Smoke from 5E3

[Bluefinger]

My amp died last night. It's basically a 5E3 with tighter filtering (100, 47, 22) fixed/kathode bias switching and a GZ34 rectifyer. The rest is more or less stock specs apart from different values on some coupling caps and kathode bypass caps in the preamp section.

It worked flawlessly with exactly the same configuration for about a year and a half now. It was played frequently and mostly for 3 to 4 hours apiece minimun.

Yesterday, during the soundcheck it developed a knock-knock sound which disappeared again after a minute. Two hours into the gig it started to sound like it had a tremolo built in and then it cut out a couple of times until it started to lose volume and then there was smoke. This all happened within less than a minute before I could react propperly and turn it off. Upon visual inspection it looks absolutely clean. I took off the back panel immediately after it happened and there was no smoke inside so I guess it came from a tranny (output?). I took some ohm readings across caps and such while it was turned off and the speaker is fine as well. I can't turn it on like it is because I guess it will start melting down again.

I guess the output tranny is toast but what could be the reason for this especially after performing without any troubles for quite a while? Any ideas?

thanks!!!!!

[Prairie Dawg]

Why do you think the output transformer is toast? What symptoms led you to that conclusion? I can tell you that checking across any capacitor with a voltmeter will not tell you anything important or relevant. Something's burned up. Go through the drill.

[Chuck H]

Ohm readings across caps aren't good for much. It sounds like a power supply failure to me. And it's worth noting that a GZ34 won't be very happy running into a 100uf cap for too long. I doubt it's the output transformer. The fading and motorboating are indicative of the power supply, grounds and maybe failed filters.

Pull the tubes and take some voltage readings on the socket pins and re post.

[Bluefinger]

Why do you think the output transformer is toast? What symptoms led you to that conclusion? I can tell you that checking across any capacitor with a voltmeter will not tell you anything important or relevant. Something's burned up. Go through the drill.

The reason I thought it was the output tranny is that the smoke did not come from inside. I took of the back panel immediately after it happened and there was zero smoke inside. By the amount of smoke that was emerging there would have been something left inside. The tubes don't show and sign of damage. I guess that if one of them was smoking there would we some sign of physical damage visible. I touched the trannies and the power tranny was warm while the output tranny was pretty hot. That's the reason. This was not a verdict based on research, just a wild guess I admit.

[Bluefinger]

Thanks! I'l try that and report ...

[Prairie Dawg]

SWAGs (scientific wild assed guesses) can sometimes be right expecially if you have a bit of tech experience but they're kind of expensive.

If the output was toasty that's a bad thing-it's your invitation to see whether it's any good.

[Bluefinger]

ok, that was probably nothing but a wild guess. The trannies both seem to be fine.

I fired up the amp with only the GZ34 installed and the voltage readings seemed normal ... roughly 500 volts across the board which seems ok with no other tubes in place. Then I tried again with the power tubes plugged in as well. The knocking started again immediately. With every pop there was a bright white spark visible inside one of the 6V6 power tubes near the socket. So I put in another pair and the popping disappeared. I tried again with all tubes and the amp worked fine in fixed mode. Voltage readings were within specs. I biased the new tubes to about 20 mA BTW. It also works in kathode bias mode but trhe reading on the bias probe goes up to over 40mV. I have a 270R kathode resistor with a 25µ bypass cap installed and the ohm readings of that resistor and all others around the output stage are correct as well.

So the remaining questions are:
1) could this be a simple tube failure or was this just a symptom of a different problem? But where did the smoke come from???? even the tubes are clean and shiny ... nothing looks burned.

2) is it normal that the voltage reading goes up this high in kathode mode? I always thought that it should be in the same range as the reading in fixed mode ...

any help is appreciated!!!! Thanx!

[Chuck H]

could this be a simple tube failure or was this just a symptom of a different problem? But where did the smoke come from???? even the tubes are clean and shiny ... nothing looks burned.

Yes. If new tubes seem to be operating within safe margins it was a tube failure. But it is possible that something caused it. A bad bias supply, an intermittant grid contact in a dirty tube socket or maybe a ground fault or cold solder joint. All should be examined.

is it normal that the voltage reading goes up this high in kathode mode? I always thought that it should be in the same range as the reading in fixed mode

The voltage drop across the cathode resistor is the bias voltage. A bigger resistor will have a larger voltage drop and therefor more bias voltage. Also, the plate to cathode voltage should be used to figure dissapation when in cathode bias. Not plate to ground. So, with the lesser plate voltage a higher current is appropriate. Also, in cathode bias the voltage across the cathode will rise with current as the plate volts drop. This effectively cools the bias. It's very common for cathode biased amps to idle hotter than fixed bias amps for this reason. Of course you don't want the amp to idle too hot. So use the plate to cathode voltage to determine the dissapation and if it's over, say, 85% you probably want to cool it down a little with a higher value cathode resistor.

[Bluefinger]

Thanks!

Everything inside looks clean and neat ... I'll check for cold solder joints and hope that the new tubes will fix it.