Weird PI voltage

[pedro]

took voltage measurements tonight.

I have 100Ks on either side of the PI plates and 10k trim between them.
Found one side measuring 329v and the other 239v

swapped resistor on the "low" side and resoldered everything - same problem.

cant balance with trim pot - voltages are just too far off each other.

what could be causing this do you think ??

TIA
Pedro

[Normster]

What is the voltage with the PI tube removed? No real experience with this problem, but it appears that it could be caused by either a bad tube or a shorted coupling cap.

[Funkalicousgroove]

have you tried a different tube? some can be terribly mismatched, also it works best to put a little higher (110K?) on one side, and a little lower(92K) on the other. the trim doesn't balance the voltage, it balances the sine wave on a scope. you have to plug your amp into a dummy load and put a scope across the output. Put a 1Khz sine wave in the front and use the trimmer to balance the sinewave on either side of Zero for max output, or unbalance it to your liking.

[glasman]

Pedro, Take a look at the voltage at the grids (1 meg resistors). This should be a -45 to - 60 volts with respect to ground. If there is a difference (they should be virtually the same), this is where the problem lies. Check the wiring around your coupling caps as well as the caps themselves. Be sure to check the 1 meg resistors as well.

This may well be the source of your popping problems that you were experiencing.

Gary

[pedro]

just replaced the 1megs for 2 matched 1 meg resistors.
replaced the .022 coupling cap as well.

still same issue but not as bad - low side now 263 and high side 344

getting +VE 40v and 44v on the grids not -40v

I'll check everything again.

thanks for the tips - let me know if you can think of anythinmg else
cheers
Pete

[glasman]

have you tried a different tube? some can be terribly mismatched, also it works best to put a little higher (110K?) on one side, and a little lower(92K) on the other. the trim doesn't balance the voltage, it balances the sine wave on a scope. you have to plug your amp into a dummy load and put a scope across the output. Put a 1Khz sine wave in the front and use the trimmer to balance the sinewave on either side of Zero for max output, or unbalance it to your liking.

The problem is that he is seeing almost 100 volts difference in the plates. This tells me that the side with 239 volts will have a grid voltage closer to zero than that of the other side. 300 to 320 is within the norm given a plate supply (V1) of 450 - 480 v.

You are correct in saying that you must use a scope. It is the only way (unless you have dogears )

Gary

[Funkalicousgroove]

If one side of a twin triode is way hotter than the other it could account for a partial voltage difference, a mis matched resistor could account for part, could even(doubt it though)be a parasitic oscillation. I would be interested to know how each half is biased, as well as what the amp does with a different tube in the PI.

[pedro]

GUYS - HELP - this is driving me nuts.... spent all Tuesday night on this and have been working away in London until tonight so trying everything in a bid to get the amp ready to gig with on Saturday night.

I replaced the coupling 0.1uF caps to the output tubes. I replaced the driver circuit 1 megs, 820 and 24K tail resistor.

Ive tried several different PI tubes.
Always the same - one side higher than the other by maybe 30-40v.
with the tube out of the socket I get 432 on both plate pins so wiring either side looks OK.

I replaced the 100k plate resistors after measuring the voltage drop across each one and working out the current draw through each .
I tried to achieve same voltage drop across each one by replacing right hand side plate load from 100K to a 75K - this improved matters but I still have 292v on right hand side as you look at the board and 334v on the left.
As far as the "tail of the PI is concerned Im seeing the following with the 100K/75K plate combination ( left/right respectively ).
- Left grid to ground 45v,
- right grid to ground 57v ( input cap side )
- PI cathode 78v
- junction of the grid 1Megs, 820 cathiode resistor and 24K tail resistor I get 76v

attached is a shot of current layout situation - forgive the mess while I rig up various tryouts. Intend to tidy up once I have this nailed. I WANT TO BALANCE THIS DRIVER !!!

any ideas - or can someone talk me through the measurements in a systematic fashion.

Heres hoping ------ Pete

[pedro]

ooops - heres the pic

[dogears]

Maybe there is a problem with the PI tube socket.....

[pedro]

redone the PI wiring after i noticed that one plate wired to opposite grid !

had to rewire output tx and feedback wire to stop the inevitable oscillation when I corrected the PI plate wiring with respect to grid. now I have 290v left hand side and 322v right hand side with the 2 original 100K plate resistors back in place.

Do ya think this is close enough fellas ?

I guess if i now go 110K one side and lower on the other I might get it properly DC balanced ? too tired to contemplate anymore tweaking on it now - been a long working week and just got back tonight.

cheers
Pete

[dogears]

I'd do 110k/100k and keep the trim straight up if you have no scope. Setting the volts to be equal means nothing!

Congrats on finding the issue!

redone the PI wiring after i noticed that one plate wired to opposite grid !

had to rewire output tx and feedback wire to stop the inevitable oscillation when I corrected the PI plate wiring with respect to grid. now I have 290v left hand side and 322v right hand side with the 2 original 100K plate resistors back in place.

Do ya think this is close enough fellas ?

I guess if i now go 110K one side and lower on the other I might get it properly DC balanced ? too tired to contemplate anymore tweaking on it now - been a long working week and just got back tonight.

cheers
Pete

[Icetech]

i had kinda the same thing last night pedro... first fire up.. one 6l6 looks like a blue fireball inside.. other is fine.. one of the big .1 caps was just open.. sending 300+volt to the imput of a 6l6.. thank god its old junk mismatched tubes for testing:)

[Funkalicousgroove]

I say rev it up!! Un-balanced amps usually sound great so long as they aren't oscillating.

[pedro]

thanks guys.

funk, is the voltage imbalance pretty typical vs say average fender output stage ?

I'll do the 110K / 100K on Rp2 and Rp1 anyway.

I'm interested in the AC balance with the trim pot. I do have an oscilloscope kindly donated to me by an Ex-EE who lives next door , and I have a cheap sine wave oscilllator so could do the set up - just need a bit of guidance. Also have marshall powerbrake I could use as dummy load.

dogears - with the 180K OD input resistor what total resistance do you set your OD trim at between relay and grid of OD1 ? Love to know how your OD input is set up to get those lovely tones I'm hearing in your clips.

cheers fellas - I'm back on track
Pete