Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: firemedic on October 04, 2011, 04:39:51 pm

Title: SE PT fried?
Post by: firemedic on October 04, 2011, 04:39:51 pm

I rebuilt my "5879 SE" yet again, & have recently updated the schem. It doesn't even have a 5879 in it any more. The input stage is a paralleled 12AY7 and the 2nd stage is a paralleled 12DW7 feeding a 6V6. It sounded real good, for about a week, with good voltages holding steady.
Today I took it out to the apparatus bay & turned it on, then walked away for a minute. When I came back the AC fuse had blown.
I replaced it, powered on & watched/listened. The pilot light seemed a little dim.
It took a minute but I started to hear some snap-crackle-pops (nothing from the speaker) and a little bit of smoke wafted out of the PT side of the chassis. No arcing in the tubes. Power off. The PT is HOT. This was @ an hour ago & it's still hot.
At this point I am just venting, all I can tell right now is that I don't see any scorch marks inside the chassis, the filter caps look good & the OT fuse is intact. The PT is from Doug (the little one), I have a hard time blaming the tranny. My test stuff ain't here so I can't know any more till I get home. Stand by for more......

Title: Re: SE PT fried?
Post by: tubeswell on October 04, 2011, 05:35:02 pm

My guesses

Either there is a short of the B+ to ground somewhere (through a shorted filter cap, or output tube or...)

Or the PT windings have developed a short

Title: Re: SE PT fried?
Post by: firemedic on October 05, 2011, 02:00:35 pm

I found 6 ohms from one of the HT windings to ground, no recto plugged in. I assume that's not good.

Title: Re: SE PT fried?
Post by: HotBluePlates on October 05, 2011, 05:41:05 pm

Quote from: firemedic on October 05, 2011, 02:00:35 pm

I found 6 ohms from one of the HT windings to ground, no recto plugged in. I assume that's not good.

Is that after unsoldering that secondary from the circuit?

If not, you may be detecting a short in connected circuitry.

Title: Re: SE PT fried?
Post by: RicharD on October 05, 2011, 08:15:04 pm

Just a guess, look for a rogue nut or solder blob.  Last amp I built, I took it out for a drive to Dummy Load's house and when we ran it, it started arcing.  It had been fine for a week before that.  Bad joint & easy to fix.  Found via a bumpy road...... ok bad driving.