vintagevoltage351

Last week arrived a new (warm) quad of RFT el34's from Sanborn!!!
Checked for maximum -voltage at pin 5(-45v) and installed them. The lowest reading i got from the 1 ohm resistors is 43mv, plate 462v.
Too high.
I removed them immediately and put in some near new Svetlanas that biased correctly so i can muck around with.

Regarding the bias resistors, there are two. From the bias pot, there is a 47k and there is a 15k(which was replaced at some stage).
I clipped my ohm wheel to the 15k resistor that was replaced and managed to reduce it by 10mv by setting it to 4.7k (-50v at pin 5)
Clipping it on the 47k directly next to the bias pot, with the wheel on 47k sent the bias through the roof, so i immediately disconnected it.

Fitted the RFT's and they now have a usable range when i clip 4.7k to the 15k resistor, so i am considering piggybacking that one with a resistor i have.

Am i correct in this?

Or should i replace the stock 47k with a much higher resistor.
Im trying to distinguish between the two resistors and what they do. Obviously they both play a part in keeping things within range, but i would rather piggyback the non standard resistor fitted(15k)

I spent all night reading the forum and i guess im on the right track.

Can someone confirm this for me please?

Cheers

Jim

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1977 JMP 100w Superlead, 1960b quad, Strauss quad, VS100 combo & ext cab, MS4 micro stack
00 Gibson SG Angus Young, 96 Gibson Les Paul Smartwood Standard
EKO Wah, JD Crybaby, Fuzz pedal, BB2
DIY SHO, LPB1 and Eternity Burst
Riviera strat copy (for bludgeoning intruders)

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Kunnz

You are playing about with a voltage division network on the bias circuit, and yes you can piggy back resistors.
Please, can you tell me what kind of amp you have and provide a schem? then we can nail it.

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plexifier

I would leave the 15k alone if it's between the bias filter caps. It balances voltage between those caps.

It would be better to increase the 47k connected to the bias pot. 56k should be enough, otherwise 68k.

You could also add a resistor in parallel (piggyback) with the first bias resistor before the diode. That may be easier.

vintagevoltage351

Thanks guys!
It is a 77, 1959 JMP Kunnz.

The yellow resistor is the one that put me back in range. Looking at it closer now, i think they were both changed at some point...
Come to think of it, i might as well replace a resistor and do the bias caps while I'm at it.

So, the first bias resistor is the 27k between the bias caps and the diodes on the right side? (printed on board 27k/220k)

__________________
1977 JMP 100w Superlead, 1960b quad, Strauss quad, VS100 combo & ext cab, MS4 micro stack
00 Gibson SG Angus Young, 96 Gibson Les Paul Smartwood Standard
EKO Wah, JD Crybaby, Fuzz pedal, BB2
DIY SHO, LPB1 and Eternity Burst
Riviera strat copy (for bludgeoning intruders)

plexifier

Yes. It won't require much reduction in total resistance to get the bias in range (probably between 150k and 270k for a parallel resistor there).

vintagevoltage351

Thanks Plexifier! I would be devastated if i blew the RFT's somehow.

150k paralleled dropped it by 10mv.

I think piggybacking is a good idea, saves lifting the board if i change different tubes again.

__________________
1977 JMP 100w Superlead, 1960b quad, Strauss quad, VS100 combo & ext cab, MS4 micro stack
00 Gibson SG Angus Young, 96 Gibson Les Paul Smartwood Standard
EKO Wah, JD Crybaby, Fuzz pedal, BB2
DIY SHO, LPB1 and Eternity Burst
Riviera strat copy (for bludgeoning intruders)