SSE popping fuses

[lousymusician]

After many years of flawless service, I turned on my SSE last week and - no sound. The main fuse had popped.

Some details on the build - I'm using the Allied 6K7VG mains transformer, EL34's, Transcendar OPT's, and GZ34 tube rectifier. The SS diodes were originally installed, but died due to the known standby-switch issue. They were left in place on the board unused. I can't see any evidence of overheating or of a blown component on the board, and haven't disturbed the wiring since completing the build.

During troubleshooting, I found that with the fuse would pop even with no tubes installed in the board. If I removed the HT (red) leads from the board, the fuse would hold. Staring at the schematic, the only thing I could think was that the bad diodes had decided to short across the secondary, so I removed them this morning. With the diodes out of the way, I reattached the secondary leads to the board and powered up the transformer without popping another fuse. That seemed like progress!

That done, I reinstalled the diode tube, and that also powered up fine. I had AC and DC where it should be. Powered down, hooked up a pair of test speakers, installed the signal tubes, powered up, and the amp worked. Hooked up a signal source, got sound through the amp, and thought I was home free. Until I put it back into the system...

When I powered up the amp again, I saw bright arc flashes inside the rectifier tube! Hit the power switch quickly, no blown fuse that time, but a nice little light show. WTH? OK, I thought maybe the GZ34 had gone bad, maybe that had caused the original problem, so I swapped in a spare. Powered up, same thing, bright flashes of arcing in the rectifier tube.

At this point, I need to buy a clue. What would be causing the rectifiers to arc? Have I blown two GZ34's? What should I check next?

Any advice would be much appreciated!

Bill Mennuti

[Alco]

I had a similarly mysterious problem with my TSE a few years ago that turned out to be an intermittent failure in one of the small caps in the b+ circuit. I was able to identifiy the problem when it became less intermittent and the cap in question began smoking.

[rmyauck]

I agree a bad cap is causing a short. The diode mod which George has added to new boards is also great for helping tube rect. life. Use UF4007 as they are a newer, faster, and quieter diode for a few cents more.

Hope that helps!


Randy

[nukaidee]

mine does the same thing.. older build about 2 years now.. It blows a fuse every 10 ish hours of listening, I've replaced all the tubes on the board and it still does it. I just ended up buying a whole bag of fuses online. Now I'm a fuse roller!

[Alco]

what size fuses are you guys using ?

I've settled on a 1.5 amp 250 volt fuse for my SSE, seems to be about right. It blew and protected most things when I had some popping and cracking from a failing GT kt88s, yet ran the kt88's just fine when they were healthy. The cathode bypass cap on the same channel as the bad tube failed as well.

The SSE is plugged in to a watt miser. In it's current configuration with no rectifier tube, a sylvania 12at7, and some sort of inexpensive chinese kt66's, it burns 108 watts at 118.3 volts. So 1.11 amps.

[tubelab.com]

I've settled on a 1.5 amp 250 volt fuse for my SSE, seems to be about right

My original SSE runs EH KT88's which are sometimes cranked up as hot as 100 mA each (430 ohm cathode resistors). A 1.5 or 1.6 amp fuse will blow randomly once every month or two. The electrical power here is subject to random brownouts, and overvolt situations, especially during the summer thunderstorm season. I have a 2 amp fuse in that amp now. The amp still has all its original parts (including tubes) except for the SS diodes which blew during a thunderstorm and were not replaced.

At this point, I need to buy a clue. What would be causing the rectifiers to arc?

A violent tube arc is usually caused by a dead short on the B+. Measure the B+ to ground resistance with an ohm meter, AFTER the amp is well discharged. Best to wait a day or so since it was last plugged in before poking around with an ohm meter. If you have a short, start disconecting things. Pull all tubes, unscrew the OPT connections, switch wiring, etc until the short goes away.

[nukaidee]

I'm running 3A 250V slow blows...

[rknize]

I use a 2A slo-blo in mine, which has an adjustable cathode resistor that let's me run KT88/90s close to 100mA or EL34s much cooler. I originally started with a 1.5A slo-blo, but it only made it through a few power up cycles before blowing. Even the 2A shows a little distress right now.

[lousymusician]

And it looks like I'm back in business. As predicted, C1 had apparently shorted out, a new cap has the amp back up and running. Apparently while I wasn't looking the original Panasonic caps have been discontinued, so the replacement is a Cornell Dublier. I bought a spare, just in case.

Thanks all!

Bill