Filmosound 385 tone stack mod question

[markmalin]

Hi guys,
This is a curiosity that turned into an obsession, so bear with me.  I modded a Filmosound 385 per Uncle Doug's video and some threads on this forum.  Essentially removed the oscillator and PEC circuits, and added a pot to dial out the effect of the NFB circuit on the power section.  I'd like to convert the tone stack to a 5E3 style, but have some questions.  Please see the cut/pasted/hand drawn schematic attached.  I thought if I take the output of the 5879 (V1), run that signal into the standard Fender 5E3 volume/tone control circuit (1M audio tone and volume pots, 500pf and 0.0047uf tone caps), then from there to V2b's triode, and leave the rest alone that might be a good place to start.  Am I heading in the right direction, or have I missed something in my design?  I'm all ears...humbly...
Mark


Regarding the schematic attached, I used someone's "FilmoSound Redraw" schematic from a forum thread, printed it out, used whiteout to remove one triode, and "penciled" in my changes, so forgive the crudeness of the attachment - I don't mean to insult anyone's intelligence...

[Adrien]

Hi Mark,

Maybe you could explain a bit about what your goal is with making this change?

More to your question, pentodes in general have a pretty high output impedance so while what you have drawn will work, I suspect it won't sound that great.  Probably a bit muffled. But I may be wrong, try it and see!  Or maybe try putting it in between the second and third stages.

Like you I have been messing around with this circuit quite a bit, it's got a great character that I really enjoy.  Your timing is funny - I was actually just playing my version of this amp today and thinking that I should post something about it.  Guess here is as good a place as any.

The attached schematic is where I ended up after much experimentation.  I think it sounds great and keeps the same overall character while being less of a one-trick pony than the original.  It's also definitely lower-gain, which is fine by me but YMMV.  I'm not suggesting you do all these mods to yours (I scratch-built mine originally so it was much easier to change) but hopefully it will give you some things to think about.  Take a look at the tone stack - I kept the values the same but split it into two individual 1M pots with a 120k resistor in between.  This works great and keeps the original character with much more flexibility.  You could probably implement this on yours with a concentric dual-1M pot if there isn't room for another hole in the chassis. 

There are also a LOT of other changes in there, I can expand on those if you like.  Hope this helps! -Adrien

[markmalin]

Hi Mark,

Maybe you could explain a bit about what your goal is with making this change?

More to your question, pentodes in general have a pretty high output impedance so while what you have drawn will work, I suspect it won't sound that great.  Probably a bit muffled. But I may be wrong, try it and see!  Or maybe try putting it in between the second and third stages.

Like you I have been messing around with this circuit quite a bit, it's got a great character that I really enjoy.  Your timing is funny - I was actually just playing my version of this amp today and thinking that I should post something about it.  Guess here is as good a place as any.

The attached schematic is where I ended up after much experimentation.  I think it sounds great and keeps the same overall character while being less of a one-trick pony than the original.  It's also definitely lower-gain, which is fine by me but YMMV.  I'm not suggesting you do all these mods to yours (I scratch-built mine originally so it was much easier to change) but hopefully it will give you some things to think about.  Take a look at the tone stack - I kept the values the same but split it into two individual 1M pots with a 120k resistor in between.  This works great and keeps the original character with much more flexibility.  You could probably implement this on yours with a concentric dual-1M pot if there isn't room for another hole in the chassis. 

There are also a LOT of other changes in there, I can expand on those if you like.  Hope this helps! -Adrien

Thanks for the input, Adrien!  I'll have a look at your schematic this evening and if reach out with questions, if that's OK with you. 


My goal?  People keep asking me that :P.  It's just for fun, but I want a usable amp.  Right now I like the sound and responsiveness, but the tone stack is super noisy.  So my thought/goal was "...what if I put a Fender Tweed tone stack in there?" and hacked this schematic together.  It may or may not be the right thing.

[markmalin]

The attached schematic is where I ended up after much experimentation.  I think it sounds great and keeps the same overall character while being less of a one-trick pony than the original.  It's also definitely lower-gain, which is fine by me but YMMV.  I'm not suggesting you do all these mods to yours (I scratch-built mine originally so it was much easier to change) but hopefully it will give you some things to think about.  Take a look at the tone stack - I kept the values the same but split it into two individual 1M pots with a 120k resistor in between.  This works great and keeps the original character with much more flexibility.  You could probably implement this on yours with a concentric dual-1M pot if there isn't room for another hole in the chassis. 

What you did with the tone stack is intriguing.  I don't mind the way it works now - I'm more of a jazz player, so I like the bass and dial the tone control only to about 2 or 3 and it sounds the way I like it.  Given that, I do have a few ganged 1M Audio pots...  I'd thought of separate Treble and Bass, but no room on the front panel, so your values with a 1M dual gang pot may be worth trying first.  To be honest, if I could eliminate the noise floor and leave it as is it would be fine, but I've narrowed the noise down to the tone control and am wondering if it's the pot itself.  There is SO little room to work in there - I end up having to remove stuff to get at the tube sockets to desolder parts which is kind of maddening. 

[shooter]

There is SO little room to work in there - I end up having to remove stuff to get at the tube sockets to desolder parts which is kind of maddening.

yep!
I tend to gut n replace when it comes to TS, to many iffy sketchy lack of quality parts to spend my time "fixing it"
troubleshooting is also a pain since even good quality test equipment "alter" your readings


to "test" your noise floor problem, simply bypass the TS, is the noise better???

[Lectroid]

I thought if I take the output of the 5879 (V1), run that signal into the standard Fender 5E3 volume/tone control circuit (1M audio tone and volume pots, 500pf and 0.0047uf tone caps), then from there to V2b's triode, and leave the rest alone that might be a good place to start.  Am I heading in the right direction, or have I missed something in my design?  I'm all ears...humbly...
Mark

I rebuilt a 179, a very similar architecture.  I also looked at the (film projector's) tone stack and choked.  Then I installed the 5E3 tone stack you're looking at and it works pretty well. It's interactive with the volume just like the 5E3 but that's part of the charm I think.

[mxrshiver]

Adrien, looks like a great circuit! curious about the dissimilar cathode resistors on the two parallel triodes in V2 - did it just end up that way, or were you going for something particular there? with those values, guessing one might clip one side of the waveform, the other clipping the other side around the same time?

Mark, what kind of noise are we talking about? hiss, crackling, white noise, hum/buzz? does it change much with the Tone control rotation? and, how did you determine it's coming from the Tone pot specifically? guessing it's popping up between the 500pF and the top of the pot? if it's just popping up at the Tone pot wiper, it may be more about the next stage's grid than the pot itself. if that's a long run between them, you might consider shielded cable and a small grid stopper, though that may be a tall order in your case.

consider the possibility of a leaky coupling cap from the first stage, check for DC. and especially if it's hum/buzz, consider the proximity of noisy power supply elements to that part of the circuit. sometimes, a little copper tape bonded to the chassy can make a big difference in isolating noisy AC runs from sensitive grid circuits. also make sure the pot's body is making good conductive contact with the chassis. when probing with the multimeter, see if anywhere you probe decreases the noise - that could be a useful clue. good luck!! love to hear about someone who actually uses the Tone control at that low settings, bet it sounds lovely besides the noise!

[Adrien]

My goal?  People keep asking me that :P.  It's just for fun, but I want a usable amp.  Right now I like the sound and responsiveness, but the tone stack is super noisy.  So my thought/goal was "...what if I put a Fender Tweed tone stack in there?" and hacked this schematic together.  It may or may not be the right thing.

Fair enough!  Definitely try it with the dual ganged 1M pot if you already have it, but it sounds like the tweed style tone control would work well for you too, and it's a fair bit simpler.

Adrien, looks like a great circuit! curious about the dissimilar cathode resistors on the two parallel triodes in V2 - did it just end up that way, or were you going for something particular there? with those values, guessing one might clip one side of the waveform, the other clipping the other side around the same time?

Yes exactly.  V2B is biased about the same as original, and V2A is a colder bias.  Just an attempt to get some clipping on both sides of the waveform and fatten up the sound.  Haven't scoped it or anything but it sounds good to me haha - it was definitely an improvement clean and overdriven compared to just leaving V2A unused.

[markmalin]

Mark, what kind of noise are we talking about? hiss, crackling, white noise, hum/buzz? does it change much with the Tone control rotation? and, how did you determine it's coming from the Tone pot specifically? guessing it's popping up between the 500pF and the top of the pot? if it's just popping up at the Tone pot wiper, it may be more about the next stage's grid than the pot itself. if that's a long run between them, you might consider shielded cable and a small grid stopper, though that may be a tall order in your case.

consider the possibility of a leaky coupling cap from the first stage, check for DC. and especially if it's hum/buzz, consider the proximity of noisy power supply elements to that part of the circuit. sometimes, a little copper tape bonded to the chassy can make a big difference in isolating noisy AC runs from sensitive grid circuits. also make sure the pot's body is making good conductive contact with the chassis. when probing with the multimeter, see if anywhere you probe decreases the noise - that could be a useful clue. good luck!! love to hear about someone who actually uses the Tone control at that low settings, bet it sounds lovely besides the noise!

There is a noise floor of white noise (hiss?) and random crackling and popping.  Rotating the tone control makes it worse.  No change in turning the volume control.  If you look at the original schematic, if I ground the grid on pin 2 V2 it is dead quiet.  I took at 500pf cap, grounded one end and touched the parts in the tone stack.  If I pul that cap on the tone control lug connected to the 390pf cap or the one connected to the 330K resistor, the noise decreases, but does not stop.  I thought I had measured DC leakage at some of the tone control caps, so I've actually replaced all of those. 

Before I move to a 5E3 tone stack, I thought I'd swap in a 1M audio gang pot like Adrien had done, leaving the other components the same.  I figured since I was going to have to remove the pot anyways, I'll drop this one in there first and see what it sounds like.


Also, to your point about noise due to part proximity, man, it's such a rats nest in there, I'm sure that contributes.  I just can't imagine Bell and Howell selling a projector that had so much background noise originally.  And honestly, when I play through it, it's not noticeable but when I stop playing you hear it.  (kind of like "...Doctor, it hurts when I do this.." "...so stop doing that!...")

[mxrshiver]

when you say rotating the Tone control makes it worse, do you mean there's a lot of noise created while rotating it? or that the noise is greater, when the Tone control is at a higher setting? i don't see a 390p cap or a 330k resistor around the tone control on your schematic, so i may be missing something there. but, random crackling and popping possibly says a bad tube to me - have you tried replacing V2? or V1 for that matter. a new pot couldn't hurt, but swapping the tubes is easier to try first.

white noise is probably coming from that 1M input resistor, i'd definitely lower that to around 10k-33k, try putting one in parallel and see if the white noise lessens.

if the pot was causing any issues in and of itself, it would be the wiper losing contact with the track due to grime, oxidation, or overuse... and in that case, you should be getting some sort of intermittent signal while playing. if the pot adds more noise while being rotated, cleaning or replacement might help. but if the tone sounds great while playing and doesn't cut out, and rotating the pot doesn't add noise, replacing the pot might not have any benefit.

[markmalin]

when you say rotating the Tone control makes it worse, do you mean there's a lot of noise created while rotating it? or that the noise is greater, when the Tone control is at a higher setting? i don't see a 390p cap or a 330k resistor around the tone control on your schematic, so i may be missing something there. but, random crackling and popping possibly says a bad tube to me - have you tried replacing V2? or V1 for that matter. a new pot couldn't hurt, but swapping the tubes is easier to try first.

white noise is probably coming from that 1M input resistor, i'd definitely lower that to around 10k-33k, try putting one in parallel and see if the white noise lessens.

if the pot was causing any issues in and of itself, it would be the wiper losing contact with the track due to grime, oxidation, or overuse... and in that case, you should be getting some sort of intermittent signal while playing. if the pot adds more noise while being rotated, cleaning or replacement might help. but if the tone sounds great while playing and doesn't cut out, and rotating the pot doesn't add noise, replacing the pot might not have any benefit.

When I dial the pot counter clockwise (toward bass) it is quieter.  The more toward treble (the more clockwise) the noise increases.  It is not caused by rotating the pot, though.  It's constant.  I have cleaned the pot with Deoxit several times, just in case.  It did initially have some effect, but not much.  I have changed out all the tubes and run the amp on a known good set (all except the 5879 which I don't have a spare of).


Thanks for the tip on the 1M input resistor.  I do need to change that.  A few others have suggested that.  I was going to set it up like a standard Fender 1/4" input jack with a 1M and a 33k.  Sorry for the confusion on the schematic.  That one I included in the original post had the original tone stack "whited out" and I hand drew a 5E3 style over it on paper.  Here's a schematic of the original circuit.


Cheers!
Mark

[markmalin]

te author=mxrshiver link=topic=33218.msg369196#msg369196 date=1766347529]
when you say rotating the Tone control makes it worse, do you mean there's a lot of noise created while rotating it? or that the noise is greater, when the Tone control is at a higher setting? i don't see a 390p cap or a 330k resistor around the tone control on your schematic, so i may be missing something there. but, random crackling and popping possibly says a bad tube to me - have you tried replacing V2? or V1 for that matter. a new pot couldn't hurt, but swapping the tubes is easier to try first.
white noise is probably coming from that 1M input resistor, i'd definitely lower that to around 10k-33k, try putting one in parallel and see if the white noise lessens.
if the pot was causing any issues in and of itself, it would be the wiper losing contact with the track due to grime, oxidation, or overuse... and in that case, you should be getting some sort of intermittent signal while playing. if the pot adds more noise while being rotated, cleaning or replacement might help. but if the tone sounds great while playing and doesn't cut out, and rotating the pot doesn't add noise, replacing the pot might not have any benefit.

When I dial the pot counter clockwise (toward bass) it is quieter.  The more toward treble (the more clockwise) the noise increases.  It is not caused by rotating the pot, though.  It's constant.  I have cleaned the pot with Deoxit several times, just in case.  It did initially have some effect, but not much.  I have changed out all the tubes and run the amp on a known good set (all except the 5879 which I don't have a spare of).

Thanks for the tip on the 1M input resistor.  I do need to change that.  A few others have suggested that.  I was going to set it up like a standard Fender 1/4" input jack with a 1M and a 33k.  Sorry for the confusion on the schematic.  That one I included in the original post had the original tone stack "whited out" and I hand drew a 5E3 style over it on paper.  Here's a schematic of the original circuit.  If you look at that you'll notice the tone stack is between the 2 triodes on V2.  If I ground the grid on the second half (in that schematic, it's pin 2), it is nice and quiet.  That's why I was thinking the tone stack itself was noisy...well...that and touching those two points with a grounded 500pf cap, but again that did not totally eliminate the noise, it just quieted it down some.

Cheers!
Mark

[markmalin]

Quick update.  I changed the gang pot to one I had on hand, a 1M Audio vs the 3M Audio.  It's much quieter now.  The only difference is in the range - it seems to have more bass and maybe it's a little less lively than it used to be.  But that was a quick, initial test.


Adrien - tell me why you added the 120K resistor between the wipers of the Bass and Treble pots?  Was that intentional?

[Adrien]

Adrien - tell me why you added the 120K resistor between the wipers of the Bass and Treble pots?  Was that intentional?

Hi Mark, yes it was.  Since I split the tone stack into two separate controls, I added the 120k to turn it into the standard “James” style tone control circuit. I think the resistor helps to make the controls less interactive. If you added that resistor on yours, try shorting it out to see what that sounds like.

Glad to hear you got the noise issue sorted out!

[markmalin]

Just to follow up, I implemented the standard Filmosound tone stack (with a 120K resistor between the wipers of the ganged pots like Adrian's schematic).  What I ended up doing as a final mod, however, was to purchase a 1M Audio dual concentric pot and this allowed me to have separate Treble and Bass controls (thanks for the tip, Adrien!).  It works great :)


Here's a tip on the 1M Audio dual concentric pots.  They are rare if not unavailable.  I purchased a 16mm version from Alpha, the only kind I could find.  It worked, but the chassis hole for the original pot is larger, just keep that in mind.  The other thing I did, which was a little crazy, but worked, was to purchase a CTS dual concentric potentiometer for a Fender Jazz bass.  250k/500k.  I then purchased 2 1M Audio CTS pots.  I dismantled the dual pot and the 2 1M Audio pots, and swapped out the resistive portion.  It worked great, but I realized later the shafts on those Jazz Bass pots are not the standard size for typical dual pots.  Because of this, I went with the smaller 16mm Alpha.


Now to find a descent guitarist who can do a video of the Filmosound :)