Incoming Amp-PRS 25th Anniversary

[JustMike]

 Does anybody have or know where I can find a schematic? I believe it's based on a 100w Marshall, perhaps with power scaling. The owner says it's been modded and he wants it restored to original. I haven't seen the amp yet so I don't know how bad it's been molested.

[JustMike]

 I got it and opened it up. First off this thing is pretty compact for a 4x EL34 amp and the iron is HUGE. Kind of reminds me of an old Mesa MK2. The circuit looks like a Plexi with the bright and normal channel inputs internally jumpered. It's got an interesting phase inverter master volume (I think) which is on a 6 position switch.  It's got a small fan on the back cover which is powered from the heater winding and thru a little rectifier board.
 The first thing I did when I got it out of the headshell is check the fuses. It has a 4A mains fuse and 4 internal fuses; one on each leg of the HT and one on each leg of the heater winding. There's a sticker on the chassis that says the  heater winding fuses are 10A 250v Type 3AG. There were a 15a and a 20a fuses in there. I replaced them and hit the power switch and those fuses instantly turned cherry red. I shut it down before they blew. With all 7 tubes and the power lamp bulb pulled, the heater ckt is open, so it's probably a shorted tube. My guess is one of the JJ EL34's. But a shorted filament? How would I check for that without a tube tester?
 BTW, if you don't know by now, I'm a hobbyist-mostly a builder, so my troubleshooting skills are a work in progress. And by this I mean if anybody has any suggestions, I won't be offended by even the most basic tips.
 Thanks,

[sluckey]

Pull all tubes. Plug one in and turn on. If it lights up as expected, pull it and put it in a "good" pile. Repeat with next tube, then the next, etc.

[shooter]

But a shorted filament?

shorted to something else like ground, cathode.......

[JustMike]

OK, so I did what Sluckey suggested and lo and behold, it stressed the fuse ( I could see the fuse bowing as the 3rd power tube was installed) but I carried on and got all of the tubes in and the fuse didn't blow. I then plugged into straight power (no lightbulb limiter) and powered it up. The fuse glowed a little at first and then settled down. Hmmm..this doesn't seem right.


 I then placed the amp on it's side like the leaning tower of Pisa, resting on one end of the chassis and the power transformer. I turned it on and the fuses blew. I suspect something is loose in a tube and it's shorting when they're horizontal.
 

[sluckey]

OK, so I did what Sluckey suggested and lo and behold

I don't think you did. My suggestion would only have one tube plugged in at any time.

[JustMike]

OK, so I did what Sluckey suggested and lo and behold

I don't think you did. My suggestion would only have one tube plugged in at any time.

Ah! I'll try that next with the chassis in Pisa mode.

[JustMike]

Tested the tubes ONE AT A TIME and moved them around thru all 4 octal sockets. All seems OK. I'm beginning to think the 15& 20A fuses were put there by a tech and not a guitar player. Even though the sticker on the chassis clearly says 10A 250V.


 Also, it doesn't say, but I assume they should be slow blow fuses due to power up inrush? And perhaps, this is a minor issue since most amps I've seen don't have fused heaters? I mean it's a nice feature, but how often does a heater short take out a transformer?

[JustMike]

 Talked to PRS tech support and Shawn said to put in bigger fuses. I did and the amp works. But this begs the question (in my mind anyway)-what happens to an amp if the B+ is turned on and there's no filament voltage? how do the tubes react to this? I'm guessing that they just don't...

[pdf64]

...what happens to an amp if the B+ is turned on and there's no filament voltage? how do the tubes react to this? I'm guessing that they just don't...

I can’t see an issue, they just wouldn’t conduct any HT current.
I guess that as PRS didn’t advise / suggest that the valves would have been damaged, maybe they couldn’t see one either.

[shooter]

what happens to an amp if the B+ is turned on and there's no filament voltage

the B+ is simply a "potential", to make it go dynamic; there needs to be a path to ground.  B+ must be sufficient to "draw" an arc, otherwise it just stays a "potential"  (assuming good working tubes)