Hum and smoke from Peavey Classic 30

[slidincharlie (Carlo P)]

Hello Ampagers!

A harp player friend of mine brought me his Classic 30.
I did a few of Steve Ahola's mods on this amp a year ago, and it has been working fine... so far.
Now it hums like crazy when I turn it on, even with all the the tubes off, and after a few seconds some smoke springs from an area roughly between the fuses and the treble pot, or thereabout.
I could not spot any burned or charred component.
I insist on the fact that the amp hums without the tubes, and that the hum is unaffected by volume setting.
I am ready to bring the amp back to the owner and suggest him to go to a pro repairman, but first I wanted to hear your advice.

TIA

[Rob Mercure]

Hey Carlo,

Yeah, I know, I owe you a few e-mails - you should get one tomorrow in fact. But back to the present issue - a transformer with a shorted turns or operating into a short circuit (the same thing actually) will sometimes draw enough current to make the core vibrate turning the entire tranny into a "speaker" (and anything it may be mechanically coupled to such as a chassis).

And in one case I observed a PT and an OT close enough to each other and "suitably" oriented for there to be sufficient magnetic coupling between them - due to the extreme current flow into a shorted load on the PT - to induce enough current into the OT to drive the connected loud speaker. "Hum" with the output tubes out - and this was a last 1950s Altec studio amp!

The first thing I would do is unsolder all of the PT secondary leads, plug the power cord up, and observe what happens - might be wise to use a light bulb as a current limiter. If the symptoms abate there is a shorted load - if not you've probably got a shorted turn in the PT. On the tube amp troubleshooting page there is a copy of a very simple shorted turns indicator using a neon bulb and a few other parts that Wayne ("Doc") Horner submitted to Ampage about 10 years ago.

Rob

[Enzo]

OK, here is my thoughts. Nothing cosmic. If the thing hums into the speaker without tubes, I still think in terms of current flowing. Disconnect the two flyback diodes by the power tube sockets and see if it stops. A leaky one of those would drag down the B+ and send hummy out the other end. A leaky one of those also might burn for you and be hard to spot.

The flyback diodes are the ones wired in reverse from plate to ground at the power tubes.

Another idea. With the amp chassis exposed, power it up with the room lights off. Watch the amp for any small sparks in the dark. They would be an arc somewhere.

[slidincharlie (Carlo P)]

Hi Rob, I'll catch you later.

Enzo,
you were right about those diodes. One of them (CR6) gets hot during operation, and it is burnt.
Now I wonder: how do I buy another one of the same type? The diode is labelled SR2873 on the schematic, and DT GP1250 on its own body. What do I have to ask at the local electronic shop to buy a new one? What are its specs?

[Steve Conner]

If it were me, I'd just replace them with two 1N4007s in series. Maybe UF4007s if you can get them. JM2C

[slidincharlie (Carlo P)]

Two 1N4007 in series? It's HUGE! The stock diode is tiny... (and we are talking of a hard-to-work PCB here)!

[Enzo]

Oh, it is the size of a common 3A diode at its largest. it is a transient suppression diode. 2kV rating PIV. only 250ma forward current. A couple 4007 in series ought to work fine.

[slidincharlie (Carlo P)]

So do I need just to ask for a 2kV PIV diode for 250mA current?
Is that all?
I hope I find it, otherwise I'll use two 4007 in series.
Thanks a lot guys.
I'll post after the replacement.

One question: would the amp work WITHOUT both diodes? In a typical Fender amp there isn't any such diode between the power tube plate and ground.

[Ray Ivers]

Carlo,

would the amp work WITHOUT both diodes?

Yes, it will work fine (assuming the diode failure hasn't damaged anything). FWIW, if I had the amp I would clip them both out, check the amp out thoroughly, and if it's OK, close it back up.

I realize these diodes are there for protection; on a scale of 1 to 10, how would you rate their success?

Ray

[slidincharlie (Carlo P)]

I am wondering if there is any issue that caused the diode burning, or if such a diode is supposed to die by itself without any apparent reason...

[R.G.]

Plate connected "protection diodes" are there to snub leakage inductance spikes under bad operating conditions.

There is a transformer between the plates of the output tubes, and the middle f the transformer is tied to B+. When one plate pulls its end of the transformer down, the other end flies up. The end of the transformer at the tube that's turning off can see as much as twice B+ minus the "saturation" losses in the tube that's on, maybe 50-90V. So you can easily see 600-110V depending on the B+ in the amp. That's when it's operating properly.

If the "on" tube then tries to turn off instantly, the current stored in the transformer core and the leakage inductance tries to keep the current flowing and produces voltage spikes. These may be up or down on either output tube plate. The reverse biased diode clamps its end of the primary to not going below ground, which also clamps the positive side to no higher than 2X the B+.

But leakage inductance is by definition magnetic flux that does not couple to the whole set of windings. So the high-side primary can see spikes from its leakage on top of the doubled B+. That gets big. The 2kV diodes probably are zenered on some spikes. The exact voltage is hard to predict, because they're just guaranteed to be more than 2kV. But they will break over somewhere, and so keep the instantaneous voltage at the plates under 2500? 3000? volts.

This prevents arc-overs inside the tubes (mostly) and arcing on the outside of the tube sockets, as well as some punctures of transformer winding insulation.

It's a try at protecting things. It works, kind of. It's not the best way to do this, but it's probably better than nothing.

[Ariel]

I am having the same problem with my Peavey Classic 30....although thankfully without the smoke (although I turned off the amp as soon as I realised the problem...ie after about 5 seconds).

Did replacing the diodes work? Or did you just solder them out?

ari

[slidincharlie (Carlo P)]

Ariel,
yes, replacing the burned diode worked.
I used two 1N4007 in series as per the advice of the guys above.
In that amp the power tubes were gone: flickering, arcing internally and making loud pops. I replaced them along with the diode, and the amp started to scream again.

[Ariel]

Hi Carlo,

Thanks for the advice, I had better get to work then

Ari

[cb.studios]

Samething here now its dead

3 questions

1. Did my diodes burn out completely? The P30 started to hum.... then it the tubes got real hot.... now there is no sound hitting the speaker nothing.

2. How can you test the 2 blocks on the P30 to see if they are bad.

3. Also I could use a great reference site for more info on Caps Resisters Trans., etc. Kind of a new guy but I have been fixing things for years. For example I still dont know how to discharge a Cap. I just hit it with a screwdriver and sometimes I get a spark. Im afraid I might get it wrong one day. Im kind of looking for the Electronic guru Bible.

Thanks
Mark