Hey friends,
I'm working on a friend's AC4C1, and it's been driving me crazy. It works and sounds good, it's just noticeably quieter than it should be.
This is the schematic I've been using, although the B+ wiring is shown incorrectly. V1A is B+4, V1B and V2A are B+3, V2B is B+2.
I've replaced the tubes, measured all the voltages, checked ESR and capacitance and resistances, and tried swapping the EL84 bias resistor.
I've run through the circuit with an oscilloscope and tone generator, and the waveform looks pretty gnarly coming out of V1 but not too bad coming out of V2.
It's a super simple amp so it shouldn't be that hard, but I'm not sure what to do next. One other thing is there's a small burn in the jacket of the wire connecting the OT to EL84 pin 7 and it appears to have made contact with the clip that holds the tube in, effectively shorting B+ to ground. Would this potentially affect the OT?
Any ideas for troubleshooting?
Thanks
I'm working on a friend's AC4C1, and it's been driving me crazy. It works and sounds good, it's just noticeably quieter than it should be.
This is the schematic I've been using, although the B+ wiring is shown incorrectly. V1A is B+4, V1B and V2A are B+3, V2B is B+2.
I've replaced the tubes, measured all the voltages, checked ESR and capacitance and resistances, and tried swapping the EL84 bias resistor.
I've run through the circuit with an oscilloscope and tone generator, and the waveform looks pretty gnarly coming out of V1 but not too bad coming out of V2.
It's a super simple amp so it shouldn't be that hard, but I'm not sure what to do next. One other thing is there's a small burn in the jacket of the wire connecting the OT to EL84 pin 7 and it appears to have made contact with the clip that holds the tube in, effectively shorting B+ to ground. Would this potentially affect the OT?
Any ideas for troubleshooting?
Thanks
Last edited:
Thread moved to instruments and amps.
What´s its output at clipping threshold?
Set Volume to 10, Treble and Bass to 7, inject 100mV 1kHz at preamp input, slooowly rise Gain from 0 until output starts to clip.
Most probably it won´t clip symmetrically so rise it a little more until second peak also *just* starts to clip.
What output voltage do you measure?
What is the speaker impedance?
Please post plate voltages at V1a/b , V2b and V2a cathode.
Plate, screen and cathode voltage at V3.
All DC voltages without signal.
Yes, see above and post results, including scope image at start of clipping.
I'm not 100% sure how to perform this test. Following along up to the point where I turn the gain up slowly, when I measure across the speaker output with my scope it looks like it's clipping immediately as soon as I turn the gain up at all. Where do I measure output voltage, and is that AC?
it's a 16ohm speaker
B+ 320V
B+1 312V
B+2 270V
B+3 200V
B+4 190V
V1A Plate 100V
V1B Plate 130V
V2A Cathode 130V
V2B Plate 180V
V3 Plate 316V
V3 Screen 310V
V3 Cathode 9.8V
Could be a lot of stuff. Have you tried checking the signal with a probe at different stages to see if there is actually a decrease in signal swing somewhere?
If the output tube has 10v at the input grid it should be giving the total output, unless there is something weird with the screen grid, which looks OK.
My guess would be shorted OT, resulting in decrease impedance. Happened to me with other kind of EL84 amp. The amp still "works" but is much quieter, since now there is an impedance mismatch.
If the output tube has 10v at the input grid it should be giving the total output, unless there is something weird with the screen grid, which looks OK.
My guess would be shorted OT, resulting in decrease impedance. Happened to me with other kind of EL84 amp. The amp still "works" but is much quieter, since now there is an impedance mismatch.