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...
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.
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.