60hz hum in "Kingston Drive" tube preamp

[plumsolly]

I built a Kingston Drive based on the schematic below (I posted in that thread in the Drawing Board but have not gotten any traction over there so thought I'd give it its own thread) I am fighting a 60hz hum problem. I have got it narrowed to V2B - if I ground V2B grid, hum goes away - if I ground V2A grid, hum remains. I have r10 and r11 right at the tube, and then I run screened 2 conductor cable to the front panel to connect to VR2 and VR3. Screen is connect to chassis at only one end. I am running AC, elevated heaters. Any thoughts?

Thanks,

Ben

 

[emrr]

What happens if you short out the EQ section to bypass it, making path as short as possible? 

C2 to R14, disconnect C7 to R12/R13.

Gain will go up, so watch that. 

 

[ruffrecords]

Are you sure it is 60Hz hum? If it is then you will get this with elevated heaters IF one of the tubes has a poor heater cathode leakage so it is worth swapping out tubes.

Cheers

Ian

 

[plumsolly]

@ EMMR - I am sure this would solve the problem. The grid of V2B and everything hanging off of it seem to be where the trouble is.

@ Ian - Its primarily 60hz, but there is some other junk in there. I switched out all the tubes one at a time. The hum does change with different V2s, but the one I already had in there (new EH) seems to be the best.

The heater wires are tightly twisted and as out of the way as possible. The power supply is external.

It looks like the eq section acts as an antenna for anything that is floating in the air, 60hz or otherwise - when I bring a hand near it or a mains cable, it blows up. Is there anything in the circuit that makes that area particularly susceptible to radiation. Below is a picture - Maybe you guys see something?


Thanks,

Ben

 

[ruffrecords]

I cannot see anything obvious in the implementation but it is a high impedance circuit so it will be prone to to pick up unless it is completely screened. Is it intended to be an open chassis as in the picture or is the chassis fitted into a metal enclosure?

Cheers

Ian

 

[plumsolly]

Hey Ian, it has a metal enclosure and it does improve with the enclosure on.

Ben

 

[ruffrecords]

Hey Ian, it has a metal enclosure and it does improve with the enclosure on.

Ben

Improve, but not go away completely??


Cheers

Ian

 

[plumsolly]

Improve, but not go away completely??

To get an idea of the levels we are talking about, with the enclosure in place, I've got ~230mV rms between the output terminals with the output cranked (more like .5V according to the scope).


If its picking up 60hz from inside the case, it must be from the heaters? I believe the ripple on my B+ line is pretty low. It's a tube PS with RCL filtering.

Thanks,

Ben

 

[Landins]

Tried moving away from workbench? External source noise? Replace heaterwinding with flashlight battery to come to conclusive results concerning heater winding.

 

[Matador]

If its picking up 60hz from inside the case, it must be from the heaters? I believe the ripple on my B+ line is pretty low. It's a tube PS with RCL filtering.

All of the supplies in the drive are full-wave rectified, which means any hum from them will be 120Hz, not 60Hz.  If you are getting 60Hz, it's coming from transformer stray fields or being picked up from outside interference in the high-impedance sections of the design.

 

[plumsolly]

Hey Matador, I am not using that supply.

It's a tube PS with RCL filtering.

Best,

Ben

 

[plumsolly]

Tried moving away from workbench? External source noise?

I have tried shutting off lights and such at the bench and it does not seem to do much. I have also taken the unit down to my studio area and still had a hum problem.

Replace heaterwinding with flashlight battery to come to conclusive results concerning heater winding.

I was just wishing I had a bench supply for that purpose. I hadn't thought of lantern battery. I will give it a try. Thanks!

Ben

 

[Landins]

To clarify. Do you have ac heaters with dc elevation or rectified dc heaters?

 

[ruffrecords]

One simple test you can do to see if the interference comes from inside the unit is to measure/listen to the hum and then turn off the mains supply. If the hum goes away  immediately then it is being picked up internally. If it diminishes slowly as the heaters cool and the HT drops then it is external interference.

Cheers

Ian

 

[Kingston]

I built the original with an external PSU anticipating problems like this. The design has absolutely no rejection for any PSU ripple and power requirements are heavy with need of big PSU transformers. They radiate pure 50/60hz which you are most likely seeing right now.

Also heavily shielded input transformers are crucial with these kinds of high step up inputs.

 

[Kingston]

Also I should point out I have never actually managed to build a unit with completely flat noise floor with AC heaters. I have also never seen the kind of flat FFT plots I personally produce with anyone doing AC heaters. I've only heard anecdotal evidence it's possible with the usual comments like "I can hear no hum".

With a kind of extreme gain that Drive-1 can do with some settings (+90dB) I wouldn't even want to play with a risk like an AC heater PSU would pose.

 

[plumsolly]

To clarify. Do you have ac heaters with dc elevation or rectified dc heaters?

AC heaters with ~75V DC elevation.

One simple test you can do to see if the interference comes from inside the unit is to measure/listen to the hum and then turn off the mains supply. If the hum goes away  immediately then it is being picked up internally. If it diminishes slowly as the heaters cool and the HT drops then it is external interference.

Thanks for the tip Ian. It goes away immediately

I built the original with an external PSU anticipating problems like this. The design has absolutely no rejection for any PSU ripple and power requirements are heavy with need of big PSU transformers. They radiate pure 50/60hz which you are most likely seeing right now.

Also heavily shielded input transformers are crucial with these kinds of high step up inputs.

My PS is external and the Input TX is shielded.


So I took Landins' advice and powered the heaters with a battery (was about 4.7V with load) and the hum is gone. So it definitely is coming from the heaters.

I would like to keep my AC heaters if possible, just because the PS is built and working and I have 6.3V winding on the power TX. I don't need it whisper quiet, just usable.

I attached a pic of my wiring, maybe someone sees something stupid I've done. The twisted blue wire is the heaters.

I was getting something like 6.8V AC with load, though. So maybe I could get away with rectifying and filtering my 6.3V winding and just under-heating the tubes a bit?

Thanks,

Ben

 

[radardoug]

You can rectify your 6.3 volts, and then use RC filters on it. The answer to your problem is to convert your filaments to d.c., so that is what you have to do. Don't try to not do it, that isn't going to work.

 

[ruffrecords]

Do you have a hum dinger pot??

Cheers

Ian

 

[plumsolly]

Do you have a hum dinger pot??

I don't. Just 150R from each heater wire to my elevated node. I'll see if I have an appropriate pot and give it a try.


Thanks,

Ben