W5M blowing fuses

[nj pheonix]

Just taking a stab
Going SS in an ST 70 makes you use a 50 ohm 10 W R to bring B+ back in line. You may need to do something like that.Perhaps an inrush CL as well

 

[Thunderbud]

Thanks to all who've posted hints and helps.

Let me get this straight, you removed the high voltage leads from the rectifier, left the others connected and the fuses quit blowing at that point? If that's the case I would expect to see shorted filter caps not a dead transformer.

Don't believe the filter caps are shorted they're new and on the board I just installed.

 

[Thunderbud]

At this point not sure where to go. Going to examine the entire circuit, every wire, component and solder joint. Perhaps that will lead me to the root cause. Doubt it's anything to do with he new board, because as I said the problem pre-dates it's installation.

 

[gadget73]

Just because a part is new doesn't mean its good. I've been bitten by that more than once myself.

 

[Thunderbud]

Just because a part is new doesn't mean its good. I've been bitten by that more than once myself.

Ok, I'll keep that in mind. Once again, this fuse blowing thing predates the new board.

 

[Thunderbud]

Try taking a step back to the old board and tube rectifier just to get back to a working amp if looking for the trouble doesn't pan out. That could give you some insight into the circuit where there might be excess load you weren't aware of since it wasn't blowing the fuses at the time.

Might just try that........

 

[primosounds]

Might just try that........

Have you checked your OPT, output trans , for a short? It is hooked up to the B+ and i had a problem with a w5 where i thought i had a bad PT when there was a short in the OPT. I am not sure that it would cause the fuse to blow. but you should try to eliminate the opt as a problem. I do all my testing with a variac and a SSR at low voltages so i can check problems before they turn into PROBLEMS.

 

[Thunderbud]

Are both amps born twins? Same power transformer on both?

Yes. Same transformer, same " vintage, " approx. 1957 builds

 

[Thunderbud]

Have you checked your OPT, output trans , for a short? It is hooked up to the B+ and i had a problem with a w5 where i thought i had a bad PT when there was a short in the OPT. I am not sure that it would cause the fuse to blow. but you should try to eliminate the opt as a problem. I do all my testing with a variac and a SSR at low voltages so i can check problems before they turn into PROBLEMS.

No, haven't tried that, but I will. Thanks for the thought......

 

[Thunderbud]

OK, is this a new mod for you that hasn't worked for any length of time, or old mod suddenly gone south?

Had to put it aside for awhile ( obviously), but to answer your question, this s a new mod, using one of Mr. Stokes's updated capboards with diode rectification. I have two of the new boards, and on the off chance that there was some kind of defect in the board, I swapped the other new one in. Same result. I have a 60 second delay timer on the HV leads. and I'm getting one, maybe two seconds after it pulls in. Then one or both of the diodes fail and the fuse blows. I'm reading approx. 500 volts from the transformer on both legs. Until I put the capboard into the circuit, that is. Then I pretty quickly read 0. Lol.

 

[gadget73]

sounds like a short in the board, or something downstream of it. The caps aren't in backwards, or rated for not enough voltage are they?

Does it do this if you install just the cap board but connect none of the leads going out to the amp? For it to do what its doing, the short would have to be fairly close to the beginning of the circuit. I'm thinking the lead heading out to the output transformer maybe, or a short on the output tube socket. Hopefully its not an internal short in the transformer.

 

[geiman]

I understood unless the SS rectifier circuit was buffered in some way, to protect it from the onrush of current at startup, blown fuses are a symptom of this problem.

 

[Thunderbud]

sounds like a short in the board, or something downstream of it. The caps aren't in backwards, or rated for not enough voltage are they?

Does it do this if you install just the cap board but connect none of the leads going out to the amp? For it to do what its doing, the short would have to be fairly close to the beginning of the circuit. I'm thinking the lead heading out to the output transformer maybe, or a short on the output tube socket. Hopefully its not an internal short in the transformer.

Haven't tried removing all the leads from the board to the chassis, just the HV leads. The others go to the preamp section. I was wondering about the polarity of the caps, but they appear to be correct. I have the transformer leads still soldered to the rectifier socket, using it as a tie point, and the HV leads from the board landed there. Doubt it's the tube socket, as at some point in my troubleshooting I had moved them to an unused terminal strip. And to answer your first question, no, all is well until I attach the HV leads from the board.

 

[Thunderbud]

I understood unless the SS rectifier circuit was buffered in some way, to protect it from the onrush of current at startup, blown fuses are a symptom of this problem.

I had one of SDS Labs older boards without the diodes installed and was using a Tube Depot plug-in SS rectifier with no problems.

 

[Thunderbud]

Haven't tried removing all the leads from the board to the chassis, just the HV leads. The others go to the preamp section. I was wondering about the polarity of the caps, but they appear to be correct. I have the transformer leads still soldered to the rectifier socket, using it as a tie point, and the HV leads from the board landed there. Doubt it's the tube socket, as at some point in my troubleshooting I had moved them to an unused terminal strip. And to answer your first question, no, all is well until I attach the HV leads from the board.

What should I see for resistance on the power xformer leads ? and how do I check them ? Lead to lead or lead -to -ground ?

 

[Thunderbud]

How does your delay relay tie into that?

It goes from one side of the choke, through the relay, then out to the center tap of the output transformer.

 

[gadget73]

Doubt it's the tube socket, as at some point in my troubleshooting I had moved them to an unused terminal strip.

I was thinking an output tube socket, not so much the rectifier tube socket. Unless you moved those leads too. Maybe the socket isn't so much bad as a stray lead shorted to ground or something like that.

 

[Thunderbud]

OK, so when you said you have the relay on the high voltage "leads" you mean the B+ output after the choke.

Yes.

 

[Thunderbud]

Take the relay out of the picture, jumper it or move its output wire to the input, and see if the fuse holds from a cold start or blows at 10 seconds as the output tubes start conducting fully.

I'm afraid to put all that HV to the tubes before they're warmed up. That's why I have the relay in there. Forgot to mention, with power tubes removed, no drama. Things go wrong when they are in the circuit. tried a pair of 6L6s, and a pair of Gold Lion KT66s. Same result.

 

[gadget73]

it won't hurt the tubes. That whole thing about cathode stripping is over-hyped. Yes it happens but a few times for testing isn't going to destroy the tubes. Its possible you have too much inrush between the caps charging and the tubes already being hot. Maybe try a resistor across the relay contacts to get the caps partly charged and reduce the current surge before the relay closes.

worth asking, but what type fuse are you using? If its not a slow-blow that may be part of the problem.

Or just put a vacuum rectifier and stock value caps in there like its supposed to have.