Bogen CHB 100 Bias Voltage goin crazy

[SirDuctTape]

Hello,

I have this 100 watt Bogen tube PA amp to repair, and doing some funky things. Schematic here: http://makearadio.com/schematics/images/bogen-chb100b-6.jpg.

The power tubes are red plating. Measuring the voltage on the fixed bias, it's pretty steady -30v ish for a while, but after 15 - 20 seconds, the wheels come off the wagon and the bias flies up and the tubes will sometimes arc. I've seen it go as high as 20v as I futz around trying to kill power.

Through experimentation, I've found that if I only put in one set of power tubes, V4 & V5 or V6 & V7, the bias is fine. Steady at -30v -ish and no issues. This works for any combination of tubes in either pair. It's only when they are all together.

This indicates something shorting out from the heat / extra current when all the tubes are in place, right?

I'm not too experienced, so is there any proper way to troubleshoot this, or any advice for this situation?

Thanks

[SoundmasterG]

Hmm, one thing off the top of my head...check for arced sockets. I had a Bassman 50 doing a similar thing and it was a socket that had arced. That doesn't explain why it won't do it when you try pairs instead of the quad of tubes, but it is worthwhile to check for it. If a socket has arced, you must replace it.


I'll think about this some more and see if I can think of some other possible causes.

Greg

[jjasilli]

Yes.  Also weird Balance Adjust shared by the power tube cathodes.  For kicks & giggles I'd temporarily bypass that, and tie all those cathodes directly to ground, or each through a 1 Ohm bias sense R.

[Willabe]

Take the power tubes out until you get a stable -dc bias voltage.

PI coupling caps that feed the power tubes might be leaking +dcv to the power tube grids.

That will cancel the -dcv bias and power tubes will run away, red plate.

Lift/unsolder the end of the coupling cap that feeds the power tube and measure for +dcv from that end of the cap to ground.

And make sure that both power tubes on each side 'see' the 100K grid return R. The schemo is probably drawn funny. But you better make sure and check it.   
 

[2deaf]

How are the power supply filter caps and the bias cap?  One of those 100uF caps charges up in series with the 50uF bias cap and aging effects might be disrupting things.

[SirDuctTape]

Well the issue turned out to be the balance pot between the cathodes. In hindsight it makes sense that the charred lookin pot that won't turn would be the issue. Especially since it started smoking a few times. 

Replaced it with some resistors and now all is fine and dandy.

 The power supply filter caps are a couple decades old so they're bad & should be changed, but I resist.

Thanks for the help!

[jjasilli]

 

[shooter]

they're bad & should be changed, but I resist.

  my kinda logic

glad you're up n runnin

[Willabe]

Well the issue turned out to be the balance pot between the cathodes. In hindsight it makes sense that the charred lookin pot that won't turn would be the issue. Especially since it started smoking a few times. 

    

And your just telling us this now?       

[DummyLoad]

Well the issue turned out to be the balance pot between the cathodes. In hindsight it makes sense that the charred lookin pot that won't turn would be the issue. Especially since it started smoking a few times. 

    

And your just telling us this now?       

right? lol!      

--pete