Fender Hotrod Deville prob.

[donegone]

I just had this in the shop. Retube/ rebias. New output tubes biased at 30mA/ 440Vp. Returned it to the customer. I get complaints 2 weeks later about how it sounds bad. So I take it back and check the tubes. They read extremely low (8/10mA). I turned the bias supply all the way up and got 10/12mA with the supply voltage at -44vdc. I put in a new pair of tubes and they biased properly. I dont know what can cause this. Screen, grid, and plate resistors all check good. Suggestions?

[Enzo]

Look VERY closely at the solder on the pins of the 6L6 sockets. ANy cracks? I just automatically resolder them when I open one of these anymore.

[donegone]

I did see some suspicious joints. I soldered them up and I still get the same readings. Did it cook the recently replaced tubes? I find that hard to believe. There isn't any burning on the tubes they just all of sudden read really low. Should I replace the tubes with new 6L6's and wait for the amp to come back again? The cust. claims it sounded great for about a week or two of moderate use and then it started "farting out" on him. That would be the low bias I can assume.

[Amp Kat]

What kind of tubes were they ? Do you see any signs of dark paint on the tube lettering showing they may have gotten cooked ?

[donegone]

The reason I replaced the tubes in the first place is because they were reading low current (12mA~18mA). The cust. at the time was still complaining about the same "farting" problem. I could get them to bias at the lowest recommended current for the plate voltage which was about 27mA but I had the bias supply pot maxed out. I didn't like that. Since the tubes were the originals from the factory and showed some burning on the envelope I decided they should be replaced. I monitored this on a scope with a 4mV input signal (sine), clean channel, tone knobs set to 50%, vol. set between 50% for 15 min. and 100% for another 15 with an 8ohm dummy load. I could see no spikes, noise or inconsistencies. Also ran it on the dirty channel for about 20 min. with no problems either.

[donegone]

Nope. Not cooked. Thats what my first guess was. I was thinking a screen resistors was intermittent. Maybe I just got some bad tubes from EH?

[donegone]

HA! Im right. Just ran it through the ringer one more time and I got it to fail. Seems the current shoots through the roof on one of the tubes. It looks to me to be a faulty screen resistor! Thanks guys for the input.

[Amp Kat]

Why do you think the screen is faulty ? 6L6's pull very little screen current and even a dead shorted screen may not red plate a 6L6. A higher resistance probably wouldn't cause thermal runnaway and an open wouldn't work at all. I would also check the coupling caps and make sure they aren't passing DC when it gets hot.

[donegone]

I didnt know that about 6L6's. You were right. It was one of the coupling caps. Thank you!

[Enzo]

Not thinking about a shorted screen or excess screen current, but an open or seriously elevated screen resistor will result in the tube not conducting.