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I’ve got a push-pull 6DN7 amp and it’s a little on the noisy side (low frequency hum). The high voltage supply has a CLC filter (47 uF/5 H/200 uF) and I’ve got AC on the filaments. There is no feedback being used and it's self-biasing. I’ve been pretty careful about routing the wires. I put my DVM across the speaker terminals and it reads in the 5 to 6 mV range which seems a little high, at least it’s higher than I would like it to be. About the only thing I can think of trying is to go to DC on the filaments. Does that seem reasonable? Any other suggestions would be most appreciated. Thanks!
Follow Ups:
I just went through a similar issue with a headphone amp I just built. It had a hum which I had trouble getting rid of. I was so busy trying to solve the hum I never tried putting the bottom plate on my amp...which solved the hum.
Tried DC on the filaments....didn't help at all, maybe even worse. Rerouted my grounding a bit, maybe helped a tad. One thing I notice is that when nothing is plugged into the input then the hum is lower (in the 3 mV range across the speaker terminals). I have not shielded the wire from the input jack (back of the amp) to the grid of the driver (front of the amp); can that cause hum in both channels? I think I'll put in a shielded cable just in case. Another thing I did which may not have been too smart was to locate the rectifier tube right in between 2 of the 6DN7's (about 1" away from each), looks cool but maybe it's imparting some hum as well. When I play music it's of no consequence so I guess I can live with the hum if I can't get it to go away. Thanks again!
Try directly grounding the grid of the input tube. You can leave everything connected and just use a clip lead (best to remove everything though). If you still have hum then look at stuff like that rectifier tube being too close and how your heater wiring is done. If the hum goes away than look at how you run from input jack to grid and how your grounding is done.
Some times you need to change the way you are grounding your set up.T Willman
If it's 50 (60) Hz, it's being picked up somehow from the AC circuit (power transformer primary or secondary windings and associated wiring). This may be hard to trace and can be due to a number of reasons such as AC-bearing leads too close to the input signal leads, poor grounding/shielding of the input socket or associated circuitry, leaky heater-cathode insulation in a tube, OP transformer too close to power transformer field (or not at 90 degrees to it), ground loop between the amp and its signal source; - just to name a few.If it's 100 (120) Hz, it's possibly due to poor filtering of the B+ (maybe one of your filter caps is not working well), a sub-optimal grounding arrangement causing the PS or output stage ground currents to mix with those of the input stage (star grounding can help to avoid this problem) or currents circulating in the chassis if there is more than one ground connection to it.
Make sure your heater circuit is referenced to ground (or another voltage of choice) and not floating.Make sure you have no dry or cold solder joints, and that the tube pins are clean and making good contact with the socket. A shaky connection anywhere in the audio or PS circuits can result in hum, dunno why.
Aloha,
It seems most people say to omit grid stoppers unless they're absolutely needed so I left them out.
The answer is no.YOu shall revise your B+ filtering section, grounding and wiring scheme, possible ground loops, transformer and tube physical orientations, etc.
Sometimes, old/worn, faulty or intermittent tubes would behave like that too..
In rare cases (usually with substantial negative feedback) you can get hi-freek oscillation, but not hum.
Trial another tube(s). Some examples are simply noisier than others due to close proximity of cathode to filament, which doesn't show up as a short. Even swapping push-pull pairs pull to push ala musical chairs may help null noise, so trial that too. IOW the culpret could be a particular tube rather than the circut.
Will DC on the filaments fix this if indeed it's the cause of the problem? I've looked through the archives and it seems most people do not think that one should have to resort to using DC. I must admit, I've had DHT amps in my system before that were quieter than this one. Bummer. I thought for sure it would be quieter.
Thanks. I'll try swapping tubes around. I seem to have the problem in both channels but I guess if it's a tube thing both channels could be effected. I'll give it a try tonight.
Is there a path from filament to ground (via a center tap or the mid. point of a resistor divider)? If not, there should be one.
Hope that helps.
Yep, I've tried the trick of connecting the center tap wire to a voltage divider so that the filament is floating at about 50 VDC. Also, I bypassed the bottom resistor with a 100 uF cap. Still got hum though.
Everyone gave you wonderful suggestions and one of those remedies should fix this problem.
Do you have access to an oscilliscope? Also I assume you did the obvious of shorting the input and feeling the woofer to see if the hum is getting thru still.
More times than not I have found hum to eminate from bad solder joints and dirty tube sockets on preamps tho.
Take your scope and look at the componets where you suspect the hum might be..If you have a frequency counter you can tell if its a 60 or 120cps hum by just putting it on the outputs.
Being its in both channels you can almost bet its a PS fault.Can you post of a photo of it so we can see the setup and that we may be able to spot somthing obvious.
mike
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