Ampeg m12 a series of unfortunate events

[nateflanigan]

Hello, I've got another humdinger of an amp problem.  I tuned up a very early Ampeg M12 for a friend recently, pretty basic stuff, no real issues.  A few days later he says the amp is dead, fuse is blown, I figure it's an old fuse that hasn't seen any current in many many years, replaced the fuse.  A few days later, again the amp is dead, rectifier has blown, parts of it are mechanically broken and rattling around inside the glass.  I don't know what to call these other than "the spring like coils at the top of the tube".  Before noticing the rectifier was actually visibly broken, I replaced the fuse, powered the amp on to check voltages, and heard a frying sound in the PT.
I replaced the rectifier and just to be sure ran a test tone into a dummy load at full blast for 6 hours.  No problems, no frying noises, a few days later, amp is dead.  Fuse has blown again. 


Something is wrong, right?


As a note: my friend is a guitar teacher/semi-professional musician and typically plays (has the amp on) for many hours at a time. 


I'm suspicious of the power transformer, one because of the frying sound, but two, the rectifier voltages are 6.2v.  I think the transformer is frying rectifiers. Now this may be confirmation bias but it SEEMS like I can see the 5y3 cooking, it seems to get too hot, it's making those sorts of creaks something makes when it's too hot.


So, my questions are, am I correct that the 6.2v is frying the rectifiers, or at least does that seem plausible? And, is there anything to be done other than replace the PT?


Thanks!

[tubeswell]

A shorted rectifier will short the PT. The mode of failure is:
1) shorted rectifier tube (cathode/heater shorting to plate)
2) dumps high VAC onto the reservoir cap*, - which in-turn will short
3) causing the HT winding to be directly shorted to ground.

* polarised electrolytic caps can only handle high voltage in the proper polarity. If they see high voltage from the opposite polarity they will go south in quick fashion.

The burning smell was your PT frying. Consider it dead I would say.


See the attached R.G. Keen article about a very cheap and relatively permanent preventative measure for shorted rectifier tubes - doesn't affect the performance (forward voltage drop/forward current supply) of a tube rectifier, but it adds protection against a shorted rectifier tube taking out the PT. I do this to every tube rectified amp I see, including all the vintage ones. Some of those PTs are expensive to replace, and some of them are irreplaceable.

[shooter]

I tuned up

Tubeswells comments probably nailed it, but, you might have a chicken egg thing also;
how long was the amp NOT used before you tuned it?

when you had it, did you do voltage checks, bias checks, swap things around the area of failure?
I keep thinking while it was stationary it worked, when you moved it to the owner it broke

[John]

Your friend plays/has played the amp a lot? So it's possible that the PT is simply giving out. However, before I replaced the PT (which seems to be "working" since the amp played for 6 hours) I'd take out that 5Y3 and tack in 1N4007 diodes in it's place. It seems to me it's possible maybe the tube socket is a little dogdy (old) and you may be getting an intermittent short.


I realize you'll raise the B+ , but since it's cathode biased I'd go ahead and try it anyway. If those diodes fix the problem, then you just have to replace the socket.

[nateflanigan]

Thanks folks, I'll address these in order.


Tubeswell, thanks for the intel and the RG Keen article, I haven't seen that one. Just to be clear I didn't smell a burning smell but heard a frying noise, which I'm sure would have been followed by a smell had I not switched off the power.


Shooter, we don't know how long the amp sat.  My friend bought from an older guy who got it from his father, it's entirely possible it had been sitting in a closet for decades, at the very least years.  It's been back and forth so many times, I could really say what I checked when. 


John, this is a great idea, I was thinking about ordering one of those solid state plugs to replace the 5y3 as a cheap fix, thinking that if for some reason it was just that the 5v windings killing rectifier tubes but the other windings working OK this would be a cheap (probably temporary) fix.

[shooter]

sitting in a closet for decades

is this a wham/bam....
or bring it back to life make it sing project?

[nateflanigan]

It's more on the bring it back to life side of the spectrum.  He got the amp super cheap, it still had what I assume was an original set of NOS spare tubes for the whole amp, the original Jensen p12. I did the standard recap and high current resistors, recapped the trem and replaced a few signal caps with some fancy pants new caps.  I think all in he's still only got about $500 in the amp.  I do think he'd probably opt for a wham/bam stop gap to get some more use out of the amp without putting much more money into it.


Also, I will always ask everyone from now on if whatever is going on is a wham/bam or something else 

[PRR]

Shouldn't be teaching in his Sunday shoes. He should have a low-price transistor amp for the teaching studio, and wedding gigs, only bring the Ampeg out for recordings and occasional jams.

This also leaves you time to play with it in all idle moments until you are sure you have burned-out all the idle corruption.