Blowing fuses

[lowell]

hey all,
working on Traynor Custom Special. here is the schem. this amp is blowing fuses. both coupling caps from PI to power tubes were leaky so I replaced them. Replaced all tubes (JJ EL34) with new. All filter caps in power supply test very good as far as leakage is concerned. I checked the 47ohm screen resistors they're good. I removed all tubes and the fuse did not blow. I removed all secondary leads on power xformer, did not blow fuse. The power tubes plates glow red 5 seconds after hitting standby switch. I moved the tubes around and it seems random as far as what tubes/positions are glowing. not all 4 are glowing, only 2 and it is 1 tube per side. When i watch the bias on my meter (transformer shunt method) the bias current on the plates fluctuates all over the map, but overall it slowly rises until the fuse goes. I checked the bias supply diode and filter cap and both test good. There is something i've never seen here. What is going on w/ the bias supply connecting to pin 1 of power tubes? Also can anyone educate me on the transient suppressors b/t the plates of the output tubes?

[Twist]

With the tubes out what voltage do you read on pin 5 of each tube socket? The schematic I found is pretty bad and hard to read. You might replace the 10K bias adjustment pot with a 25K Linear to grab some more negative voltage.

[Enzo]

Do what Twist said, pull the power tubes and make sure the voltages are right FIRST. Then we can worry about tubes.

Is there B+ voltage at all pins 3 and 4 on the power tube sockets? Is there reasonable bias voltage on ALL pins 5 on them? (-50v more or less) Due to the unusual wiring, is there good bias voltage on ALL pins 1 of the sockets?

Interesting way to set bias - adjust for screen current.

If the tubes are red plating within seconds, I tend to think it is not a matter of bias adjustment range. I suspect a lack of bias altogether. Possible problem in the amp, or could be a shorted tube squashing the bias.


Once the amp itself seems OK, then we can stick your power tubes back in ONE tube at a time. Pick a socket and then stick each power tube in it, one at a time, and monitor current. We are looking to see if any tube or tubes are bad. Once we have eliminated any bad tubes, take a good tube and try it alone in one socket. if it behaves, move it to another socket, then another then the last socket. We just verifyied all the sockets work. Note we were not playing the amp or concerned with powr output or tone or anything. Those tests were strictly to find trouble in the tubes and sockets.

After finding all the bad tubes and bad sockets, the amp ought to work, unless the transformer melted too.

The transient protectors on the output transformer are there as flyback protection. This is in case some dimwit cranks the amp without a speaker, or if someone has an intermittant speaker cord. These things help protect the transformer and tubes.

[lowell]

thanks for replies guys.... pulled all power tubes. there is correct B+ on pins 3 and 4. on standby the - bias voltage is twice (-80v) what it is when I put full power on (-40v). There are a few seconds b/t the change when I flip the switch as the bias changes. something is leaking positive voltage into the bias supply right? or something else? It seems the bias is there and working, but something else is offsetting it.

[Bruce / Mission Amps]

thanks for replies guys.... pulled all power tubes. there is correct B+ on pins 3 and 4. on standby the - bias voltage is twice (-80v) what it is when I put full power on (-40v). There are a few seconds b/t the change when I flip the switch as the bias changes. something is leaking positive voltage into the bias supply right? or something else? It seems the bias is there and working, but something else is offsetting it.

I'm not sure about this but think I've seen this happen before in amps with a FW rectifier and half wave bias supply... with the center tap of the PT Hi-V secondary used as a standby switch.
With that switch open, I think the bias supply charges up to twice the derived bias voltage due to the high voltage cap's bleeder resistors in the power supply being referenced to ground... with respect to the other side of the transformer being connected to the other half of the FW rectifier leg... or something like that! ha ha

[lowell]

ok guys... just thought I'd try another set of tubes... guess what, yup, it's fine now. i had retubed this previously and lowered the bias to run the tubes hotter. the customer played it 3 hours straight that night at rehearsal and the fuse blew. i found some leaky coupling caps b/t the PI and power tubes as well as a VERY leaky bias supply cap. i replaced those and it was still redplating... now w/ the new tubes it is fine. Do you think that rebiasing the first time w/ the new tubes could have caused some old components to fail resulting in this redplating of the tubes, and then the new tubes failed because of that? Then when I fixed first problem (bad components) the tubes were now bad still causing the problem? Kinda like a snow ball effect? Any thoughts on this? Has anyone dealt w/ this situation? I'll feel better if this is just sometimes the way it goes... some amps need to be fixed twice if they haven't been into the shop for a while? I am going to answer that question and say that from now on I will test these components in every amp on my bench:

-the two coupling caps from the PI to power tubes
-the bias supply filter cap and diode
-screen grid resistors

[Enzo]

I think you had a tube failure, plain and simple.

No, I don't think adjusting the bias had anything to do with it.

Tubes fail. That is why I suggested you test them in my post above.

[lowell]

Yeah, I tried the tube one by one and the fuse never blew. i guess I should have monitored the current of each tube AS i substituted them. next time for sure. thanks for the help!