fuses popping on carver tube amp

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nooshinjohn
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fuses popping on carver tube amp

Post by nooshinjohn » Wed Apr 08, 2015 2:09 am

Tonight, my left channel started popping the fuses on the input transformer. I have been using 6a 250v slo-blo fuses since the amps were moved into the new audio room, which has a 20 amp circuit. The amps have been plugged directly into power , and after the fuse popped, I moved the amp onto my power conditioner.

After ten to 15 seconds, another fuse blew. I watched the output tubes get an eerie Emo-blue glow to them before they went dark. I popped the bottom off of the amp and found no sign of blown components or other damage, and I do not want to risk causing any. Without a variac setup, I am unable to run any testing, so I am dead in the water for now. I am afraid to power them up again until they get checked out.

Just wondering what you guys think could be the cause... bad input tube(s) maybe???

I will likely need to send them to Bob and have him sort through it, but it sucks that now is the time for them to act up... is it possible that the amps do not like seeing a 20 amp service?
The Gear... Carver "Statement" Mono-blocks, Polk Audio "Signature" Reference Series 1.2TL with complete mods, Pass Labs X1 two channel preamplifier Signed by Poppa himself, Pioneer PDD 9MkII SACD Player, Triangle Art Reference SE prototype, with Walker Precision speed controller and Origin Conqueror tonearm and Dynaudio 20x2, Pass Labs Xono twin chassis phono preamplifier, Pioneer Elite SC-65, Polk audio AB700/800 "in-wall" surrounds, Oppo BDP-93

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BrianT
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Re: fuses popping on carver tube amp

Post by BrianT » Wed Apr 08, 2015 5:34 am

nooshinjohn wrote:Tonight, my left channel started popping the fuses on the input transformer. I have been using 6a 250v slo-blo fuses since the amps were moved into the new audio room, which has a 20 amp circuit. The amps have been plugged directly into power , and after the fuse popped, I moved the amp onto my power conditioner.

After ten to 15 seconds, another fuse blew. I watched the output tubes get an eerie Emo-blue glow to them before they went dark. I popped the bottom off of the amp and found no sign of blown components or other damage, and I do not want to risk causing any. Without a variac setup, I am unable to run any testing, so I am dead in the water for now. I am afraid to power them up again until they get checked out.

Just wondering what you guys think could be the cause... bad input tube(s) maybe???

I will likely need to send them to Bob and have him sort through it, but it sucks that now is the time for them to act up... is it possible that the amps do not like seeing a 20 amp service?
hooshinjohn

I had a similar problem on my Silver Sevens, it ended up being a bad input tube.
I took the input tubes from the working amp and put them in the faulty amp, problem went away, so I
ordered 8 new input tubes and changed them all.

Good luck Brian
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radioeng2
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Re: fuses popping on carver tube amp

Post by radioeng2 » Wed Apr 08, 2015 7:02 am

Fuses that blow immediately on throwing the switch on would have me looking at the rectifier diodes.

15-20 seconds is well after the filaments have got going and more like about the time of the tubes starting to ramp up the current draw. If no other signs of heating are observed, then I would be looking at trying the tube changing.

Also eyeball the main filter caps. If they're run to close to the edge and have some breakdown internally at this stage, they might take about that long to start to heat and start to try to pull current through them in about that kind of time.

Mark
I was there "At the Fest!" Hope you were too!

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Robert R
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Re: fuses popping on carver tube amp

Post by Robert R » Wed Apr 08, 2015 9:01 am

This is, I assume, one of your "Statement" mono amps. I don't have any data for those - no schematic, nor even the tube complement. But there are a couple of things you can try.

First, forget the AC connection. I assume everything else in the home still works OK. 15 amp, 20 amp, or 200 amp feed - it doesn't matter. As long as the AC voltage runs 110-125 you have no problem there.

If you suspect tubes, try running it with them removed. If the fuse doesn't blow under those conditions, measure the high B+ voltage Knowing Bob's designs it should be at least 600 VDC. Be careful there - the caps will likely stay charged way up after shutdown, especially with no tubes. And 600 volts will not be appreciated by your body...

Next, test that left amp with known good tubes, borrowed from the right channel. This is how you narrow things down. Those tubes on the right are all presumably OK.

If it works with the good tubes, put the original (now suspect) set of tubes into the right channel amp and try that. Clearly, if that amp now blows its fuse, you have a bad tube. If you have enough fuses, you can swap tubes one by one until you isolate the culprit.

If you run into the apparent paradox where the right channel works with the tubes that were in the now-working left, one would suspect a flaky connection between the tube pins and a socket, cured at least temporarily by your removing and reinstalling them.


If the left amp doesn't work with the other tubes, the next step would be to measure the bias voltage; maybe at this point you'd want some help. But at least you would send it out knowing that it's not just a bad tube.

RobertR
"You know what folks? I'm happy."

nooshinjohn
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Re: fuses popping on carver tube amp

Post by nooshinjohn » Wed Apr 08, 2015 12:53 pm

I swapped the front end (12ax7, 12at7, 6al5) from the good amp to the problem child and got the same result. Ten to fifteen seconds and the fuse pops. I did not think an output tube would cause problems with the input side of things, but I will swap those tonight when I come home with new fuses.

I don,t know why but my gut suspects the input transformer...
The Gear... Carver "Statement" Mono-blocks, Polk Audio "Signature" Reference Series 1.2TL with complete mods, Pass Labs X1 two channel preamplifier Signed by Poppa himself, Pioneer PDD 9MkII SACD Player, Triangle Art Reference SE prototype, with Walker Precision speed controller and Origin Conqueror tonearm and Dynaudio 20x2, Pass Labs Xono twin chassis phono preamplifier, Pioneer Elite SC-65, Polk audio AB700/800 "in-wall" surrounds, Oppo BDP-93

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Robert R
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Re: fuses popping on carver tube amp

Post by Robert R » Wed Apr 08, 2015 2:45 pm

My first suggestion, to run it with no tubes, should be a pretty good test of the power transformer. If it has shorted turns or internal arcing the fuse would still pop. But the same problem could just as well be caused by a defective output transformer. But I don't equate those sorts of problems with a delayed fuse, unless the current draw is by chance just around the fuse's specified value. A shorted xfmr. usually kills the fuse almost instantly.
Other possibilities include what Mark said - filter caps. That"s why I suggested measuring B+ voltage.
Then we consider bias voltage - there should be a negative DC supply in there, fed to the grids of the KT-whatevers. If that supply fails, the outputs will draw way too much current, and something has to give.
RR
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radioeng2
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Re: fuses popping on carver tube amp

Post by radioeng2 » Wed Apr 08, 2015 4:46 pm

If the color in the finals as they start to light up all look different, then that would tend to point to a voltage being different. (I realize the time is probably short and it's tough to tell).

I've had filtering out in high powered stuff before (big chokes gone) and it just ate the tubes way fast. Like instead of two years, about two months and gone. When you're talking a couple grand each, it's noticed. But the chokes effect ripple filtering and don't go to ground. But if the filter caps go out, then they can quickly warm, conduct, pull enough to blow the fuse. Could match the symptom. I've seen that before.

Bias out could too. It's usually not fused cause you always want it there. Not too much to that circuit to check in a powered down condition. It's a very low current draw but check resistors with a VOM for any opens.

If one tube went bad then you should go real hot on a big resistor feed from B+. Maybe a short on and off and then check for sign of one hotter than others. Probably easier to play swap on tubes though. Resistors can go to almost red without opening. Usually will show signs of hot though like discoloration and you then need to check and see if the value has changed.

I'm used to physically WAY bigger components and voltages, though I never have multiple output tubes in transmitters. Filaments are always separate, so you can turn those on and check that, and the bias comes on then too so you can check it's E and I. Then the B+ on is a quick two stage up to lessen surges coming up. In these amps, assuming a single switch on, you can't separate functions near as much as nice during troubleshooting.

Mark
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nooshinjohn
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Re: fuses popping on carver tube amp

Post by nooshinjohn » Thu Apr 09, 2015 12:57 am

An update... I came home from work, plugged in an old speaker and removed all the tubes. I powered it up and it stayed on without a problem. I reinstalled the input tubes, then followed with the output tubes. I found one pair that popped the fuse in 15 seconds. They are the two that are on the rear right as you look at the amp from the front.

I have two new tubes on the way...
The Gear... Carver "Statement" Mono-blocks, Polk Audio "Signature" Reference Series 1.2TL with complete mods, Pass Labs X1 two channel preamplifier Signed by Poppa himself, Pioneer PDD 9MkII SACD Player, Triangle Art Reference SE prototype, with Walker Precision speed controller and Origin Conqueror tonearm and Dynaudio 20x2, Pass Labs Xono twin chassis phono preamplifier, Pioneer Elite SC-65, Polk audio AB700/800 "in-wall" surrounds, Oppo BDP-93

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BrianT
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Re: fuses popping on carver tube amp

Post by BrianT » Thu Apr 09, 2015 5:12 am

nooshinjohn wrote:An update... I came home from work, plugged in an old speaker and removed all the tubes. I powered it up and it stayed on without a problem. I reinstalled the input tubes, then followed with the output tubes. I found one pair that popped the fuse in 15 seconds. They are the two that are on the rear right as you look at the amp from the front.

I have two new tubes on the way...
Nooshinjohn

Glad you got it worked out.

Brian
VPI Classic Turntable (black)
Transfiguration Phoenix Cartridge
Devialet 440PRO Mono Blocks
Audioprana SP/DIF-AES/EBU cable
Auralic Aires G2 Music Server
Audience AU24 USB cable
B&W 803D3 Speakers (black)
Auditorium 23 Speaker Cables
Raysonic CD-168 CDP (silver)
Carver TX11

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Robert R
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Re: fuses popping on carver tube amp

Post by Robert R » Thu Apr 09, 2015 7:56 am

Absolute first rule when dealing with tube amp problems: be sure the tubes are all good. That is what my procedure was designed to test. Glad you found a bad one - everything else is harder.

Final step after replacing a bad output tube: check out the pertinent operating voltages to be sure all is well, lest you sacrifice performance and/or maybe damage another tube. It's not really enough to listen and decide it "sounds great" - it might well sound quite normal with a pair of paralleled output tubes completely missing, though the peak power is reduced somewhat. If you have burned out, say, a plate resistor, you might never notice.
The plate (pin 3) and both grid voltages (pins 4 and 5) should be about the same in all the output tubes.

RobertR
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