help with bias voltage doubler for peavey butcher?

[xjbear]

Playing around and using a peavey butcher to learn some things. So far ive added a filament tranny to the power tube board and set it up for 6550's and everything is good BUT I do not have enough bias on tap. Im measuring -55 which is giving me about 85 ma @ 500v.
Does anybody have some practical knowledge for getting this boosted up where I need it? or some suggestions of where to pull off the power board and hell just tell me what to do. And no im not gonna backtrack im going to complete this hopefully with some help

Ill try and post the butcher schematic below

peavey-butcher.pdf

[g1]

Have you tried adjusting the values of R52 or R53?
Reducing R52 or increasing R53 should get you more voltage. You could also put a pot in series with R53.

[xjbear]

All that can be done with r53 and r52 is drop it less negative and warm up the bias.. Really cant squeeze any extra out of whats there unless im missing something. Ive culled thru every post that has pertained to the butcher already so im thinking vl doubler.

[nashvillebill]

Isn't something like -30 volts typical for a 6550/KT88 running 500 volts on the plates? -55 volts seems mighty far out.

[g1]

All that can be done with r53 and r52 is drop it less negative and warm up the bias.. Really cant squeeze any extra out of whats there unless im missing something.

No you have that backwards. R53 is loading the voltage supply, so decreasing it's value or adding a pot in series will lighten the load on the supply and you will have more negative voltage. Likewise, R52 is in series with the supply and dropping voltage across it. Decreasing the value of R52 will result in more negative voltage at R53.

[g1]

Isn't something like -30 volts typical for a 6550/KT88 running 500 volts on the plates? -55 volts seems mighty far out.

The schematic shows -56V, but I am wondering if that 85mA was for a single tube or a pair?

[xjbear]

The schematic shows -56V, but I am wondering if that 85mA was for a single tube or a pair?

-55-56 is all its making. Even if I pulled resistors and jumped straight wire thats all id get. That feeds grids on all 4 tubes so I need moar

[Jazz P Bass]

My personal feeling is that the -55Vdc bias will be fine.

My main concern would be the heater winding.

As the 6L6 has a current draw of 0.9 A's & the 6550 will pull 1.6 A's per tube, you may want to call Peavey & verify that the transformer can supply that much current.

[xjbear]

In my first post I described that ive already installed and filament tranny 115/6.3v/8amp for the 6550's. The stock power tranny is handling pre tubes only. -55 is certainly not enough for the 4 6550's, with a bias rite im getting 500v and 85ma biascurrent which it should be around 40ma.

[xjbear]

Ive had to do this before with a Sound City 50 just to run modern el34's safely. That was easy but this power board has a ground to the heater and a reference off the b+ into a transistor or fet maybe to regulate it. I dont know anything about design just repair

[Jazz P Bass]

Pop in a bias tranny.

[xjbear]

[g1]

-55-56 is all its making. Even if I pulled resistors and jumped straight wire thats all id get.

"Even if" sounds like you did not try it and are not willing to, or am I reading that wrong?

[xjbear]

It wont. 470 ohm is stock. It would probably get it to -60 but I doubt it. Its gonna have a doubler

[nickb]

It wont. 470 ohm is stock. It would probably get it to -60 but I doubt it. Its gonna have a doubler

If you really want to have a doubler, something like this should do it. But it will be tricky to do it neatly on the power PCB, especially as the 220uF 160V are quite large. Be careful as I see Peavey used the "CR6" designation twice on the same PCB.

EDIT: As noted later, I put polarity indication on the wrong side of C1 on the attachment below - thx Loudthud I didn't change it as that might have caused confusion.