Power section hum?

[dehughes]

Well, I tried isolating the OT jacks from the chassis and moving the ground around to see if I could get things quieter (just in case this was the trouble...), and wasn't able to find a better ground point that was quieter. So, I assume the OT ground isn't the trouble.

HOWEVER, I noticed that when I stretch out my fingers on my right hand and touch both the OT casing and the tube socket shield for V1, there is a POP and then a really loud hummmm/buzzzzz until I remove my hand from either the tube shield or the OT casing, or just touch another finger to the chassis. What's that about? Touching the OT casing and then the shield for V1 shouldn't introduce noise/hum/popping into the circuit, should it? I have the tube sockets isolated from the chassis with rubber washers....and none of the tube socket pins are touching either the chassis or anything connected to the chassis.

This gets me thinking that maybe all this noise has something to do with the preamp ground and/or V1's interaction with the OT...

Am I correct in this, or way off?

THANKS GUYS!

A) You should solder a piece of desolder braid from the tube socket to chassis, otherwise the socket shield can be an antena
B) The only way to figure out the ground path is to completely rebuild it correctly, the ground path needs as much thought as the whole circuit layout

Interesting...so a socket shield can actually transmit noise/etc.. into a tube stage? I wouldn't think that there would be contact between the tube pins and the socket itself....and if the socket is not making contact with the chassis I'd think it wouldn't be part of a ground path.

What is it that makes the socket act like an antenna and then transmit into the gain stage?

Thanks!

[pila]

DZR400 Do you think it's OK to ground the backs of the pots separately to ground? The tabs on the back of the pots aren't always that great a ground. I know that grounding them in guitars helps.

PILA

[dehughes]

I've done some tinkering and moving of grounds, and that doesn't seem to help anything. I do know that the problem is in the preamp somewhere as when I pull V1 and V2 the noise disappears, just like when I turn all the tone knobs down to zero. So, I guess I'll just take another look at the preamp wiring and see if something there jumps out at me...

[drz400]

I've done some tinkering and moving of grounds, and that doesn't seem to help anything. I do know that the problem is in the preamp somewhere as when I pull V1 and V2 the noise disappears, just like when I turn all the tone knobs down to zero. So, I guess I'll just take another look at the preamp wiring and see if something there jumps out at me...

But did you try and totally re- layout the ground path???
OF course pulling V1 will get rid of the hum even if it is a ground issue.
It still all points to heater hum and ground path to me.
Dont forget that some preamp tubes are freaking hummy so you should rule that out as well

[drz400]

DZR400 Do you think it's OK to ground the backs of the pots separately to ground? The tabs on the back of the pots aren't always that great a ground. I know that grounding them in guitars helps.

PILA

I let the pot case be grounded to chassis.
I would never solder a ground to the pot, I take the ground for that volume pot for instance to the stage ground that it is feeding alone with the PS decoupling cap and cathode ground, then take that to the appropiate spot on the buss

[dehughes]

I've done some tinkering and moving of grounds, and that doesn't seem to help anything. I do know that the problem is in the preamp somewhere as when I pull V1 and V2 the noise disappears, just like when I turn all the tone knobs down to zero. So, I guess I'll just take another look at the preamp wiring and see if something there jumps out at me...

But did you try and totally re- layout the ground path???
OF course pulling V1 will get rid of the hum even if it is a ground issue.
It still all points to heater hum and ground path to me.
Dont forget that some preamp tubes are freaking hummy so you should rule that out as well

I've lifted some of the grounds and done some tube swapping, but none of that seems to help. I've tried lots of different 12A*7 tubes and they all produced nearly identical results, in all three positions. What I may do is just totally lift the ground (buss bar...) for the PI/Cathode follower section and see if moving that helps, as I'm pretty sure the noise is coming into the circuit in the preamp. The amp itself is pretty stinking quiet with either the tone knobs turned off or with the preamp tubes pulled, so I'm confident that the noise isn't entering the circuit through the power section.

The things that are keeping me from tearing this thing apart and star-grounding everything is 1) the faceplate is too thick to allow isolation washers and grounded input jacks, 2) same thing for the output jacks, 3) the turret strips are too close to the front panel to allow for the deeper input jacks, 4) I have it all wired up and am too lazy to start hacking about trying things that I'm not sure will work, and 5) I've seen this exact grounding layout work on a steel chassis of the same type of amp, same dimensions, same component layout. This leads me to think that my wiring is the culprit. So, I'll just take things one at a time, being as I'm not certain what the problem is exactly, and try to go about this in as orderly and controlled a fashion as possible... I'll gradually re-ground things and see what happens...as I think my ground wiring from the tubes to the strips isn't the greatest, and my isolation of the various stages' wiring isn't all that hot either.

I'll let you all know if I find out anything...

[rhinson]

i'm betting the source of your hum is part wiring and part just the way things are---you're gonna have some noise with the second stage fully bypassed with a 22uf cap. try this---put a 470k series resistor right on the second stage entry grid(ie--right up tight on the tube socket just like you do the 1.5k's on the power tubes) and see if some of the noise doesn't go away---and use shielded wire to the grid if you haven't already. rh

[dehughes]

i'm betting the source of your hum is part wiring and part just the way things are---you're gonna have some noise with the second stage fully bypassed with a 22uf cap. try this---put a 470k series resistor right on the second stage entry grid(ie--right up tight on the tube socket just like you do the 1.5k's on the power tubes) and see if some of the noise doesn't go away---and use shielded wire to the grid if you haven't already. rh

Interesting. What would this 470k resistor do, then?

[dehughes]

So, I've tried connecting the ground buss for the preamp to the ground for the filter caps on that stage (star-ish ground, a la Aiken, etc...) and this didn't help anything. I've also tried isolating the input jacks from ground and this didn't help anything either. I've tried poking and prodding various wires on the sockets and to the pots and haven't discovered anything either. One oddity, though:

I've found that this hum/hiss is present on also when the EF86 channel is turned up. This channel connects to the opposite side of the PI as the 12AX7 channel, and only has a volume and a 6-way rotary tone switch. What's more, when I removed the EF86 tube and turned up the volume pot for that channel, the low-level hum and a measure of hiss was produced! Weird? With the tube gone that channel should be dead, right? I found that removing V1 and V2 (leaving only the PI) on the 12AX7 side killed the tone pots and volume controls and the noise, so why wouldn't the EF86 side to the same when that tube was removed? Pulling out the PI tube killed all preamp noise for both channels, so it is definitely getting in before or at the PI.

Also, before I pulled the PI, but with V1 and V2 pulled, the 12AX7 side exhibited some odd behavior. With all the tone controls off, and the master and preamp gain at noon, bumping up the mid control produced some pops...odd. What's more, with the PI still in there and the tone controls up at about noon, I noticed that the master volume when turned all the way off or all the way on was very quiet, EXCEPT that the sweep in between off and on produced a fairly loud hum!?! Weird. Man, this thing is getting more funky every time I look into stuff....and I haven't really changed anything yet!

Finally, I noticed that when I reduced the voltage to the amp as a whole (switches in a 121ohm resistor before the 1st filter cap...), things are much quieter, but with the resistor out of the circuit (standard AC30 voltages...) the amp produces more low-level hum. This makes me think that something from the power supply is getting into the preamp, and when the power supply is running hotter then there is "more" seeping into the preamp, causing hum and hiss....

So, that's that. I'm done tinkering at this point as I'm totally clueless as to what's up. Maybe I'll get back to it later in the week...eesh.

Thanks for all your help so far, though.

[drz400]

Well, I tried isolating the OT jacks from the chassis and moving the ground around to see if I could get things quieter (just in case this was the trouble...), and wasn't able to find a better ground point that was quieter. So, I assume the OT ground isn't the trouble.

HOWEVER, I noticed that when I stretch out my fingers on my right hand and touch both the OT casing and the tube socket shield for V1, there is a POP and then a really loud hummmm/buzzzzz until I remove my hand from either the tube shield or the OT casing, or just touch another finger to the chassis. What's that about? Touching the OT casing and then the shield for V1 shouldn't introduce noise/hum/popping into the circuit, should it? I have the tube sockets isolated from the chassis with rubber washers....and none of the tube socket pins are touching either the chassis or anything connected to the chassis.

This gets me thinking that maybe all this noise has something to do with the preamp ground and/or V1's interaction with the OT...

Am I correct in this, or way off?

THANKS GUYS!

A) You should solder a piece of desolder braid from the tube socket to chassis, otherwise the socket shield can be an antena
B) The only way to figure out the ground path is to completely rebuild it correctly, the ground path needs as much thought as the whole circuit layout

Interesting...so a socket shield can actually transmit noise/etc.. into a tube stage? I wouldn't think that there would be contact between the tube pins and the socket itself....and if the socket is not making contact with the chassis I'd think it wouldn't be part of a ground path.

What is it that makes the socket act like an antenna and then transmit into the gain stage?

Thanks!

Trust me, it does. Just like having a shield on a cable without terminating it at either end is worse than no shield at all. The metal picks up the signal and injects it into the tube thru the glass. If you look at old Fenders that had gromets you will also see a ground wire attached to the shield base.

Do you have a shematic on this amp? I still say your issue sound like ground path especially when describing the Volume controls. You could try and add some filtering to rule out that.

[dehughes]

A) You should solder a piece of desolder braid from the tube socket to chassis, otherwise the socket shield can be an antena
B) The only way to figure out the ground path is to completely rebuild it correctly, the ground path needs as much thought as the whole circuit layout

Interesting...so a socket shield can actually transmit noise/etc.. into a tube stage? I wouldn't think that there would be contact between the tube pins and the socket itself....and if the socket is not making contact with the chassis I'd think it wouldn't be part of a ground path.

What is it that makes the socket act like an antenna and then transmit into the gain stage?

Thanks!

Trust me, it does. Just like having a shield on a cable without terminating it at either end is worse than no shield at all. The metal picks up the signal and injects it into the tube thru the glass. If you look at old Fenders that had gromets you will also see a ground wire attached to the shield base.

Do you have a shematic on this amp? I still say your issue sound like ground path especially when describing the Volume controls. You could try and add some filtering to rule out that.

Cool. Will do on the filtering....but first I'm going to see about re-grounding the cathode resistors/caps on the gain stages and see if I can clean things up.

I have a schematic but it is an older one....I've made some changes since then and haven't updated it. I still have a few things to try before I'm out of ideas, but I'll keep trucking along... Thanks for your help, man.

As for the shields, the amp was just as noisy without the shields on, so I doubt that's the source. You know, my amp is just as hummy as my little Top Hat Club Royale (built back in the day before they became absolute construction gurus...), so maybe I'm making a bigger deal of this. I mean, my little CR always has had a low enough noise floor for my uses, and this amp of mine ONLY is noisy when cranked, and I'll never play it at that level....AND, even if I did, you'd never hear the noise as the amp would be sickeningly loud by that point.

Actually...wait....my little Top Hat also has the same tube shields as this amp...the Fendery type with the spring top and twist-on base. Do you think that the bottom piece of metal on the tube shield base would cause hum to be induced into the tube, even with the top shield part removed? I mean, even on those bases you have the lower half of the tube surrounded by metal. I mean, the PT and OT are situated to be flat-mounted, so the laminations would directly bisect the tubes as they are on the chassis....maybe that little bottom metal wrap on the socket is inducing hum....wouldn't that be something....I guess I could replace the sockets.... What do you think....a possibility???

[drz400]

I think it would probably be ground path
Class A amps will hum more than others but you can still make them pretty damm quiet if you are real careful and even at a volume of 10.
I would ground the shield base just cause you should.

[dehughes]

I think it would probably be ground path
Class A amps will hum more than others but you can still make them pretty damm quiet if you are real careful and even at a volume of 10.
I would ground the shield base just cause you should.

Okay, I'll try that again and see.

Thanks.

[lf353]

Have you tried a 1 ohm power resistor after the bridge but before the first filter cap?

A story years ago I modded a vc30 I changed the power supply caps to Poplypros. The amp hummed the transformer made noise. I realized the surge currents caused by the very low ESR caps were causing a DC inbalance in the torroid PS transfomer the PS diodes are not matched so they conduct a little different at higher currents. I installed a 1 ohm power resistor and problem went away.

The 1 ohm came from measurements of power supply caps. Al electros at 450VDC to 500VDC I like have less than 1.5ohms ESR per 20uf.

An EI transfomer is less sensitive to this but it still matters. The first caps in the PS if they are good will have very low ESR and the more cap the lower the ESR.

[dehughes]

Have you tried a 1 ohm power resistor after the bridge but before the first filter cap?

A story years ago I modded a vc30 I changed the power supply caps to Poplypros. The amp hummed the transformer made noise. I realized the surge currents caused by the very low ESR caps were causing a DC inbalance in the torroid PS transfomer the PS diodes are not matched so they conduct a little different at higher currents. I installed a 1 ohm power resistor and problem went away.

The 1 ohm came from measurements of power supply caps. Al electros at 450VDC to 500VDC I like have less than 1.5ohms ESR per 20uf.

An EI transfomer is less sensitive to this but it still matters. The first caps in the PS if they are good will have very low ESR and the more cap the lower the ESR.

Regarding the low, power section hum...now this is interesting... I'll need to re-read this post, but I think I'm understanding it. Where would the resistor go, exactly.....right between the recto and the 1st filter cap? I'm using 450v Sprague Atoms...


Also, regarding the hissy/static-y preamp noisenoise, I think it may be a function of the preamp tube sockets. I've run the socket shields all to ground and there was no reduction in the noise (it sounds like a tube going bad...but doesn't diminish when other tubes are swapped in...). HOWEVER, when I physically pushed V1 a little bit and rocked it in its socket, I noticed that the noise dropped by about half the volume. Interesting. This leads me to believe that there might be some issue with the socket, either in the wiring of it or something else... These Tung Sol 12AX7s are pretty fat tubes and they touch the sides of the tube shield...can this be a potential problem, do you think?

THANKS all! I'm getting sooooo close to fixing this...