Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BobL on June 27, 2020, 12:36:32 pm
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Playing my PR a while back and the volume faded from normal to dead over the course of about 5 - 10 seconds...
Replaced the power tubes, no difference. Unplugged it and put the original power tubes back in.
A day or so later I turned it on again to see if I could figure out what was up... and it worked... for a couple of minutes... then faded out again.
Since then it has been dead...
I'd had some crackles from what I thought was a bad joint on the .02uF cap coming off of the plate of either V1 or V3 (I can't recall exactly which at this point) months back, and retouched the joint, which didn't seem to fix it, but then did seem to fix it on a subsequent attempt, so I do wonder if the problem there was actually with that cap. I have a single .0223uF Mallory that I just tried in place of both of these caps with no difference.
Voltages aren't good...
Filament is showing 3.6V AC on V4/5 and 3.3 on V1 - 4.
Everything else on the preamp tubes is showing from -1v to 60mv DC, basically nothing...
V4/5 are showing -1v at pin 3... this has me concerned that maybe the output transformer is dead?
Rectifier has 330v DC at 4/6, and 5.6v AC at 2. 8 shows basically nothing (which as I recall, has always been the case).
Transformers are both Mojotone - OT is a MOJO772 and PT is a MOJO760.
What else should I be looking at?
Thanks in advance...
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Sounds like the rectifier tube filament is not lit up. What do your eyes say about that?
If rectifier filament is lit then replace the tube.
BTW, pins 4 and 6 are AC, not DC.
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My eyes tell me that you are in fact correct, and it is not lit... thought it was, but was just a reflection from the tube next to it.
This was a Valve Arts 5AR4... I threw a 5U4GB in and the amp is working again. Would there be any reason to worry about anything beyond the tube just having gone bad?
Appreciate the help, as always.
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Use your ohm meter to check resistance between pins 2 and 8 on the tube. Should be low resistance unless the filament is open. If you measure low resistance then put the tube back in the amp and wiggle around a bit. Does it light up? I'm thinking that the tube is probably OK and that it was just a bad connection in the socket. I've had some new production tubes that had slightly skinnier pins than old tubes and they sometimes don't make good connection in the socket.