• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

7868 tubes in Bogen CHB100

NOW I get it! Got blocked by the "search" requirement twice! OK. I recapped and revalved this amp and have an intermittent "tube glow" issue with the 7868s. All the voltages stay normal and this problem existed before I started working on it. Even if I bring it up on a variac, it will have scary excessive glow after 2 minutes. Only one time out of three, though. Gotta be a cap issue but I have changed the 370/355 volt caps twice. New ones, not reformed old-stock. COOL site!! How many of you sound freaks are there out there, anyhow? A couple million?
 
Could be a tube, more likely a socket or solder connection problem. Caps don't USUALLY go intermittant, though resistors sometimes do... old coupling caps are ALWAYS the first suspects, though.

Either tube is losing negative bias (have you cleaned the bias pots, if any?) or it is running away with grid emission or oscillating. The 7868 was specified for a maximum grid return resistor of 300K with fixed bias, and they often used 330K... asking for trouble. 150K-220K is safer. Does this use parallel output tubes? Are there grid and plate series stopper resistors? Do you always, always, ALWAYS have a speaker connected when powering up?
 
7868 glow

OK. The grid return res. is 200k. Has a 30 ohm "balance" pot that has been cleaned. Parallel output. I ALWAYS have a speaker hooked up. You got me thinking about a socket. This uses a 6C4 splitter. If that socket has gremlins that I can't catch, could that cause this? The 7868s are a fresh matched quad. All other tubes are fresh and I tested the old ones and tried them.
 
If ALL the tubes go red, it's either loss of bias or oscillation. Suspect a bad solder joint in negative bias supply. The phase splitter is AC coupled to outputs, so it can't affect bias.

Parallel output tube like to oscillate, especially high gain ones like 7868. There should be series resistors to each grid and screen grid pin (usually in 1-2K range), and 50 Ohm 1 Watt resistors in series with each plate lead.
 
Bogen tube glow

Bavis, you got it right the first time. Sorry it took me so long to get back but it has been a VERY busy summer. Cracked bias resistor. To get to the bias circuit, I had to loosen and move two terminal strips and when I loosened the second one, the resistor fell it half. All is well in the Bogen world, as far as I am concerned, now that the new resistors are put in with some breathing room and no strain on the leads. Thanks so much for the help everyone!! Karma, are you playing guitar through yours? A small amount of tweaking makes these things a BEAST for soft-tube distortion.