[Drache713]

Anybody know how to adjust the bias on a Peavey Ultra Plus? I know they are "fixed bias" amps, but I want to adjust the bias to get the tubes running right. I haven't opened up the amp yet to look, but do they have an adjustable trim pot in them or do I need to modify a resistor to change the bias, like the 5150 Bias Mod?

[WarriorOfMetal]

"fixed bias" usually means that there's no trim pot

[zimbloth]

Matt just call up Bob @ Eurotubes and ask. Any 'fixed-bias' amp can be converted to adjustable bias. Look over his website too, I believe there's an article about this.

[Drache713]

Originally Posted by zimbloth

Matt just call up Bob @ Eurotubes and ask. Any 'fixed-bias' amp can be converted to adjustable bias. Look over his website too, I believe there's an article about this.

I haven't called him yet but I looked on his site and watched the bias video. I also read instructions and schematics for how to do the 5150 Bias mod, so I know how to do it. I just have to find out where the resistor I need to replace is.

[Drache713]

Hmmm well so much for that, he said it would take too long to explain in e-mails or over the phone. So...anybody know how to mod an Ultra Plus to adjustable bias?

[Emiliano]

i'm working on the same head and i can give you some advice, maybe :P

the ugly part is that the resistor that you need to change to tune the bias is under the power amp pcb, so you have to unsolder teh tube socket from the pcb to get under the pcb, or dremel away the nuts that keep the tube socket attached to the metal case


a very long and hard work!


if i remember right you can ask for tuned quads of tube from bob at euro tubes tuned specifically to drain more or less current, altering the bias to some extent, he talked to me about this chance for my 20/20 mesa power amp, is fizxed bias but with a set of HOT tubes you can warm it up!!!


i hope this helped you!

do you have the schematics? i just asked at the peavey site and they emailed them, try to take a look!

[Drache713]

Originally Posted by Emiliano

i'm working on the same head and i can give you some advice, maybe :P

the ugly part is that the resistor that you need to change to tune the bias is under the power amp pcb, so you have to unsolder teh tube socket from the pcb to get under the pcb, or dremel away the nuts that keep the tube socket attached to the metal case


a very long and hard work!


if i remember right you can ask for tuned quads of tube from bob at euro tubes tuned specifically to drain more or less current, altering the bias to some extent, he talked to me about this chance for my 20/20 mesa power amp, is fizxed bias but with a set of HOT tubes you can warm it up!!!


i hope this helped you!

do you have the schematics? i just asked at the peavey site and they emailed them, try to take a look!

Yeah I just got the schematic from Peavey also, and looking at the schematic I'm almost certain that R99 is the bias resistor that needs to be replaced with a potentiometer, ala the 5150 bias mod. Then it's just a matter of locating the damn thing, which if what you say is true about where it is located then that may be more of a pain in the ass than I'd be willing to go through.

[Emiliano]

R99 is in the preamp section, between the first and the second tube ( counting from above ) it is on page four of the schem, however

if you are talking about power tube bias i find strange that it's located on the preamp board, then
looking at the schem for the power section, it seems that every tube has its own set of resistor to make 'em work

but

following every jumper number it seems to turn back to that resistor for bias, but i find it very strange, and if what i saw is correct
you're changing power tube bias along with preamp tube bias

please, double check! i'm just awake!

[LordOVchaoS]

anybody have a .pdf file of the schematic they could email me?

[Drache713]

Originally Posted by Emiliano

R99 is in the preamp section, between the first and the second tube ( counting from above ) it is on page four of the schem, however

if you are talking about power tube bias i find strange that it's located on the preamp board, then
looking at the schem for the power section, it seems that every tube has its own set of resistor to make 'em work

but

following every jumper number it seems to turn back to that resistor for bias, but i find it very strange, and if what i saw is correct
you're changing power tube bias along with preamp tube bias

please, double check! i'm just awake!

Well I asked Bob @ Eurotubes and he said that yes R99 is the one that you can replace with a 25K ohm resistor in series with a 25K ohm pot, so that should be the one. I'm pretty sure I found where it is too on the schematics. I don't know where you get that it is located between between the first and second tube though (if you're talking about being located between the preamp tubes). On the bottom board on page 4 there are a bunch of cylinder shaped capacitors on the right side, and to the left of those are capacitors C64, C65, and C66. R99 appears to be located directly inbetween C65 and C66 towards the top, the resistor below it appears to be R98. Why would it be strange that it's on the preamp board? The resistor to do the 5150 Bias Mod on 5150's is located on the preamp board of that amp as well.

LordovChaos, I sent you the PDF to your e-mail.