If it happens without tubes in (but you think the PT is still alright? - Are you sure??) then it could be filter cap(s) shorted out (or something else shorting out the B+)
Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
Nah - 260VAC vs 272VAc for your HT windings is the same difference (if you know what mean)
Check to see whether you are getting (approximately) 325VDc on C38, 335VDc on C39, 355VDC on C40 and 360VDC on C41/42 (in that order). I would guess it is possibly C40 (if R52 is blowing out)
Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
Duh - Sorry I was looking at the power supply back-to-front! - C39 (Not C40) must be shorted causing too much load on that little 1/2W resistor. I bet if you replaced C39 with a new 450-500V 20-22uF cap it would kick it back into life.
Tubeswell....Amp is now working and wow...does it sound GREAT .....but there is a hum. What I found...see picture...a very tiny thread-like piece of solder between 2 of the 500v caps. The first thing I do on amp repairs is a complete visual and somehow I missed it. Double DUH.
As for the hum...What I put in was a smaller 450v, 22mfd cap (1/2 the actual size) and a 47ohm 1/2 watt in R52. If the R52 is the problem would you give me the color code for the right resistor for 52? Thanks
Doc
Tubeswell....Amp is now working and wow...does it sound GREAT .....but there is a hum. What I found...see picture...a very tiny thread-like piece of solder between 2 of the 500v caps. The first thing I do on amp repairs is a complete visual and somehow I missed it. Double DUH.
As for the hum...What I put in was a smaller 450v, 22mfd cap (1/2 the actual size) and a 47ohm 1/2 watt in R52. If the R52 is the problem would you give me the color code for the right resistor for 52? Thanks
Doc
Well its good that its working again. The schematic I have states 1/2W 2k2 for R52. 47R won't do anything much there. Try putting in a 2k2 (red, red, red) first and see if the hum is reduced.
The other thing is how old is the amp? If one of the filter caps was bad, chances are you should replace them all - if they were about the same age. Could be that old caps aren't filtering properly - and that might be where the hum is coming from. (Although I'm only guessing where the hum could be coming from at this stage)
Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
You were right. I found the last 2k2 in a parts amp and also put back the 500v cap I thought was bad ???...............and BINGO .....the hum is gone. I sure wish I had seen that small thread of solder on the bottom of board the first time. That was probably the only problem. Sounds like a new amp except for an occassional scratchey sound on channel 1. Thanks for your help and the resistor site.
Doc
Sounds like a new amp except for an occassional scratchey sound on channel 1.
Well that scratchiness could be just a dirty pot, or it could be DC on the pot from a leaky coupling cap. Or it could be a dodgy triode on that side or a dodgy plate resistor on that triode (I had a dodgy plate resistor in V1 do that to me once). Try swapping V1 first and see if that improves anything. Otherwise try cleaning the pot and then if that doesn't work - testing the prior coupling cap for DC.
Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
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