Single-Ended Hum

[TD_Madden]

I was thinking about trying this on my Gibsonette (2- 6V6 in parallel).....might help.

[slajeune]

Or,

use 2 100ohm resistor to form artificial center tap on the heaters (if your transformer doesn't have a center tap for heaters) and ground the 'center tap' to the cathode of the power tube. Since the champ is a cathode biased single ended design, the cathode will already be above ground, not 40V but enough to eliminate a lot of potential hum. Look at the AX84 single ended designs for examples ( http://www.ax84.com ).

In all my cathode biased single ended designs, this is what I use and it is very quiet.

Cheers,
Stephane.

[Sir Cuitous]

Yes, I believe it should be connected from B+. MWJB, I have a question. Can that come straight from the transformer or should it be the first filter cap node?. Second, my friend insists this mod is deterimental to tone. His reasoning is that if it were that easy to reduce the hum, then all the old single-ended amp designs would have included this feature. I can't see how this would effect tone.

[Enzo]

Are we really certain it is heater hum? Hum is inherent in the nature of SE amps when the power tube is run off the first filter. Drawing the power tube off the second node cleans up B+ ripple.

Is the hum 60Hz or 120Hz?

All the heater hum abatement in the world won't affect B+ ripple.

connected from B+. MWJB, I have a question. Can that come straight from the transformer or should it be the first filter cap node?.

There is no B+ at the transformer, only AC. Any DC offset voltage you want to mess with must come from a filter cap somewhere.

Second, my friend insists this mod is deterimental to tone. His reasoning is that if it were that easy to reduce the hum, then all the old single-ended amp designs would have included this feature. I can't see how this would effect tone.

Me either, only affect on tone would be to remove hum from it. Your friend is not reasoning so much as rationalizing. The reason all the old SE designs don't include this feature is that they worked well enough without it, not because it didn't work. Leo Fender was a master of "good enough." it ain't hifi. An extra stage of filtration, an elevated heater supply, whatever, each thing adds to the cost of an amp in terms of parts and labor. Why add it in if sales are already fine without it?

I have in my files a service bulletin from Fender involving the CHamp. The bulletin states that hum is NORMAL in the Champ and service centers should not waste time trying to "fix" it. It would not be reimbursed labor under warranty.

[Sir Cuitous]

You're right - I can get DC from the first node. This amp has a filter cap then a choke. It looks like the power tube is run off of the second node. If I understand you correctly, then biasing the heaters won't make a difference if the hum is B+ ripple. I am not entirely certain of the source of the hum only to say that it is not loud and could be the natural SE hum.

But, I am learning here. I have the parts and I will try the bias trick tomorrow. I should know soon what effect it has. Thanks so far for a good discussion. I believe you are correct about the frugal Fender design ethic.

[Sir Cuitous]

I checked and do not have the parts. My local electronics store does not have them. So I have ordered them. I will share results here in a week or so if anyone is interested.

[DirtyGeorge]

Sir Cuitous, I don't think the resistor values are critical as long as the ratio is correct. Since this is just a voltage source with insignificant current requirements you could use much larger values and reduce the wattage rating. I would try 1M/100K 0.5W resistors.

MWJB, do you see any problem with that?

DG

[bob p]

But I thought Randall Smith created every innovation in amplification.

I doubt that either Weber or Torres would agree with you on that.

[Enzo]

But they are not patenting everything up to and including sliced bread.

[Sir Cuitous]

Resistors

So far all I have done is to rewire my ground which didn't change anything and wire a 200 ohm pot into the filiment circuit. This seems to work as well as using two 100 ohm resistors. Adjusting the pot doesn't have any effect.

I have obtained 2watt resistors from two different sources and they did not measure to spec - both way off. Does anyone have a reliable source for components? I'm still interested in trying the positive bias filiment.

[Chuck H]

Good Sir, (had to)

Dirty is right about being able to use another value R for the false ctr. I prefere to use the smaller (100 ohm) value because it has the added benefit of acting like a fuse in the event of a short. Anyplace that sells resistors should have a pair that will work. Most resistors have a % tolerance. At 10 cents a piece you should buy several and test two that are relatively close (though I think +/- 10% should be fine without matching).

I agree with Enzo about also checking elsewhere for hum. Lotsa places it could be.

O.K...As for the earlier reference to Gerald Weber taking credit for old ideas (which in this case I guess he didn't but...) I have a tale:

If you read his hype in the back of "Tube Amp Talk for blah blah etc." He mentions modeling his Blackframe speakers after the Jensen P12R but with upgrades . Including a special cone shaped like a trombone bell. Which he absolutely does take credit for developing. Well, as it turns out, I grabbed an old VOM (Voice of Music) phono with a "FREE" sign tapped to it off of a nieghbors lawn. I figured to make a cool little practice amp from the salvage. Upon removing the speaker I was surprised to find it had a trombone bell shaped cone. O.K. I've seen that before. Not often, but I have. So I check the code on the rim...It's a P12R!...With a bell shaped cone!..From 45+ years ago. So I called Gerald... Yes, I was polite. I left a mesage with the short story, ie: "Hey you might be interested in this. I've got a P12R with a bell shaped cone. My number is..." He never called me back .

I posted this story about 6 years ago on ampage and even though I specifically mentioned "Gerald", a gagle of posters massed en force to kick my ass for daring to accuse TED weber of lying about speaker design . At the time Weber speakers were new to most of us and Gerald wasn't as commonly recognized. Some people refused to make the distinction. I was tarred and feathered for something I had not done. I was despised and talked about in other posts, disrespected, and actually had to leave the forum for almost a year. When I returned there was still one stragler who asked "hey, are you that guy who called Ted Weber a liar?" "No...That must have been someone else." I replied.

And I will accept any appologies now

Not claim jumping the post. Just wanted to share that tale now that it might be better appreciated.

Chuck

[Sir Cuitous]

That's a great story, Chuck. Unlike your experience, I feel I have been helped immensly by the comments and interest shown in this forum. But I understand your prediciment.

The hum is a low hum that is not amplified with the volume. I am simply interested in experimenting with different wiring. I'm still interested in the positive bias wiring and Dirty George's proposal that different resisitor values could be used (1M/100K 0.5W). Has anyone tried different values?

[Chuck H]

"Unlike your experience, I feel I have been helped immensly by the comments and interest shown in this forum."

Don't get me wrong. I love this forum. I've been with it for well over a decade. what happened broke my heart. But it's all water under the bridge. I need these guys (Bruce, Enzo, and all, wheres Wild Bill???) more than ever right now. I'm Glad to be back very recently since the change in format.

If you don't have a ctr tap on the filament winding then there is nothing to lose in using almost any value to reference ground on the ctr tap. But if you go higher in resistance (above the 100r value) it is likely that in the event of a short any current will find an easier path to ground, ie: through the transformer winding or arcing across the socket if it's a high enough voltage, no fuse like properties in the circuit. If you're going to trouble with a false ctr, you may as well get the most out of it.

FWIW I'm not 100% on this theory but it seems to make sense... Corrections accepted.

JM2C

Chuck

[loudthud]

Two things I wanted to mention on this thread.

I worked at an amp repair shop for a short time and got stuck repairing one of the boomerang (it keeps coming back) amps, an SVT. The problem was that the hum balance control was burned out and at a certain signal amplitude, the thing would break into an oscillation only on part of the waveform. The heater wires were forming a path from the output back to the input because they were not a low impedance to ground. That's why I like to put a cap to ground on a heater bias network.

As an experiment on an amp that really needs a complete re-do of the ground to get all the buzz out (old Sunn PA), I installed a heater bias network. The difference between biased heaters and grounded heaters is 9dB.

[nico.verduin]

One other reason could be that the OT is picking up "hum" from the magnetic radiation of the PT through the chassis. You can verify this by disconnecting the primaries and B+ form the OT and listen with a small earphone on the primaries with the amp swtiched on. If you hear hum, you will be searching forever without resolving the problem. You might try disconnecting the OT and move it around on the chassis to see if there is a location where no hum is picked up. Put the OT there. Should make the amp a lot more silent.