Tube amp blowing fuses and rectifiers when on for a while

[Blair]

I have these Baldwin amps that I dropped some A-470s on and they sound great. These did the same thing with the stock output iron and the A-470s.

I got them with a blown fuse in one amp, popped in a pair of 1N4007s, popped right back to life. Power cycled them several times and let them run for several hours each time. Turned them on one day, and the left amp went cold after about 9 hours of running. Dropped in another set of diodes and a fresh fuse, just blew after 8 days of running.

Any ideas?

- Not power transformer because the diodes blow
- Not power tubes I would think because they come right back up and bias perfectly

Thanks!

Blair

[Geek]

A 1N4007 is pretty meek for tube power amp power... unless you string say, 3 in series, each leg.

Diodes short when they go, that's what's likely taking out the fuse.

If you can afford it, string a pair of MUR4100 in series each leg. That will easily outlive the amp.

Cheers!

[Blair]

Right Gregg,

The issue is that it is the same amp every time and the other is running strong. Otherwise, I would just beef up the diodes and call it good.

Blair

[20to20]

edit

[Geek]

There's peak currents in there I bet juuuuuust on the edge of 4007 failure and one amp will live and the other won't for some reason.

Cheers!

[nyazzip]

could a faulty/out of spec filter cap be to blame...?

[burnedfingers]

could a faulty/out of spec filter cap be to blame...?

It could be the filter cap has a problem. You stated the bias does stay stable? You might want to monitor the bias each tube if possible just to make sure they are doing something stupid. Are the coupling caps fresh?
Also watch the voltage in the power supply and if you can monitor that it would provide some information to provide the cure. There are some salvage stores that sell VOM meters for like $3 a piece and they can come in handy in situations like this.

[Blair]

Thanks guys!

The filter and coupling caps were brand new when I installed them.

Gregg, the diodes I installed originally are 5A. That's what was in it when I got it back. Kind of a bubble looking job with an aluminum ring on it.

These are good starting points.

Too bad I don't have 5 or do meters with current probes so I could just set the peak hold and find the general area.

One thing I did was add a "V dump" to these to drop the B+ with a high current Zener. With the UL operation of the A-470, I should be safe to remove the Zeners and go with the roughly 465-75V of B+. On the Baldwin output iron, I had the tubes strapped in triode so I added the Zener to not exceed the B+ in triode.

Thanks!

Blair

[Geek]

It could be the filter cap has a problem.

How could I have not thought of that.. D'OH!

[Blair]

Really?

I have never had a filter cap last 20+ hours and then short, then open back up again and function normally.

Think I should replace the can?

Blair

[Geek]

You can test it... place a 100K, 2W resistor in series and use a bench supply at its maximum rated voltage. Monitor the voltage across the cap. It should be steady. If it fluctuates, drops, or doesn't fully charge, she's a gonner.

Cheers!

[DeathRex]

I'm wondering what size is the fuse and what's the current through it? Maybe the fuse is too close to the actual current.

[Blair]

Hi,

It's a 3A slo-blow. One a single channel EL34PP amp.

Also, I doubt it is a fusing isse because that wouldn't cause diode failure.

Blair

[20to20]

Edit...

[burnedfingers]

A Quicksilver 8417 or KT88 mono block amp has a 3A fuse. I did manage to blow the fuse after I installed some 6550's in the amp. The amp ran fine for a while and then poof the fuse was open. What had happened was the tubes were steady at 55mA per tube and then went up in current draw due to the fact I believe that the screen voltage was boarderline maximum. I lowered the bias and haven't had a problem. Haven't gotten around to purchasing some zeniers to lower the screen voltage.