Filmosound Conversion to 5E3

[Leif H]

I've aquired a Filmosound amplifier, stock it's ok, but its a bit hissy. I gather that as it is 50 years old, its going to need filter caps, I was going to remove the whole oscilator portion of the circuit, and... I figured while I'm at it, I'd like to neaten it up as it's a rats nest.

After thinking long and hard, I've got the goods for a 5E3, and want to go that route, Its about $20 for caps, resistors, jacks and pots plus a home made turret board (free). I'm going to turn it into a head for the time being, and see how it goes.

My question is regarding the OT, and it's abilities. I can't find anything on it, as it has a B&H part number, but no specs seem to be available for it. Should I keep the output section resistors / caps to what are in B&H chematic, and connect the 5E3 preamp section to it, or just go for it and tear it all apart and go with a full 5E3 end to end.

Any input would be appreciated.

Thanks,

Leif

[cminor9]

For the OT...does the amp use 6V6 tubes in a push-pull configuration? If so, great. What is the impedance of the speaker? Anywhere with 6-12 ohms? If so, I'd say you could probably use it. Worst case scenario, a brand new 5E3 OT can be had for $30-$40.

[atmars]

I've worked on a few of these. The OT will be fine. Your problem is going to be the voltage supplied by the PT will be way to high for a stock 5e3. Also, i don't think there is any way you are going to be able to fit a turret board in that chassis. The tube sockets take up all the real estate in there.

[Leif H]

I've worked on a few of these. The OT will be fine. Your problem is going to be the voltage supplied by the PT will be way to high for a stock 5e3. Also, i don't think there is any way you are going to be able to fit a turret board in that chassis. The tube sockets take up all the real estate in there.

I'm making the turret board, I think I can get it to fit. It's a piece of insulation board from a switchgear, even if I have to break it out into two pieces.

Could you explain he PT being too high? is that what is giving me more overdrive?
Looking at the schematic, and comparing it to a 5E3, would putting a 2w resistor in the value of 470 - 500 ohms between the grids on pin 4 of each output tubes drop my voltage enough?

I suspecting that this would give me more clean, and later breakup?

[atmars]

is that what is giving me more overdrive?

You're saying you have too much overdrive? Explain.

Could you explain he PT being too high?

What I am saying is that if you build a 5E3 into this chassis you are going to have voltages that are much higher than what is called for in the 5E3. It's going to be high right off the bat because today's wall voltages are higher, but you will also be pulling the third 6v6 which will kick it up higher still. It's my understanding that voltages measured at the plate (pin 3) on the 5e3 should be 350v-ish. With the amp you have it will be more like 450v.

would putting a 2w resistor in the value of 470 - 500 ohms between the grids on pin 4 of each output tubes drop my voltage enough?

I'm not an expert, but this is not a technique I have ever heard of to lower b+. I'm not sure how it would work. Is this something you read about or thought of yourself?

I suspecting that this would give me more clean, and later breakup?

If you are looking for clean, the 5E3 might not be the circuit for you. There is not going to be very much no matter what you do.

[cminor9]

measure the dc voltage at the first filter cap. That should be about 370vdc.

More voltage won't result in more overdrive. It'll result in more headroom, more volume, and if you don't use a proper bias resistor on the power tubes, you'll drive the tubes too hard (a stock 5E3 is running the 6V6 tubes close to the line as is.) In my experience, too much voltage make the tone too harsh.

To lower the voltage, here's what you do: measure your voltage at the first filter cap. It should be around 370 vdc. If you have, say, 420 vdc on the first filter cap, you want to shed about 40-50 volts. Get some Zeners like these: 1N5358BG ON Semiconductor Zener Diodes and put them in series on the power transformer's HV center tap (which connects to ground) before the ground connection.

IIRC the stripe points to ground, but I may remember that wrong (if you wire it up wrong, nothing bad happens...but your voltages aren't lowered.) A couple of zeners will lower your voltage fairly accurately.

For what it's worth, I have an on-off-on standby switch on my 5E3. Since I use the method of disconnecting the PT's center tap from ground for my standby switch (as opposed to the break the B+ connection method), I have it wired up one way so that the amp runs at the full 420vdc that my PT puts out, and the other way I can run it at stock voltages. The lower voltage sounds a lot better actually. To my ears, it made more of a positive difference to lower the voltage than I expected.

[Leif H]

cminor9
According to my schematic, it's 310V I'm lower or should have lower voltage... I haven't had the chance to Dig into it, and check with a meter, it's a real rats nest in there. I checked with our supplier, if need be I have access to a Hammond "classic" Teed Deluxe PT on the cheap through work.

Looking at the whole circuit, I think all my voltages are lower.

My gut is telling me that this is lower gain than the 5E3 and had it wound up some, it was clean up to about 1/3 volume, then it starts to break up, not as creamy, I think new filter caps are in order. I have a recap kit on the way, so I'll try that next weekend.

Amtars, Ive measured out the chassis, and I should have room to fit the turret board, I'ts going to be tight, and I'm cutting it to fit, so I'll make it, allowing for the the pot's and Jacks, which are going to be on the sides.

The "idea" I had about the 470ohm resistors on the grids wasn't mine, it was from the 57 deluxe schematic, it's the only one I had found with voltages.
Again looking at my schematic for the Filmosound, the plate voltage on pin 3 is supposed to be 335V.

Again, I need to rip into this with a volt meter when I have some time.

Right now it's decent clean, the overdrive gets by, but wound up it's a little harsh.

[atmars]

Again looking at my schematic for the Filmosound, the plate voltage on pin 3 is supposed to be 335V.

Maybe you'll get lucky, but when you get in there and measure it will probably be much higher, especially with the two extra tubes pulled.


So the plan is to gut, or to modify?

it was from the 57 deluxe schematic

Can you post a link to this schematic?

[Leif H]

Maybe you'll get lucky, but when you get in there and measure it will probably be much higher, especially with the two extra tubes pulled.


So the plan is to gut, or to modify?



Can you post a link to this schematic?

Gut, I have the bug to do it... I think if I can get another on the cheap, I'll modify. I'll see if I can find a Canadian one with El84's in it and maybe I end up with a "Bernie".

For the schematic, it's the PCB version. They didn't seem to have a link for the PTP, reissue. Here's the link, right from Fender's page http://www.fender.com/support/amp_sc...0JUN%2007).pdf

[atmars]

would putting a 2w resistor in the value of 470 - 500 ohms between the grids on pin 4 of each output tubes drop my voltage enough?

I thought you might be referring to the grid stoppers. They are not there to reduce voltage. Check out this link.

Grid Resistors

[Leif H]

I thought you might be referring to the grid stoppers. They are not there to reduce voltage. Check out this link.

Grid Resistors

ah... I see! Thanks.

Forgive me, your'e dealing with an electrician here. resistor = load = Vdrop for me, so I seem to look at that way first...

[atmars]

Forgive me,

Forgive ME - I pointed you to the wrong article. Pin four is the screen grid, not the input grid

This has some info on screen grid resistors:

Tales From The Tone Lounge; Marshall Myths

as do other places I'm sure.

The part I was right about, however, is neither is there to reduce your b+.

Sorry for making you read some good info for the wrong reason.

[Leif H]

Forgive ME - I pointed you to the wrong article. Pin four is the screen grid, not the input grid

This has some info on screen grid resistors:

Tales From The Tone Lounge; Marshall Myths

as do other places I'm sure.

The part I was right about, however, is neither is there to reduce your b+.

Sorry for making you read some good info for the wrong reason.

No problem. The input grid was a bit easier to understand... I have been attempting to digest the screen grid info on the second page you pointed me to, that is more complex than I could understand in my no coffee yet state.

It's going to take some time to understand that one better.

Thanks amtars

[swaptronics_tech]

Filmosound to 5E3 amp

Maybe you'll get lucky, but when you get in there and measure it will probably be much higher, especially with the two extra tubes pulled.


So the plan is to gut, or to modify?



Can you post a link to this schematic?

I got lucky and scored an old Filmosound projector with amp on e-bay a few months back. I just had the seller send me the power transformer and amp chassis and he kept the old projector. This model had the (3) 6AQ5 tubes which are the narrow bottle equivalent of a 6V6 tube except for pinout. I kept the 5Y3 rectifier, 12AX7 tube and the PT and OT and a few octal sockets. Otherwise I scrapped the rest of the junk--except for the bumblebee caps. I bought a 5E3 chassis, turret board, tweed cabinet, hardware, and caps/resistors from sources on the internet. The PT has high current and delivers a perfect 340V DC after the NOS rectifier. I used 5V6GT tubes (black plate RCA) found on e-bay for $4.25 each with a 3-ohm 3-watt dropping resistor on each heater (pins 1,2 of the 5V6GT sockets). The amp sounds wonderful as a 4-input 5E3, but doesn't come close to the awesome sound of your converted amps. The cool thing is I can use 12AV7 in the V1 or V2 position. The best sound for me now is a 12AY7 in V1 and a 5751 in V2. I ended up having to buy a hammond OT because the leads of the filmosound OT were brittle and short--1 lead broke off.

I'm totally blown away by the sound of Lance Keltner playing through you builds--nice work.

The phrase "two pounds of shit in a one pound bag" doesn't even begin to describe the components Bell & Howell packed into their tiny chassis--I totally agree with you!

I will build you one of my fuzz pedals if you share with me how you got one input and one tone control to sound so fantastic in your builds.

Best regards,

Dave Adams

a.k.a. "swaptronics"