Filmosound 202 conversion question

[travisking51]

I got me one of these at an antique store for roughly $30, and decided I would convert this into a guitar amp for funs. I switched out one of the speaker outputs with a 1/4 jack, and I hear crackling and such when I turn the knobs, but I am getting no sound from the input jacks. All the tubes seem to be working, I switched out the 6AQ5s since two were shorted and I also switched out the 5Y3 and 12AX7. My question is mainly about the 5879: do I need to keep this in the circuit to work? I read somewhere that it was not needed, but I am unsure. I looked at the schematic and it seems to be bypassed? Any help is appreciated.

 

[travisking51]

Here's the link to the schematic, it is the Tech 202 Diagram.

Bell & Howell

 

[ok_state_blues]

The 5879 is part of the pre Amp and not bypassed. Based on what you are going for tone wise will dictate how/if you want to use it. My At Mars specialist is a filmosound conversion that utilizes that same tube. Are you going for high gain?

 

[travisking51]

The 5879 is part of the pre Amp and not bypassed. Based on what you are going for tone wise will dictate how/if you want to use it. My At Mars specialist is a filmosound conversion that utilizes that same tube. Are you going for high gain?

Ah I see, maybe that’s why I am not getting any signal from my input jack. I am going for somewhat moderate gain, somewhere around blues/classic rock. Do I only need 1 of the 2 in the circuit to operate or both?

 

[ok_state_blues]

I would just get it operational first before customizing to taste i.e. getting it to signal passing state. That chassis must be tight to work in. Have you seen any of Uncle Doug's vids?

 

[travisking51]

I would just get it operational first before customizing to taste i.e. getting it to signal passing state. That chassis must be tight to work in. Have you seen any of Uncle Doug's vids?

I've seen a bit. I plan on changing out the capacitors sometime this week. I managed to get signal with one of the 5879 tubes in the circuit, but the output was weak.

 

[printer2]

Do you have any electronic experience, what do you have for test equipment?

 

[ok_state_blues]

From what I have heard I believe you need a gain stage before and/or after the 5879 to really maximize it. If you take a glance at the old Gibson amps that used that tube, they did have a stage after the 5879. Also, search the EL34 website for uses with that tube. There have been many a solid Amp built on similar bones.

 

[travisking51]

Do you have any electronic experience, what do you have for test equipment?

I got a bit, I fix up some old radios and built some guitar amps in the past. For test equipment, I have an oscilloscope, signal generator, DMM, variac, current limiter, etc.

 

[travisking51]

From what I have heard I believe you need a gain stage before and/or after the 5879 to really maximize it. If you take a glance at the old Gibson amps that used that tube, they did have a stage after the 5879. Also, search the EL34 website for uses with that tube. There have been many a solid Amp built on similar bones.

I'll take a look at it sometime this week. I replaced most of the electrolytic capacitors; there were quite a few leaky ones. Replacing them managed to get the sound back up to reasonable volume. Do you have any tips on replacing the capacitor cans? They seem like a real hassle with this circuit since it's a rat's nest in there.

 

[printer2]

I got a bit, I fix up some old radios and built some guitar amps in the past. For test equipment, I have an oscilloscope, signal generator, DMM, variac, current limiter, etc.

Good we know at what level we are talking at here. I am assuming the transformer on top is a power transformer and it is not line powered. Depends on what you want for an amp. Lot of stuff to strip out of it. The NFB loop, the low pass filter section. What do you want for tone controls? You could make a Tweed Bassman if you want. Actually most amps with the number of sockets you have.

 

[travisking51]

Good we know at what level we are talking at here. I am assuming the transformer on top is a power transformer and it is not line powered. Depends on what you want for an amp. Lot of stuff to strip out of it. The NFB loop, the low pass filter section. What do you want for tone controls? You could make a Tweed Bassman if you want. Actually most amps with the number of sockets you have.

Yes, it is a power transformer. I hooks up like a socket, which I haven't seen before. I want something Tweed-y, possibly something like a 5F6. I'll probably do some research and find something that I might favor. I'll also probably strip out the NFB, but I am a bit confused on what I am looking at. On the schematic, it appears that there are 3 resistors in play?

 

[printer2]

Yes, it is a power transformer. I hooks up like a socket, which I haven't seen before. I want something Tweed-y, possibly something like a 5F6. I'll probably do some research and find something that I might favor. I'll also probably strip out the NFB, but I am a bit confused on what I am looking at. On the schematic, it appears that there are 3 resistors in play?

They are using the NFB as a tone control, nothing wrong with that given the product but not quite what we are used to. Personally I would strip it all if I were to make a 5F6, Maybe use SS diodes to get the voltage up a little although the 5Y3 would make for a more softer feeling amp.

 

[travisking51]

They are using the NFB as a tone control, nothing wrong with that given the product but not quite what we are used to. Personally I would strip it all if I were to make a 5F6, Maybe use SS diodes to get the voltage up a little although the 5Y3 would make for a more softer feeling amp.

So I decided to actually keep the circuit how it is, and just modify it. I am getting some sweet tones on it with it cranked up! Sounds like an overdriven tweed/brownface. I do have a small problem though: whenever I get near 0 with the volume-switch knob, there is this loud oscillation that kicks in, could this possibly be a leaky cap? I haven't got all of them yet, about 3 more to go.

 

[printer2]

So I decided to actually keep the circuit how it is, and just modify it. I am getting some sweet tones on it with it cranked up! Sounds like an overdriven tweed/brownface. I do have a small problem though: whenever I get near 0 with the volume-switch knob, there is this loud oscillation that kicks in, could this possibly be a leaky cap? I haven't got all of them yet, about 3 more to go.

Hard to say why the oscillation. First thing I would do is try replacing the 12AX7's or at least swap them around. Try pulling the oscillator tube.

 

[travisking51]

Hard to say why the oscillation. First thing I would do is try replacing the 12AX7's or at least swap them around. Try pulling the oscillator tube.

I had it out originally. The original 12AX7 that was in there was microphonic, so I replaced those. Now it seems that the 5879s are also microphonic, but I tried 3 of them and they all seemed to be. Maybe put the oscillator back in there and try it?

 

[printer2]

I had it out originally. The original 12AX7 that was in there was microphonic, so I replaced those. Now it seems that the 5879s are also microphonic, but I tried 3 of them and they all seemed to be. Maybe put the oscillator back in there and try it?

Basically a search and destroy mission. You isolate sections to see if they are the cause. Just for kicks disconnect the NFB just to eliminate one possibility.

 

[travisking51]

Sorry I have been busy with school. I replaced all but the filter caps and so far all is good except the hum and idle (most likely the filter caps) and the oscillation at zero volume. I think the volume switch is faulty, as it doesn’t happen at zero but rather between turn off and zero, if that makes any sense.