Shorted PI coupling caps. Why?

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Post Reply
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Shorted PI coupling caps. Why?

Post by sonicmojo »

I just ran across a Mesa Son of Boogie with a pair of shorted PI caps, of course causing massive runaway bias. What might have been the trigger for something like that? Amp works fine now with replacements.
---------
Bryan
tubeswell
Posts: 2337
Joined: Thu Mar 27, 2008 6:42 am
Location: Wellington. NZ

Re: Shorted PI coupling caps. Why?

Post by tubeswell »

Underrated caps/over-voltage across them - due to some modding idiocy/experimentation with voltage doubling inappropriately raising the HT/B+ supply?

Or something like a screen-to-control-grid short on 1 tube causing over voltage on one side - followed by failure of the coupling cap, followed by someone swapping the bad tube around to see if it was 'a tube? or a socket?' problem, causing the same fault on the opposite coupling cap. Could have been caused by plugging in the wrong type of output tubes (with a different pin-out) , or plugging in some tubes incorrectly (if the key/spline was knocked off)?

Or just an unfortunate coincidence of a bad pair going at the same time? - but much more likely to be one of the other faults.
He who dies with the most tubes... wins
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Shorted PI coupling caps. Why?

Post by sonicmojo »

Hmm....this customer's amp came to me with 6L6 compatible 6N3C Russian tubes which I have used before too but I know they can be inconsistent and not rated for higher plate voltages. I wonder if I should recommend him changing them out although I can't think that was the cause. It may have been something prior. The caps were both the original Orange Drops - 600V.
---------
Bryan
tubeswell
Posts: 2337
Joined: Thu Mar 27, 2008 6:42 am
Location: Wellington. NZ

Re: Shorted PI coupling caps. Why?

Post by tubeswell »

The 6P3S is supposedly only rated for 250Vish and I've seen Russian sites that say to keep the voltage on these under 300. But then again, I've seen some SF amps with these in them idling at 500V perfectly functional and happily running (although admittedly woefully coldly-biased). I think the reliability of these is really hit and miss. It could well have been the cause of the problem.

The mil-spec 6P3Se (wafer base) is a much better/more-reliable (and completely different) tube. But even that should definitely not be run at 500V - because of potential arcing (but is perfectly happily run at 460, and biased at 70% - I have several of these running like this in my own amps).
He who dies with the most tubes... wins
Stevem
Posts: 4750
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: Shorted PI coupling caps. Why?

Post by Stevem »

Keep in mind that caps have a max AC voltage rating ( as in audio signal ) also, and this is close to 1/2 of the caps D.C. Voltage rating!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
User avatar
Leo_Gnardo
Posts: 2585
Joined: Thu Sep 27, 2012 1:33 pm
Location: Dogpatch-on-Hudson

Re: Shorted PI coupling caps. Why?

Post by Leo_Gnardo »

FWIW I've had a couple of cases of orange drops gone bad in Mesa/Boogie amps 25+ years old, despite the reputation of OD'd being "tough as boots." Some shorted, some making pop/sizzle noises. For some OD's, their time is already up, repair people be aware of this, don't assume "nawwww.... that can't be."
down technical blind alleys . . .
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Shorted PI coupling caps. Why?

Post by sonicmojo »

Who knows....I didn't check the socket wiring while on the bench but there were also some EL34s stuffed in the back of the amp. I think I'll recommend he keep standard 6L6s in this SOB from now on. I'm pretty sure these were made for just 6L6 and there is no bias adjust pot or anything in this one.
---------
Bryan
Stevem
Posts: 4750
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: Shorted PI coupling caps. Why?

Post by Stevem »

The 34s also need a full 1 extra amp of heater current to boot, which the PT may not have on hand!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
User avatar
sonicmojo
Posts: 669
Joined: Thu Mar 03, 2011 3:41 am
Location: Oahu, HI

Re: Shorted PI coupling caps. Why?

Post by sonicmojo »

Funny enough but I had a broken MB Bass 400 come through with another shorted Orange Drop yesterday. Luckily I thought to check them as an early step!
---------
Bryan
Stevem
Posts: 4750
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

Re: Shorted PI coupling caps. Why?

Post by Stevem »

That's a cheap fix that puts a smile on the customers face until you tell them the bad news that they also need over a quad of new output tubes, lol!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Post Reply