filmosound again

[NotSure]

Hey yall,
I am concerned about a hiss that is not something I've heard from any tube amp. Can anyone help with trouble shooting this 1955 B&H 385 Film-O-Sound circuit. This thing sounds cool, but not sweet, with the hiss. Should I start replacing all the orange-drop caps? There is a boat load of em. Or is it more related to the can electrolytic caps? I dread really taking this thing apart. Looks complicated beyond my expertise. It sounds pretty good as is, but not really good compared to what I've heard on YouTube stuff. Anybody with experience with these 385 conversions, please give me some tips.


Thanks!

[shooter]

I am concerned about a hiss

here's my usual suspects list;
pull tubes to "guess" at which stage
DC volts bleeding through coupling caps
plate Resistors
bad grounding, especially un-grounded shielded cabling

1st thing though, a schematic is golden for fixing broke things

[NotSure]

Shooter,
I do have a reprinted manual with schematic. I can identify stuff, but trouble-shooting is an art that I don't have.


Can you give me some detail about pulling tubes and measuring, or listening, or what am I supposed to do?


This thing is, as you probably know, A/B with 2 6v6's and 12ax7 preamp and 12ax7 phase inversion. I know for a fact the variable resistor is not really functioning. No matter what I set it on pin 4 for V4 is way hot. The resistor does get hot and smell bad if I try to aggresively reduce the voltage. I cant find a replacement for this component either. Any repair ideas for this component would be greatly appreciated.


I've pulled the components that produce oscillation for pulling sound from the film. All this from the Uncle Doug video. Installed a  three prong plug, capped the plug thing so I don't electrocute myself. All the easy stuff. The big electrolytic cans don't have anything oozing out of them. So I assume they're okay.


I know this is not the typical guitar amp thing. There are parts of the schematic that makes absolutely no sense to me compared to any guitart amp circuit I've traced out.


Any FilmoSound buffs out there, please chime in.


Thanks Shooter. Don't spend too much time  on this. I know its kind of a weird deal.


Dave

[shooter]

I know its kind of a weird deal.

Odd and weird are in my wheelhouse

I made my career troubleshooting broke stuff.

figure out how to post a pic of your schematic or point me to a link and life gets 100% better.

don't have anything oozing out of them. So I assume they're okay.

unless they're bone dry and turning to dust

get me a schematic and you should be rocking soon.

guessing you have a meter and a license to drive it 

[shooter]

Forgot to mention, there's like 20+ folk here that also like to solve broke amp problems

[NotSure]

Thanks Shooter.


I'll take some pics and get back


D

[kagliostro]

To me hiss can easy come from resistors

old resistors are prone to those exploits

Franco

[NotSure]

Shooter et al.


Thanks for the tips on the old carbon comp resistors. Here is the schematic. I noticed Ratgon posted this in the Filmosound thread. Same thing.


The coupling caps seem a little brittle. There are edges at the leads broken off, but generally they seem in tact. Who knows about old carbon comp resistors.


I can't measure the required voltage at per the schematic at Pin 4 of V4. 325 volts recommended. I'm measuring around 400 V.  Adjusting the position produced very noticed heat and smell. I put it back where it was. How do you replace this weird thing?


Thanks

[shooter]

conversions,

you're not doing a restoration correct?
this will become a guitar amp?

for starters I colored a couple points, the 1st, R40 500ohm 10W, I would replace that with a fixed resistor of the same value.
then let us know what the 2 voltages are I have circled.  the 19vdc is your bias, will also need the resistance value of the resistor that goes from pin 8 to ground (V4/5)(measure ohms with power OFFFFFF and caps discharged).  Also the plate voltage, BE VERY CAREFULL, verify both tubes (V4/5) pin 3. I use gator clips with power OFF, making sure nothing is touching ANYTHING it ain't supposed to.  That way you can power up quick, and down quick and your hands are clear.

for your hum/hiss, pull V1, still there?, yes, pull V2, yes, pull V3.

[kagliostro]

..... R40 500ohm 10W .....

I've seen some guitar Boutique Amp that has such rheostat, the purpose is regulable SAG

(I don't remember if those were SS or Vacuum rectified, nor I remember the brand)

What about to maintein it and put on a bypass switch ?

--

V6 ... don't see an use as is (for guitar amp use), I think you can take it off, may be one of our skilled friends can find a use for it

Franco

[shooter]

What about to maintein it and put on a bypass switch ?

When he adjusted it "that smell" showed up, my guess is it's coming from the rheostat, or cathode R.  I think IFFFF, the PS rail is good, there is more B+ because of the modern wall AC.

might confirm that by checking filament volts (V4 or 5 pins 2 & 7)

[shooter]

I re-drawed the schematic, I like how they were really creative, but it hurt following along
****NOT completely verified**** so ALWAYS go to original.  Hopefully i'll get it nailed down soon.

[NotSure]

thanks shooter, and kagliostro,


Yes, this is a guitar amp project, no restoration.


 I attached some pics of the cluster of ceramic coupling caps. They seem like they are not crumbling but do have some edges broken at the beginning  of the leads. Also another pic "Area 2" you can kind of see how far I moved the setting for the variable resistor. There is actually a dark spot, that may show the heating to the component. I don't see why the resistor would behave this way. It is clearly built to be adjusted. I tightened the set screw about how I found it. The heating issue does not occur when I put the metal ring back to its original position. I dunno.


The Area Pics are just general views so that you can get some idea how tight everything is and what, if any, info they can provide.

[shooter]

That is a thing of beauty 

he's what you have to decide, fix it - as is, very rewarding, but possibly a fools errand
OR
take all the tubes and iron and build in a new home
not as rewarding especially if you leave the "old box" where you can see it often, but way less possible head-aches, frustrations and the like. 

EDIT: I updated the re-drawn schematic still checking

[PRR]

> I re-drawed the schematic, I like how they were really creative

Thanks.

I dunno about "creative". They pasted the tube pin-out and then connected the dots. This worked OK for single-element tubes; gets pretty eye-crossed with dual tubes. Turning the one socket for better connections is creative, but still a pain in the eye.

[NotSure]

I like the idea of getting rid of the variable resistor. In the Area 3 pic you can see an empty tube socket in the lower left and that connects to some components attached to the side. There is a connection from that group to R40. That tube is not used for a guitar amp and I'm thinking those components and the connection to R40 are not necessary either? Still, looks pretty easy to pull R40, and replace with a 500 ohm 10 watt resistor. I  have an old 3 lug terminal strip that looks like it might fit nicely.


I did pull tubes, and listen. The hiss remained, no matter what configuration I was in. The v1 5879 tube is a mystery. Looks like the metal cap is pop riveted on? How the heck do you replace something like this without drilling out the pop rivets?


I'll take some measurements per your instructions when I get R40 out of there.


Thanks!

[shooter]

The v1 5879 tube is a mystery. Looks like the metal cap is pop riveted on? How the heck do you replace something like this without drilling out the pop rivets?

I'm confused, a metal cased tube still plugs in like a glass one, the socket might be popped, I still do that on my builds.

Everything inside the red box can go.

[NotSure]

Replaced R40 with a 500 ohm 10 watt resistor. The circled voltage: at Pin 3 for V5 is 300.0. The voltage circled on the connection from Pin 8 to Pin 8 is 17.80.


The hiss noise issue still remains, maybe more pronounced now. But the tone is more musical. Break up is happening a little sooner. That's ok. Even order harmonics are more noticeable, and really sweet feedback is jumping out more than before at high volume. I feel like the tone control provides greater variability. BTW, this amp does not produce a lot of hum. When I crank it up, of course, there is a definite hum that is not heard once the guitar makes sound. But, the hiss noise is there even with the volume at zero. I can over power it playing the guitar, but to my ears it is noticeable in the background. The conversion has great potential if I could get rid of the noise.


See the pic for the V1 5879. It is rubber mounted via pop rivets. Uncle Doug says something about this tube being micro-phonic? I can twist the metal cover but I'm afraid to pull on it hard. The rubber insulators look pretty dry and cracked.


Thanks again for any advise.


David

[PRR]

If the hiss goes away when you turn-down the Volume, that first tube is hissy. Replace it.

> rubber mounted via pop rivets.... I can twist the metal cover but I'm afraid to pull on it hard.

Twist and pull-- that's how it comes off. Look at the dents in the cover so you know which way to twist to get the socket bumps into the cover comes-off dents.

Yes it can be tight. Yes, you may damage the rubber... you could try rigging something to hold the socket-ears down while you work the cover off, but that rubber is perished in any case, and this could be a time to drill the rivets, install new rubbers and nuts/bolts.

[shooter]

Listen to PRR, he IS one of the bright bulbs here

here's your #'s

Vk = 17.8vdc / 225 (Rk) = .08A
Vp = 300 - 17.8 = 282.2vdc
282.2 * .08A = 22.58W (divide by 2) 11.3W dissipation per tube

there are a few MMF caps used by V1, when you order parts, order them, you'll find them under pF, same value