Peavey Classic 30 bias adjustment

[Philr228]

Hi All,

Firstly, can anyone tell me, with conviction, whether the classic 30 is Class A or Class AB?

Also, generally if it is Class AB it would be sensible to adjust the bias when replacing the output valves. The original circuit design uses fixed bias, but I'm considering changing this to Steve Ahola's adjustable circuit and maybe adding some 1 Ohm resistos to the cathode grounds where there are some jumpers on the board.

Has anyone done this and have you any current values that ran well with specific valve manufacturers glass.

I've had this amp for years and never bothered with this and changed valves - maybe it's not even worth doing so I thought I'd ask for your opinions.

Thanks

Phil

[tubeswell]

Hi All,

Firstly, can anyone tell me, with conviction, whether the classic 30 is Class A or Class AB?

AB1 I believe

Also, generally if it is Class AB it would be sensible to adjust the bias when replacing the output valves.

The stock C30 is set up at the factory with a nice cool bias, so that you can just chuck a new set of tubes in.

The original circuit design uses fixed bias, but I'm considering changing this to Steve Ahola's adjustable circuit and maybe adding some 1 Ohm resistos to the cathode grounds where there are some jumpers on the board.

Has anyone done this and have you any current values that ran well with specific valve manufacturers glass.

Not sure that I understand your question. Even putting a bias adjustment pot in still makes it fixed bias. The 1R from each cathode to ground is not going to change the effective 'zero sum' potential of the cathodes. Besides that, the values for the bias pot etc in Steve's mod are good for getting a decent bias adjustment range

[Enzo]

I suspect he wants 1 ohm resistors for taking readings across.


Many people have done the mods on Steve's Blue Guitar page.

It is not a trivial matter to insert 1 ohm resistors into this board.

[tubeswell]

It is not a trivial matter to insert 1 ohm resistors into this board.

Indeed not

[Philr228]

Thanks for your replies Tubeswell and Enzo

You're correct regarding my wanting to use 1 Ohm resistors to measure the cathode current across, but I'm happy too if the consensus is that Peavey's original design is sound and that bias adjustment is not necessary with the circuit as it is. If it is indeed biased "cool" then maybe I'd be happy to leave well alone.

I've had mine apart a number of times and I know the pain that the circuit board can be although if it were really to improve things I can do it!

Tubeswell, I was asking what bias current others may have found good if they had done a similar mod.

Cheers

Phil

[tubeswell]

Hi Phil

If you go to the article on Steve's site here,

http://www.blueguitar.org/new/articl...y/c30_mods.pdf

he states he measured the stock bias at 24.5mA per tube FWIW

[CharlieP]

My advice is to get a Bias Rite or similar and don't mess with the 1 ohm resistors. Much easier and safer for you and your amp in the long run. For what they cost they are well worth the assurance and safety.

[Philr228]

Charlie,

Thanx for that, seems like a nice product to have if you're doing it regularly but not just for a feew offs. Not sure also, where I would get hold of one in the UK.

Could probably make up a few adaptors with taps so I could measure cathode current, plate and screen volatges etc.

Thanks any way

Phil