6505+ blows output fuse on standby. Does not with no power tubes. (

[g1]

Yep.

[Biffstar]

Edit (erased): I'm an idiot and I measured these all wrong... see post below...

[Jazz P Bass]

If R54 / R55 junction does not have -50Vdc, then the connection back to the power supply is bad.
Could be a burnt copper track or a faulty solder joint.

You will have to flip the preamp board to find out.

-phase-inverter.pdf

[Biffstar]

So I erased the last post that I made about an hour earlier... turns out that I measured the resistors, capacitors and pins backwards... You guys have kept stressing NEGATIVE voltage... I was measuring as if positive, and kept seeing zeros... I'm sorry for wasting your time with my retardedness...

But I've gone back and measured them again with the correct polarity. Turns out the resistors, caps, and pins are NOT zeroed...

Here is the bias junction for the two resistors (J170/J172), which goes out/in to the external bias adjustment knob:



It reads proper voltage (a few volts high, but I was playing with the knob to see if it still worked):



So with that, the resistors DO provide a reading, a difference of 10 volts between input/output. I'm not sure if the output voltage is supposed to be around 10 less than the input, but the input voltage is the same as the bias junction/bias adjust.



The capacitors on the input side are seeing around 300 volts, which I'm guessing is ok?



And the capacitor output side is seeing around 45 volts:



And finally, the J57/J58 pins are seeing the same as what the capacitors are putting out:



So what does this all mean? The voltages around here seem to be around the bias voltage mark, but I'm not sure if that's what the capacitors and resistors SHOULD be doing... Can one of you guys confirm?

[alexradium]

10 volts its too much,a good working circuit is less than one volt.

Is that your best meter?
Not very safe for 500V,there are good meters around for 50 bucks.

[Biffstar]

^ My tester works fine; I've been using it for years. Those readings are powered-on readings.. not what the tester itself was passing through the circuits.

So curiously, after testing and seeing what are somewhat normal readings last night, I couldn't help but plug some tubes in, and lo and behold, the stupid thing WORKS! Wtf!? I didn't even DO anything! Did something short itself back into working condition? I have no idea...

With a full complement of tubes, comes out of standby. No arcing, no red plating, all normal:



I plugged in a guitar, and got clean sound. The EQ controls on the clean channel work too. I'm not sure if the voltages are healthy enough to keep it as is, or if I should still swap those resistors and caps anyway, but in any case, I get proper sound finally.

However, there appears to be a new problem... The amp is now stuck on the clean channel only. Back when I could only turn it on with no tubes installed, I could flip between green/red (clean/dirty) no problem. Now, flipping the channel switch just sits on the clean/green channel.

I have no idea where to begin on this one... I tested the switch, and the switch itself is fine still. (it worked before, recall). All 6 of the preamp tubes are lit, but I'm not sure if a dead dirty tube would prevent it from channel switching? Bad relay somewhere maybe?

[Biffstar]

Actually, disregard the above. On top of the power board is the little daughterboard that handles the footswitch for channel switching and effects loop send/return. I found a toasted transistor on that board that I thankfully had a replacement for kicking around. Soldered it in, and bam... channels switch just fine now. Amp is fully functional. Still not sure how it happened... But the parts I did replace earlier on were necessary.

I still think I might replace those two resistors R54/R55 though. They're still doing their job, but being as discoloured as they are, who knows what life they have left in them.

If I had to guess what had originally happened to this amp with the previous owner, I'd bet that he had a tube go bad, fully short out and arc internally, zapping half the shit on the powerboard, and whatever else further down the chain, namely the bias resistors and caps (which still seem to be ok in the end now strangely). Not sure how it went from 500+ volts in the pin5 bias pins of two tubes, 0 volts in the other two, and now back to 50 volts in all four without me actually swapping any parts related to the bias... Something must have shorted itself somewhere back to normal. No idea how or what, but given that, I still question the long-term reliability of the amp. Guess we'll see.

In any case, it's up and running. I want to thank all of you who chimed in to give me a hand! In particular Enzo, Jazz P Bass, Pedro, g1 and The Dude for really taking the time to break down the exact resistors, capacitors and other components and their readings for me. I'd have been completely lost otherwise, and this was a fantastic learning experience with all of your guidance. Very much appreciated.

[Biffstar]

Actually, one last question. My B+ voltages are still high side, near 550 volts. In doing some digging, everything I've read says that these amps should be around 490-500 volts (including Enzo mentioning earlier that it should be between 480-500), so I'm quite far up there. Not sure if this will cause problems, or if the amp will eat tubes because of it, but I'd prefer to have this thing back to at as-close-to-stock values as possible.

What controls this voltage? Is it a couple of resistors that I can check out? Perhaps one of them is on its way out?

[Enzo]

Nothing "controls" it. Is your 550 with no tubes in it? 500v or so ought to be when the tubes are all in it and running. All amps have B+ rise without tubes.

[Biffstar]

Nothing "controls" it. Is your 550 with no tubes in it? 500v or so ought to be when the tubes are all in it and running. All amps have B+ rise without tubes.

You are correct, that was without tubes.

So I just tested it again with all tubes in and powered on off of standby, and each tube is reading 522 volts.

That's still a tad high, no? Is this nothing that can be reduced/changed?

[Enzo]

It's fine.

[Biffstar]

It's fine.

Haha ok. I hope so.

Thanks again, Enzo.

[Biffstar]

So in case anybody's still interested, I believe I've figured out why the amp just suddenly started working again without me doing anything else beyond replacing the initial blown components. I wasn't content with just accepting "hey it works!" and leaving it at that; I wanted to find the source of the problem so that it wouldn't happen again.

The problem, it would appear, was the small effects loop/footswitch board all along. Look at how little clearance there is:



Hard to tell in the photo, but those pins on the end are nearly touching the board, and that's with no cables plugged in. The downward weight of some heavy effects loop cables in and out translates into an upward bend of this board, right into the power board. Add a bunch of heat, and over time it looks like it has bent permanently. Both boards on the sandwiched side have some pretty high solder pads and pins, and while it's hard to tell what actually touches, something does (or did at one point or another) causing some shorts.

It's also not a coincidence that this board is right overtop of the two tube sockets that were giving me grief, and that a transistor on the small board was fried... It probably got zapped with 500+ volts.

As a quick fix, I've put a few layers of electrical tape to keep them separated. Yeah yeah, it's ghetto, I know.. but it's better than nothing. Kind of a piss-poor design to have these two boards that close together, let alone with no insulating material between them.

[glebert]

Way to stick at it! Really good information for others.