Restoration
1965 VOX AC50 MKIII
This 1965 VOX AC50 came to us via ebay as "not working / defective". For the most part in usable, but battered condition. And for some reason the output transformer was lost...
Age assessment
the first thing on the "To Do" list was determining the exact age.
At first one is tempted to use the burnt-in pencil inscription "23.1.69" near the output transformer for dating. However, this quickly turns out to be "unlikely", since AC50's were only produced under the JMI label until 1967 and later versions also have a different mix of components.
Interestingly, there is an identical label on the cathode resistances of the power tubes, which have apparently been changed from 47 ohms to 10 ohms in the past. Probably on 1/23/69... ;-)
So our AC50 must have been built well before 1969 and this is a very interesting example indeed. In 1965 VOX changed the production of the AC50 from the MKII to the MKIII version, which differed mainly in that instead of the GZ34 rectifier tube, a diode rectifier with Mullard diodes was used. At the same time, a somewhat more complex bias system was introduced, with which the quiescent current can be set separately for both power tubes.
We already find the rectifier diodes and the new bias circuit in our amp, but the chassis still has the cutout for the GZ34 rectifier tube. So it must be a very early AC50 MKIII from 1965 in which the existing chassis of the MKII version were used up.
Housing
Magic potions and what you should "NEVER" do.
The housing itself was in usable condition, but the old tolex was very dirty and someone had written "Bass Objects" on the back.
After a lot of trial and error, for me, Meguiar's Convertible Top Cleaner turned out to be a wonder-weapon for worn-out tolex. This has also proven itself with the AC50. Spray on... leave to soak in for a short time... scrub well with a brush that is not too soft... rub off with a terry towel. Hey voilà!
The removal of the extremely stubborn and dried lettering on the back turned out to be a real Sisyphean task in which the paint had to be removed piece by piece with a scratching needle. Unfortunately, this also affected the tolex and you should really consider whether you really want to remove such old inscriptions or whether it is better to live with it.
Power supply & Power amplifier
what do we do with the missing "Brimistor"?
With a vintage amp it is important to keep as much of the original amp as possible.
When it comes to safety-relevant components, however, one must not make any compromises...
The power supply mainly had old "Hunts" capacitors which surprisingly still had good measured values in terms of capacitance and ESR. Nevertheless, these capacitors were exchanged for new JJ types.
The complete BIAS circuit for the power tubes has been refreshed with new capacitors from F&T.
The power amp's grid stopper, grid leak and screen grid resistors have been replaced with modern metal film resistors with oversized power and high heat resistance.
The failure of one of these components would lead to serious damage to the amplifier, in which case the mains transformer and/or output transformer and/or power amp tubes could be destroyed. Original condition or not, at this point it is better not to experiment with components that are more than 50 years old.
The 10 ohm cathode resistors on the EL34 (which, according to the label in the chassis, someone had installed on January 23, 1969 instead of the intended 47 ohms) were left as they make measuring the quiescent current easier and were still "spot on" in terms of their values.
The "Brimistor"... a tip for anyone who is restoring an AC50 MKIII:
The nice thing about the previously used GZ34 rectifier tube is that it powers up slowly. This means that the remaining tubes have enough time to heat up and become conductive. Therefore, you can do without a standby switch.
With the diode rectification of the MKIII, the high voltage is there immediately, even before the remaining tubes conduct. This means that the high voltage shoots well over 500V when switching on and only levels out at approx. 450V after the tubes have heated up. This is very dangerous for the power supply capacitors, which normally have a dielectric strength of 500V!
Since VOX obviously did not want to change the chassis and the front panel with a standby switch, a so-called "brimistor" was integrated into the power supply. In this case, a brimistor acts as a kind of automatic standby switch in which the high voltage is only switched through with a time delay, after the internal heating element has heated up.
Of course, the brimistor was no longer present in this AC50 and these components are virtually no longer available these days. A good trick to ensure that the AC50 does not over-shoot the power supply capacitors when switching on is to insert a load resistor to ground directly after the rectifier diodes. With this the transformer sees a certain load even with unheated tubes and the B+ voltage does not shoot up uncontrollably. The insertion of a 100 kOhm / 5W resistor has ensured that the B+ voltage is not higher than 475V immediately after switching on and thus remains within the dielectric strength of the power supply capacitors. Caution: During operation, 2W are wasted in this resistor. It should therefore be placed in such a way that it gets enough cooling and does not heat up other components.
Output transformer
our "problem child".
The original output transformer of the AC50 was very high and had a lot of metal (= lamination runes). This made the amp extremely attractive, especially for bass applications, since the AC50 only saturates very late, even at low frequencies. It's a pity that this transformer was missing in the present amp.
Unfortunately, there are almost no replacements for this transformer nowadays.
Only Mercury Magnetics still offers replicas of the original transformer, but in Germany at a price that outweighs the purchase price of this amp.
In the AC50, an output transformer was used that corresponds to many common amps with 2xEL34, for example a Marshall 2204. This works and sounds excellent and speaker impedances of 4-16 ohms can be mapped. If you want to use the AC50 specifically as a bass amplifier, you should consider investing in the replacement from Mercury Magnetics in order to achieve maximum performance in the bass range.
Preamp
renovation work... and "did you really want to do it that way?".
In the preliminary stage, the approach was to keep as much of the original circuit as possible and only replace faulty components or correct inadequacies in the original circuit.
Despite their age, all cathode electrolytic capacitors still had good values and were left in the circuit. While this is a risk, these are not safety-critical components.
An old carbon film anode resistor was noisy and got exchanged for a metal film type.
The input sockets were heavily corroded and no longer shorted to ground when unplugged. These have been completely renewed.
The 500pF Lemco coupling capacitor of the "Brilliant Channel" was finally exchanged for a NOS Mullard Mustard type with 2.2nF. 500pF is simply too low at this point and cuts off too much bass to be able to use the channel with a guitar in a meaningful way.
From the factory, the wiring of the tone controls was reversed from the normal standard. The treble and bass controls had their maximum position at "0" and their minimum position at "10". The reason for this remains a mystery and inexplicable to me, since due to the linear 500k potentiometers, these can also be wired the other way round just as well. A comparison with pictures of other AC50's on the internet has shown that the "reversed" wiring was probably no exception, but rather the rule.
So the wiring was corrected to a standard that normal people are used to. "0" = minimum.... "10" = maximum.
And since the potentiometers were already unsoldered anyway, I immediately took the opportunity to remove particularly dirty specimens, open them and thoroughly clean them from the inside. This saved 2 original pots (which still showed quirks even after the usual DeoxIT cure) from going into the "Box of Shame" and being replaced.
Tubes
Puzzling and playing Tetris.
The AC50 contained 2 EL34s labeled "Ultron" and in the preamp 2 Mullard ECC83, 1 Raytheon ECC83 and 1 ECC82 where the manufacturer was no longer recognizable.
One of the two Ultron EL34 had become weak over the years. Thus, both power tubes were exchanged for a well-matched pair of NOS Siemens EL34, which were adjusted with 40mA quiescent current to a plate dissipation of approx. 17.5W. Here we are pretty much on the 70% of the maximum plate dissipation intended for EL34s in push-pull with fixed bias.
Be careful with the height of the tubes! Many new EL34 are too tall and don't fit into the tightly dimensioned housing.
Many thanks also go to Stephan Mayer from the Tube Amp Doctor in Worms for the exchange regarding the VOX AC50. If you don't have a NOS pair of Siemens EL34s at hand (like I do), this should be a good replacement that also fits into the AC50 in terms of height:TAD EL34L-Cz
Both Mullards and the unlabelled ECC82 were still in good condition, had excellent readings and will also find their home in the AC50 in the future.
Unfortunately, the Raytheon ECC83 (12AX7) had very poor measured values and was exchanged for a new TAD ECC83 with very well matched systems as a phase inverter tube.
Conclusion
underdog in the shadow of the AC30?
The VOX AC50 is definitely a "legend" that tries to transport the classic sounds of the AC30 to a higher performance class. He does this excellently and flexes his muscles with very high anode voltage and 2x EL34, even if he doesn't quite achieve the fine detailing of a good AC30.
Things like vibrato/tremolo have been removed to create a "straight-forward" 50W amp. This is very good for the AC50.
A great 50W/EL34 amp in which only the "normal" channel is really fun in its original condition.
With the small modifications (described above), the Brilliant channel also becomes a real weapon and one is torn into which channel one wants to plug into.