New clone....hummm

[soje]

I've build my first Dumble, aiming for a 50W version of #124.

The clean "channel" is great, but kicking in the OD I get hum. Guitar unplucked and everything on max, the speaker almost dances
There's also hum on the clean "channel" but it's faint.

I've read thru posts here, and tried the following:
- ground grid of V1a to input - hum increases
- ground grid of V1b to input - hum decrease
- ground "bass cap" (.1 Uf)/turn down bass - hum decrease more
- turn down volume - hum almost gone
- ground cap from V1b cathode - hum gone
- move presence ground from busbar to PT bolt - no change
- hardwire OD chanel, no relays - no change
- elevate heater to ~20v - no change
- remove relay power from relay board - no change on clean channel (behavior as on OD channel, only hum is faint)

I'm running out of ideas. Maybe some of you can spot the problem?

Other than that, I havent adjusted anything but bias, but from what I can tell, the amp deliveres

Thanks, soje

[odourboy]

Hum problems can be a bear. Is it 60Hz or 120Hz (or something else)? What you've described suggests maybe a problem in the tone stack wiring (just a guess based on the symptoms).

It would be very useful for you to measure and post your dropping string voltages, cathode and plate voltages and a couple of really good gut photos (make sure you have one that captures the pots and wiring.

Cheers!

[soje]

I think the hum is 50 Hz (I live in Denmark). It's very low, on the brink of audible.

Here's the volts:

B+1: 450V
B+2: 449V
B+3: 430V
B+4: 297V
B+5: 288V

V1:
pin 1/6: 188V
pin 8/3: 1,5V

V2:
pin 1/6: 188V
pin 8/3: 1,65V

V3:
pin 6: 289V
pin 8/3: 59V
pin 1: 286V

V4:
pin 3: 448V
pin 4: 447V
pin 6: -50V

V5:
pin 3: 446V
pin 4: 446V
pin 6: -50V


I'll check up on the TS wiring tomorrow, and see if I can get some pictures taken of the thing. It's past 2 here, so I really need to find my bed


Thanks for looking into this!
Regards
Soje

[Structo]

You probably know what you are doing from what I can see but, did you try different tubes?

How is your grounding scheme? Did you follow #124?

I found in my amp that when I had the standby B+ wire too close to any of the AC wires on the PT that it would pickup hum.

So I separated that out and ran it by itself to the filter board.

How did you run your heaters? In the air? Chassis floor?

Overall I suspect it is a grounding issue.

Good luck sorting it out.

[soje]

I got some gutshots of the amp, please don't scream

http://www.box.net/shared/3fzietoizz

I have the relay sitting up against each other, but I've tried to hardwire the OD channel, and disconnect poewr to the board with no change.

I've also noticed, that there's quite a hiss from the local feedback circuit. It's gone if I ground the cap (switch side )

I started out with a grounding scheeme similare to some of the layout pictures from Normster I found in here. Today I changed it to what's in the pics, but it didn't really help

Any ideas?

soje

[talbany]

Here is a simple test...If you can isolate where the noise is being generated...output section or the preamp.. To do this pull all the preamp tubes first..no hum.. then install the PI tube.. if it's quiet move on to the clean channel tube.. if the noise comes back disconnect the wire coming out of the .047 clean channel side going to the relays V1b and run it straight to the input to the PI.. .02 cap bypassing the relays all together...If after that the noise is still with you it's more than likely in the tone stack or the relays..That's what I would do first...... This way you can narrow it down somewhat...Report back...Good Luck
Tony VVT

[soje]

Talbany:

I did the tube test and bypassing, and ended up with the noise still there

- I desoldered power from the relay board without any change.
- If I switch to jazz, the hum is gone.
- When I turn down the bass knob, the hum is gone, so I tried desoldering the wire from the bas cap (.1 Uf) with the same result.
- So does this mean it's generated in the first stage, and if so any ideas?


Thanks for helping!
soje

[talbany]

When I turn down the bass knob, the hum is gone, so I tried desoldering the wire from the bas cap (.1 Uf) with the same result.
- So does this mean it's generated in the first stage, and if so any ideas?

Looks to me like you have the bass pot hooked up wrong.... From the Jazz rock switch should go to the top of the pot and the wiper should go to the relays (center)... Is that how you have it hooked up... Looking at the pics it's hard to tell... Looks like you don't have anything going to the wiper...


Tony VVT

[soje]

The wire from the wiper is hidden up under the edge of the chassis. Here's how the pots are hooked up to the relays:

[talbany]

Are you sure if there is a wire attached to the wiper I don't see it?


Tony VVT

[soje]

I'm sure, but it is hard to see on the pictures. You can see the solder on the lug thou

....yes, I did check it in RL, I even desoldered it to see if that did anything....it didn't

[Structo]

Something that is causing me some confusion is the fact that you built the amp in reverse of the common layout.

Is this so you can have the tubes up?

It has to have something to do with your tonestack or relay wiring if you can change it with the bass pot.

[heisthl]

If you run out of things to try, remove the input jack from the chassis, clip lead it's ground to the cathode ground of v1a to see if its a ground loop problem.

[talbany]

I've also noticed, that there's quite a hiss from the local feedback circuit. It's gone if I ground the cap (switch side )

1 This seems strange if you are talking about the LNFB on V2b...producing noise...Hiss..I would disconnect it see if it helps...

2 I would then disconnect the shielded wire @ the 22k grid input V1a
after trying Henry's ground loop trick...

3 Disconnect the relay input wire coming off the .047 cap v2b take it straight to the phase inverter input..

At this point you have totally isolated the tonestack and boost circuit..and filtering for V1..
If you think you have the the tonestack wired correctly and everything works properly then get out the scope and start tracing it back through the stack...move some wires see if it gets worse or better..

Flip the tone switch to Jazz and dime the bass pot and see if it's still there?

I assume you tried different tubes...


I had this issue several years ago ended up scraping the amp and salvaging the transformers...The next amp I built using those transformers had the same issue slight hum in the clean channel..Turns out it was a noisy power transformer Heybor...I think I put a hole in the back of the dumpster when I threw it in there... LOL!!

Running out of ideas!!

Tony VVT

[odourboy]

Just thinking out loud here...

Given that the Bass pot and the Jazz/Rock switch have an effect - I'm speculating it's a low frequency oscillation rather than hum. Oscillation comes from feedback. Given the proximity of your relays, could you have crossed a connection on the PAB relay with the Clean/OD relay?