Hum in Ac30-style amp

bluemonk · 10-11-2009, 11:22 PM

I built a sort of Ac30 TB clone based on Hoffman layout with a couple of Matchless suggestions.
It does hum. Not very loud but louder than any other amp of mine.
I used shielded cable for inputs and elevated DC heaters to power tubes cathodes (10V idle) thru a pair of 100R resistors (is that correct?).
Hum isn't affected by pots' action.

Any suggestion?
Thanks in advance

loudthud · 10-13-2009, 04:30 PM

Do you have a way to monitor current in each output tube? Imbalanced currents can cause hum. Any change to the hum when you pull the phase inverter tube? Got any spare tubes to try? Try different combinations of output tubes in different sockets. Verify voltages on all output tubes at the tube sockets.

Does the amp have a standby switch? Hum from the heater supply will fade out when when you put the amp in standby mode. Hum from grounding or supply ripple will die immediately when the amp is put in standby mode.

Elevated heater bias reduces 60Hz hum from low level preamp tubes. I think you have done this correctly with the two 100 ohm resistors. You can still get 60Hz hum from signal wires being too close to heater wiring. This usually is affected by tone and volume controls. 120Hz hum is usually an issue with the high voltage power supply or grounding.

bluemonk · 10-13-2009, 11:04 PM

I double checked my wiring following your helpful tips.
Finally I found incorrect grounding

Now hum has come back to "inherent" ranking IMHO

Thanks a lot

10-11-2009, 11:22 PM	#1
bluemonk Member Join Date: Aug 2006 Posts: 71	Hum in Ac30-style amp I built a sort of Ac30 TB clone based on Hoffman layout with a couple of Matchless suggestions. It does hum. Not very loud but louder than any other amp of mine. I used shielded cable for inputs and elevated DC heaters to power tubes cathodes (10V idle) thru a pair of 100R resistors (is that correct?). Hum isn't affected by pots' action. Any suggestion? Thanks in advance

10-13-2009, 04:30 PM	#2
loudthud Supporting Member Join Date: May 2006 Location: Texas USA Posts: 922	Do you have a way to monitor current in each output tube? Imbalanced currents can cause hum. Any change to the hum when you pull the phase inverter tube? Got any spare tubes to try? Try different combinations of output tubes in different sockets. Verify voltages on all output tubes at the tube sockets. Does the amp have a standby switch? Hum from the heater supply will fade out when when you put the amp in standby mode. Hum from grounding or supply ripple will die immediately when the amp is put in standby mode. Elevated heater bias reduces 60Hz hum from low level preamp tubes. I think you have done this correctly with the two 100 ohm resistors. You can still get 60Hz hum from signal wires being too close to heater wiring. This usually is affected by tone and volume controls. 120Hz hum is usually an issue with the high voltage power supply or grounding. __________________ WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel. Last edited by loudthud; 10-13-2009 at 04:37 PM.

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10-13-2009, 11:04 PM	#3
bluemonk Member Join Date: Aug 2006 Posts: 71	I double checked my wiring following your helpful tips. Finally I found incorrect grounding Now hum has come back to "inherent" ranking IMHO Thanks a lot

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