Filmosound conversion hum

[ElPositivo]

I have converted two filmosound projector amps into guitar amps. I wanted to keep the 6J7 preamp tube in V1 and the 6SL7 as a phase inverter, so I designed a hybrid schematic of a 5C3 deluxe but keeping the 6J7 and 6SL7 tubes. The problem I am having is that both amps have the same issue: a very loud hum that is gone with the volume control at zero, but getting worse when opening up the volume. This makes me think the cause of this lies before the volume control.

It's been a couple months since I worked on troubleshooting the hum in one of the amps, I basically tried everything I could think of, including:
- bypassing tone stack
- true star grounding
- adding screen and grid resistors on power tubes
- replacing all filter caps
- adding an extra separate filter stage for V1
- adding a 47nf couple cap from volume pot into the grid of v2
- tried a different output transformer
- using 2 x 100 ohm resistors instead of the center tap for the filament wiring.
- Converted V1 and V2 for use with 6SL7 tubes: hum remained.
- changing all tubes
- cleaning sockets

I have fixed quite a few amps in my life but now I am stuck.

All voltages seem correct (I can post them later).

I'd appreciate some input on the schematic I used, do you spot any obvious mistakes around V1 / V2? Am I missing a path to ground somewhere? Possible problems with impedance? Anything else? I am stuck.

 

[ElPositivo]

Some chassis pics just for the heck of it, but on such a small chassis it is tough to get clear pictures.

 

[ElPositivo]

This ground connection appears not to be soldered properly but it is soldered good on the other side. Will resolder it anyway.

 

[YellowBoots]

Have you tried bypassing the cathode resistor of V1 just to see if it made a difference? Use something large that would bypass hum frequencies.

 

[YellowBoots]

On your schematic, that 0.047u V1 screen bypass cap isn’t really doing anything. It should be connected to the screen.

 

[Peegoo]

If it's ripple hum, consider installing a 10-20 Henry choke before your first node (B+1) after the rectifier.

 

[ElPositivo]

Have you tried bypassing the cathode resistor of V1 just to see if it made a difference? Use something large that would bypass hum frequencies.

I think I did but I am not sure anymore, will try this again tomorrow.

On your schematic, that 0.047u V1 screen bypass cap isn’t really doing anything. It should be connected to the screen.

Yeah I took this from a GA-30 (see attachment). Perhaps I could rewire it to be like a 5C1 champ’s first stage.

Edit: I see the GA-30 schematic does have the 047 uf cap directly in the V1 screen. I will have to check if I drew the schematic wrong or if I wired it like my schematic… might be on to something.

If you guys have any more suggestions for 6J7 or 6SJ7 circuits that’d be great.

If it's ripple hum, consider installing a 10-20 Henry choke before your first node (B+1) after the rectifier.

Yeah I could give it a shot. Although the volume control clearly controls the amount of hum, so I think it might be the circuit preceding the volume control. I will try to make a recording of the hum so you get an idea what I am hearing.

 

[Peegoo]

Some chassis pics just for the heck of it, but on such a small chassis it is tough to get clear pictures.

You have what looks like a filter or coupling cap really close to V1 and the input jack. Especially if that's a filter cap: move it to a location farther from V1. Filter caps and resistors that do power supply duties can act like RF transmitters and any adjacent gain circuitry will pick up hum from them.

 

[ElPositivo]

Turned out I made an error on the schematic, but I soldered the V1 screen bypass cap correctly in the amp.

I made some good progress today though. Until now I was looking for a single cause of the hum but today I realized I have a couple of issues which are affecting each other. As @andrewRneumann suggested, bypassing the cathode resistor on V1 with a capacitor helped a good deal. For now I have it bypassed with a 33uf. Also bypassed the power tubes cathode resistor with a 22uf cap.

Next to that I installed an aluminium cap over top of the 6J7 so it doesnt pick up hum through the input grid connector. This was another step in the right direction.

As @Peegoo suggested I moved the filter cap away from the input. This made a slight change in hum and every little bit helps.

Now that the hum resided some, i could hear the amp has way too much gain and suffers oscillation, which doesn’t surprise me too much considering the small chassis. I lowered the gain of the 6J7 by using a 68k plate resistor and a 1M screen resistor. This helped a whole lot. I also used shielded cable on some key locations and this helped as well.

At the moment the amp is stable until halfway on the volume knob, then it starts cutting in and out like a tremolo and I hear some motorboating. I think the next step I take is to use a different PI circuit and see how that goes…

To be continued…

 

[YellowBoots]

I made some good progress today though. Until now I was looking for a single cause of the hum but today I realized I have a couple of issues which are affecting each other. As @andrewRneumann suggested, bypassing the cathode resistor on V1 with a capacitor helped a good deal. For now I have it bypassed with a 33uf. Also bypassed the power tubes cathode resistor with a 22uf cap.

Now that we know it is mainly heater hum on V1, I would try elevating the heater center tap by connecting it to the 6V6 cathodes instead of ground. (Even better would be a humdinger pot connected to the 6V6 cathode, forgoing the heater center tap altogether.) This may or may not help, but it’s worth a shot. If it does help, you may be able to to dispense with the bypass cap and that will return the gain to a more reasonable level.