Heater center tap ground or thru .1 cap

[Normybaby]

Hi guys, I have an 1950'ish Stromberg Carlson PA Amp (see previous post) and the heater supply (6.3VAC) center tap goes thru a .1 mf cap to ground. I'm getting a little 120 hz hum. Will by-passing the cap and going directly to ground help?
Normybaby

[Tom Phillips]

It shouldn't hurt to give it a try. However, if you truly have 120Hz hum, that's not the problem. If the cap has gone bad, you may be able to improve the 60 Hz hum by replacing or bypassing it. 120 Hz hum is most likely caused by the old filter caps.
Regards,
Tom

[Normybaby]

Ooops, I'm not sure if it is 120hz; now that I listened to it again, it probably is 60hz; I didn't put a scope on it to measure. I did a complete cap job w/tubes on it so I'm thinking it is the filament/wire. This amp is old, 1946 Stromberg Carlson; it has the old two wire (single coil) filament heater 6sc7 preamp tubes. The heater hookup wire is not twisted. I really love the way this amp sounds with guitar plugged into it so I’m thinking I try the center tap to ground and see if it improves; I'll post a response with the results
Thanks for your advice. you're right on
Normybaby

[stokes]

If you have cathode biased power tubes,connecting the heater CT to the cathode of the power tube will eliminate the 60hz hum from the filament.

[Bruce / Mission Amps]

If you have cathode biased power tubes,connecting the heater CT to the cathode of the power tube will eliminate the 60hz hum from the filament.

No it won't. It will reduce it's effect on preamp tubes though and a good thing to try.

[stokes]

No it won't. It will reduce it's effect on preamp tubes though and a good thing to try.

Maybe it wont completely eliminate it,but it will go a long way to reducing it.Like you say,it will reduce its effect on the preamp tubes which is where it is introduced and then amplified with the signal,virtually eliminating it.

[Bruce / Mission Amps]

Maybe it wont completely eliminate it,but it will go a long way to reducing it.Like you say,it will reduce its effect on the preamp tubes which is where it is introduced and then amplified with the signal,virtually eliminating it.

Here's what it will do:

http://www.zwengel-amps.com/heater.html

Using the cathodes of self biased power tubes to supply some DC bias to the filaments is probably as old as tube amps are.
I built my first tube amp around 1964 and discovered old timers had been doing the biased filament trick many MANY MANY years ago.
So long ago I don't even know when that was! ha ha.

Between the two of us, I bet Carl Z and I have been doing this type of mod to tubes amps for +35 to 40 years.
To be honest, I can't remember the last time I built and amp that didn't have biased filament supply.
One thing is for sure, I wouldn't waste a minute of my time building a DC filament supply for a guitar amp and I wouldn't build one with out a DC biased filament supply.
Regardless... it will not eliminate all the hum. But it really helps.

[Normybaby]

Thanks guys, I will definitely try this fix. I did run the center tap to ground and it did improve the hum but take a look at the picture; the volume controls are mounted outside the chassis, Although the wires are shielded, I think this might contribute to the buzz/hum. Also, the heater wires run along the bottom inside the chassis parallel with the other wires. The signal path and components are mounted on small boards up off the bottom about an 1” with stand-offs. The wire from the input jack is about an 1 ½” long and go right to the grid w/ a 1 meg res to ground; it is not shielded. Any suggestion on how to improve this without changing too much? I really love the sound of this Amp and I want to keep it as close to stock as I can; besides it looks really cool
Thanks again

Normy