Filmosound 385 issue

[lowell]

I've recapped all coupling caps, filter caps in this amp as well as removed all unwanted "film" circuitry. Tried new tubes. I changed the 1st two filters to 16uf, and the other 3 preamp filters to 10uf. I added a 1 meg across the input jack as well. I cannot figure out this noise issue. When I turn up the amp to where it starts to distort, there is a "bad solder joint" kind of sound. It is bad when the note is first picked, then decays away. I don't remember if this issue was here before I overhauled the components. The amp sounds good played gently and at lower volumes. I've narrowed the issue down to the cathode side of the concertina. I've checked the bias of the PI and the grid is about 1.5v negative of the cathode which is around 45v. That doesn't strike me as being off or causing this issue. The PI biasing resistors are indeed off spec but the biasing still seems ok, unless someone thinks I'm wrong here. Again the plate and grid signals don't have this issue, just the cathode. I know this because I used a signal tracer to locate it. And the bad sound is pre and post power tube coupliing cap. Power tubes are biased at 100%. I'm questioning the biasing resistors in the PI. Maybe they're so old that they're causing this? Any suggestions welcome. I may dig in there and replace those resistors to see but feel it may be a wasted effort.

Schems are here http://www.film-tech.com/warehouse/manuals/AMPS.pdf and this one is pg. 217-218.

[Raja]

Hi Lowell,
I have just started a conversion of the same amp, so I have great interest in your progress. Have you removed any components from the circuit? Is the PEC tube still in or have you blinded it?
I had a terrible hum issue before and I'm doing a cap job now, along with changing the mains input to a grounded three prong plug. I also removed the oscillator circuit to make room. What I have noticed is that everything being 50 years old makes the wiring brittle so you may have loosened a joint when you replaced the filters.
I'm a noob at this so take anything with a grain of salt!!!
Good Luck
Raja

[Euthymia]

Just off the top of my head, those 100K resistors in the concertina seem on the high side.

Gotta remember when converting film projector or PA amps to guitar use, they were not necessarily designed to clip gracefully, so when we slam them guitar-style, we can hear odd stuff like blocking distortion, etc.

[lowell]

Raja,
I've now converted about 6 of these to great sounding amps. To be honest I can't remember how I fixed the problem from this original post. It's been a long time.
Without any mods, just new components, these heads sound fantastic. Actually I've used slightly different filter cap values (30/16/16/16/16). These amps have lots of compression and even-order harmonics! I run mine through an 8" weber speaker in the studio and it's by far one of the best overdriven guitar sounds I've ever recorded. For fun I plugged it into my power soak and into the input of a solid-state peavey TNT amp... great if you want to use this head in a live situation.

[Raja]

Lowell,
Thanks for the reply, I may need your help later.
I'll chime in with a progress update after I finish the cap job and crank her up!
Raja

[loudthud]

The problem might be a bad or dirty tube socket, but I would expect any kind of intermittent connection on the cathode to affect the plate of the PI. If there was a problem on the grid of the output tube, the coupling capacitor might make it look like it's coming from the PI when in fact it's going backwards from the output tube to the PI.

Carbon composition resistors can become intermittent after 50 years, they are usually out of tolerence or change resistance when you wiggle the leads so they should be replaced. Clean the tube socket with contact cleaner and make sure there is good tention agains the tube pins.

[olddawg]

I am far from being as experienced as many on this board are, but IMHI it is much easier and less time consuming to gut a conversion and build up a new turret board with all new components. Also new ceramic tube sockets are cheap and you know there will be no issues. You already have the expensive parts: transformers, chassis, maybe some good old tubes, etc. It saves a lot of time and frustration in trying to modify a weird circuit that was designed for something else with 50 year old components and strange unintended consequences.

[iiebill]

For that matter OLDDAWG why not make it real easy and just buy a new tube amp from anywhere and be done with it. Sometime its better to just keep your thought in your head. Think about what JFK said about going to the moon…What he has is already there and maybe, just maybe he will end up learning something about building amps and actually feel good about himself no matter how it turns out.
Where is the "MOJO" if all he does is start to buy new parts left and right ? Most amp guru's like to only replace things on an "as needed" basis.
I know this thread is old, but I read it today for the first time , so its new to me. I'm currently restoring the BH 399 and I think what "BERNIE did was great but maybe all we need to do it modify out what we don't need and only change what has to be changed. Then we have the old mojo that these amps have in them with a new life in this century. That in itself is impressive enough. AMP on!