Excessive hum on Bell and Howell 179 amp

[samgraysound]

Hi all,

I've been modifying this Bell and Howell filmosound amp to be a guitar amp and boy have I had problems...

But currently I'm dealing with excessive hum. It seems to be from the preamp stage, as it is controlled by the volume knob.

I have about 100mv AC on the plate of the preamp tube. The weird thing is that there is very little ripple on the other side of the bias supply resister A349, it is less then 10mv there. So that says to me it is not an issue with the power supply. What could this be? Schematic attached.

Other info:

M equals K in this schematic

This amp had a horrible parasitic oscillation issue that I eventually "solved" by placing a .01uf cap to ground at the plate of the preamp tube.

I've removed a couple non-audio related parts of the circuit. The 6v6 in the bottom left of the schematic and its associated components, and the lamp right at the beginning of the circuit path

I've added 10k grid stopper resisters to the power tubes

 

[Wildfara]

What kind of hum? HAve you done any work to it at all? You may have to alter the input stage to make it work well for guitar etc. It's very likely the bad hum is from poor power supply filtering from ancient capacitors. I live in Port Townsend and work on stuff like this alot. Could maybe help you figure it out with more info.

 

[Wildfara]

Also that one tube is a light sensitive photocell tube. In these filmosound amps the sound is played back from the film from an edge of the film that has a waveform exposed onto it. It shines a light thru it onto the photocell tube which turns it back into an electrical signal. That extra 6V6 is used as a bias oscillator. Remove the photocell tube and see if the hum goes away.

 

[samgraysound]

It's a 60hz hum but with lots of overtones. More of a buzz. It's kind of a jagged triangle wave on the oscilloscope.

I've replaced all the power filter caps. I've also removed and tested all the signal and eq caps and replaced all the non functional ones or ones that had drifted out of tolerance. Of course one of one's leftover could still be leaking or being weird.

Also have replaced the preamp and power tubes. Replaced the volume pot. Replaced the output jacks with 1/4" jacks.  Ive also replaced a significant amount of resisters that were out of tolerance. And in doing all that have desoldered and resoldered most of the wires replacing some. Oh and I added a 3 prong cord, removed the death cap, and rewired the fuse and power switch.

I removed the oscillator circuit and the photocell circuits though not the tube sockets for them.

 

[samgraysound]

And as mentioned in the first post. I've added a .01uf cap to ground at the plate of the preamp tube, and 10k grid stoppers at the power tubes.

 

[Wildfara]

Haha so I see your frustration...wasnt sure how much you had been in it...What is the filament supply like? Is it centertapped to ground? If not try a 100 ohm resistor from each leg to ground. This can remove TONS of hum.

 

[Wildfara]

OK check that capacitor on the Filament center tap ground you can see it on the schematic....

 

[samgraysound]

Ok I'll check it. That is one I've already replaced but doesn't hurt to check.

 

[samgraysound]

Looking at that spot on the schematic, what do you think it has happening with the power tube cathodes? Are they connected up to that same spot? There is no dot, but I think that is how it set up.

Do you think it would make a difference if I referenced the filament supply to ground instead?

 

[Wildfara]

I just thought it was set up strange. Sometimes its elevated like that to reduce noise. Oh ok I just realized that is the ground for the CT from your Rectifier tube but not sure why it is also connected to there thru that cap as well as its own ground. MAybe for the same reason but I havent seen that before

 

[Wildfara]

You probably know you could put a cap across that cathode resistor on the power tubes and switch it in and out for a bright/gain switch right there...super easy. Maybe after the noise issue is resolved

 

[Jacobysmalls]

Hey Sam, did you ever get the hum figured out?
I'm about to dive into a 179 myself, any tips?
Thanks!
Would love to hear a clip of your finished 179. Is it on any of your recordings on bandcamp?

 

[radardoug]

That .01 uF on the preamp plate will pretty much kill the frequency response.