Power trans hum heard through speaker

[bigguy12321]

Hello all.
Just finished a Champ style amp, point to point, and am experiencing something new to me.
The power trans hums a little. That wouldn't be too bad but it transfers through the speaker.
What follows is a list of what I've tried in no particular order.
Tightened P.T.
Two different P.T's.
One different O.T.
Different set of new filter caps.
Checked and re-soldered all grounds.
Pulled preamp tube to isolate.
Unhooked 22k to 12AX7 tube to isolate noise.
New tubes.
Unhooked filament/ lamp lines.
Unhooked H.T. lines.
New diodes.
Shielded O.T.
Moving O.T. farther from P.T.
Turning O.T.
Tried a few different outlets. Even one across town...

The amp works otherwise, just hums... noticeably.
I'd say bad P.T. as it hums under no load, but I've tried two others with exact same sound.
I basically rebuilt the amp a couple times and cannot get the sound to even change.
It's definitely the same hum from the P.T. just amplified.
I have built several of this style amp and this is the first time any trans hum has been amplified.

Thanks in advance for any tips or suggestions.

Aaron

 

[corliss1]

When you say "hum from the PT" we need more details. If there was any type of hum induced from the PT into the speaker, the issue would happen *without* the power tube installed, as the signal is going straight to the OT.

Is this a known-good layout, such as a Fender-style clone and not a custom build?

What frequency is the hum?

Does pulling the 12ax7 kill it? Does pulling the 6v6 kill it? Is it affected by the volume control?

 

[dogmeat]

when I build an amp the power tranny is pretty much in a fixed location. to locate a favorable position for the output tranny, reverb tranny, or choke, I energize the power tranny and listen through a headset attached to windings in the other tranny. you can move it around to find what angle and distance it needs to be to get quiet. I'm in the process of building a Princeton right now and I could not get a spot where it went silent... until I turned it on it's side. apparently something about the way that tranny is made makes it more of a receptor. right side up I had to move it over a foot away from the PT, flipped 90 degrees on its side I could put it almost side by side with the PT. I bent up a 90 degree bracket to mount it

 

[Jon Snell]

Remove the valves and does it still hum?
If it does the hum maybe caused by induction from the mains transformer into the output transformer.
Disconnect the speaker with no valves , hum gone?
If it has, it is not induced directly into the loudspeaker but through the output transformer. If it hasn't, move the chassis out of the box and see if that makes a difference.
Not easy to explain without doing a few basic cheskc; position of transformers etc.

A few photos may help you get a constructive answer.

 

[bigguy12321]

Ok. Thanks for the replies. It was late last night when I posted.
The"hum" is actually P.T. buzz. It buzzes even when not installed in the chassis. Very slightly, have to have ear on it to hear it but it's there.
The noise (buzz) is amplified to the speaker as 6V6 starts up.
No valves, no amplification of buzz but P T. is still buzzing.
I checked a couple other P.T.'s and they all buzz similarly.
I took it to work and the same thing occurs so it's not my power.
The layout is not "standard" but it's the same one I've used for at least 10 others like it.
See pics. Not pretty but I've done several others and had no issues.
The guitar sound through it is fine, just humming at no volume.
Oh ya, it is not volume affected. Guitar gets louder, not hum.
This isn't a do or die piece for me, I am mostly trying to understand what is happening. I build my own chassis so if I have to re-do it, I'd be out about 2 hours and 6 bucks...
Thanks again!
Aaron

 

[corliss1]

Can you tighten the bolts holding the laminations on the PT at all? Not the screws holding the PT to the chassis, but just the PT itself. Sometimes transformers will "sing" if those get loose.

 

[bigguy12321]

Yep.. did that.
Rechecked another new trans that I have. Buzz is not as loud there.
I emailed AES for info also
Thanks!
Aaron

 

[58Bassman]

Ok. Thanks for the replies. It was late last night when I posted.
The"hum" is actually P.T. buzz. It buzzes even when not installed in the chassis. Very slightly, have to have ear on it to hear it but it's there.
The noise (buzz) is amplified to the speaker as 6V6 starts up.
No valves, no amplification of buzz but P T. is still buzzing.
I checked a couple other P.T.'s and they all buzz similarly.
I took it to work and the same thing occurs so it's not my power.
The layout is not "standard" but it's the same one I've used for at least 10 others like it.
See pics. Not pretty but I've done several others and had no issues.
The guitar sound through it is fine, just humming at no volume.
Oh ya, it is not volume affected. Guitar gets louder, not hum.
This isn't a do or die piece for me, I am mostly trying to understand what is happening. I build my own chassis so if I have to re-do it, I'd be out about 2 hours and 6 bucks...
Thanks again!
Aaron

I see AC wires very close to components that are connected directly to the output tube and that won't change by rotating the volume control. Some wires (like the heater wires) are parallel to component leads going to V1, others are close to and parallel to the bundle of caps/resistor next to the transformer.

The problem with induced hum is that we can't see it, but we can find it by using an inductive pickup like the wand in the link- it's used for identifying cables in AV, Cable TV and network installations but for this, you wouldn't need the box with the leads coming out, just your guitar and the amp. Leave the volume on the guitar set low and place the tip close to the power transformer wires, to get an idea of how much hum they produce, then move it to any tube pin, cap, resistor, or wire in the amp. Many amp builders are using shielded wire for the leads connecting the controls to V1- if you find that these wires are picking up the hum, maybe shielded wire will help.

Proximity to a hum or noise source is your enemy and if a signal carrying wire or component has to be placed near wires that produce hum, make sure they're positioned at a right angle- that reduces the magnetic field's affect from the transformer wires on the signal carrying wires/components.

https://www.tequipment.net/Extech/TG20/Tone-and-Probe-Kits/

 

[King Fan]

You've tried lots of fixes, so I'm gonna speculate, throw pasta at the wall.

Usually PT buzz is something you can feel, with your fingertips. Can you?

As noted by @corliss1, the usual attempted fix is to 1) tighten the stack bolts (using the nut on the *outside* of the chassis) and 2) insert a fiber (or possibly neoprene) washer *between outer stack nut and chassis*, then tightening *that* down with the mounting nut on the *inside* of the chassis.

But you say it happens with *several* PTs. Are they the same exact model? same mass? In any case this could suggest it's not a 'bad' extra-buzzy PT but one with a standard amount of PT hum/vibration.

In any case we gotta ask what's transmitting that vibration into the circuit. You say you've moved the OT -- like all over the place, different rotations? Actually, that might make sense -- that's a case of electronic coupling, not vibration.

You make your own chassis. Good. Steel or aluminum? Slightly rounded folds (no harm, usually) and a small form factor -- could this one (by chance) be 'tuned' to the PT vibe frequency? Vibration that gets into the chassis, board, tubes etc. can get amplified. Don't ask me how I know.

If that was the case, more damping between the PT and chassis might just help. Or maybe you could 'de-tune' the chassis -- JBWeld a big weight somewhere off center? Can you suppress the amplified buzz by pushing hard on the chassis somewhere? Can you feel the buzz in the chassis?

Again, just thinking out loud. OK, electronically. Good luck.

 

[bigguy12321]

Ok so... Here goes the pasta!
Cannot feel the buzz. Have to put my ear ON the trans to hear it without power tube installed.
I have unhooked the heater wires and moved them out of the way with no change. H.T. removed the same way also. Trans buzzes with no load.
Moved wires around while listening to noise through speaker with amp on, with no change.
Steel chassis, 16 ga. Not incredibly tight corners but better than most I've done...
It is a little smaller than the others I've done but only by a half inch or so. Maybe that's it???
All trannnies are about same mass and have similar buzzing. As I said, it doesn't seem to be abnormal buzz but this is the first one that gets amplified.
I did move and turn the O.T. and isolated it. No change.
This leads me to believe it's something about the chassis that's causing it.
There are small plastic washers on the bolts holding the transformer together. I did tighten those bolts before installing trans. Maybe isolating the trans with rubber washers is a good thing to try.
I think I'll remove the trans from the chassis and try it again. There should be enough wire to get it not touching or at least I can get some cardboard or something insulating between it and chassis.

This will give me something to try before ripping it all out and starting over... Actually I'm pretty impressed with myself for not doing that already. I really want to know what is causing this so I can avoid it in the future.

Thank you to all!
Aaron

 

[bigguy12321]

I just removed power trans from chassis leaving it hooked up. No change.
Isolation didn't help.
This is starting to get old...

 

[archetype]

Ok so... Here goes the pasta!
Cannot feel the buzz. Have to put my ear ON the trans to hear it without power tube installed.
I have unhooked the heater wires and moved them out of the way with no change. H.T. removed the same way also. Trans buzzes with no load.
Moved wires around while listening to noise through speaker with amp on, with no change.
Steel chassis, 16 ga. Not incredibly tight corners but better than most I've done...
It is a little smaller than the others I've done but only by a half inch or so. Maybe that's it???
All trannnies are about same mass and have similar buzzing. As I said, it doesn't seem to be abnormal buzz but this is the first one that gets amplified.
I did move and turn the O.T. and isolated it. No change.
This leads me to believe it's something about the chassis that's causing it.
There are small plastic washers on the bolts holding the transformer together. I did tighten those bolts before installing trans. Maybe isolating the trans with rubber washers is a good thing to try.
I think I'll remove the trans from the chassis and try it again. There should be enough wire to get it not touching or at least I can get some cardboard or something insulating between it and chassis.

This will give me something to try before ripping it all out and starting over... Actually I'm pretty impressed with myself for not doing that already. I really want to know what is causing this so I can avoid it in the future.

Thank you to all!
Aaron

Here's an old mechanic's trick for finding the source of a sound or vibration. Get a wooden dowel, length of broomstick, or whatever that's 12-14" long. You can put one end to your ear and the other end to an object or surface to hear the vibrations that are occurring at that point. Use this to listen to the transformer, all transformer mounting points at the chassis, etc. It works.

 

[corliss1]

So even completely disconnected, the part still hums too much? Are all the PTs from the same supplier? If so, could it be a bad batch?

 

[bigguy12321]

No definitely different manufacturers. Two were old stock. Two are new.
Just did a "Kids don't try this at home" moment.
Hooked up yet another O.T. completely separate from chassis. Same thing.
Also found that bias was way off so I adjusted accordingly. Same.
I'm getting tempted to swap output tube socket... More pasta...
This is really odd. I know there is an answer but I've already swapped 95% of the parts of the amp and not even a change in the tone of the hum.

I don't have my noise finder probe thingy here. I'll have to take this back to work on Thursday. I'm retired but I do amp repair at the local guitar shop a couple days a week. Thursday is my next day there.

Thanks for looking y'all!!!
Aaron

 

[Lowerleftcoast]

More pasta:
Recently we seem to have come across some poor QC pilot lamp holders. Weird hum as a result.

Move the heater CT to pin8 of the power tube to elevate the heaters. The AC heaters are not part of the DC power supply. They don't need to terminate with the HT CT.

Tuff time seeing the tube socket pins from this side of the room. Double check each connection for each pin is correct. Also have a look at the power tube socket. Maybe there is no solder on a pin.

How about a voltage chart for those of us that look at those things.

 

[2L man]

I have kept this link TubeCad page open because about 1/3 down there is very interesting "Aikido SE OPS article" where one additional electrolyte is used to cancel the HiFi amp hum. If it work in HiFi SE it should work in guitar SE as well. Perhaps you could test it?

BCF-2 & Aikido Single-Ended Output Stage

 

[King Fan]

OK, trying to use logic. Stand back and hold my beer.

We have an amplified hum (or IIRC you said buzz) after the tubes warm up. No change with volume knob. You can hear a hum/buzz when you put your ear up to the PT, but you can't feel it through the bell casing. It happens with *several different PTs from different makers.* It does it *with the PT removed from the chassis.*

My fuzzy logic says the PT hum you hear may not be the sound that's getting amplified. IOW, true, true, and unrelated. All PTs may hum to themselves.

I've had more than a few hum/buzz problems, from lots of sources -- and here's the thing: most all of them seemed to come "from the amplifier." Stuff I've used to hunt for them: Tap tests (I use a beefy plastic stick to get some mass). Damping tests (I use a hand on the outside of the chassis, a length of dowel with a fat pencil eraser inside). The dowel-phone or a stethoscope. Chassis upside down, right side up, etc. Hmm, somewhere I have a list of conceivable sources a friend drew up. OK here we go. Give me my beer, then stand back again. Sorry, many of these go beyond the chassis or your symptoms, but you'll see many that don't.

Tubes can do it, for sure.... tube pin holders in the sockets do harmonic buzzing very nicely if they aren't quite snug enough, and re-tensioning them with an awl or a jewelers screwdriver fixes that well (be sure to bend both sides in slightly, not just one).

Other sources:

• little washers in the chassis or between the speaker surround and the speaker frame,
• loose board mounting screws or component mounting screws (trannys, tube socket nuts, jacks, etc.)
• unused solder lugs on tube sockets (especially the rectifier),
• lugs in use but with enough play and metal-on-metal action at the bottom and a long enough wire to vibrate,
• ceramic sockets with little broken chips of ceramic or loose lugs
• leads on the board that look fine, have tight joints, etc. but vibrate just like a snare drum snare on the right note,
• loose speaker baffle screws,
• loose amp badge screws,
• loose handle t-nuts,
• space between the back covers and the furring strips they screw into,
• loose aluminum or screen shielding on the underside of the cab top where the chassis snugs up to it,
• long flying leads that rattle against a component on the board,
• the lamp holder bayonet against the chassis,
• the lamp bulb and / or the spring electric contact that pushes the bulb's two little lugs up against the holes in the lamp holder (sometimes filing the solder blob on the bottom of the bulb smoother helps keep the spring in contact with it);
• the lamp jewel and / or the bezel that screws down on it to clamp it to the outside of the chassis,
• those bear claw tube clamps,
• bubbles in the tolex or tweed that allow the shellaced or hardened glue on the inside of the bubble to rattle against the cab
• loose faceplates

Try not to fix or change anything just on speculation without an accurate diagnosis first, or you'll spend a long time and introduce a bunch of potential new problems. I'd start by taking the chassis and shaking it, tightening every fastener on it, plucking every wire and component to see if it can mechanically resonate, put a dab of silicone over each unused socket soldering lugs, tighten every screw, etc.

 

[Lowerleftcoast]

 

[corliss1]

At this point we really need to know the frequency of the hum. I don't see how it could be the PT at this point.

 

[bigguy12321]

View attachment 1116060

Good eye. That contact was relative new. It has since been corrected. Last version of O.T. removal.
Thanks!

King Fan, great list. I'll be checking the relevant ones for sure but also good for future troubleshooting.

So far I've been able to toss and slam this thing all around without the slightest but of tone change.

Also have yet to try the taste test...

I really don't like the pilot lamp holder and I will eliminate it when I get back out there.

Tube socket. I'll look closer at the holders.

I'll also elevate heater center. I've been meaning to try this anyway.

At this point we really need to know the frequency of the hum. I don't see how it could be the PT at this point.

Ya. Me too neither...

Sounds like a bad filter cap. Or the exact tone of power transformer buzz amplified through a 6V6GT...

I'll keep trying. Now that others are involved I feel I have a responsibility to the group.

Aaron