I think I've blown up my supersonic 22 power transformer

jackbart1960

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Hey all I just tried to bias a new set of power tubes in a Fender Supersonic 22 after power tube failure . I can get voltage of 24mV but no current 2mA at best. Should I be looking for new PT? Any help much appreciated, J.B.
 

pdf64

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Use your meter's mV range.
The reading is across a 1 ohm resistor (R140 or R141), so 1mV means that 1mA is flowing through the resistor.
When you set your meter to current, the mA reading you got was invalid; to measure current, the meter must be in series with the circuit in question, whereas your meter had a 1 ohm path in parallel.
http://bmamps.com/Schematics/fender/Super-Sonic22_schematic.pdf
 

zenas

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It's not blowing fuses or smoking so it's probably not the power transformer.
 

jackbart1960

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I was trying to read current from D35/36 TP40/41 to ground. I was getting 24mV but 2mA at best. Is my measurement method of current incorrect? Thanks so far.
J.B.
 

pdf64

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Note that the schematic previously linked indicates 24mV at TP40 and TP41.
A direct measurement of current is not possible without interrupting the cathode circuit.
 

Blue Strat

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ONLY use the mv reading. If you're getting anywhere near 24mv you're ok. Is the amp working?
 

zenas

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I'm not familiar with that amp but I'd be checking secondary voltages if I suspected a problem with the PT. It's possible to have a winding go bad but the bad ones I replaced have been a dead short. Fuse blows instantly.

I'd be surprised if it's a PT here. I've been suprised before though.
 

Blue Strat

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I'm not familiar with that amp but I'd be checking secondary voltages if I suspected a problem with the PT. It's possible to have a winding go bad but the bad ones I replaced have been a dead short. Fuse blows instantly.

I'd be surprised if it's a PT here. I've been suprised before though.
Agreed, but it seems as though he's getting the right cathode current (by way of measuring mv across the cathode resistors) which would seem to prove that everything is ok. I guess we'll know soon enough.
 

jackbart1960

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Hey all this has been helpful. I do get sound and all but the low end sounds like ass. I've swapped out all of the pre-amp tubes also except for the AT's (didn't have spares). My thought was that I just replaced the power tubes and it's sounds like sXXt and I couldn't get a mA reading so it must be the PT. So much for jumping to conclusions. Any other thoughts would be greatly appreciated.
J.B.
 

kevmin

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1,439
I have never seen replacement of power tubes of the correct type take out a power transformer. Can you describe your original power tube failure? What exactly happened to make you think the tubes had gone bad?
 

pdf64

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There's hundreds of components that could screw with your amp if they go bad; transformers are about the least likely suspects. There's no reason to suspect the worst without strong indications that's the case.

Assuming that the new power tubes are good (hope you got them from a vendor that tests them at realistic guitar amp voltages and provides a warranty) then V5 the LTP phase splitter uses a 12AT7. If one side of that is down, either due to a bad triode section or component, eg plate resistor, the result will be similar to a bad power tube, eg thin tone with nasty distortion.

If the reverb seems ok, we can assume that the 12AT7 in V3 is ok; try swapping the tubes in V3 and V5 over and report back. Or a 12AX7 should work fine in V5.

If the sound is still bad, and you're competent to work inside a live amp chassis (there's no shame in deciding not), check the numerous Vdc noted on the schematic against your amp.
 
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jackbart1960

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1,167
PDF64,
Will do. I work as a tech., just totally different stuff, so poking around a live circuit is no problem. i'll check out the AT swap and let you know.
 

jackbart1960

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1,167
All,
First, thanks for you guys help. It was the PI. A bud sold one out of his stash and I found an RCA four star(if the stencil means anything. I was told it did) 12AT7 in my pile that sounded awesome in that spot. I feel like a putz now for trying to measure current in parallel, but a lesson learned so not a waste. There was a tube in V2 that wasn't helping matters any also. Biased at 24mV sounds freak'n so cool. All be well J.B.
 

Blue Strat

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30,183
The lesson here is not to change more than one variable at a time. If you do, and something goes wrong, you have multiple issues to chase instead of just one. In this case you ignored one of them.

Congrats.
 

jackbart1960

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Messages
1,167
Got another for you guys check your fuse holder. Was still getting this buzzing. Checked everything again. Poked at the fuse and the ¥%=^'n thing was loose in the socket. Tightened socket and no buzz. Even after all this it still a pretty cool tone. Oh and I put an Eminence Canis Major in it. Now that's pretty neato :0) Be well all
 
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