my theory.
Prof. Nestler would certainly have a different opinion on this... ;-)
