so i am working on an amp that has a JCM800 50w power supply/power amp section with a completely different preamp section and when I finished, all worked well. The amp worked for about 20 min and sounded awesome.
I then went in for a mod on the preamp section (add bypass cap to tube 1) and when I turn on the amp there is this very loud hum/buzz happening.
After much investigation here is what I have found......
- hum/buzz is unaffected by preamp section, volume, tonestack, etc. It still amplifies the preamp signal, though quieter.
- the hum goes away when the power tubes are out
- the B+ is half of what it should be from the first rectifier diode (which doubles as a bias supply....220k resistor on). In other words, power to the B+ is only coming from one of the transformer leads once it hits the rectifier...the diode with the 220k resistor reads (post diode) 2v while the other diode reads normal HT.
- the bias circuit resistors are all functioning fine as is the bias pot
- the EL34's are not bad...tested several sets
- the el34's are reading a super high bias....like over 200....i turn it off very quickly!
- replaced the caps from the PI in case they were bad...nothing changes
- checked every connection i can think of
- heaters are fine
- the B+ to ground resistance is over 2M with all tubes and OT connected........huh?
i'm probably missing something easy but am at a loss and losing my mind. I assumed it was the bias circuit, and will probably switch out the 2 caps there to see if it helps, but the B+ to ground resistance being near infinity is confusing to me.
any thoughts?
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I then went in for a mod on the preamp section (add bypass cap to tube 1) and when I turn on the amp there is this very loud hum/buzz happening.
After much investigation here is what I have found......
- hum/buzz is unaffected by preamp section, volume, tonestack, etc. It still amplifies the preamp signal, though quieter.
- the hum goes away when the power tubes are out
- the B+ is half of what it should be from the first rectifier diode (which doubles as a bias supply....220k resistor on). In other words, power to the B+ is only coming from one of the transformer leads once it hits the rectifier...the diode with the 220k resistor reads (post diode) 2v while the other diode reads normal HT.
- the bias circuit resistors are all functioning fine as is the bias pot
- the EL34's are not bad...tested several sets
- the el34's are reading a super high bias....like over 200....i turn it off very quickly!
- replaced the caps from the PI in case they were bad...nothing changes
- checked every connection i can think of
- heaters are fine
- the B+ to ground resistance is over 2M with all tubes and OT connected........huh?
i'm probably missing something easy but am at a loss and losing my mind. I assumed it was the bias circuit, and will probably switch out the 2 caps there to see if it helps, but the B+ to ground resistance being near infinity is confusing to me.
any thoughts?
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Guitar amplifier being discussed so I will move this thread to Instruments and Amps per forum policy.
Are you using the recommended power transformer or something close with a center tapped high voltage secondary? If you are the transformer could be defective.
i am using the very same transformers suggested by the layout....i bought them from the company that posts these layouts.
are you thinking i am indeed dealing with a bum trafo?
i removed all tubes and powered up briefly.....all B+ voltages appeared normal and the bias feed looked normal too.
B+ = 520v at first cap
bias circuit = -37v at second cap
just put the output tubes back in out of frustration and holy crap they work! All B+ and bias is normal but the tubes seem super microphonic...i tap the chassis (not hard) and the sound rings through the speakers.
so i've been dealing with partial output tube failure? Why did my other known working set have the same problem?
so i've been dealing with partial output tube failure? Why did my other known working set have the same problem?
re-solder the center tap of the heater winding that goes to dc ground. make sure its connected to a power cap.
re-solder 1 ohm resistors on base of output tubes that are not on the schematic.
Note: this unit doesn't bleed the voltage down when power is removed ideally. leaky caps is the culprit.
B+ should not be more than 485V since 500V caps are used in the power supply. Check your line voltage power setting.
too high of a B+ makes this amp really sensitive (microphonic).
re-solder 1 ohm resistors on base of output tubes that are not on the schematic.
Note: this unit doesn't bleed the voltage down when power is removed ideally. leaky caps is the culprit.
B+ should not be more than 485V since 500V caps are used in the power supply. Check your line voltage power setting.
too high of a B+ makes this amp really sensitive (microphonic).
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