Hum in a sortof 18W amp

[TexasTone]

Guy brings me his amp, saying that there's a hum only on the normal channel and not on the high gain channel.  It's a channel-switching amp.  Okay.  I pull the chassis and fire it up and think, where's the hum?  It's only on the normal channel, but only when you crank the volume past Noon.  It's not bad, but there's a noticeable 60Hz hum that's only on the default normal channel.  When you switch it to the high gain channel, there's no hum.

The normal channel has only one gain stage and a volume and tone control.  The channel switching is between each volume control and the LTPI.

I've gone over the grounding and it all looks good.  Since the two channels are out of phase, due to a cathode follower on the high gain channel, would that cause this hum?

I've done a bit of troubleshooting and haven't gotten to the source yet.

Thanks in advance.

[tubeswell]

Roll another tube through each channel (including the CF just in case)

[TexasTone]

Rolling tubes has no effect.  Even pulling V1, first stage for both channels, has no effect.  The volume control still raised the hum.

[shooter]

C20 could be sketchy. 


EDIT;
maybe lift the relay out-of-circuit for test.

[TexasTone]

C20 could be sketchy. 


EDIT;
maybe lift the relay out-of-circuit for test.

Two things will eliminate the hum.
1.  Removing power from the relay.  It still passes signal from the Norm Vol to the LTPI due it the N.C. default.
2.  Removing V3, the LTPI.  Removing V1 and/or V2 had no effect.

Here's the latest.

The manual switching is a 1M-A pot with a SPST Pull-on Push-off (Normally Open) switch. When the switch is pushed in (Off or N.O.) then the open/closed (off/on) footswitch activates the channel switching relay. When the manual switch is pulled out - On - then the relay is bypassed and not operable.

- Relay OFF (or manual switch in) = Default; Normal channel active - volume control output passes to the LTPI through C13. TMB volume output is open. This is the case even if the relay is powered off (see no, 4 below).
- Relay ON (or manual switch out) = TMB channel active - volume control output passes to the LTPI through C13. Normal channel volume output is open.

Whenever the Normal channel is active, via either the manual switch "in" or the relay switch in the Default position, there is a 60-cycle hum when the Normal volume is turned up. The hum goes away when the Normal volume is down. Whenever the TMB channel is active, via either the manual switch 'out' or the relay in the Switched position, the hum disappears.

1. Pull V1, the 1st preamp for both the Normal and TMB channels. No change, hum still there.
2. Pull V2 the TMB tone stack driver cathode follower. No change, regardless of whether or not V1 is in place. No change, hum still there.
3. Pull V3, regardless of whether or not V1 and V2 are in place. Hum disappears.
4. With all tubes in place, removing the low voltage power from the relay makes the hum disappear, even though the signal passes through the N.C relay switch.

So, with the relay powered, there is a hum. With no relay power, there is no hum, even though the relay switch, being NC, passes the signal from the Normal channel volume to the LTPI. O, there is no hum with the LTPI pulled.

I am including a marked-up schematic.
The preamp grounds are daisy-chained to one point near the isolated input jacks.
- LTPI R26 and C14 ground to C3.
- C3 ground connects to C4 ground lead
- Mid (R22) & Volume ground to TMB Gain, which grounds to point "A".
- R20 and R16 ground to point "A".
- Input jacks ground to point "A".
- Point "A" connects to C4 ground lead.
- Normal Volume and Tone ground to C4 ground lead.
- C4 connects to both V1 cathode RC grounds.
- V1 cathode RC grounds go to chassis, so everything ends up at chassis ground.

All the power grounds - isolated output jacks, bias circuit, heater elevation circuit, and C1/C2 can, ground to the chassis ground next to the cap can.

The earth ground in near the power cord input.

[tubeswell]

How's your channel switching relay grounded?

[TexasTone]

How's your channel switching relay grounded?

The relay is not grounded.  It's powered by the 6.3VAC heater supply, and diode rectification to DC.

[tubeswell]

diode rectification to DC.

Hmmm, that's not what your schematic shows

[TexasTone]

diode rectification to DC.

Hmmm, that's not what your schematic shows

The Mojotone relay channel switcher gets its supply voltage from the tube heater string.  The thing that's bugging me is that this amp is wired exactly like Mojotone's Studio One amp schematic and yet we have a 60Hz hum on the normal channel only.  Removing the supply voltage will stop the hum.  Removing V1 and/or V2 will not.  Removing V3 LTPI will.

This leads me to think there's a ground loop between the Normal preamp and the LTPI, but I haven't found any, so far. 

I appreciate the responses.

Thanks

[Willabe]

Removing the supply voltage will stop the hum. 

Removing what supply voltage?

The relay supply voltage?

[Willabe]

I'd try moving the heater elevation/stand off dcv ground from R38/C20 over to the preamp ground star.

Just disconnect that ground wire and use an alligator clip wire lead to temp it in at the preamp ground star.   

[Willabe]

Where is the shield grounded at from the wire that goes from the normal channel volume pot's wiper to pin 6 on the relay?

And is the shield grounded at only 1 end of the shield?

[Willabe]

Are the power supply filter caps 3 and 4 single caps? Or are they part of the cap can?

[TexasTone]

Are the power supply filter caps 3 and 4 single caps? Or are they part of the cap can?

Thank you for the replies

1. Removing what supply voltage from the relay stops the hum.  Since the default (N.C.) position is the normal channel, the normal channel stays on, but the hum goes away.

2. I will try moving the heater elevation/standoff dcv ground from R38/C20 over to the preamp ground star.  This amp uses elevated heaters for the high voltages on the cathode follower.  I replaced R38 with a larger value to up the elevation from 50 to 70 volts, which did not eliminate the hum.

3. Where is the shield grounded at from the wire that goes from the normal channel volume pot's wiper to pin 6 on the relay?  The shield for the wire that goes from the normal channel volume pot's wiper to pin 6 on the relay is shielded at the ground terminal of that volume control.
  The TMB channel volume control shielded cable is grounded at the volume control pot casing.
  The shielded cable from the TMB Treble to the TMB volume is also grounded at the TMB volume pot casing.
  The shielded cable from the relay switch to the LTPI is grounded at C3, along with the LTPI tail resistor and the AC shunt cap.
  The shielded input cable to V1 is grounded at the input jack, which ties to the normal tone control, along with the normal volume control.
  From the normal tone control, a ground wire goes to the preamp chassis ground.

4.  The power supply filter caps 3 and 4 are both single caps, F&T 22µF @ 500V.

[Willabe]

What's the value of R38? It shouldn't be more than 100K.

Make R38 100K, for the least heater current waste to ground, and use R37 to adjust the dc stand off heater voltage.

[Willabe]

3. Where is the shield grounded at from the wire that goes from the normal channel volume pot's wiper to pin 6 on the relay?  The shield for the wire that goes from the normal channel volume pot's wiper to pin 6 on the relay is shielded at the ground terminal of that volume control.
  The TMB channel volume control shielded cable is grounded at the volume control pot casing.
  The shielded cable from the TMB Treble to the TMB volume is also grounded at the TMB volume pot casing.
  The shielded cable from the relay switch to the LTPI is grounded at C3, along with the LTPI tail resistor and the AC shunt cap. 

I wouldn't trust grounding by soldering to a pots casing.

Take a gator clip jumper cable and try grounding the pot ground to the ground star it's going to.

See if that helps.

If it does get rid of the pots casing ground and hard wire the shield to the ground star.

[Willabe]

Is anything else grounded to any pot's casing besides the shield?

How many chassis ground connections to the chassis do you have?

You said all the jacks are insulated/isolated from the chassis.   

[TexasTone]

Is anything else grounded to any pot's casing besides the shield?

How many chassis ground connections to the chassis do you have?

You said all the jacks are insulated/isolated from the chassis.   

The only things grounded to pot casings are the two shields.  I checked all pots casings with my DVM to ensure grounding.

All jacks (two input, two output, and one footswitch) are Cliff jacks, isolated from the chassis

There are two circuit ground points, on preamp ground and one power ground, in addition to the earth safety ground.

The power ground is attached to the 50/50 can cap.  The output jacks, 290-0-290 HV center tap (fused), bias circuit, and heater elevation/6.3VCT voltage divider, since it gets its voltage from C2.

The LTPI tail resistor and AC shunt cap ground to C3.  The C3 ground connects to the C4 ground.  The TMB mid resistor connects to the TMB volume control ground tab, which connects to the TMB gain control ground tab, which connects to the turret board point "A", along with the V2b cathode (DC couple cathode follower driver) and the V2a (CF) grid resistor.  The input jacks also connect to point "A".  Point "A" connects to C4.  The Normal channel tone and volume controls connect to C4.  C4 connects to both V1 cathodes (normal channel stage 1), and then to the preamp circuit chassis ground.

Please see the annotated schematic on Reply #4 above.

[BrianS]

Try re-flowing the solder joints on the jack/relay board.  Or replace relay board if cost effective and available.  Seems likely your problem is there.  Good luck.

[Willabe]

Please see the annotated schematic on Reply #4 above.

I saw that.

[Willabe]

Try what BrianS said about the relay board. Somethings funny about removing the relays dcv and it removes the humm.
 
If that doesn't work;

Since you can't find the humm source and after looking at the layout you posted, I'd try a few things. Their small adjustments to the grounding wiring but they/1 of them might fix it.

1. This could be important, remove the black ground wire that goes from C5 to the turret boards corner bolt for the preamp chassis ground. That preamp chassis ground wire should be run from C4's ground lead to chassis ground.

Does that help any? If so hard wire it in like that.

2. I'd disconnect 1 end of the black ground wire that goes from the input jack with R13 on it to the normal channel tone pot's lug 3. Now disconnect the black ground wire going from that tone pot's lug 3 over to C4's ground lug at 1 end and gator clip it over to where C8/C6/R14/R6 all meet. So that now the normal channels tone pot lug 3, volume pot's lug 3 and the input tubes grid return 1M/R9 all 3 go together over to C8/C6/R14/R6, that forms a ground star that you bring over (with the dotted black wire that's already there) to the filter caps ground lug that feeds the tube that those R's and C's are from.

Does that help any? If so hard wire it in like that.

3. Next I'd try separating the TB (you don't have a middle pot) channels gain control's ground from it's volume controls ground, and maybe separate R 20 from R16. I don't trust those grounds being together.

I'd disconnect the black ground wire on the TB gain controls lug 3 and leave that gain pots ground going over to R16/R20.

Now use a gator clip lead to go from the TB channels volume pot lug 3, that also has R22 that sets the mids, over the C4 filter caps ground lead.

These changes might look to be pretty much the same as it's wired now, but their not. You want to 'form stars' with all the components that belong to a tube stage and the filter caps ground that feeds those components. You don't want to mix those together. Ground is not ground in that it's all the same, it's not.

If those changes don't kill the humm, I'd go in and and separate V1's triode 2 cathodes (K) C6/R6 - C8/R14 and ground them with their own input jacks ground that they feed. Then run them separately, they each get their own ground wire, over to C4's ground lug.     

[Willabe]

I checked all pots casings with my DVM to ensure grounding.

How?

Did you check for resistance to chassis or continuity?

Have to test for resistance NOT continuity. Continuity wont be accurate.

[Willabe]

What's the value of R38? It shouldn't be more than 100K.

Make R38 100K, for the least heater current waste to ground, and use R37 to adjust the dc stand off heater voltage.

This is important. Going past 100K for R38 will exceed a tube rating, I can't find it in my books right now.

And the lower you go the more heater current you throw away to ground/waste. That just heats up the PT more needlessly.   

[Willabe]

Have you read this from Merlin on grounding?

Look at how he shows multiple star grounding using a buss to connect the ground stars. It works great.

https://www.valvewizard.co.uk/Grounding.html

[TexasTone]

Try what BrianS said about the relay board. Somethings funny about removing the relays dcv and it removes the humm.
 
If that doesn't work;

Since you can't find the humm source and after looking at the layout you posted, I'd try a few things. Their small adjustments to the grounding wiring but they/1 of them might fix it.

1. This could be important, remove the black ground wire that goes from C5 to the turret boards corner bolt for the preamp chassis ground. That preamp chassis ground wire should be run from C4's ground lead to chassis ground.

Does that help any? If so hard wire it in like that.

2. I'd disconnect 1 end of the black ground wire that goes from the input jack with R13 on it to the normal channel tone pot's lug 3. Now disconnect the black ground wire going from that tone pot's lug 3 over to C4's ground lug at 1 end and gator clip it over to where C8/C6/R14/R6 all meet. So that now the normal channels tone pot lug 3, volume pot's lug 3 and the input tubes grid return 1M/R9 all 3 go together over to C8/C6/R14/R6, that forms a ground star that you bring over (with the dotted black wire that's already there) to the filter caps ground lug that feeds the tube that those R's and C's are from.

Does that help any? If so hard wire it in like that.

3. Next I'd try separating the TB (you don't have a middle pot) channels gain control's ground from it's volume controls ground, and maybe separate R 20 from R16. I don't trust those grounds being together.

I'd disconnect the black ground wire on the TB gain controls lug 3 and leave that gain pots ground going over to R16/R20.

Now use a gator clip lead to go from the TB channels volume pot lug 3, that also has R22 that sets the mids, over the C4 filter caps ground lead.

These changes might look to be pretty much the same as it's wired now, but their not. You want to 'form stars' with all the components that belong to a tube stage and the filter caps ground that feeds those components. You don't want to mix those together. Ground is not ground in that it's all the same, it's not.

If those changes don't kill the humm, I'd go in and and separate V1's triode 2 cathodes (K) C6/R6 - C8/R14 and ground them with their own input jacks ground that they feed. Then run them separately, they each get their own ground wire, over to C4's ground lug.   

Thank you for the thorough reply.

1. C4 negative side connects underneath the board to C5.  C8 is linked to C5, and then C5 to the chassis ground.  C5 is the normal channel cathode and C8 is the TMB channel cathode, and since the hum is only on the normal channel, I did not deem this a likely cause.  However, I can reroute that by clipping the wire and running a link from C4 directly to chassis ground.  BTW, I found an error in the layout. 

2.  "I'd disconnect 1 end of the black ground wire that goes from the input jack with R13 on it to the normal channel tone pot's lug 3."   The two jacks are typical Fender Hi/Lo jacks, the leftmost with R12 being the high and the rightmost with R13 being the low jack.

If I'm following you correctly, you're saying they're already linked to C4 instead via the under-board wire and having the chassis ground only connect to C4. of the chassis ground.

3.  "Next I'd try separating the TB (you don't have a middle pot) channels gain control's ground from its volume controls ground, and maybe separate R 20 from R16. I don't trust those grounds being together."

"These changes might look to be pretty much the same as it's wired now, but they're not. You want to 'form stars' with all the components that belong to a tube stage and the filter caps ground that feeds those components. You don't want to mix those together. Ground is not ground in that it's all the same, it's not."

My thinking there is that the entire DC coupled cathode follower is fed by C4 so I return all the ground to it.  They can be separated but they should still all go C4.  I daisy-chained them and you're saying to individually ground them to C4.   Point "A" is the ground for R20 and R16, and ties via buss wire to C4.  So, point "A", the buss wire, and the negative terminal of C4 should be equal, even though each one does not have a separate path.  The TB channel doesn't hum, only the Normal channel.

"If those changes don't kill the hum, I'd go in and separate V1's triode 2 cathodes (K) C6/R6 - C8/R14 and ground them with their own input jacks ground that they feed. Then run them separately, they each get their own ground wire, over to C4's ground lug.  The two jacks are typical Fender Hi/Lo jacks, the leftmost with R12 being the high and the rightmost with R13 being the low jack."

The only thing grounding to C3 is the LTPI, from where it gets its supply voltage.

[TexasTone]

I checked all pots casings with my DVM to ensure grounding.

How?

Did you check for resistance to chassis or continuity?

Have to test for resistance NOT continuity. Continuity wont be accurate.

Both, measure with a Fluke 117
To Preamp buss chassis ground lug.
TB volume lug 3 to chassis ground lug- 0.4Ω
TB gain lug 3 to chassis - 0.5Ω
C3 to chassis - 0.6Ω
R25/C14/ LTPI cable shield to chassis ground lug - 0.6Ω
Mid resistor, TB volume, TB gain, R16, R20 (point "A") to chassis ground lug - 0.5Ω
Input jack ground buss to chassis ground lug - 0.4Ω
Normal volume ground lug to chassis ground lug - 0.3Ω
Normal Tone ground lug to chassis ground lug - 0.3Ω
C3 to chassis ground lug - 0.3Ω
R14/C8 to chassis ground lug - 0.2Ω
R6/C5 to chassis ground lug - 0.1Ω

Power amp chassis ground lug.
Output jack to chassis ground lug - 0.3Ω
C1/C2 ground terminal to chassis ground lug - 0.2Ω
Bias circuit tail resistor to chassis ground lug - 0.4 Ω
Bias cap grounds to chassis ground lug - 0.4 Ω
Heater elevation ground to chassis ground lug - 0.3 Ω

[TexasTone]

What's the value of R38? It shouldn't be more than 100K.

Make R38 100K, for the least heater current waste to ground, and use R37 to adjust the dc stand off heater voltage.

This is important. Going past 100K for R38 will exceed a tube rating, I can't find it in my books right now.

And the lower you go the more heater current you throw away to ground/waste. That just heats up the PT more needlessly.

R38 is 68k.  It was originally 47k, which yielded bout 53 V elevation.  R 37 is 270k.  At about 360V B+it yields about 72V.

[TexasTone]

Have you read this from Merlin on grounding?

Look at how he shows multiple star grounding using a buss to connect the ground stars. It works great.

https://www.valvewizard.co.uk/Grounding.html

Yes.  That's why I ground each circuit to its supply cap, and only have two grounds, power and preamp.

[TexasTone]

Try re-flowing the solder joints on the jack/relay board.  Or replace relay board if cost effective and available.  Seems likely your problem is there.  Good luck.

I have checked the voltages and operation of the relay board and been in dialogue with Mojotone.  The relay switching appears to be functioning.

I did R&R the heater supply wires and swap and replace signal wires and cables.  I was thinking that having all three shielded cables might be causing a loop. 
The normal channel volume shield is terminated at the volume control ground lug.
The TB channel volume shield is terminated at the back of the volume pot.
The LTPI shield is terminated at the same turret as the LTPI tail resistor and AC shunt cap, on the C3 buss.

[BrianS]

Yes, the relay switch functions "properly", in that is switches...but there is an issue that is causing hum, and in my reading of your description, it seems to be related to the relay/relay circuit.  It looks like on their current website, the circuit needs to be soldered together by whomever is the installer.  This introduces the chance for a bad solder joint...not that a bad solder joint couldn't happen at Mojotone if they are the ones that soldered it together, either.  Some other minor anomaly or damage on the board or to a component could cause a slight issue as well. 
 

[Willabe]

That's why I ground each circuit to its supply cap, and only have two grounds, power and preamp.

No you didn't. You have some from 1 triode mixed with some from another triode then you ran a wire over to the filter cap. You have it only part right.

I wrote it all out for you and explained what needs to go where and why.

You make a ground star with all the components grounds from a single triode then run a wire from that star to the filter caps ground lug that feeds that triode.

Do what ever you want, but you still have humm and your running out of things to try.

[Willabe]

1. C4 negative side connects underneath the board to C5.  C8 is linked to C5, and then C5 to the chassis ground.  C5 is the normal channel cathode and C8 is the TMB channel cathode, and since the hum is only on the normal channel, I did not deem this a likely cause.  However, I can reroute that by clipping the wire and running a link from C4 directly to chassis ground.  BTW, I found an error in the layout.

I don't know why you wrote all that out, yes, I know, I understand this, I saw it on the 2 drawings. I know you can rerout it, I said to do that.

..... since the hum is only on the normal channel, I did not deem this a likely cause.

 

At this point what does seem like a likely cause for the humm?     

2.  "I'd disconnect 1 end of the black ground wire that goes from the input jack with R13 on it to the normal channel tone pot's lug 3." The two jacks are typical Fender Hi/Lo jacks, the leftmost with R12 being the high and the rightmost with R13 being the low jack.

I know their hi/low input jacks. That's not the point. The point is to separate those grounds as I wrote out exactly how to do it.

If I'm following you correctly, you're saying they're already linked to C4 instead via the under-board wire and having the chassis ground only connect to C4. of the chassis ground.

Yes.

You don't want the ground current at C4 going over to V1's a/b triodes cathodes (K) C6/R6 - C8/R14 and possibly modulating those 2 triodes K's. So you run the ground from those 2 K's over to C4's ground. Then you run a wire from C4 over to the chassis preamp ground. This isolates V1's K's from the ground current at C4. I wrote all this out for you. Please go back and re read it all until you get it. 

3. "Next I'd try separating the TB (you don't have a middle pot) channels gain control's ground from its volume controls ground, and maybe separate R 20 from R16. I don't trust those grounds being together."

The TB channels gain and volume are feed from 2 different triodes. They don't get grounded  together. They get grounded with the ground star from the triode their tied to. Then you run the ground star over to C4's ground lug. I wrote out what goes in which ground star. It makes a difference. 

"These changes might look to be pretty much the same as it's wired now, but they're not. You want to 'form stars' with all the components that belong to a tube stage and the filter caps ground that feeds those components. You don't want to mix those together. Ground is not ground in that it's all the same, it's not."

My thinking there is that the entire DC coupled cathode follower is fed by C4 so I return all the ground to it.  They can be separated but they should still all go C4.  I daisy-chained them and you're saying to individually ground them to C4.   Point "A" is the ground for R20 and R16, and ties via buss wire to C4.  So, point "A", the buss wire, and the negative terminal of C4 should be equal, even though each one does not have a separate path.  The TB channel doesn't hum, only the Normal channel.

No. See above.

Your randomly tying these components from different triodes to a buss wire. They should be tied together with only the ground leads from the components from their triode. That forms a ground star, then you run a single wire from the ground star over to the filter caps ground lead that feeds that triode it's B+dcv. It's the same thing over and over.

[Willabe]

I checked all pots casings with my DVM to ensure grounding.

How?

Did you check for resistance to chassis or continuity?

Have to test for resistance NOT continuity. Continuity wont be accurate.

Both, measure with a Fluke 117
To Preamp buss chassis ground lug.
TB volume lug 3 to chassis ground lug- 0.4Ω
TB gain lug 3 to chassis - 0.5Ω
C3 to chassis - 0.6Ω
R25/C14/ LTPI cable shield to chassis ground lug - 0.6Ω
Mid resistor, TB volume, TB gain, R16, R20 (point "A") to chassis ground lug - 0.5Ω
Input jack ground buss to chassis ground lug - 0.4Ω
Normal volume ground lug to chassis ground lug - 0.3Ω
Normal Tone ground lug to chassis ground lug - 0.3Ω
C3 to chassis ground lug - 0.3Ω
R14/C8 to chassis ground lug - 0.2Ω
R6/C5 to chassis ground lug - 0.1Ω

Power amp chassis ground lug.
Output jack to chassis ground lug - 0.3Ω
C1/C2 ground terminal to chassis ground lug - 0.2Ω
Bias circuit tail resistor to chassis ground lug - 0.4 Ω
Bias cap grounds to chassis ground lug - 0.4 Ω
Heater elevation ground to chassis ground lug - 0.3 Ω

I just asked if you tested for resistance at the pot casing to the chassis.

We didn't need all these measurements.

[Willabe]

I still think the grounding could be better, but...

It might just be that relay is bad because the humm stops when you remove the dcv from the relay. Try a different relay.

BrianS might be right.

[Willabe]

I did a search for relay humm in guitar amps found this from our forum;

https://el34world.com/Forum/index.php?topic=20838.msg220304#msg220304

And I found this from geofex, but I can't get it to open. Is his site down?

Excessive Hum - geofex.com

[Willabe]

Your layout drawing in reply #17 doesn't show a rectifier for the heater to relay power supply.

And it shows a dc fan. Are you using a dc fan powered from that same supply?

If so, have you tried disconnecting it to see if the humm stops?

[Willabe]

Here's the link to Doug's relay board and relay power supply board. Many guys have used these here with no humm problems.

I thought Mojo used to sell these same boards, but I don't see them on their web site.

https://hoffmanamps.com/MyStore/catalog/parts20.htm

[TexasTone]

I still think the grounding could be better, but...

It might just be that relay is bad because the hum stops when you remove the dcv from the relay. Try a different relay.

BrianS might be right.

Willabe, I appreciate your input and patience.

"Your layout drawing in reply #17 doesn't show a rectifier for the heater to relay power supply." 
This is built into the relay board, which takes the 6.3vAC and turns it into ~6 to 7 vDC for the relay.  This is shown in Reply #8 above.

" Are you using a dc fan powered from that same supply?  If so, have you tried disconnecting it to see if the hum stops?"
There is a separate terminal strip to tap off the heater string for the fan.   I disconnected the fan.

"I just asked if you tested for resistance at the pot casing to the chassis."
1Ω this morning.

"You don't want the ground current at C4 going over to V1's a/b triodes cathodes (K) C6/R6 - C8/R14 and possibly modulating those 2 triodes K's. So you run the ground from those 2 K's over to C4's ground. Then you run a wire from C4 over to the chassis preamp ground. This isolates V1's K's from the ground current at C4."
I did that this morning.  The hum is now louder!

"The TB channels gain and volume are feed from 2 different triodes. They don't get grounded together. They get grounded with the ground star from the triode their tied to. Then you run the ground star over to C4's ground lug. I wrote out what goes in which ground star. It makes a difference. "
I haven't done that yet; it's next.

I checked all the solder pads on the relay board; I haven't tried a different board.

Thank you again.

[Willabe]

It's probably your relay.

I did a search for relay humm in guitar amps found this from our forum;

Read this link;

https://el34world.com/Forum/index.php?topic=20838.msg220304#msg220304

And I found this from geofex, but I can't get it to open. Is his site down?

Excessive Hum - geofex.com

[Willabe]

"You don't want the ground current at C4 going over to V1's a/b triodes cathodes (K) C6/R6 - C8/R14 and possibly modulating those 2 triodes K's. So you run the ground from those 2 K's over to C4's ground. Then you run a wire from C4 over to the chassis preamp ground. This isolates V1's K's from the ground current at C4."

I did that this morning.  The hum is now louder!

Impossible.

It would either not change the humm or it would lessen the humm. It can't increase the humm.

You must have done something wrong. 

[Willabe]

"You don't want the ground current at C4 going over to V1's a/b triodes cathodes (K) C6/R6 - C8/R14 and possibly modulating those 2 triodes K's. So you run the ground from those 2 K's over to C4's ground. Then you run a wire from C4 over to the chassis preamp ground. This isolates V1's K's from the ground current at C4."

I did that this morning.  The hum is now louder!

Impossible.

It would either not change the humm or it would lessen the humm. It can't increase the humm.

You must have done something wrong.

Did you remember to disconnect the ground from C6/R6 - C8/R14 over to the chassis ground bolt?

If you didn't you made a ground loop and that would cause it to humm. 

[TexasTone]

"You don't want the ground current at C4 going over to V1's a/b triodes cathodes (K) C6/R6 - C8/R14 and possibly modulating those 2 triodes K's. So you run the ground from those 2 K's over to C4's ground. Then you run a wire from C4 over to the chassis preamp ground. This isolates V1's K's from the ground current at C4."

I did that this morning.  The hum is now louder!

Impossible.

It would either not change the humm or it would lessen the humm. It can't increase the humm.

You must have done something wrong.

Did you remember to disconnect the ground from C6/R6 - C8/R14 over to the chassis ground bolt?

If you didn't you made a ground loop and that would cause it to humm.

The only thing connecting to chassis ground is C3.

Here's the way it sits now.

• The LTPI tail resistor and AC shunt cap, and the shield ground for the Relay-to-LTPI cable all tie to C4.
• The TB Mid and Volume grounds tie to C3.
• The Treble to Volume cable and Volume to Relay cable grounds tie to the Volume pot case.
• C4 ties to C3.
• The input jacks' ground and the TB Gain control ground tie to both V1 cathode grounds.
• The V1 cathode grounds tie to C3.
• The Normal Volume and Tone grounds tie to C3.
• C3 ties to chassis ground.

None of these changes reduced the hum.

I ran the relay coil directly off of 6 VDC from the chassis fan tap of the filament string, bypassing the relay board diode. No change.
I inserted a grounded aluminum shield between the relay case and the channel and LTPI signal cable.  No change.
I bypassed the heater elevation and grounded the 6.3 VAC center tap directly.  Hum much worse.

Only two things eliminate the hum:

• Turning the Normal volume off.
• Zero power to the relay, and then there's no channel switching.

I ran a 100Ω resistor from the relay board 6.3 VAC pad that connects to the two relay-board capacitor negative terminals and the footswitch ground (which is also connected to the push-pull switch). That reduced the hum a few dB.  Putting a 100Ω resistor on the 6.3 VAC pad that connects to the F/S tip and relay coil made the hum worse.

[TexasTone]

When I take all the signal cables out of the relay board, and hook each channel up to the LTPI, there is zero hum.

[HotBluePlates]

When I take all the signal cables out of the relay board, and hook each channel up to the LTPI, there is zero hum.

Looks like the half-wave rectification for the relay DC Volts is not clean enough.  There's a high chance Ripple on the relay coil was sneaking into the circuits being switched by the relay, which seems confirmed when you bypass the relay.

You could bridge-rectify the heater supply, which gets you to 120Hz instead of 60Hz.
You could add a resistor-cap after your existing cap to get cleaner DC.

I don't tinker with relays, so I'm not certain that will solve the hum, but it's where I'd go next ("cleaner DC Volts for relay").

[TexasTone]

Using 4 AA batteries in series quieted the hum about 4 dB, from a -36 to a -40, but it also induced a new 120Hz hum at -45 dB (measured on my Spectroid Android app.

Here what dropped the 60 Hz hum down to bout -50 dB: hanging a 100Ω resistor to ground off of one side of the 6.3 VAC filament pair (HS2) that connects to the C1 negative leg, the C2 negative leg, the D2 anode, the jack sleeve (footswitch ground),and R1.  Putting a 100Ω resistor to ground off of the HS2 filament pad increased the hum.    Putting a 100Ω on each side to ground (false center tap) increased the hum.

The 6.3 VAC filament string is: twisted pair to the five tubes, from V5 to V1, then to a terminal strip where a 1N4007 diode / 220µF capacitor taps about 6 V for a 12 VDC fan.   The twister pair continues on to the relay board.  The 6.3VAC center tap connects to a voltage divider heater elevation circuit.   The heater elevation is for the standard reasons of hum reduction and high cathode voltages on the cathode follower.

I disconnected the fan.  No hum reduction. I bypassed the fan tap off.  No hum reduction.  I ran the relay from the fan DC to the relay board after removing the filament pair. No hum reduction.  I ran the relay off of a 6V batter array. No hum reduction.

The only thing that eliminated hum was no power at all to the relay.   Connecting the Norm Vol directly to the LTPI results in no hum.  The relay board fully connected but no power results in no hum.

I did not try creating a false center tap from the V5 heater to the heater elevation circuit, and leaving the center tap open.

I don't know why but putting a 100Ω resistor to ground off of one specific wire of the 6.3V filament pair reduced the hum.

​

[BrianS]

Replace the relay board.

[Willabe]

I did a search for relay humm in guitar amps found this from our forum;

https://el34world.com/Forum/index.php?topic=20838.msg220304#msg220304

And I found this from geofex, but I can't get it to open. Is his site down?

Excessive Hum - geofex.com

Did you read this?

[TexasTone]

I did a search for relay humm in guitar amps found this from our forum;

https://el34world.com/Forum/index.php?topic=20838.msg220304#msg220304

And I found this from geofex, but I can't get it to open. Is his site down?

Excessive Hum - geofex.com

Did you read this?

Yes, I did.  Specifically, this: "I think Geezer figured a way he hung a 100 ohm resister from one side of the reley and it killed the hum."

[Willabe]

Reply's #2 and #4 say the relay itself can be noisy.

Since you tried the 100 ohm R and it only brought it down a little, it could be like HBP's said, use a FWB rectifier, and/or it could be the relay itself is leaking noise from the coil.   

[Auke Jolman]

I was having hum problems with a relay board. My solution was using a stereo plug instead of a mono plug. I'm not sure what you are using, but in my case the hum went away when using a stereo plug.