Single-Ended Hum

[Sir Cuitous]

Hi, nube here. I just finished a 5E1 with Mercury Transformers/Choke and a cab with a P12R Jensen. It's my first amp and I am very happy with the result. I have noticed that it doesn't matter whether I wire the filament circuit with single wire or a double wire (artificial center tap) - it still hums. It's the normal hum and I realize I should just leave it alone. But, my question is if anyone has tried a mod suggested by Gerald Weber that adds a 40v positive bias voltage to the filament circuit with a dropping resistor and a load resistor? He claims this will "elimnate" the hum. Sounds promising.

[MWJB]

Yes, that works a treat, absolutely silent amp at full vol, w/no signal. If you still have hum after doing this, it's not the filament supply.

[Jag]

+1

[Bruce / Mission Amps]

But, my question is if anyone has tried a mod suggested by Gerald Weber that adds a 40v positive bias voltage to the filament circuit with a dropping resistor and a load resistor? He claims this will "elimnate" the hum. Sounds promising.

A mod created by G. Weber? That's a joke, isn't it?
HA ha ha...
That trick is as old vacuum tubes themselves and found on lots of old hi-fi amps from before most us were even born.

[TD_Madden]

Bruce, I tend to join your amusement...but the mod was only "suggested" by G. Weber, not "created".

[DirtyGeorge]

But I thought Randall Smith created every innovation in amplification.

Sir Cuitous, If your output tube is cathode biased you can tie the heater center tap to the cathode to raise the heater DC bias.

If you use the voltage divider instead don't forget to add a filter cap from the resistor junction to ground.

DG

[Sir Cuitous]

Yes, I wrote that he suggested it. Weber specifically doesn't take claim for it. But I couldn't find much about that circuit anywhere on the Internet. I'm probably not searching right. Thanks for confirming.

[MWJB]

Hi DG,

Why the filter cap?

[DirtyGeorge]

MWJB, I don't know the theory behind it but if you don't use a cap to decouple the voltage divider from the B+ node (even if a well filtered preamp node) you get a nasty buzz. It's sounds like the power supply cap charging pulses are coupled to the voltage divider node.

DG

[MWJB]

Hi DG,

I've installed the "old tube hi fi" mod "suggested" by Gerald (should keep everyone happy), exactly as he describes (no filter cap from voltage divider juction, DC voltage derived from 1st filter cap node) and not experienced any buzz, or any other unfortunate symptoms. In fact, the amp in question is so quiet that the pilot light is the only clue that it's turned on (with no instrument connected).

[DirtyGeorge]

MWJB, The first time I installed the voltage divider I got the buzz. Since the cap fixed it I have just used a cap on every build since then. Maybe it was just masking some other problem in that build and was not needed on those later builds. Or maybe it has something to do with placement. You put yours on the first node while I put mine on a preamp node.

It sounds like some experimentation is in order.

DG

[MWJB]

In effect you are creating an additional preamp node, I would expect additional noise without a filter cap. Try it from the B+.

[DirtyGeorge]

MWJB, thanks for the info. I will try it on my next build.
DG

[TD_Madden]

boy, do I feel stupid...I have no idea how this is connected!

[MWJB]

On a SE amp wire heaters as a twisted pair, no CT, no 100ohms, no ground reference at all. If you do miss a ground reference you will fry your PT!

From 1st filter cap run a 2W 220K to one side of the heaters. From the same side of the heaters run a 0.5W 27K to ground. You should now have 35-40vdc biasing the heaters. Tweak 27K to make sure that you don't have less than 35v or more than 40v (SF voltage champs might need nearer 22K?).

Shouldn't be necessary on push pull amps.