Randomly popping fuse

[OC75]

Amp: Late 90's Peavey Classic 400 bass amp. it has 5 12ax7,1 12at7 and 8 6550's. 400w output.

The amp is randomly popping fuse when powering on. Doesn't do it everytime. Once it 's on you can use standby switch without problems and has never popped the fuse while playing. Just while powering on. Tried switching out all of the tubes with known good ones. Still randomly pops the fuse.

Where do I start looking for problems? Any ideas what's the cause of this?

[oc disorder]

Could be any number of things. The power switch arcing, a worn fan,
faulty capacitors.. a process of elimination.
By the way a great resource with schematics etc here:-
http://www.classic400.com/
Hmm just saw the back panel fuse is a 12 amp.
Seems to use a triac for turn on maybe that's too obvious,
and doesn't seem to have that for the export market.
All the supplies seem to be fused perhaps you need to disconnect them one at a time and try turning it off and on .
The power switch looks to have a second pair of contacts try using those and maybe the quenching capacitor across the switch has failed causing the switch to arc.
9 times out of 10 its power tubes but you seem to have eliminated that.
Also I see the standby switch only switches the screen grids so that the whole HT circuit power's up and all the big caps charge and only the ones after the switch can be discounted c109 , c111.

Not my recommended procedure but you could fit a timed or slow blow fuse.
Eventually i would imagine it would get worse but if you are convinced it is not any of the valves with 675volts ht rail I would look at the 6 470uF 400v caps.

Doubt an iffy component in the 15v rail circuit would pop a 12 Amp fuse.

Unless you are confident I would take it to someone competent in high voltage valve amps. May be tricky to trace.

[Amp Kat]

Also try just running one tube on each side and see it it still blows. Then add 2 more and so on . Have you tested each tube seperately with a tube tester. That was my best weapon when working on SVT's and it would point out a bad one in a batch of 6. Sometimes tubes won't fail until they really start conducting current.

[OC75]

Thanks for the input. I'll spend some time w the schematic and amp. I'll post back then.

This amp belongs to my bass player, and luckily enough I have an identical amp for him to use while I trouble shoot the malfunctioning one.

Thanks again.

[Enzo]

Are we talking about the mains fuse blowing? Are we using the proper fuse type there?

The power switch turns on th triac. There is almost no current through the switch, since all it controls is the gate to the triac. The contacts therefore should not be arcing at all.

If the triac fails, it shouldn't blow fuses. if it fails open, then the amp doesn't power up. if it fails shorted, the power switch will have no effect, the amp will be always on. Neither should blow fuses. The gate could short and run excess current through the switch, but the gate has a 1 amp fuse, and that should prevent the switch from stressing.

The triac is there because the amp uses a lot of current from the 120v mains, and especially when switching. At 240v mains, the current is only half. I suspect that is what determined the use of the triac or not.

SInce the amp survives power up OK and will play reliably once powered on, the fuses only blow at turn on. Right? Insert an ammeter and monitor mains current draw. SInce this pops fuses at power on, am I right in assuming you don't get as far as turning the standby switch to the on position? The standby turns on the separate screen B+ supply. The power tube plate supply is always on. The main filters for the screens are before the standby switch as well.

SO the charging power supply cap inrush is one potential cause. Of course all those cold heaters firing up at once is as well.

BE VERY CAREFUL, the B+ on this amp is 675v at substantial available current. IT CAN KILL YOU.

By watching the mains current we might find clues. Does the mains current ride higher than expected in general - meaning any additional draw would be that much closer to the fuse blowing? Turn on peaks might be hard to measure easily.

F100 is a 2A slow fuse for the plate B+. Check for voltages, then remove that fuse, and then try cycling the amp on and off a few times. Wait between times to let the amp discharge fully. That will remove the high voltage B+ cap inrush.

Since different tubes don;t affect this... Removing a couple tubes will reduce the overall current as well, so that doesn;t really tell us if a tube is the culprit.

And after all, this is a 12A mains fuse, right? Try a 15A.

[OC75]

Are we talking about the mains fuse blowing? Are we using the proper fuse type there?
Yes sir. Main fuse, and it is the proper spec.

SInce the amp survives power up OK and will play reliably once powered on, the fuses only blow at turn on. Right?
WHEN the amp survives power up, it will be fine until the next power up. The standby seems to have zero affect on the problem.

I will go through the steps you outlined tomorrow. And get back to you on this...

Thanks very much for your input and help.

I'll also grab a handful of 15a fuses tomorrow.

[Mars Amp Repair]

Just a thought,
I agree with Enzo I have had issues with the triac being bad & either shorting or opening...either way, the amp either runs all the time or not at all...

HOWEVER, I wonder if the triac could get flakey & either do the equivalent of a bad relay & chatter at power on or even be slightly resistive at power on creating the effect of a carbonized switch.

OR, possibly the trigger voltage is not consistent causing it to 'chatter'. there is that 1/4watt resistor in series with the gate...

Crazy, yeah I guess. I'd just be very suspicious of that triac as I've had problems with them being flakey before. glen