Transformer transplant

[ynor]

In the quest of getting rid of humming noise, I finally got the C-lator to work properly and started working on the amp. I got the Mercury Magnetics transformers and choke. The reason I wanted to change transformers was that the amp made an AC hum through the speakers even in standby mode and also with the output tubes pulled. Also, the power transformer made a vibrating mechanical noise. So I started with changing the power transformer first as I was suspicious that flux from PT leaked into the OT. Oddly, changing the PT did not make that hum go away. The vibrations went away though, but hum in standby persisted. So I changed the choke and OT. Now, the amp is completely silent as it should in standby. I really can't explain why changing the choke and/or the OT got rid of the standby speaker hum. Any explanations?

Also, I would appreciate if you could check if my voltage readings are as they should be (quality check )

OTS MM voltages.jpg

I'm adding some post transplantation photos. Does wiring seem ok? Any hints for improvement? There is a slight AC hum when in operate mode, when all volumes are turned down. It is small compared to the previous, but still, if you guys can see anything in the wiring that could improve, I'll put in the effort. I'm pretty sure that the last remaining hum is from the power amp section, because when I break the fx loop (bypasses the preamp) the hum is still there. When I run the preamp into another power amp, it is silent, so I don't think the hum is injected into the preamp signal.

XT transplant 1.png

XT transplant 2.jpg

[martin manning]

It's possible that the magnetic field from the choke was inducing current in the OT, as it receives a pretty noisy DC voltage with a sawtooth waveform on it, or maybe more likely that the original OT was not doing a very good job of cancelling the AC on the B+1 feeding into the primary center tap. That noise is normally cancelled in the OT since the currents in the two halves of the primary coil create equal and opposite flux in the core.
The wiring looks fine as long as the extra wire can't flop around. I guess the MM PT has a relay supply winding included?
Voltages look good. You might try raising the V1 plates just a bit by reducing the 2k7 resistor on the main filter board to see what that sounds like. The easiest way to experiment with that would be to clip or tack solder another resistor in parallel, maybe try another 2k7 in parallel to get 1k35, and see where the voltage goes and what that sounds like. If you measure voltage across that 2k7 you will see it is not much, so you can't influence the V1 voltage much. I would not reduce that below 1k to maintain decoupling of the power nodes

[ynor]

It's possible that the magnetic field from the choke was inducing current in the OT, as it receives a pretty noisy DC voltage with a sawtooth waveform on it, or maybe more likely that the original OT was not doing a very good job of cancelling the AC on the B+1 feeding into the primary center tap. That noise is normally cancelled in the OT since the currents in the two halves of the primary coil create equal and opposite flux in the core.
The wiring looks fine as long as the extra wire can't flop around. I guess the MM PT has a relay supply winding included?
Voltages look good. You might try raising the V1 plates just a bit by reducing the 2k7 resistor on the main filter board to see what that sounds like. The easiest way to experiment with that would be to clip or tack solder another resistor in parallel, maybe try another 2k7 in parallel to get 1k35, and see where the voltage goes and what that sounds like. If you measure voltage across that 2k7 you will see it is not much, so you can't influence the V1 voltage much. I would not reduce that below 1k to maintain decoupling of the power nodes

Thanks! Yes the MM has relay supply windings as well. They are direct drop-in for the Ceriatone trannys. But the PT leads does not have quite the same placing as the C-tone's.

Wonder if the filaments wiring are too close to the board on the left, and if that's what's causing the last remnants of hum?

[martin manning]

Wonder if the filaments wiring are too close to the board on the left, and if that's what's causing the last remnants of hum?

I doubt it. They are twisted, and that side of the filter board is not sensitive. I suppose you could run the heater leads around the other side of the PT if you want to.

[martin manning]

I would appreciate if you could check if my voltage readings are as they should be (quality check :)

BTW, with 35 mV on the cathodes your bias is pretty cool at 0.035A X 432V = 15.1W, or just over 50% of max anode dissipation (30W). You could bring that up to 60%, around 42 mV.

[ynor]

I would appreciate if you could check if my voltage readings are as they should be (quality check

BTW, with 35 mV on the cathodes your bias is pretty cool at 0.035A X 432V = 15.1W, or just over 50% of max anode dissipation (30W). You could bring that up to 60%, around 42 mV.

Thanks! I didn't do the calculations based on the actual voltages, I just aimed at 35 mV based on what "stock" voltages would be. I didn't think of that. I will adjust the bias accordingly.

[ynor]

Ok, so I started chasing that last remnants of mains hum. 100 Hz hum in the speaker when all volumes turned down. Also present with preamp tubes removed, I concluded that the problem is not in the preamp. I measured AC ripple with my scopemeter before and after the choke.


Before choke

Before choke.bmp

After choke

After choke.bmp

The ripple measures around 2,4 Vac before the choke, and around 0,2 V after the choke. I think that is good? So I concluded that the B+ line is probably noise-free.

So if the B+ is noise-free and the noise is not from the preamp, I started looking at grounding as a possible cause. I found one grounding I found somewhat suspicious: The speaker jacks (also from the OT secondary) are grounded at the same point as the bias adjustment pot. I thought this might cause grounding hum. The speaker jacks are isolated from the chassis, so that point is the only grounding for the speaker jacks. See the original layout where the jack is grounded together with the bias pot, and the picture where I have removed this grounding:

Speaker jack grounded with bias pot.jpg

Speaker jack grounding unsoldered.jpg

So I temporarily moved that ground to the front of the chassis (not soldered, only mechanical connection):

Speaker jack grounding moved.jpg

This removed that 100 Hz hum!! Look at the spectrogram from my phones recording in front of the speakers before and after moving that ground connection:

Spectrogram new grounding.jpg

And after moving that ground:

Spectrogram original gounding.jpg

Those spectrograms do not convincingly show the real audible difference in hum, but still, there is no noise in this amp now when all volumes are turned down using this new grounding.


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?? 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?

[martin manning]

The ripple measures around 2,4 Vac before the choke, and around 0,2 V after the choke. I think that is good? So I concluded that the B+ line is probably noise-free.

Note that the ripple voltage on the B+ is 100 Hz due to the full-wave rectification. It's nice to check that, but you could have shifted focus to 50 Hz sources right away. Those would be the heater circuit and the bias supply (which is half-wave rectified, and may have some ripple voltage).

So if the B+ is noise-free and the noise is not from the preamp, I started looking at grounding as a possible cause. I found one grounding I found somewhat suspicious: The speaker jacks (also from the OT secondary) are grounded at the same point as the bias adjustment pot. I thought this might cause grounding hum. The speaker jacks are isolated from the chassis, so that point is the only grounding for the speaker jacks. See the original layout where the jack is grounded together with the bias pot, and the picture where I have removed this grounding:...
So I temporarily moved that ground to the front of the chassis (not soldered, only mechanical connection):
...This removed that 100 Hz hum!! Look at the spectrogram from my phones recording in front of the speakers before and after moving that ground connection:
...Those spectrograms do not convincingly show the real audible difference in hum, but still, there is no noise in this amp now when all volumes are turned down using this new grounding.

I see reductions in the fundamental (50 Hz), and a significant reduction in the third harmonic (150 Hz).

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

Original Dumbles have the OT secondary common grounded at the speaker jacks, and soldering the secondary common lead directly to the speaker jack sleeve lug is the best practice. If the jacks are isolated (as yours are), the OT secondary ground is sometimes connected to the chassis at the main filter ground. You have isolated speaker jacks, so you can try a couple of places and see what works and what doesn't. Nice work finding a solution to your residual hum!
The #124 schematic here has ground locations called out and a ground map: https://ampgarage.com/forum/viewtopic.p ... 006#p55006

[ynor]

Thanks again, Martin! Yes the OT secondary common is grounded at the speaker jacks. So I'll try connecting the speaker jack/OT secondary common to the mains filter ground first, and if that doesn't work, I'll try different locations.

It's has been quite an extensive but ultimately satisfying process, eliminating hum problems in my new amp: New transformer in C-lator made it completely quiet after extensive troubleshooting. New transformer in the amp made it almost quiet, and now moving the grounds seems to complete the process. The C-lator and amp are wired from the manufacturer and should have been done right in the first place.

[ynor]

I see reductions in the fundamental (50 Hz), and a significant reduction in the third harmonic (150 Hz).

Yes indeed. It just sounded much worse than it looked

[Guy77]

Grounding speaker jacks at or near the jack is always the best option. If you are using jacks that do not self ground just place a terminal solder lug at the floor of the chassis near the speaker jacks.
Cheers
Guy

[ynor]

Grounding speaker jacks at or near the jack is always the best option. If you are using jacks that do not self ground just place a terminal solder lug at the floor of the chassis near the speaker jacks.
Cheers
Guy

Thanks. Actually, that's where the speaker jacks were grounded. Maybe the bias pot grounding needs to move in direction towards the PT instead of moving the speaker jack grounding. Will try.

I tried moving the speaker jack grounding to the mains filter grounding. That made a horrible hum, where the closer the long lead were to the PT, the worse it got.

[martin manning]

On the snip you posted above, there is an "OD trim pot" connected to the OT common ground. I assume that is the OD entrance trimmer, and I suspect that is the reason it is so sensitive. I would separate that and ground it with the preamp grounds.

[martin manning]

Can you post that whole C-tone diagram?

[ynor]

OTSFM50-ModernEagleMod.jpg