Transformer transplant

[martin manning]

Yea, not a good idea. I'd ground the shields and the OD trim pot at the input ground eyelet at the center of the board near the 220k. That's probably why grounding the speaker/OT common at the front of the chassis worked well.

[ynor]

Ok, will separate and ground the OD trim pot where you suggested.

But what are the "shields"?
And should I leave the bias ground where it is, or move to filter ground?

[martin manning]

Shields are the outer braid on the coaxial cables going to and from that pot. I think you can try this as an experiment. Disconnect the wire at the red X, and add a jumper on the main board as shown by the red arch. This seems to be the least invasive way to test the theory. Once you do that, you can try different locations to ground the OT common. The location shown in the diagram may be fine after separating the preamp ground. Keep the bias pot ground where it is, next to the bias pot between the output tubes.

[martin manning]

There is one part of the above experiment that won’t be optimum, and that is the phase inverter ground, at the junction of the 390 ohm resistor and 2.2 uF cap. That belongs to the power amp, and should be grounded where the OT secondary common is grounded. The short under-board jumper tying that eyelet into the preamp ground isn’t easily accessible, so it would be easiest to lift the resistor and cap leads out of that eyelet and run a ground wire from those leads to the speaker ground. With that you will have the grounds arranged as in the original Dumbles.

[bepone]

So my questions are:
That original wiring of the speaker jack, grounded with the bias pot, is that correct, or is it a design flaw from the manufacturer??

yes

If so, this makes me question the rest of the grounding scheme in this amp, so I'll then have to take a deeper look at the rest.

Could you point me to the correct grounding schemes that are verified noiseless? Where should those speaker jacks be grounded?

you should beleive only to yourself not to some random grounding methods from the internet.. i never use any of the proposed gnd methods because they are often way out from optimum or full of mistakes.

ha, now question how to do that? you are on the good way i see already.. and one day when you will be expert, there will be no questions at all

[ynor]

I think you can try this as an experiment. Disconnect the wire at the red X, and add a jumper on the main board as shown by the red arch. This seems to be the least invasive way to test the theory. Once you do that, you can try different locations to ground the OT common. The location shown in the diagram may be fine after separating the preamp ground. Keep the bias pot ground where it is, next to the bias pot between the output tubes.

As you point out, I can now see that the OD part of the preamp section is grounded at the speaker jack and bias pot. So I connected the OD ground to the V1 ground at the board, and isolated that wire that connected the OD ground to the speaker ground. This removes most of the hum, simular to the experiment I did. Did some live testing, and I believe you are correct that the hum is caused by the bias and preamp OD sharing the same ground. Once separated, I could not hear any significant differences in where I grounded the output jack. The noise level is then the same regardless of if it's grounded with the bias or at a completely different part of the chassis. But once the bias and OD ground are connected, some hum appears. I do believe it is 100 Hz though, at least according to the "tone" from the speaker. Or is it because the first harmonic of 50 Hz is more prominent that the fundamental frequency?

[ynor]

There is one part of the above experiment that won’t be optimum, and that is the phase inverter ground, at the junction of the 390 ohm resistor and 2.2 uF cap. That belongs to the power amp, and should be grounded where the OT secondary common is grounded. The short under-board jumper tying that eyelet into the preamp ground isn’t easily accessible, so it would be easiest to lift the resistor and cap leads out of that eyelet and run a ground wire from those leads to the speaker ground. With that you will have the grounds arranged as in the original Dumbles.

Every bit of noise reduced is appreciated!

Just tried lifting the resistor and cap leads and run a wire to the speaker ground as you suggested. That actually made the hum worse; back to more or less the same level as when the OD section of the preamp ground was connected to the speaker/OT ground.

[ynor]

you should beleive only to yourself not to some random grounding methods from the internet.. i never use any of the proposed gnd methods because they are often way out from optimum or full of mistakes.

ha, now question how to do that? you are on the good way i see already.. and one day when you will be expert, there will be no questions at all

A process of guided trial and error, I guess

I do have some mods I would like to try eventually. But first, I think putting this amp together as close to an original Dumble layout with confirmed optimal solutions is essential before doing any mods. It must work 100 % right before I start experimenting with it.

[martin manning]

Just tried lifting the resistor and cap leads and run a wire to the speaker ground as you suggested. That actually made the hum worse; back to more or less the same level as when the OD section of the preamp ground was connected to the speaker/OT ground.

Not sure why that would be, maybe some other aspect of the C-tone ground scheme. Does the hum level change when selecting OD vs. clean?

[ynor]

Just tried lifting the resistor and cap leads and run a wire to the speaker ground as you suggested. That actually made the hum worse; back to more or less the same level as when the OD section of the preamp ground was connected to the speaker/OT ground.

Not sure why that would be, maybe some other aspect of the C-tone ground scheme. Does the hum level change when selecting OD vs. clean?

I don't think so, but I can check for that again. I soldered the cap and resistor back to the board as I wanted to play. Now I have soldered a temporary ground lead between the OD and V1 grounds on the board, and have isolated the speaker jack/OT secondary from the preamp grounds and grounded those together with the bias. I do see that the #124 has 5 grounding points, but the OTS has 4 (not including the safety mains ground).

[martin manning]

Another common source of noise/hum is the loop jacks. If the switching contacts are dirty, you will get hum in the speaker. A good way to test for that is to put a patch cord across the send-return jacks.