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abhyg 7th November 2006 06:32 PM

hum in 1 channel
I built a stereo tube amp with no controls. It uses, per channel, two 6550's and one 12AX7. The chassis is a steel plate with four bolts as legs. All components except the tubes and the O/P xformers under placed under the chassis. I've tried to keep the layouts of the two channels as identical as possible. All ground connections are made to a copper bus which is attached to the chassis at only one point. Although one channel is quite silent the other has a slight hum. There is no visible difference between the AC voltages across both the speakers, either with a DVM or a HP VTVM. Any and all ideas will be greatly appreciated.


HollowState 7th November 2006 09:15 PM

The most obvious place to start is by exchanging the tubes, one at a time, with the tubes from the quiet channel to see if the hum follows. If this doesn't pinpoint a hummy tube, then perhaps moving the ground point may help. Also try grounding the inputs to see if it reduces. It's hard to be more specific without being there and seeing the circuit and layout.


abhyg 8th November 2006 02:06 AM

Thanks for your reply. I changed the tubes and shorted the input , to no effect. I'll start on moving the ground point. Do you have an idea how different the voltage across the hummy speaker terminals should be as compared to the quiet speaker terminals. I also measured the ac voltage across the cathodes of the PP tubes of both channels and couldn't see anything significant.


HollowState 8th November 2006 04:08 AM

Hi A,

A good speaker will produce an audible sound from just a few millivolts, so it's difficult to give you a specific level. Let your ears be the measuring tool.

I suppose it's conceivable that you may be picking up hum do to transformer placement or orientation. If you are using one PS and one amp section is closer to it then the other. Turning transformers once they are mounted ain't fun, but may be necessary. The use of headphones instead of speakers to listen with may prove helpful if you want to try reorienting transformers.

You might also want to try adding extra filter capacitors while listening for hum reduction to see if it's coming from the PS, or being picked up elsewere.

Right now my brain is whacked because the enemy within has struck a serious blow this election night. :mad:


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