Weird bias loss!

[Satamax]

Hi everybody!

Well, i'm working on a sound city B100. And i get this weird bias loss.

Let say V7 goes in thermal runaway. Plate starts to glow. When iddling with no B+ on, bias is there, no prob. Then i connect the B+, bias starts to drop ever soo slightly, from -31 to -27, then drops to 0 and even seem to go positive, you bet i switched before i could take a proper reading. It's socket dependent, i swapped the tubes around, and it doesn't follow the tube. But both tubes should loose the bias, the way it's aranged. Tho, V8 doesn't start to glow before V7, no mater which tube i chuck in there.

I checked for oscilation, exept if it's above 5MHZ, there seem to be none. Then again, switched quickly and may be the autoranging tekmeter didn't have the time to pick it up. Thought, same again, both V7 and V8 should glow.

I've reflowed the solder on V7 pin 6 where the 100k bias res is connected to the 10K res going to pin 5. I haven't checked the resistor thought, nor the soldering on pin 5. But this wouldn't make me loose bias, would it?

Thanks for any help you can give.

Max.

http://www.soundcitysite.com/sc_webpages/B100_schem.jpg

[52 Bill]

What happens if you pull V7 and power up the amp, does V8 start to redplate? What happens if you pull all 4 outputs and power up, does the bias voltage disappear without the tubes? Have you checked all of the 10K grid resistors? Also be sure to check the socket itself.

[g1]

Also check all the screen resistors. If the one at V8 was open, it might not redplate as soon as V7.

[Enzo]

Pull the power tubes, does tha bias still drop on that socket? If so, explore the coupling caps.

[Chuck H]

And check for faults in the grid lead for that socket. If it's connected via a grid resistor check that too.

[Satamax]

Pull the power tubes, does tha bias still drop on that socket? If so, explore the coupling caps.

Hi Enzo, the bias doesn't go away when there's no tubes. But it's socket dependent.

Thanks a lot to all of you guys.

[Steve Conner]

That is how thermal runaway works. The tube draws lots of grid current and eats the bias voltage. If both sockets lose bias but only one redplates, the one that doesn't probably has a blown screen resistor or other loose connection preventing plate current from flowing.

If the problem doesn't follow a particular tube, all of the tubes may be shot, and any of them would go into thermal runaway when put in a socket that had power.

[km6xz]

Second that, the tubes are unbalanced either by screen open or one just worse then the other because they are under biased causing thermal runaway. Why is the bias so low? The tubes are probably damaged from heat, fix the bias before putting new tubes in it.