Blowing fuses

[martin manning]

That test might not work here since it will run until there is current demand from the HT.

[Firestorm]

Not that you can trust Fender schematics, but this one shows 315-0-315 on the PT secondaries and 405 plate volts. You have 335-0-335 and 467. I wonder if when the negative bias voltage goes sky high if the B+ does, too.

[JazzGuitarGimp]

Are you certain that your bias filter cap is really connected? If it has an open connection at either end, you will be applying raw DC to the output grids, not to mention the measured voltage will be 1.414X lower than expected since the cap isn't there to maintain the peak voltage. Just a thought.

[gui_tarzan]

...a problem with the PT, where the HT shorts to the bias winding and creates an internal loop.

Time for this? http://www.premierguitar.com/articles/t ... r-tester-1

Did that, the bulb flashes so there is probably not a short.

*sigh*

[martin manning]

It could be that there is no short unless there is high voltage present. I'm back to trying to lower the bias voltage. Or, another test you could try is disconnecting the PT bias tap and connecting the bias curcuit to the HT through a large resistor.

[Stevem]

With that low of a bias voltage the output tubes will be pulling fuse blowing current before they show that they are red plating even in a darkened room!

You need a minimum of -46 volts on pin 5.
Get a 200 ma fast blow fuse and wire it into the feed to the center tap of the OT and I will bet you that it will blow before the main primary fuse does, that amp at idle should be pulling no more than 44 ma per tube, so 88 ma total. If a 200 ma fuse kicks then you have found the issue!

[martin manning]

If the plate voltage comes down to around 400V under load, then -36V Vg1 should result in idle current around 60mA (80% of Pa max), so the four output tubes would be drawing 240mA. That's high, but not enough to cause immediate red-plating.

You have 465V on the plates in standby, which is a bit higher than the schematic's 450V. That's not out of line, though, and it shouldn't drive the idle current up too much.

An oscillation is a possibility as I suggested above. How about pulling the PI tube to isolate the power amp? Before trying that, I would still recommend paralleling another 1k across the one on the bias supply board to get the bias voltage down another 5-6V, just to be safe. You could also remove two of the output tubes to reduce the current draw, then if that works add the other two. If you can get the power stage working by itself then that probably means the PT is ok.

[Stevem]

Sorry I forgot it's a 4 output tube amp, not two!

[Firestorm]

It's the sudden increase in magnitude of the negative bias voltage just before the fuse blows that seems so odd. If that is not a DMM artifact, it's high enough to shut all the tubes off unless it is accompanied by a corresponding rise in B+, implying an interturn short in the primary. Of course, I've never seen a primary short that only manifests under load, but...

[gui_tarzan]

Sorry for the late return, it's been a very busy week.

I replaced the 1K resistor in the bias circuit with a 470 ohm and have -66v on V7 & 8, -61v on V9 & 10 at pins 5. It still blows the fuse after 15-20 seconds when I flip the standby switch off.

This is with no pre-amp tubes installed, just the four 6L6s.

I have not yet pulled the bias tap off and run it from one of the HT leads with a dropping resistor, that's probably the next step.

[Stevem]

You now need to rig up my PT test I posted up!

[gui_tarzan]

With that low of a bias voltage the output tubes will be pulling fuse blowing current before they show that they are red plating even in a darkened room!

You need a minimum of -46 volts on pin 5.
Get a 200 ma fast blow fuse and wire it into the feed to the center tap of the OT and I will bet you that it will blow before the main primary fuse does, that amp at idle should be pulling no more than 44 ma per tube, so 88 ma total. If a 200 ma fuse kicks then you have found the issue!

Well the mystery continues. I wasn't able to get a 200ma fuse from the only local place that carries fuses, all they had was a 1A fuse so I put that inline with the bias tap and it did not blow but the main fuse did. The odd thing about this whole situation is when I power on I have -51v at V7 & V8 (pins 5) but when I go to V9 & V10 it's always well over -100v and climbs for about twenty seconds before the fuse blows.

And this is with a different, new set of power tubes. So I can rule out the tubes I just wish I had a lower amp fuse to test the bias lead. The plates aren't glowing anymore and there is a beautiful blue glow and sound comes out of the speaker.

The wire that feeds the 1.5K resistors to pins 5 on V9 & V10 comes from a .1/600v cap that goes back to pin 6 on the PI. I do not understand why the voltage is so high there on the power tube side of the cap. I replaced the cap with a new one and still have the same reading.

[martin manning]

If you take V9 and V10 out does the fuse blow? If it will hold that way then maybe you can get some more measurements.

BTW, this is where you need a small circuit breaker that you can clip across the fuse holder.

[Phil_S]

I'd take a hard look at the bias circuit and check all the parts carefully. I might swap a pair of 5K fixed resistors for the pot to see if that's the problem.

[gui_tarzan]

No, but after about 15-20 seconds V7 & V8 start red-plating. The bias voltage is at -100v too, which I didn't expect. I also have +54v at pins 5 on V9 & V10 sockets.