Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on February 04, 2021, 04:24:41 pm
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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.
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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,
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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.
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But a shorted filament?
shorted to something else like ground, cathode.......
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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.
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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.
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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.
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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?
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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...
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...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.
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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)