capacitor failure?

[jk]

So I'm most of the way through the first set on Saturday night, and my 36w TMB's volume drops out, fizzles, then quits completely. No sound at all.

I pulled the chassis last night and discovered the last cap on the power end of the board (connects to cathode pin 3 of the power tubes) looks like it failed; melted plastic cover, etc.

Suggestions, anyone, on how to resolve correctly?

Thanks,

Jim

[Plexi]

seems funny -(well not funny) but i was going to post that i have been having alot of "New" bad sprague caps popping up. I just built a Bassman wreck.. and one of the new bypass caps was shorted.. A while back i had a big hum in a preamp due to a bad bypass cap.. These have been different values..but they were the black body sprague.

A few things to always watch out for.. keep the bypass cap away from the cathode resistor..it gets very hot.,and can melt the plastic coating.. Its also best to use a 5 watt for each pair. The bigger wattage resistor will run cooler. You might also check your tubes.make sure one of those didn't poop out on that end and short something. Use a 50v bypass cap.

Richie

[rjgtr]

Obviously replace the Cap. Also check the cathode resistor and make sure it has not drifted. If the cap was too close to the resistor, then the resistor could have caused heat induced failure. Make sure the cathode resistor is of a sufficiently high rating (5 or 10 watts).

[jk]

Richie & Richard-

thanks for the words. Yep, it's a black Sprague. I'll check the resistor (it is 5W) and leave some distance for cooling, and check the tubes.

Jim

[Plexi]

Jim.. measure the resistor without the cap across it. If the cap is shorted,it will not read the resistor value correctly. So measure the resistor with the cap removed. I have had other of the Black sprague also bad..those were the 2 latest that have popped up recently. If you are useing one cathode resistor per pair of tubes..and the cap shorted..it may have really sent those tubes into red plateing. So the tubes that are used on the side of that cap /resistor..be sure to check those.

Richie

[jk]

Richie- excellent advice as always. Both the cap and the resistor were bad. I'll check the tubes too. Jim

[rjgtr]

If both the Resistor and Cap are bad make sure you check your tubes. They could have been destroyed by a bad output tube. When you replace them make sure you keep an eye on the tubes, and make some measurements to be sure the tube isn't having issues. Otherwise you could burn up a perfectly good cap and resistor again.

[jk]

RJ- thank you for the guidance, I'll do that. I've got four more quartets in inventory. Any advice or links as to how to measure the tubes?

Jim

[Plexi]

as rjgtr was saying.. it could have been one of the tubes that tookout the resitor or cap..or a bad cap or resistor could have took out one or both of the tubes on that side. Also look at the socket,make sure there is no burnt mark or arc on the sockets for that side,look on the top and bottom side.. may sound funny..but i also sniff the socket. You can smell the burnt smell even when the arc is inside out of sight. Your probably ok..but never hurts to check.

When you have everything ready to start.. clip your volt meter to the cathode connection at the board. on the side that had the problems. As the tubes warm up the voltage will rise..should go to around 11v or so..
If it starts to read much less or weird happenings..shut it off.

There are only a couple other things that come to mind that might cause the tubes on one side to have problems. A bad OT..or leaky cap off the plate resistors of the PI/Driver on that side. Or some small piece of stray wire clipping, or solder blob shorting the tube pins..
Lets hope its just the easy fix..
What kind of tubes were you running?

Richie

[jk]

Richie-

Thanks for the troubleshooting roadmap, I'll use it for sure.

Further to the saga: I pulled off both the failed cap and resistor last night - the underside of the resistor was scorched, and it must've really heated up. because it put a 1/4" by 1/2" burn mark into the turret board. It melted the G10 sufficiently that it exposed the fiber threads.

Not sure if this gives more insight to the root cause, but I thought it would be worth mentioning.

Re: tubes - they are the NOS russians. I'll use a different set to bring it up once I retrieve a new cap and resistor tomorrow.

I'll check that voltage as it comes us and post a report.

Jim

[Plexi]

man that sucks.. the burnt spot,you can try to remove that. Or mount the cap and resistor on a terminal lug in its own spot..or off the board.
The burnt spot or carbon can make straight contact through it. So if its not removed..it can cause problems. Must of had a good short to have it heat up like that.

Richie

[jk]

Thank you, Richie and Richard for all the insight.

I put the power tubes on a tester at lunchtime. Turns out one from the side with the cathode cap/resistor failure was fully shorted.

So I think it was the tube, not the cap, resistor, or socket that caused the meltdown.

I'll report back after I grind the board, put in the replacement components, and light her back up.

Jim

[EBE]

Thank you, Richie and Richard for all the insight.

I put the power tubes on a tester at lunchtime. Turns out one from the side with the cathode cap/resistor failure was fully shorted.

So I think it was the tube, not the cap, resistor, or socket that caused the meltdown.

I'll report back after I grind the board, put in the replacement components, and light her back up.

Jim

I will relate my 36 watt experience....

When I finished my 36 build I had some noise that I was having trouble with so I sent the build down to Richie to take a peek. Richie tuned up my layout and sent it back. ( Great Job Richie, as always ). When I got the amp back I put in a set of the Russian 7189 equivalents that I bought in a big batch off of Ebay. The amp lasted about 5 minutes and then one tube red plated and then went dead and took the fuse with it. Back at the house I put in another set and they redpated at full volume. I think a lot of these Russian tubes floating around as 6N14EB or the like are just junk.

Norm

[jk]

Update: I ground the carbon off of the turret board, and replaced the cathode cap, resistor, and all four power tubes:

- First, I want to make sure I'm measuring that pin 3 voltage correctly. I clipped the positive lead from the voltmeter (DCV) to the board at the positive side of the cap/resitor, and the negative to ground. I'm seeing no voltage.

- The pilot light is glowing, and the tubes are too.

- Still no sound whatsoever from the speaker.

Any suggestions on the next step to diagnose the problem? Could it be the OT?

Jim

[EBE]

Do you have voltage on Pin 7 and Pin 9 on the power tubes? Have you checked the B+ as it moves across the board?

Norm

[jk]

Norm-

I haven't checked the pin 7 and 9, but will. When you're doing that, where do you attach the black probe from the voltmeter?

Also, I haven't checked the B+ as it moves across the board - how do you do that?

I appreciate the advice and help,

Jim

[EBE]

Norm-

I haven't checked the pin 7 and 9, but will. When you're doing that, where do you attach the black probe from the voltmeter?

Also, I haven't checked the B+ as it moves across the board - how do you do that?

I appreciate the advice and help,

Jim

Attach the black probe lead to the chassis. If you are right handed put your left hand in your back pocket and then use the red probe to check against pin 7 and 9. B+ across the chest = one dead DIYer.

The B+ leaves the rectifier and goes to the 32/32 cap can and then on to the board. A lot of the layouts mark the run with red dots.

Norm

[jk]

Richie / Norm-

You guys rock the house. Turns out the no voltage culprit was the CT fuse. I was able to measure everything, so we'll have some more baseline numbers for this circuit.

I went through everything, the OT's fine.

Thanks again for the thinking and lesson.

Jim