Amp smokes but keeps playing

[alvarezh]

Yes, the amp is cathode biased. Now, that bias arrangement surpasses my knowledge, it's somewhat unorthodox, I have never seen that circuit.

Bad schematics are known to run around the web, although I am not in any way or form implying this one is bad.

The caps on the bias circuit usually are there to bypass high frequencies and increase efficiency on the tube and they are placed negative to ground, but that 47K resistor connecting to 260 vdc really confuses me.

Anyhow, as we wait for a proper and accurate explanation from another forum-nee, I would proceed as follows:

Leave the tubes out for the moment. Lift one end of all the electrolytic caps (remember to discharge all of them before proceeding) you replaced and check that none are shorted on the DC scale of your tester. This because one could of been shorted from new or could of have shorted the first time you fired it up.

If you find none, then reinstall all the caps, and proceed to place the tubes back one by one (turn it off for each installation) into the amp starting with your preamp tubes. Observe for smoke.

Report back.

All the best.

[Tillydog]

Can somebody expand on the topic of bias supply? I've attached the schematic I'm working from....

It is cathode biased - ignore my previous comment. That schematic is highly dubious, though, IMHO.

[Structo]

Smoke was white

Hey! Another new pope!

BAWAHAWAHAWA!

I'm glad I didn't have a mouth full of coffee when I read that.

Anyway, pliers and a soldering iron. (what more do you need, maybe a blow torch?)

Get all medieval on it!

[img:175:135]http://3.bp.blogspot.com/-UabO9OdDU3k/T ... g_Gimp.jpg[/img]

Seriously though, OP please don't go beyond you skill level with a high voltage tube amplifier.
They can bite hard and stop your heart.

Ask for advice before going any further.

[artlaugher]

Smoke was white

Hey! Another new pope!

BAWAHAWAHAWA!

I'm glad I didn't have a mouth full of coffee when I read that.


Seriously though, OP please don't go beyond you skill level with a high voltage tube amplifier.
They can bite hard and stop your heart.

Ask for advice before going any further.

Sorry to sound like such a terrible newbie, I do in fact appreciate the dangers of high voltage tinkering. This being my first tube project I'm working on an 8 watt amp, the biggest cap in it is 10uf.

the schematic is from a publisher called "Photofact". I bought it for a few dollars on eBay, it seemed legit compared with the way the thing was wired when I first looked it over.

Anyways you guys were right, it was smoking because a ground connection involving one of the electrolytic caps wasn't wired properly. The amp now powers up beautifully, but it sounds viciously awful - the output cuts in for only a few moments at time, punctuated by weird and horrible sounds and an unsettling "pulse" when the the resistance on the bass control is turned up.

I believe I've created a monster , I think I should cut my losses and buy a fender excelsior, at least then I could reuse my JJ 6v6's...

[Reeltarded]

The symptom mimics a bad output transformer, or at least one failure mode of an OT.

Don't let an 8w amp fool you. Approach rectified power as if it is scary and dangerous. It is. Be suspicious of it's intention at all times. Pretend you are working with a rabid bobcat that has a head on both ends. It wants to shred you to little bits.

Rawwwr.

[Colossal]

Sorry to sound like such a terrible newbie, I do in fact appreciate the dangers of high voltage tinkering. This being my first tube project I'm working on an 8 watt amp, the biggest cap in it is 10uf.

Hi Art,

Keep in mind that it's not about the wattage or the size of the capacitors. It's the potential (voltage) your amp is operating at. You could have a 1W amp that operates at 500V on the power tubes that will kill you just as dead as a 100W amp operating at the same voltage. Whenever you probe an amp, always do so with one hand in your pocket. This is so if you do take a hit, it is localized and you stand a better chance of survival if you do not use your heart as the path to ground down through your other arm.

[Tillydog]

Anyways you guys were right, it was smoking because a ground connection involving one of the electrolytic caps wasn't wired properly. The amp now powers up beautifully, but it sounds viciously awful - the output cuts in for only a few moments at time, punctuated by weird and horrible sounds and an unsettling "pulse" when the the resistance on the bass control is turned up.

Looking at the schematic more carefully, I think it's ok - the screens are fed from a potential divider from B+ to cathode - means you need to use HV rated caps for the cathode bypass.

Are you *sure* that all the caps are connected properly, because you could get that sort of instability if one or more of the smoothing caps are u/s.

[surfsup]

Tilly, the screens are fed from the PS rail, imo. The 47k connected to the cathode should create a current loop and feed some ripple into the cathode as well?

[Tillydog]

Oops, I think you're right

So the 47k feed just enough ripple to cancel out hum from the minimal smoothing? Neat.

[surfsup]

Could be to reduce hum in the output stage me thinks. Wonder what the real voltages are at the cathode and screen.