Would installing a choke help SE hum?

[captntasty]

I have a homebrew Champ - basically a 5f1. It is ~8W with 2 parallel 6V6's. It has what I believe to be normal single-ended hum (low freq, not a buzz). Would installing a choke, a la 5e1, reduce hum? Would this choke work? http://store.triodestore.com/22707.html

Thanks......
Patrick

[captntasty]

May have answered my own question.

This post answers my question and also brings up the option of running the center tap 6.3V to the cathode resistor. (I have a center tapped tranny)

http://music-electronics-forum.com/s...80187#poststop

Is this the right place to run the center tap to?

[tubeswell]

Yes the choke will reduce hum but you might need a more robust one (~90mA or so) one in that circuit if you are going to use the choke in any filter between the B+ and the plate supply - as that circuit uses two 6V6 output tubes, which will run about 36mA each, and with the 12AX7 on top of that you'd have about 76-80mA (and the 22707 is only rated at 40-50mA I believe), otherwise it will work fine if you use it in a CLC filter supplying the screens onwards (as the current draw would only be about 12mA at that point), and yes that is the right place to elevate the heater

[captntasty]

CLC?

Please pardon my lack of knowledge - I don't know what a CLC filter is

otherwise it will work fine if you use it in a CLC filter supplying the screens onwards

Is R2 where the choke would go? Would this be a better candidate http://store.triodestore.com/ch90ma4hfe.html

Or as a CLC do you mean where R3 is?


Thanks for your advice...
p

[Enzo]

CLC - capacitor inductor capacitor.

You could replace R2 with a choke. For my money to get hum out of a champ I would add a filter stage. I'd put the choke before C1 and add another cap from pin 8 of the rectifier. Then the WHOLE amp runs through the choke.

[captntasty]

new cap and choke

Thanks for your replies and help tubeswell and enzo. Is this what you are getting at Enzo?



What value cap would be appropriate - if memory serves me I have a 22uf for C1 - would the new cap be the same?

[Enzo]

Exactly. Cap? I'd use whatever C1 was.

My thoughts on elevating the heaters to the cathode voltage of the power tubes. In my opinion, the main hum problem in these single ended amps is the lack of B+ ripple cancellation. Making the B+ rock solid cures that. The point of elevating the heaters is to eliminate unwanted heater/cathode coupling, mainly in preamp tubes. If that hum is not present, then there is nothing to eliminate. See how the hum is after making the powr supply more filtered. The grounded center tap works pretty well. On the other hand it is nothing more than moving a wire, so as long as you are in there anyway, why not?

The only reason I could think to not do it intitially would be to have the educational experience of finding out how much hum is B+_ and how much is heater related.

[captntasty]

Choke values

Which values on the choke would be appropriate?

[captntasty]

A few choke options

Which one of these would be suitable?

Hammond 156M 3H 100ma 86ohm 400V

Hammond 157Q 3.5H 150ma 98ohm 400V

Hammond 158Q 5H 150ma 105ohm 400V

Fender(Triode Elec.) 125C1A/22699 4H 90ma 110ohm 2500V

[stokes]

Exactly. Cap? I'd use whatever C1 was.

My thoughts on elevating the heaters to the cathode voltage of the power tubes. In my opinion, the main hum problem in these single ended amps is the lack of B+ ripple cancellation. Making the B+ rock solid cures that. The point of elevating the heaters is to eliminate unwanted heater/cathode coupling, mainly in preamp tubes. If that hum is not present, then there is nothing to eliminate. See how the hum is after making the powr supply more filtered. The grounded center tap works pretty well. On the other hand it is nothing more than moving a wire, so as long as you are in there anyway, why not?

The only reason I could think to not do it intitially would be to have the educational experience of finding out how much hum is B+_ and how much is heater related.

And a good lesson in discerning 60 cycle hum (heater supply) from 120 cycle hum (B+ ripple).

[captntasty]

Elevating heaters

Elevating the heaters had no effect on the hum - it is still present.... what now?

[tubeswell]

Which one of these would be suitable?

Hammond 156M 3H 100ma 86ohm 400V

Hammond 157Q 3.5H 150ma 98ohm 400V

Hammond 158Q 5H 150ma 105ohm 400V

Fender(Triode Elec.) 125C1A/22699 4H 90ma 110ohm 2500V

Any of those'd be okay.

[jrfrond]

CLC filter??? After over 30 years in this business, I haven't seen that as an official term. It's called a "Pi Filter". CLC would really just describe the layout. Sorry to be so anal, but.....

Anyway, concerning hum: like Enzo said, if it isn't there, then it won't eliminate it. The issue with power supply ripple in SE designs is that there is no OT common-mode cancellation happening as there is in push-pull designs. You CAN drastically increase filtering, but the tradeoff is feel and possible motorboating if you go TOO high. Floating the heater center-tap, whether it is on the power tube cathode, B+ line or whatever, is a great way to POSSIBLY eliminate heater hum, but you won't know until you try it.

In factory-made amps, the hum debugging has already been done, and hum can occur when mods are made if the tech doesn't know what they are doing. In new builds, the majority of hum problems come from grounding and layout errors, and you can wrestle with the for weeks on end without getting rid of them. Truth is, most design do not require it.

[Enzo]

SOmewhere I have a Fender service bulletin. It tells us that the hum in their Champ amps is normal, so don't try to "fix" it, the warranty doesn;t cover such work.

[tubeswell]

Elevating the heaters had no effect on the hum - it is still present.... what now?

Well it most likely is B+ ripple, which better filtering will improve (like Enzo said)