Two Stroke = Smoke

[bclick]

I built the Two Stroke class A amp based on Dave Hunter's design in the Guitar Amp Handbook. It turned out nice and I have been playing it here and there for about 9 months.

At the very end of a long and loud set last night my audio quit completely and I observed some smoke rising from the amp! Opened it up today and can see no visible distress and it plays like usual.

Thanks in advance for any: Ideas?

[Cliff Schecht]

When I see smoke the first thing I suspect is something in the power supply, usually an underrated or otherwise bad resistor. I'd start there. If any other parts are underrated or of questionable quality then I'd look there as well. Gutshots would help us..

[bclick]

Thanks Cliff. I will double check ratings on PS components. The PT is a Weber W022772, rectifier is 5Y3 and was using a single KT66 power tube. I have the PT switched so I can use the 120 or 125 primary winding, and had it on the 125.

[Structo]

Modern production rectifier tubes are sometimes short lived.
I have read the 5Y3 are particularly unreliable.

I have a few 5U4GB tubes that are over 50 years old that are still good.

Put your sniffer real close to the output transformer.
They have a unique smell when they have overheated, same with PT.

Do some voltage checks. If you have recorded them in the past, compare readings.

Make sure you are not too hot on the bias.

Look at the power resistors carefully for signs of excess heat (colors look funny or scorched)

How is the match up between your output transformer and speaker load? (primary to plate resistance to speaker impedance)

[bclick]

All good points Tom, thanks. Will do some checking after work.

I do have the OT switched to 8 ohms and running into a single 8 ohm speaker. The OT is a Weber WSE15, I believe 5K on the primary.

[M Fowler]

Were you pushing that Two Stroke maybe a little too hard on that set?

It could be a combination of things as the guys have suggested; hopefully the OT and PT survived.

Got any sound at all?

Mark

[bclick]

<Were you pushing that Two Stroke maybe a little too hard on that set? >

Ha ha! Come on Mark - this is rock and roll! The volume was around 30% which was pretty loud, but I have played other sets where I had it dimed and my poor little 10 watts was not keeping up with the other guys!

It was dead at the time last night, but now plays like normal. I ran a keyboard demo through it for several hours, but can't reach that volume here at home. Guess I maybe need a dummy load for testing.

[rdjones]

Did the smoke come from inside the amp, for certain ?
The smoke should have left a bit of trace behind where it originated.

It may have been something in the incoming line voltage, look at the fuse/holder, iec connector, power switch(s), anyplace a contact may have opened just enough to spark.

rd

[M Fowler]

It could have been the beer can rattled over knocking the cigarette off the amp causing a swoosh of smoke Or not.

Up in smoke my a$$

Mark

[Andy Le Blanc]

you can tell by the smell she isn't feeling very well

The transformers will smell waxy if there potted or acrid if the insulation layers are shorted stick your nose in there

resistors have a particular stench too

[bclick]

Ha - you guys are harsh! I was only drinking Mountain Dew & not even smoking anything! The last chords of One Way Out were ringing but I had nothing. Did not inspect closely at that moment, but there were definitely some wisps of smoke from the amp, and quit as soon as I turned it off.

Sniff test & close inspection reveals nothing.

Just did some voltage readings and they are higher than when first assembled. See attachment.

So the question in my mind is what component(s) can fail under duress, enough to stop the signal, and then recover?

[Andy Le Blanc]

look on the ends of that first filter cap, see if there's any signs that it popped

On the positive side if its axial, there's always a little safety vent
might only be visible as a little bump

if you were pushing it, it could be the PT, it still runs?, check for heat.

and yank the power tubes, get a close look at the socket and tube base
looking for any sign of arcing,

[M Fowler]

I'm wondering if one or both of the dropping string resistors overheated?

Mark

[Structo]

How close is the power transformer to being maxed out?

[bclick]

First 2 filter caps are axial Sprague Atoms; 20uf/500v & 16uf/475v. No signs of distress on either.

Sockets look clean. Resistors look and measure ok right now.