This is a push pull amp with a pair of EL84s. One tube has 70 volts 'missing' from the plate. I have changed the tube and it makes no difference. Bias voltage and screen voltage are normal but.....where do those 70 volts get lost?
320 volts depart B+ on their way to the plates. One plate gets 317V, the other one only gets 250V. Schematic says there's only two parts between B+ and the tube's plate. The OT and a diode to ground. The diode seems fine, the OT measures 190 Ohms across the primary winding, 95 Ohms between B+ and each plate. Looks OK too. I have no idea what's going on here, with no tubes both plate pins show the same readings. The tube is not the problem.
http://www.blueguitar.org/new/schem/fender/blues_jr.gif
320 volts depart B+ on their way to the plates. One plate gets 317V, the other one only gets 250V. Schematic says there's only two parts between B+ and the tube's plate. The OT and a diode to ground. The diode seems fine, the OT measures 190 Ohms across the primary winding, 95 Ohms between B+ and each plate. Looks OK too. I have no idea what's going on here, with no tubes both plate pins show the same readings. The tube is not the problem.
http://www.blueguitar.org/new/schem/fender/blues_jr.gif
Could you see if the amplifier is oscillating? In this case, a VOM can give a false reading.
Good luck.
Good luck.
Thanks. I don't have the gear to check that out but when I attach the VOM probes to the circuit this strange noise comes up, it could be oscillating....but why everything reads normal except at the plate of that tube? Could be the OT?
It would be odd but maybe with current through the windings the OT starts misbehaving. A wild guess, I know.
It would be odd but maybe with current through the windings the OT starts misbehaving. A wild guess, I know.
With no signal applied, measure the voltage on pin 2 of each EL84 they should both be aprox -10 V dc. If not check the values of R20 through R23. One tube is drawing more current than the other. You could also reverse the position of the two outputs and see if the low voltage follows the tube. Frankly it sounds like a bad tube. Good luck.
Try removing the previous tube stage and re-read the voltages. This way, the amplifier must be stop oscillating. The reason you see only one plate at abnormal voltage depend of the waveform of the oscillation and its amplitude.
I'm not going back. Amp plays fine and it's not mine. It only oscillates when my hand gets near the circuit. Not very stable. 🙂 A friend plays a gig next week and he wants no surprises so he ask me to take a look and see if everything was OK with the amp. I did take a look and took some measurements and that's it. He's not getting any more work from me. 😀 Again, thanks for the help.
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