BF Super Twin Reverb - Burnt resistor

[cjlectronics]

I'm fixing a super twin reverb that has a burnt 33k resistor that connects to the output tubes matching pot. I have attached a .pdf of the schematic with the highlighted resistor that blew up. I am going to replace the 33k and the two 4uf/450v caps (one is leaking white powder) then power the device up with the power tubes removed.

Can I do voltage checks (and compare to the schematic) with the power tubes removed?
Any ideas why the 33k resistor burnt to a crisp?

Thanks,
CJ

[Twist]

It fine to power up the amp without the tubes. The high voltage will show higher without the power tubes. I would bet that you have a shorted output tube on the side with the burnt resistor. Check all the 470 ohm resistors too.
Make note of which tube came out of which socket because you may find a blown screen grid resistor to a particular tube. That may be your culprit.
Replacing leaking electrolytics is always a good idea.

[cjlectronics]

I replaced the burnt 33k resistor (and one other 33k resistor that measured 55k). Then replaced a 20uf/500v cap and 2 of the 4uf/450v caps. I found one of the 470 ohm resistors had a hairline crack and I replaced it. Now, no fuses blow. The problem is that after about 2 to 3 minutes will full power ( 120VAC - I am using a variac and monitoring the AC current draw) one of the tubes start to glow very bright and get very hot. When I swap tubes around the one tube that was getting hot no longer gets hot but another tube in a different spot gets hot. It does seem to be isolated to the V9, V10, and V11 set of 6L6's.

I've check lots of resistors but nothing looks out of the ordinary. Could filter caps be causing this problem? Could the 220uf/285v caps be doing this?

CJ

[Enzo]

No bad tubes cause this problem. Loose sockets cause this problem.

Bad filter caps would affect all the tubes.

There is only one path the current can take to burn out that 33k, and that is through the tube. One of your 6L6s on that side shorted internally. This would also make the tube lose its bias and red plate.

Now when you aer moving tubes around and hte problem is moving around, all the more reason to suspect the tubes. Next time a tube goes nuts, just remove it. now how does the amp act with the remaining 5 tubes?

Now leave all of them in the same places except one. Move just the one into that vacant socket. Does it behave there?

Power off and discharge, UNPLUG THE AMP, now inspect the socket pins with power tubes removed. Tighten any that are loose gripping.

DO you have a bias probe?

[cjlectronics]

No bad tubes cause this problem. Loose sockets cause this problem.


DO you have a bias probe?

Now that you say that...one of the pins on one of the tube sockets pulled out of its socket.... Sounds like a total 6L6 tube socket swap is in order.

Bias probe....? Not sure what that is but I have an O-scope, multimeters, HV probe (it connects to my DVM), function generator, LCR meter, 7A digital power supply. Does this sound close to a bias probe or is this a custom made test probe? Oh, I have an 8 ohm/1000W dummy load as well.

[Enzo]

A bias probe is an adaptor for your DMM that is plugged un between tube and chassis and it allows the meter to measure tube current. You can get just the adaptor, or you can get them with dedocated metering units as well.

Read this:
https://taweber.powweb.com/biasrite/br_page.htm