VOX AC30 mods

[Valvehead]

Anyone have any tweaks they like to do to the bias section of these amps ?

They sure run "on the edge" .....

[trevorus]

The AC30CC series have switchable bias resistors that allow you to run it a little cooler. You could probably easily clone that into yours

[Valvehead]

what value do those use ? cant find a scem.

[Pedro Vecino]

82 Ohms/10 watts + 220uF
150 Ohms/5 watts paralleled (53 Ohms total)

Regards

[Robert M. Martinelli]

AC30s are already biased very hot!

Hi,
My 1964 AC30/6 TB had a 50 Ohm/5W as a cathode bias resistor, I changed it to a couple of 100 Ohm/5W paralleled only because the original resistor, probably due to its age, was out of spec; I would not recommend to tweak the bias on a "made in England" AC30 ( the only REAL ones ) due to the bias being very hot already; the quiescent plate dissipation is already "borderline", the amp is biased pretty hot; someone would like to bring it to work in pure class "A" ( the AC30 normally works in class "AB1" ) but to do so the cathode resistor should be lowered to 27 Ohm, and it would then be necessary to go way down with the HT voltages to bring back the tubes into their SOA ( 6BQ5s / EL84 have only a max plate dissipation of 12 W ), losing power and efficiency, and since crossover distortion is not an issue, ( at least not when the bias is so hot ) why bother? There are a lot of terrible-sounding amps out there waiting to be modded, but ( IMHO ) I can' t see the point in modifying something that stood the test of time so well....
Best regards
Bob

[Bruce / Mission Amps]

The idle current of a power tube doesn't set the class operation of a power stage.

[Robert M. Martinelli]

Hi,
well....the class of operation of a final stage can be determined looking at the angle the output devices ( I' m not necessarily talking about tubes ) are conducting, so, if they' re on all the time ( 360 degrees ) we talk about class "A" and one device can suffice, if they' re on half of the time ( 180 degrees ) we talk about class "B", and at least two devices are needed; since the devices do not switch on and off instantaneously, we keep 'em on for more than half of the time ( more than 180 degrees ) to get rid of cross-over distortion.....and we are in the "AB1" class realm.

The way we do so is biasing the tubes ( cathode bias or grid bias, but the purpose is still the same ), so, IMHO, it is indeed the amount of quiescent current that changes the class of operation, for example from class "B" ( WITH cross-over distortion ) to class "AB1" ( without cross-over distortion ). If the quiescent current is zero in a push-pull stage then it will operate in class "B", and we will have cross-over distortion, if it is more than zero it' ll be "AB1" because the tube will be on already when the zero ( which, angle-wise, is found at 0 and 180 degrees ) is crossed, eliminating the cross-over distortion.

I found an excellent article on this subject on Randall Aiken' s ( Aikenamps ) site, in which he analyzes the AC30 real class of operation, with simulations, voltages, currents and pictures of the waveshapes, and he also discusses about how to bring AC30 push pull to work in pure class "A". I encourage everyone to read this article because it is very interesting and the matter is clearly explained.

( If I' m wrong about what I stated above, please someone enlighten me )

Peace

Best regards

Bob