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Hi gurus.
This is my first post…and I’m hoping you can help me out. This problem has me stumped. The amp is a Fender Twin of the ProTube era of 2003, sometimes called the Evil Twin. Here’s the schematic from Ampwares: http://ampwares.com/schematics/Twin_...Amp_SchE45.pdf .
The amp's volume is very low, (less than a Champ). I have put a sinewave/scope on it and the signal looks all OK until the output of V5 to going to Preamp Out. The voltage at the test point at the plate of V5B is a little low (0.9Vac instead of 2.3Vac) but at the voltage at the junction of the Zener D8 and R79 is real low (4-6mV), so almost nothing is going into the PI, V4.
I am suspecting the JFET Q5 as slamming the output of V5 to ground. The test point at the collector of Q4 is right on -16vdc. This Q4/Q5 circuit must be a protection of some kind, but I can’t see what it’s meant to be doing. Any clues, my guru comrades?
Any suggestions for what else could be causing this very low volume ? All tubes tested OK. Voltages on power tubes all measured good.
I am dreading the prospect of removing that main PCB. It’s going to be an hour minimum to remove the board and front panel. To test it, will mean reassembly, and perhaps the need to remove again. The customer will freak when he gets the invoice.
The Tremolo was also not working, and with Enzo’s help I sorted that out pretty quick. I’m hoping I can impose you’re your combined experience again.
Thanks guys
This is my first post…and I’m hoping you can help me out. This problem has me stumped. The amp is a Fender Twin of the ProTube era of 2003, sometimes called the Evil Twin. Here’s the schematic from Ampwares: http://ampwares.com/schematics/Twin_...Amp_SchE45.pdf .
The amp's volume is very low, (less than a Champ). I have put a sinewave/scope on it and the signal looks all OK until the output of V5 to going to Preamp Out. The voltage at the test point at the plate of V5B is a little low (0.9Vac instead of 2.3Vac) but at the voltage at the junction of the Zener D8 and R79 is real low (4-6mV), so almost nothing is going into the PI, V4.
I am suspecting the JFET Q5 as slamming the output of V5 to ground. The test point at the collector of Q4 is right on -16vdc. This Q4/Q5 circuit must be a protection of some kind, but I can’t see what it’s meant to be doing. Any clues, my guru comrades?
Any suggestions for what else could be causing this very low volume ? All tubes tested OK. Voltages on power tubes all measured good.
I am dreading the prospect of removing that main PCB. It’s going to be an hour minimum to remove the board and front panel. To test it, will mean reassembly, and perhaps the need to remove again. The customer will freak when he gets the invoice.
The Tremolo was also not working, and with Enzo’s help I sorted that out pretty quick. I’m hoping I can impose you’re your combined experience again.
Thanks guys
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