Bogen CHB-100 project weirdness

[AmberB]

Hello all!  It's been a while since I've been on here.

I'm working on a Bogan CHB-100 conversion to guitar amp.  I've been working on this thing intermittently for a while now.  These amps normally have 4 of the 7868 power tubes, I decided to change to EL-34s for this project.  A while back it was pointed out to me that the EL-34 requires twice as much heater current per tube as the 7868.  That left me with 2 possibilities, add another heater transformer to supply the extra 3 amps of heater power, or just use a pair of EL-34s rather than a quad.  I chose to go with the pair of EL-34s for this build.  Of course, that meant that I had to change the speaker connections for the output, using the transformer's 2 ohm output for 4 ohm load, and the transformer's 4 ohm output for the 8 ohm load.
Then I had to modify the amp's negative bias circuit to put out a more negative voltage for the EL-34.  The amp was set up to put out about -22.5 volts, the EL-34s need about -45 to -55 volts with a plate voltage of close to 500 volts in order to bias in the 50 to 70 percent range at idle.  Of course, I figured this out after red plating one of the tubes pretty much right away...
I have an old pair of Sylvania 6CA7s that run pretty well in it right now.  With about 498 volts on the plates, I was able to balance the idle current at 28 ma.  I have a more or less new pair of TAD EL-34B-STRs that are supposed to be matched, I can't get them to balance.  The closest I can get is 24ma and 29ma, and that's with the bias balance pot turned as far as it goes toward the under biased tube.  The bias voltage on the low bias tube is -49.7 volts, the higher current tube bias voltage is -51 volts.  I would have to have more than a 2 volt differential to bias those tube close to the same.
I built the bias balance circuit using a design that I found on the internet by Rob Robinette.  The design also has a bias voltage adjust pot, but I didn't put that in, I just used a fixed resistor for that part.  I wanted to get the bias balance circuit to work before I put in a pot for the voltage adjustment.  I thought there was something wrong with the circuit until I put the 6CA7s in and they balanced nicely.  I'm guessing that the "matched pair" of TAD EL-34s aren't very well matched in that regard.
I still have some refinements to do to the preamp section.  It works, but it's pretty basic.

[PRR]

> 24ma and 29ma

Close enough to rock and roll. Who plays at idle?

[AmberB]

Good point! 

[AmberB]

I know that I would need a much wider voltage difference from one side of the balence pot to the other side if I wanted to balance those 2 tubes.  I have about a 2-3 volt difference from one side to the other when the balance pot is adjusted all the way to one side.  I think I would need a much higher resistance pot than the 30k pot that's in there now.  I tried a 250k linear pot in there and it didn't make much more of a difference than the 30k pot, so I'm not sure how I would go about getting more voltage swing if I were to want to do that...

[AlNewman]

Would the resistors you have between the wiper and the ends of your 30k pot reduce each side's resistance to less than whatever value the resistors are?

[scstill]

Noticed what looks like O-gauge train track in your picture.
I once was deep into collecting restoring and running post-war Lionel.

[AmberB]

SCSTILL, that's actually G gauge track.  :-)
I do have some Lionel shelf queens on display on a couple of wall shelves...

"Would the resistors you have between the wiper and the ends of your 30k pot reduce each side's resistance to less than whatever value the resistors are?"
Those are 470k resistors across a 15k resistance, so they should not make much difference in voltage drop across the pot.  Those are there to protect the tubes in case the balance pot fails.  If the pot were to fail, you would still get some bias voltage to the tubes.