Replacement Valves AC30

[Mark Burton]

Hi All

I was going to email Amanda off line but thought others may benefit from this question.

My AC30 HH has a lot of hum on the TB channel so I'm considering changing the pre-amp valves. Then I thought - well maybe I should carry a complete spare set of valves when gigging, as no doubt at some time somewhere, a value will need replacing !

That set me thinking about output valves (EL84's) and biasing. Am I right in thinking the AC30 is a cathode biased amp and therefore requires no bias changes when installing new EL84's - is it simply a plug and play scenario ?

I know other amps need a visit to a valve whizz to be correctly biased when changing output valves.

think I've confused myself

Mark.

[Charlie Hall]

Hi Mark,
You are right in that the cathode biasing allows the circuit to adjust itself and there is no way of biasing other than to change the cathode resistor which isn't normally done. The current can still vary somewhat depending on the valves themselves. It is essential to have a matched set of output valves in these amps because the one cathode resistor has to control all 4 output valves.
I would suggest Watford Valves for replacement output sets as they can supply cooler running sets for hot running amps like the AC30.
The hum problem could be the preamp valves but it could equally be something else. Trying different valves in the top boost channel is the only way to find out.
Output valves can also hum if there is a mismatch but this would affect all channels and would be heard even if the volume controls are at zero.
Regards,
Charlie