Hum issue - please help

[fabiomayo]

Hi, everyone.

Finished building my amp but I have a serious noise issue.
The amp is a 50w HRM, 2x6L6. Individual bias adjustment.
Tubes work fine in other amps. Voltages check ok (CL1: 188v/1.73v;
CL2: 190v/1.67v; OD1: ~190v/1.80; OD2: 199v/1.57v; B+3: 432v;
B+2: 460v; B+1: 460v). I triple checked all grounds and tried
connecting presence ground to the pot ground buss. Nothing changed.

Noise tonality changes with settings. Input volume and drive
control (when in OD) seems to have no effect on the amount
of noise. Setting clean master at minimum will cease the noise,
same is true for the OD Master when in OD.
Jumping a wire to ground at the PI entrance capacitor, kills the noise
(and sound, obviously). I started jumping every point in the preamp.
Jumping each master output (middle lug) to ground will kill noise in the
respective channel. These are the only points where I get no noise.
Grounding the grids has no effect on noise.

Relays seem to be working fine. PAB doesn't pop when switched,
Mid boost pop a little bit, channel switching pops big time.

Here's the layout (HRM omitted for clarity only) and measured schematic.


I'm thinking it's the relay supply or a fault at the OD relay.

[Bob-I]

How about some pictures of the build, especally close ups of the grounding.

It appears like you've grounded the cathodes to the input jack. I'd move that to closer to the PS on it's own ground.

[heisthl]

Could be anything but here's some things:
1. You sure the heater resistors are the correct values (100 ohms) and are working? Or better yet if you have a heater center tap use it instead.
2. keep the relay supply ground off the pot bus, either ground it at the B+1 ground point or have it totally isolated. I've had better luck just grounding it at the B+1 ground lug.
3. Try moving the presence ground off the pot bus or on the pot bus (I know its strange to have it on the pot bus but sometimes this works).
3. ground B+4 and B+5 filter caps to the ground lug by the input jack if possible.

[heisthl]

How about some pictures of the build, especally close ups of the grounding.

It appears like you've grounded the cathodes to the input jack. I'd move that to closer to the PS on it's own ground.

I think that's a standard Dumble method to ground anything(including cathodes) on V1 and V2 to the input jack ground lug (also including the filter caps).

[fabiomayo]

Guess I should've tried everything before posting.
Found the culprit. I had not grounded the relay power supply and
was using a virtual ground (taken from the regulator) to operate them.
Grounding this point made the amp much quieter and better sounding (!!!).
The relays don't seem to pop anymore (I'll test it better later) and
tone is a little "cleaner". I was suspecting of heavy oscillations but the
amp is more defined.

I made a different layout because of my donor amp but tried to take
everyone's advice. Grounding is close to Dumble. I'll try moving things
as the amp is not as quiet as I think it should be.

Could be anything but here's some things:
1. You sure the heater resistors are the correct values (100 ohms) and are working? Or better yet if you have a heater center tap use it instead.

They measured 97.2 ohm each on my Fluke meter. I clipped the center
tap. Voltages around the CT weren't that centered anyway. Maybe that's
someplace for improvement.

2. keep the relay supply ground off the pot bus, either ground it at the B+1 ground point or have it totally isolated. I've had better luck just grounding it at the B+1 ground lug.

Since I was using a virtual ground, the realy grounds are actually tied up
directly to the relay power supply ground, not to the chassis.

3. Try moving the presence ground off the pot bus or on the pot bus (I know its strange to have it on the pot bus but sometimes this works).

Tried that prior to solving the problem. Made no difference. I'll try again.

3. ground B+1 and B+2 filter caps to the ground lug by the input jack if possible.

I suppose you mean B+5 and B+4? If so, B+5 B+4 and B+3 are all
grounded near the input jack as I'm using Funk's power supply board.
(it's the green wire on my layout - I see now that it's confusing).


Here's a pic of my build (I'm a bit ashamed but that's that... )

[heisthl]

"I suppose you mean B+5 and B+4?"

Yeah - I edited my previous post so it doesn't confuse the newbies.

[fabiomayo]

Thanks so much guys. I'm a newbie BTW.

I forgot to add the pics. Now that the noise issue is solved, do you
have anymore tips on how to make the amp quieter? It hums just
a tiny bit when not plugged in (odd humming as it's not constant -
appear to be a grounding issue. One of my amps do this when the
input jack is a little loose). However, all grounds checked ok.
The amp has a health hum when using P90s or singles and whenever
PAB is on or I'm on the OD.

Just saw that I routed presence wiring wrong. It's supposed to go
behind the power supply board if I'm right. Even though the layout is
tight and differs a lot from the real deal I tried to follow lead dress
guidelines best as I could.

[Structo]

it's hard to tell from the photos but if the heater wires are anywhere near wires carrying signal, it will hum.
Unless you are running DC heaters.
That is a tight chassis so lead dress is even more important there.

[fabiomayo]

Hey Structo, thanks for the reply.

Yes, it is tighter than I expected.

Also, the heater wires for V1 and V2 do run close to
the preamp, especially the V2 which is close to the
HRM board. No way to run them anywhere else.

I'll try to use a shielded two-conductor wire in those
spots. Hope it helps.

Also, today I tried a 1k pot in place of a 100 ohm on
the heater wiring. Adjusting it had no effect on noise
(none that I could hear anyway) except the noise got
bassier when the knob was turned to no resistance.
1k doesn't allow me enough finesse of adjustment I guess...

[Bob-I]

Heaters suspended like that in this style chassis is not the best. I'd lay the heaters down on the chassis like Fender did in the Tweed amps. I'm not sure it would help but it is a best practice.

[ic-racer]

Not much to add here, other than I notice two bias controls. I suspect it would hum if they were set quite a bit different. Have you tried tweaking the bias of one tube to see if it improves the hum?

[fabiomayo]

Thanks everyone.

I'll try the heaters down, Bob-I. Problem is once again space, especially
since there are a lot of parts and wiring around the tube sockets.

Each tube is biased to draw the same current (~35ma). Different bias
settings didn't make any difference in hum.

It's fair to say that the amp is up to a high but normal level of hum.
My other amps are much quieter though.

[jelle]

Where does the hum come from? is it from the poweramp? Locate the problem first by disconnecting the pi input. Good luck!

jelle

[fabiomayo]

If I ground the PI input no sound will result. Do I have to disconnect it?

[jelle]

Yes, just ground the PI input. Hum?