Tweaking the negative voltage on fixed bias power tube

[kitspeller]

Hi all,

New member, first post! 

I am currently getting into the world of tube amps and am loving it so far.  I'm building a Princeton Reverb clone and have a question:

On the power tubes (2x 6V6) the negative voltage to Pin 5 on the octal socket is supposed to be around -40V according to the manual. My reading is showing -24V and the manual says turn off immediately if it's this high, recheck wiring on bias board etc.  Well I've done all of that several times, checked for cold solders etc. and printed out a schematic and crossed everything off but it is still showing -24V.  The voltage to the Pin 4 screen grid is 391V, manual says 386V.

So how do I adjust this?  Perhaps this is a painfully obvious question to those experienced in this field.  As it is a fixed bias I'm wondering do I change some of the resistors in the bias board?  For context it is the Mojotone Princeton Reverb clone, I've attached a picture of how it is supposed to be wired and my build follows this exactly with same resistor values etc.

Power tubes are x2 JJ electronic 6V6S.

I can't find an answer to this question anywhere, but my research in finding an answer has taught me a lot about how these things work so....could be worse

Thanks,
Kit

[pdf64]

At the little bias supply circuit board, are you sure that the electrolytic capacitor is the right way around (positive end to chassis 0V) and properly soldered?

[kitspeller]

I did take it all apart last night to check and everything seemed fine.  The cap is definitely the correct way around

[pdf64]

Maybe the cap is bad, what is the VAC across it?
Also check the bias VDC at the bias board.
At the tube socket pin 5, make sure to set the trem intensity control to minimum.
Make sure not to plug the 6V6 in until you get the bias right.

[sluckey]

Surprise!!! Another pretty mojo drawing with errors. The number just keeps on growing.

First, replace that 2.2K resistor with the correct value of 27K like the original Fender schematic. This is probably all you have to do. But if you need to tweak the negative bias voltage even more, then decrease the 100K to increase the voltage at pin 5 of the 6V6s, or increase the 100K to decrease the voltage at pin 5 of the 6V6s.

[2deaf]

What resistor do you have in there for the 2.2K resistor shown on the layout?  A 2.2K resistor would only give you single-digit bias voltage and you're showing double-digit.

Here is a table with a 100K dropping resistor.

[kitspeller]

There is some great help here thanks everyone.

I will check the VAC on the cap and VDC on the board this evening. 

The resistor that straddles the cap is actually a 22k, so I can swap that for a 27K as mentioned.  Perhaps if this is successful that's all I need?

[sluckey]

Replacing the 22K with a 27K is a move in the right direction. If that still doesn't get you where you want to be then change the 100K until you are happy. Just remember, bigger = less voltage and smaller = more voltage.

[kitspeller]

UPDATE:  I switched out the 22k resistor to a 27K half watt resistor.  It decreased the negative voltage to about -30V which I think will be ok.  However, it has increased the voltage to pin 3 of the power tubes so they're running at about 425V, which seems about 40V too high.  Is this way too much?  I assume the fix is going to be putting in a bigger resistor in place of the 100K resistor as previously mentioned but just seeing if this is necessary before I actually do it.

Also, the voltage over the cap on the bias board was about -36V and the rectifier tube was reading about 340VAC which is about 12V high.  Thanks

[sluckey]

When you increase the negative voltage on the grid, the tube conducts less and the plate voltage will increase. This is all normal.