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.

[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. :p

[Enzo]

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

[Sir Cuitous]

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 :rolleyes:. 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 :confused:. 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. :mad:

And I will accept any appologies now :D

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.

[Sir Cuitous]

Thanks for all suggestions - good info. I believe this is a "normal" level of hum to expect from this amp. Though I could be wrong, I don't think any kind of instability is involved. It's a very low level hum that increases in the first few seconds as the power tube warms, but it does not amplify as the volume is turned up. I have to listen carefully for it. I'm not trying to waste anyone's time here. I am only seeking to learn and improve what I can. If my efforts do not yield a better result, I will have at least explored many options on an amp that I built and not some vintage masterpiece.

There is no center tap. I have gone from a single wire heaters to artificial center tap with 100ohm grounds and now I have a 200 ohm "balancing" pot to ground. No difference in the hum. I will try more of these suggestions - heater bias then OT placement.

[Sir Cuitous]

That's an interesting theory Nico. I looked at my placement of the trannys and the Champ chassis does not leave a lot of room to move the OT around to check on a better location. If the OT were "picking up hum from the magnetic radiation of the PT through the chassis" then any place near the chassis would be a source of hum for the OT. I am also not sure about your method of checking this. I'm am no expert, but if I disconnect the OT and listen on the primaries, I am just going to hear what the OT would be "hearing". If the OT were picking up hum "magnetically", I should be able to hear it right out of the speaker even with the primaries disconnected, right?

I am now have the artificial center grounded to the cathode of the 6V6. Still humming. I could continue with connecting a positive bias voltage from a B+ node, but I now believe this is something else. I would not have been able to reach this conclusion without everyone's help. Thanks.

[teleamp]

That's an interesting theory Nico. .....

Not a theory. It works

MikeY

[Steve Conner]

I believe that Champ-style circuits hum by default. The single-ended output stage with no negative feedback is lousy at rejecting power supply ripple.

I modified my Champ-style amp to have a CLC filter with two 47uF caps and a choke, which shut the hum up nicely.

[MWJB]

Most champs do have NFB. Interesting to note though that Fender did flirt with choke filters on 5E Champs & Princetons but dropped it again (most likely due to cost). One possible draw back with this method is raised screen voltages, a larger than usual screen grid resistor may be required to drop the screen voltage below plate voltage (if that bothers you, most folks would fit a 470ohm and be done with it - SF champs typically have screens running a little higher than plates).

If filament hum is a problem though, DC biasing is quick, easy & effective.

[Sir Cuitous]

Every Champ I ever heard had some hum. Isn't there about 3dB of negative feedback due to the unbypassed cathode resistor on second triode of the pre-amp?

The filter caps are 16/8/8. I'm sure a 3x16 would have been a better design. I did DC bias the filament to 40v which gave a very slight improvement on the hum. At this point I am enjoying the amp. As my first build, it's decent sounding, but I believe the hum is due to my sloppy wiring technique. At some point I may change the filter caps to higher values along with some other improvements.

[jyoung]

Good Sir, (had to)

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.

+1 Chuck -- I can back you up on this one - 1969 VOM console record player 6V6 PP amp 4 ohm P12R trombone shaped cone - You speak the truth.;)

[tubeman]

So you are getting hum on your Champ. I built mine using my own layout (Hoffman like). My amp is dead quiet cranked all the way up with only alittle hiss. I suspect you are getting hum from a ground loop. To quiet my amp, I seperated the preamp / power amp caps from sharing a common path to ground. My preamp stage is grounded to the input jack ground lug which is in turn grounded to the volume pot. The power amp caps ground are run to the nearest PT bolt. one last thing, when you are checking your amp for hum make sure nothing is plugged in the input jack. You may be picking up noise from your guitar pickups..

Good luck

[Chuck H]

Thanks. Wow this is an older thread. I never expected anyone would actually READ it.

FWIW I've heard from guys in the know that these old bell cone p12r's are the $h!t. Supposed to sound great for clean and OD but get funny in between (where most gutar players live). Oh well, you can't have it all (unless you pay for vintage Bulldogs). But I'm told that using them is tragedy in the making. The old animal glue will have become so brittle that it will crack up the first time you crank it up. Nothing to lose I guess. Who knows about these speakers anyway...Besides you and I and a select few. Mine is still sitting in a box.

Chuck