Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Cirrus on July 03, 2022, 04:08:15 am

Title: AC30 cathode bias scheme - how much does it matter?
Post by: Cirrus on July 03, 2022, 04:08:15 am

Hi, first post here!

I've got two Vox AC30CCs.

One's a 1x12, and last week I replaced the 60r common cathode resistor/ 220uf cap with two 120r/100uf - one for each P/P pair, so I could run it with 2 el84s at 15 watts (running a 16 ohm alnico blue into the 8 ohm tap to get 8k on the primaries, and subbing in a 5y3 rectifier to counter the voltage increase of pulling 2 tubes).

At practice it was perfect volume running at the edge of clean sweet spot.

I'm thinking about doing the same to my 2x12 combo, but having it on a switch so I can set it to half power to take the edge off at gigs. Might switch off one speaker too, in an open back combo I'm not too worried about it becoming a passive radiator.

It got me to wondering, though - does the biasing scheme make any real difference in tone/ response from the power section?

I imagine 4 individual cap/resistors would make a difference, because then each side of the P/P signal would be free to bias shift independently. But two shared resistors vs the usual common one for all four? The bias is still being shared across the two P/P pairs in that case. I'd appreciate any experience or thoughts, before I pull the amp to bits and break out the soldering iron!  :icon_biggrin:

Title: Re: AC30 cathode bias scheme - how much does it matter?
Post by: shooter on July 03, 2022, 05:47:41 am

Rk alone, if biased close to the original shouldn't have much, if any, tonal effects.
IF you change Rk enough to change the bias point from original, then maybe


Ck will play some roll in effecting tone, especially if you use 4 different values, what that sounds like, only your ears and lots of swaps will tell.


I did a quad SE, gave each tube it's own Rk, even messed with each enough to get a close "match" on all 4 tubes.
It was a HiFi build so I kept the caps small but equal.  sounded very good.

Title: Re: AC30 cathode bias scheme - how much does it matter?
Post by: pdf64 on July 03, 2022, 08:43:45 am

My understanding is that sharing between push pull pairs should be fine. Merlin advises that can add 2nd harmonics, though I’ve not been able to work out why that might occur theoretically, or to verify it practically.
I’ve tried individual R//C for each EL84, and I don’t notice any degradation in tone  :dontknow:

http://www.valvewizard.co.uk/pp.html