Do I need to BIAS my Peavey Classic 30?

[alex_toney]

Hi everyone, I just bought a new JJ re-tube kit from www.eurotubes.com for my Peavey Classic 30. I am just wondering (because there seems to be mixed feelings about this on here) whether I need to bias it or not? Can anyone help me out?

[mds]

One of the many cool mods for the Classic 30 is an adjustable bias pot...running the amp a little hotter opens it up quite a bit. Do a search for Peavey Classic 30 mods on google and you'll find TONS of info.

[alotas123]

As I undertand it, the answer is technically "yes" but Bob selects and matches sets of tubes that will work correctly with the factory bias settings on a Peavey. You can select ones that run hotter or colder and he will furnish those. If the stock settings have been modified then of course you can't count on the same results.

[ruger9]

Official word from Peavey is "no: plug & play."

I've had a C50 for 16 years now, not problem one with buying new tubes & "plugging & playing."

These amps have a fixed bias. Yes, over time that fixed resistor (or whatever it is) can apparently drift off-value, and that would need to be remedied. I'm not an amp tech, just telling you what the official word from Peavey is. They did BUILD the amp! So if anyone disagrees, take it up with Peavey, not me. Thanks.

[GuitarsFromMars]

I believe the amp is cathode biased,so technically,the correct answer would be-yes,take it to the tech.If it needs a certain bias resistor replaced you aren't going to be able to do it,much less bias the tubes.If the amp is less than a couple years old,I would guess(and notice I said guess)that you could do it without problem.If it fails,you're going to the tech anyway.

[wixedmords]

I believe the amp is fixed bias, non-adjustable. Meaning it needs to be set to a certain bias, but you have no way to do that unless you have a bias pot installed or change a resistor. Cold biased tubes would be the ones to specify when buying or specify the amp model.

You should check the bias, as you should with any amp. You are blind to what the amp is doing otherwise.

I had what I thought was a cathode biased amp measure a really high bias on me once and if I hadn't been looking at the bias meter it wouldn't have been a good thing.

Biasing isn't hard. If one wants to use tube amps. One should learn how to bias.

[John Phillips]

It's a non-adjustable fixed-bias amp as wixedmords said. In order to re-bias it, you would have to change a fixed resistor or fit a trimmer pot - and it's really not an easy amp to work on.

Given that Peavey designed it sensibly and conservatively to not run the tubes too hot, chances are you won't have to rebias it for most sets of tubes, so I would leave it alone unless there's clear evidence it's cooking them.

It will probably kill them through vibration long before they wear out in actual use anyway :).

[alex_toney]

Ok! Thanks for all the help guys, I'll probably leave it for now and then check it out again next time I re-tube again.

[phsyconoodler]

It is indeed fixed bias.If you look at the schematic it shows the bias supply of about -14v.A bias pot is a good idea.Then you can use different types of EL84's and bias them accordingly.
But.....the classic 30 is a pain to work on.