Traynor YGM-4 Hum

[hearforever]

I just purchased a Traynor YGM-4 Studio Mate.
The tone is good but it has annoying hum.

The circuit has been modified in several places- C1 has been changed to .01 and the brightness cap value was changed. Also there are 1/2 watt 1 ohm metal film resistors tied from the cathodes of the power tubes to ground. R39 in the bias circuit has been changed to a 10k resistor & 10k pot in series.
Finally a 100k resistor was added beween the pilot light and PT secondary wire, the side that doesn't have the 470k resistors in parallel.

With all of the 12ax7 tubes in I hear a faint pulsing that sounds like tapping which makes me think something is amiss with the bias or in the tremolo circuit. The hum gets much better without the tremolo oscillator/reverb recovery 12ax7 but stays the same if I replace it with another tube. I got a reading of -17V at C29 and -30V at C28

Also I get a pop if I flip the stand-by switch off after the amp has been running for awhile


Schematic below-

[olddawg]

When I get an amp with iffy mods and weird problems, I usually put it back stock and start over.

[hearforever]

When I get an amp with iffy mods and weird problems, I usually put it back stock and start over.

Yes that's reasonable enough, but I can't see what in the mods could cause extra hum. Changing C1 seems to be a very common mod on these amps as does changing the brightness cap value.
Looking at the schematic though, a 10k resistor + 10k trim pot is less than the 22k called for on the schematic

[Enzo]

One thing that often changes when amps are modded is the lead dress, and that can affect hum and noise for sure. A heater wire pushed aside and left there can couple to a signal wire. Just one possibility.


10k + 10k is 20k. Compared to 22k, that is no more off value than a 22k 10% resistor on the low end. And unless you measured it, pots are notoriously loose in values.


So, close enough.

[hearforever]

Yes, thats true, but then it doesn't give you much adjustment does it? it only lets you lower the resistance from the circuit–
I measured the pot value.
I will check the lead dress but it looks pretty normal. Heater wires are elevated running away from everything
I will try to post a picture of the guts

[Enzo]

OK, looking at the schematic now. Here is an easier to see copy:
http://www.webphix.com/schematic%20h...nor_ygm3_4.pdf

A 22k replaced by a 10k and 10k pot makes adjustable from 20k down to 10k. How much range you need? As far as I can see, R39 variable would be used to trim up the trem intensity control. That might adjust out the pulsing you hear.

I'd wager someone was either trying to cure the pulsing thing, or perhaps make the bias adjustable. I don't think that appraoch would be very effective as a bias adjust, but maybe. In any case, VERY few amp tweakers want to run their amps colder, so a bias control that only reduced the bias voltage from stock is usually what you find.

In any case, that doesn;t seem like it would be involved in a hum problem.

You got hum? Isolate the problem. Does it still hum with ALL controls at zero? If not, then turn up the controls, and try turning each one. Do ANY of the controls affect the HUM in ANY way? Can they affect the hum volume, or change the tone of the hum, or does the reverb control affect the amount?

Is the hum 60Hz or 120Hz. 120Hz is the same note but an octave higher than 60Hz. Sound a lot alike.

Is the heater center tap properly grounded?

You said removing the reverb tube helped the hum. Are you using the reverb footswitch? If so, unplug that, any change? WIth no footswitch, short the reverb return to ground. ANy help? The FS jack is parallel to the reverb return jack, so also shoprt across that, just to be sure. I see a shielded wire from that jack. Is the shield still grounded? The jack?

[hearforever]

You got hum? Isolate the problem. Does it still hum with ALL controls at zero? If not, then turn up the controls, and try turning each one. Do ANY of the controls affect the HUM in ANY way? Can they affect the hum volume, or change the tone of the hum, or does the reverb control affect the amount?

Is the hum 60Hz or 120Hz. 120Hz is the same note but an octave higher than 60Hz. Sound a lot alike.

Is the heater center tap properly grounded?

You said removing the reverb tube helped the hum. Are you using the reverb footswitch? If so, unplug that, any change? WIth no footswitch, short the reverb return to ground. ANy help? The FS jack is parallel to the reverb return jack, so also shoprt across that, just to be sure. I see a shielded wire from that jack. Is the shield still grounded? The jack?

OK here are the results-
This is all with Volume/Bass/Treble set at zero
I thought that with V6 removed (thats the tube that controls both reverb recovery & tremolo osc) there was less hum but I'm not actually sure that's the case. I have a 1/4" male plug that I used to short the tremolo footswitch. This makes the faint pulsing disappear. If I then turn the intensity to 10 the hum decreases.

Removing the reverb tube V5 or shorting the footswitch does not seem to make any difference.

The reverb knob increases hum as it is turned up

I think it is 120hz hum comparing it to a tone generator app on my phone

Thanks for your help

[Enzo]

Reverb involvement then? The schematic shows a shielded wire over to the footswitch jack. Is that jack grounded to the chassis? And is there a shielded cable to it? Try dismounting the jack from the chassis, any diffrence?

120Hz hum is from power supply ripple. That CAN be a weak filter cap or caps. But it also can be sensitive circuits - like a reverb return - sharing a ground path with a filter cap or higher current stage.

60Hz hum is from basic grounding, ground loops, heater noise, radiated fields like from the power transformer or your flourescent room lights.

Have you or anyone else moved any ground connections? For example replacing a can cap with individual caps, and not grounding all of them to the can cap point.

[hearforever]

Reverb involvement then? The schematic shows a shielded wire over to the footswitch jack. Is that jack grounded to the chassis? And is there a shielded cable to it? Try dismounting the jack from the chassis, any diffrence?

120Hz hum is from power supply ripple. That CAN be a weak filter cap or caps. But it also can be sensitive circuits - like a reverb return - sharing a ground path with a filter cap or higher current stage.

60Hz hum is from basic grounding, ground loops, heater noise, radiated fields like from the power transformer or your flourescent room lights.

Have you or anyone else moved any ground connections? For example replacing a can cap with individual caps, and not grounding all of them to the can cap point.

Power filter caps are replacement Weber 2 section caps 50-500V each. They were replaced 2 owners again...not sure when that was
The 2 caps are grounded at same point as 1 ohm resistors from cathodes of power tube the other 2 cap ground at the same point as various preamp components. It looks this follows the original layout

Most previous owner was the one who added the adjustable bias pot. I don't know anything else about the amps history besides that

The reverb footswitch jack is grounded to the chassis and has a shielded wire to it. If I dismount the jack hum gets real bad

Ok here's another weird detail. I get what sound like 60hz hum towards the middle (5) of the bass pot that rolls off if I turn either direction

Here is a photo of the inside-

[hearforever]

Here's a better (larger) photo
Ignore the fact the one of the leads from a filter cap seems to go nowhere. I had that lead disconnected for a minute but it is soldered back in place where it should be in the circuit

[Enzo]

My bad, for got to say: clip a wire to the jack bushing and ground it to the bottom end of the cathode resistor of the tube. The tube the shielded wire goes to the grid of. Otherwise, yes, you get big hum because now the shielded wire is a big antenna. It has to be grounded, I just wanted it not grounded to chassis to see.

Does the volume control turn the hum down? Look at the schematic, at the BASS control. RUn the wiper to the right, and the pot is grounded at that end. Makes sense that would drop the hum coming through it. And going to the left, you hit the wiper of the volume control. If the volume is at zero, then that end of the bass control is grounded too. Even if the volume is up, that end of the bass control is closer to ground than at its middle. And while we are there, we'd expect the hum to respond through the bass control rather than the treble, eh?

[hearforever]

My bad, for got to say: clip a wire to the jack bushing and ground it to the bottom end of the cathode resistor of the tube. The tube the shielded wire goes to the grid of. Otherwise, yes, you get big hum because now the shielded wire is a big antenna. It has to be grounded, I just wanted it not grounded to chassis to see.

Does the volume control turn the hum down? Look at the schematic, at the BASS control. RUn the wiper to the right, and the pot is grounded at that end. Makes sense that would drop the hum coming through it. And going to the left, you hit the wiper of the volume control. If the volume is at zero, then that end of the bass control is grounded too. Even if the volume is up, that end of the bass control is closer to ground than at its middle. And while we are there, we'd expect the hum to respond through the bass control rather than the treble, eh?

OK I will try that-grounding to pin 8 of V6A

Is it likely that the bias pot would be screwing up the tremolo or causing any hum? When the intensity pot is fully clockwise the hum decreases. And with the tremolo footswitch grounded the faint pulsing disappears. Could that be caused by signal wires picking up the tremolo LFO?

[hearforever]

Ok so here's my update.

The reverb is the small pan type screwed directly to the chassis. I ordered a 4FB3A1B thinking I'd have to somehow modify the grounding scheme since the replacement has a grounded output (the original did not). Testing the tank insulated from the chassis I have no reverb hum. The quality of the reverb is much better but it is quiet (the old one was too)

I am measuring 375V on the plates of V5 versus 330V indicated on schematic. That seems in line with modern 125V increasing the B+ voltages
The hum is much better on the amp
Now I guess I'm on to thinking about making a slight tweak for more reverb