Traynor YBA-1 popping circuit breaker

[Beyer160]

No, pins 1 and 8 should be connected together (usually to ground) regardless.

Power it up with no power tubes. You can leave in the small tubes if you like. Does it still pop fuses? Also, does it pop a fuse even if in standby, or only if the standby switch is in operate mode?

get out an AC ammeter and measure current draw while it idles. How much current is it using?

A visual inspection is always important, but the vast majority of bad parts look exactly like good parts. And your problem may not be a bad part, but could be a short between two adjacent things.

So I hooked my meter up inline with the breaker, with the min/max setting engaged. At startup it drew 2a, but quickly settled down to .1a at idle. I started playing through the amp, and the current draw gradually increased to .5a as I turned the volume up. At this point it was quite loud. I gave the volume another bump and hit my best Pete Townshend power chord, and the breaker blew. The meter revealed an impressive 12.23a draw at the point of failure. At this point there was a distinct burning smell.

This is well beyond my troubleshooting experience- any suggestions where to go from here? My first thought is to just shotgun the power section and start over, but that's only because I can't think of anything else.

[The Dude]

Did you check the bias before you started romping on it?

[Beyer160]

Did you check the bias before you started romping on it?

Yep, when I installed the new tubes. Dissipation is in the mid- 30% range... I forget exactly, I'd have to check my notes, but the point is the bias is fairly cold.

[TimmyP1955]

Put a .5A fuse in the B+. See which fuse blows first. This may tell you if you have a power tube or OPT issue, Vs a PSU issue.

[The Dude]

Some of the YBA series amps were fitted with flyback diodes that are not shown in the schematic. Does your amp have them? If so, maybe one or both of them is failing after warmup. If it has them, temporarily unhook one side of each and see if your problem goes away.

[g1]

If you can figure out where that burning smell is coming from that would probably help narrow things down.

[Beyer160]

Some of the YBA series amps were fitted with flyback diodes that are not shown in the schematic. Does your amp have them? If so, maybe one or both of them is failing after warmup. If it has them, temporarily unhook one side of each and see if your problem goes away.

Nope, no extra diodes except for the rectifier, which I already replaced.

This is the schematic that came with the amp-
http://www.thetubestore.com/lib/thet...-Schematic.pdf

If you can figure out where that burning smell is coming from that would probably help narrow things down.

I know, right? I did stick my nose in the chassis as far as I dared but couldn't locate it. I was just glad nobody saw me huffing my amp.

[mozz]

Check for the diodes on the output tubes, output trans primary, as in this schematic. Traynors have many changes and many schematics don't match.

[Beyer160]

Check for the diodes on the output tubes, output trans primary, as in this schematic. Traynors have many changes and many schematics don't match.[ATTACH=CONFIG]45086[/ATTACH]

There's only one other diode in the amp, D1 on my schematic. That one I hadn't replaced earlier, though I should have.

[1ampman]

Check the ohms of the speaker cab & check the speaker cable for a short.

[Randall]

Could it have an arcing transformer or tube socket?

[g1]

There's only one other diode in the amp, D1 on my schematic. That one I hadn't replaced earlier, though I should have.

Those ones in the schem mozz posted are thyrectors, they look more like a brown rectangular box around the size of a 10W sandbox type resistor. Maybe post some pics?
Another possibility is a failing coupling cap between the PI tube and the power tubes.

[Enzo]

Could it have an arcing transformer or tube socket?

Absolutely, would be consistent with working OK up to a certain level.

[AL30]

If you can figure out where that burning smell is coming from that would probably help narrow things down.

Check the power supply resistors. Located on the board? (it's been a while). Those guys love to burn up.

[Beyer160]

So after screwing around with this on and off for the past few months, I think I solved the problem. I wound up replacing the sockets, two big 470 ohm 10w resistors, and another resistor that looked suspect. Oh, and I realized that although the power cable had a 3-prong connector, it was still wired 2-wire, so I replaced that, too.

And thank Flying Spaghetti Monster for my lightbulb current limiter- I managed to cross two heater wires when wiring up the sockets, but was saved from disaster by the "you dumbass" light.

Now I can crank the bejeesus out of it, and damage my ears to my heart's content without tripping the breaker. Thanks for all the help, everyone!