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So, I have wonderful sounding AES Six Pac monoblock amps, and one just started eating the 0.5A tube fuse within 5-10 sec of power up.
I've swapped tubes from the good side to the bad and the fuse still blows.
I even tried a new set of kt77s in place of the el34s, and still, the fuse blows. I'm fairly sure this is not just another "bad tube" failure.
Right before the fuse blows a couple of loud pops are heard through the speaker.
I believe this is a failed coupling capacitor. Could anyone be so good as to tell me which of the capacitors in this image are the coupling caps?
Does anyone have experience with or advise a high quality, good sounding replacement to try here? Thanks so much!-Michael
Follow Ups:
It was the Jensen PIO coupling caps at fault. Easy soldering job, and back in business with great sound again. Thanks all for the help.-Michael
Will be interested in knowing what you find. I had a similar situation with blowing fuses in my SixPacs a while back. and also have the Jensen PIO copper caps. Swapping in a new set of power tubes seems to have kept the problem at bay, but I sent the previous set of tubes back to Jim McShane for testing and none of them came back with an issue, so I still suspect that my issue was due to something else in the amp.
Tube testers test tubes at a single voltage and bias level, which is most likely different than how the tubes are used in your amplifier. And when the tube is tested, after it warms up, the tube is only tested for a few seconds. I'm not suggesting to not test tubes, but for the problem you described, the best test is to swap the tubes from one channel to the other, and see if the problem follows the tubes.
A common design mistake made with tube amplifiers with solid state rectifiers is that the capacitor voltage ratings are selected at some percentage above static voltage. During the first 5 or 10 seconds, before the tubes start to conduct, the B+ is higher. This is the voltage that should be used, not the lower B+ when the tubes are conducting. One simple solution is a thermistor to the power transformer primary.
If you want to troubleshoot, there are a lot of people on this forum to help. The sockets are usually numbered, grid on an el34 is pin 4. Is pin 4 of the outputs connected to the Jensen caps? If so that is the coupling cap. And your starting point is a good one, leaking coupling cap is a possibility.
A tech would check out the voltages with a variac at about 60 VAC, but you can probably get away with checking voltages as it powers up, before the fuse blows (leave it on only enough time to check voltage with tubes conducting). And compare the bad channel to the working one, and post the results. Caps will stay charged, so use insulated tools and clips or leave it off for few minutes to discharge before changing lead. You can connect voltmeter to EL34 pin 3, anode, highest DCV setting, and leave it connected, power on and off and you can see how long it takes for the DC voltage to bleed down.
Thanks for the suggested troubleshooting tip. With due caution, I will give this a shot. Thanks!
If you feel like you can do the coupling cap replacement, i.e., the amp is out of warranty, it is not a bad place to start. Since the bias voltage is injected just downstream from the coupling caps, a bad cc will affect that reading. If you have no experience doing this kind of repair you should have a competent tech do it. BTW, if it is just the cc that needs to be replaced it should not take more than a half hour to do it. good luck, Dak
Thanks for the vote of confidence. I've build a couple chip amps and Hypex amps, and do a lot of automotive electrical. I don't imagine that soldering in a new cap will be too much of a challenge! Thanks again.
The coupling capacitors are the large copper cylinders, Jensen paper-in-oil capacitors.
I absolutely agree, though, that this is a job for a competent technician for a proper diagnosis and repair.
I recently needed to replace the capacitors in a Cary Sli-80 and took it to a local shop that is an authorized repair facility for several high-end manufacturers but not for Cary, who don't have any authorized repair stations and wanted me to send it directly to them. The local shop claimed they couldn't get technical information from Cary and returned a ridiculously high estimate with all sorts of disclaimers.
Instead I shipped it to a Cary dealer, The Audio Connection in Seattle, with whom I'd had good dealings in the past. They replaced the Jensen caps with Cardas Golden Ratio caps, along with the power supply caps, turned it around in a week, charged half what the local shop wanted and the amp works perfectly. The irony there is that John, the owner of The Audio Connection, had tried to talk me into replacing the Jensen caps a couple of years ago because they have such a high failure rate.
You really need to get the amp to a tech. If you can't identify the coupling caps, and don't have the proper test equipment you are setting yourself up for either more damage to the amp or possibly electrocution to yourself.
BTW, you could be right about the coupling cap, but it could also be a number of other things.
Dan Santoni
There maybe other issues, like PSU caps, which maybe at fault. What is the idle bias measurement of the amp in question?
Good question, FenderLover. I am not sure it is the coupling caps at all. Bad answer, I know, yet I don't have the proper equipment to test, and this is just an educated guess.
Idle bias was at 230mA, although now reads 0mA no matter where the dial is adjusted. The bias output now also reads ~24mV and ~16 Ohms no matter where the adjustment dial is placed, whereas typically the mV can be adjusted.
Thanks much for any insights on how to troubleshoot!
Kind regards,
Michael
Hi, I'm trying to understand your problem.Can you tell me what is meant by, "The bias output now also reads ~24mV and ~16 Ohms no matter where the adjustment dial is placed, whereas typically the mV can be adjusted."?
Thank you.
You said "one just started eating the 0.5A tube fuse within 5-10 sec of power up. "
Here the thing, I don't think the power tubes have even started to conduct in 10 sec. I think they have a longer warm up time than that.
Something is drawing to much current through the .5amp B+ fuse and I don't think it could be an over biased output tube caused by a leaky coupling cap because I don't think the output tube's cathode is heated up enough after just 10 sec for the tube to conduct.
That fuse is probably between the power supply and the output transformer's primary center tap.
If that fuse is blowing within 10 sec then it almost has to be a shorted output transformer.
Have you tried turning on the amp without the output tubes installed?
If the fuse stills blows I think all that leaves is the output transformer.
I wish I could see a schematic.
Caution, the B+ voltage will be higher than normal when the output tubes are pulled. Don't shock yourself!
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/10/14 07/10/14
One that is familiar with your product will help.
Good luck with the unit!
Usually the coupling caps are the most expensive ones in the amp. Those copper foil caps are expensive. That would be my guess.
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