Filmosound dual pot

[atmars]

http://s294.photobucket.com/albums/mm109/atmars/?action=view&current=filmosoundL-2.jpg

I am trying to convert this filmosound amp to a guitar amp with limited abilities, so I'd appreciate any advice.

My most pressing issue is the volume pot is a dual pot that is seen better days. It's marked 200m/25m on the schematic but I believe it's really 200k/25k. I'd like to replace this pot but I don't think this type of pot is available. One half the pot feeds the grid of the preamp tube the smaller half feeds a third 25L6 (lower left of schematic) that I believe is not needed in the circuit. My understanding is that removing the circuitry for this tube is not possible without major modifications to the rest of the circuit.

My question is: Is there a way I can leave the third 25L6 in the circuit but not have the grid attached to a volume pot so it is doing it job in the circuit but just not "hooked up". Maybe a resistor on the grid? What value? Attached to what? Somme other idea? If this could be accomplished it would allow me to replace the volume pot with a standard pot.

Thanks for any input.

[Enzo]

The power tube in the lower left is an oscillator. You don't need it, remove the tube. Now the 25k pot is irrelevant.

Yes, 200M means 200k. M for thousand in roman numerals. No one uses that terminology these days. A common 250k pot will be fine, it is just a voltage divider, use a 500k if you have no 250ks.

If you have not already, remove the photo tube.

[atmars]

Thanks for the help

Photo tube is gone. I tried pulling the oscillator tube a while back after having read that it can be done. However in this version of the filmosound this doesn't appear to be the case. Pulling this tube kills the circuit (same with the 5879).
I am guessing since the heaters are wired in series this is the reason (part of the reason at least). Can one pull the tube and add a resistor value that would take the place of the heater, or conversely remove all elements essential to this tube and just leave the tube in the circuit to have it's heater running? The other issue with this amp is space. Anything that can be remove from the chassis would be a blessing. This amp was engineered without and a square inch to spare.

[Enzo]

Oops, my mistake. The Film o sound amps I have seen used 6v heaters. So put that tube back in.

You could remove anything not related to its heater, but considering the effort, it might be easier to just leave the tube in there.

[atmars]

Thanks

I'm fine with leaving the tube in, but I gotta solve this dual pot issue.
Do you think if i disconnect everything but the heater all will be well? This kind of stuff is in the "tube theory" category and is a bit over my head. I'm more of a "substitute values" kind of modder. I need some advice as to what will happen if I try to take a giant chunk out of the circuit.

[Shea]

Thanks

I'm fine with leaving the tube in, but I gotta solve this dual pot issue.
Do you think if i disconnect everything but the heater all will be well? This kind of stuff is in the "tube theory" category and is a bit over my head. I'm more of a "substitute values" kind of modder. I need some advice as to what will happen if I try to take a giant chunk out of the circuit.

Another way to do it would be to leave everything connected but remove the tube and wire a resistor across the socket to replace the heater. Look up the amount of heater current used by that tube. Divide the heater voltage (6.3V, I'm guessing) by that amount of current, and that will give the value of the resistor. To figure out the power rating for the resistor, multiple the heater voltage times the current, and increase that by at least 10% for a safety margin - I would double it if feasible. This resistor will dissipate as much heat as the filament, so figure out a way to keep the heat away from the other components. If it doesn't dissipate any more power than a typical screen resistor, then maybe it'd be OK to just solder it across the terminals where the heaters usually connect. If it's more than that, then perhaps you can get one of those tube base sockets (if it's an octal tube) and solder the resistor to the appropriate pins to replace the heater, so that the resistor will be outside the chassis. If that won't work, use a resistor that you can mount on the chassis in a spot away from the other components, so the chassis acts as a heat sink, and run leads from the resistor over to the socket terminals where the heaters usually connect.

Shea

[atmars]

Thanks,

heater voltage for this tube is 25V and heater current is 0.3A

Resistor Value = 84 ohms?

Power rating + 10% = 8.25 watts?

My guess is this resistor is going to get pretty hot if not quite large, as the tube itself gets crazy hot quick.

My question would be: Why not leave the tube and let the tube heater keep doing it's job and disconnect the plate, grid, etc? Does this cause some sort of havoc in the circuit? Also, what happens if just the grid is disconnected? Does the tube just hum nicely along, or does some runaway meltdown or some other catastrophic event occur?

[Shea]

Thanks,

heater voltage for this tube is 25V and heater current is 0.3A

Resistor Value = 84 ohms?

Power rating + 10% = 8.25 watts?

My guess is this resistor is going to get pretty hot if not quite large, as the tube itself gets crazy hot quick.

My question would be: Why not leave the tube and let the tube heater keep doing it's job and disconnect the plate, grid, etc? Does this cause some sort of havoc in the circuit? Also, what happens if just the grid is disconnected? Does the tube just hum nicely along, or does some runaway meltdown or some other catastrophic event occur?

I just thought it would be easier to stick a resistor in there than have to remove every lead or component that is connected to the cathode, plate, and screen terminals. But if it has to dissipate that much heat, maybe not.

Shea

[atmars]

Thanks,

In this case it's better to remove stuff because there is so little room inside the chassis that anything that can get thrown over board makes a big difference.

You could remove anything not related to its heater, but considering the effort, it might be easier to just leave the tube in there.

This quote was in a previous post and seems to imply I can remove anything non-heater. Anyone want to verify?

[Enzo]

The only thing you need the tube for is its heater.

If you were to remove just that multisection coil, there would be nothing getting to the rest of the tube. You can just leave the whole thing in place, it won't really get in the way electrically. But if you want to replace the pot assembly then I would take the wire going to the wiper of that section of the pot so the grid of the tube is grounded. That will keep the tube from runnign away.

ANother alternative would be to SHORT across that tubes heater pins, and adjust the already existing fropping resistors, instead of adding a resistor just in place of that tube. Looks to me like there is already a 20 ohm and a 78 ohm series resistor happening. You could increase one of them to compensate for the missing 25v heater.

[atmars]

Thanks for the advice, might need a little clarification. I'll take the easy part first.

ANother alternative would be to SHORT across that tubes heater pins, and adjust the already existing fropping resistors, instead of adding a resistor just in place of that tube. Looks to me like there is already a 20 ohm and a 78 ohm series resistor happening. You could increase one of them to compensate for the missing 25v heater.

i understand this idea, but the resistors mention are HUGE. They are ceramic, about 4 inches long and 2 inches wide, shaped like a surfboard and contained in their own little cage. I probably can't realistically mess with them.

If you were to remove just that multisection coil, there would be nothing getting to the rest of the tube. You can just leave the whole thing in place, it won't really get in the way electrically. But if you want to replace the pot assembly then I would take the wire going to the wiper of that section of the pot so the grid of the tube is grounded. That will keep the tube from runnign away.

When you say multisection coil I am guessing you mean the (inductor?) section below the tube. The first pair of wires labeled black/black-yellow are the pair that was connected to what I believe was the old input for the projector. I removed that stuff a while ago and tied off the leads. Are you saying I can remove the blue/red pair? I'm assuming the pot can't work without yellow/green. If I could do that and remove the rest of the components feeding the tube that would be great.

If I am misunderstanding, it also seems you are saying attaching the grid to ground instead of the dual pot (but leaving the rest of the tube circuitry intact?) will work.

Thanks in advance for any help you can offer.

[Enzo]

Sure they are large, they are dissipating what, 10 watts or more, so they should be like 20-25 watt resistors. I would guess adding the 83 ohms to one of them would work. Approximately 150 ohms is close for the 78 or 100 is close for the 20 , so for example this Mouser part:

284-HS25-150F
284-HS25-100F

Check their dimensions for fit though.

Data sheet for the types: http://www.arcol.co.uk/uploads/products/hsseries-10.pdf

But leaving the old tube in there is simpler.


Yes, the inductor under the tube. Disconnect the red and blue wires and the whole circuit goes dead. The tube screen would keep some voltage but who cares? By the way those black and yellow wires are the oscillator output. Or were...

At that point the pot doesn't matter, nor do the yellow and green wires. The pot won't "work" because the tube circuit it is in is now disabled. So you can mount a new single section pot for the audio portion above without regard to this pot section. I would still ground the wiper wire though.

And if you just left the whole thing as is it would likely be fine, except you want to remove the old dual pot and install a new one for the audio section. That would leave the old pot wiring hanging for this oscillation section. FOr that reason I would ground the wire that had gone to the wiper of this extra pot section so as to turn off the grid of the oscillator tube. That would prevent the tube from doing anything. The cathode of that tube is ground as a potential spot to conect it.

[tubeswell]

Another simple mod that might work is to clip all the pins off the tube bar the heater pins (find out which ones are the heater pins first), then pop the tube back in (making sure it lines up the right way). That way the heater should still work fine, and you might even have a bit of extra in reserve in your B+ afterwards (seeing as how there would be no B+ voltage on the tube in question).

The National Valve Museum might have data about pin layout on that type of tube

http://www.valve-museum.org

[atmars]

For that reason I would ground the wire that had gone to the wiper of this extra pot section so as to turn off the grid of the oscillator tube. That would prevent the tube from doing anything. The cathode of that tube is ground as a potential spot to connect it.

Maybe I not understanding this, but it's dawning on me that shunting the grid straight to ground is like the pot on zero or OFF. Is that right? So am I right in thinking you are saying I can attach the grid wire to the cathode pin to ground it?

Another simple mod that might work is to clip all the pins off the tube bar the heater pins (find out which ones are the heater pins first), then pop the tube back in (making sure it lines up the right way).

It sounds like this idea is different way to disconnect the tube from all inputs without removing components, so am I right in thinking I could remove the resistors and caps associated with the tube as long as the red and blue inductor wires intact?

Thanks for the help

[tubeswell]

It sounds like this idea is different way to disconnect the tube from all inputs without removing components, so am I right in thinking I could remove the resistors and caps associated with the tube as long as the red and blue inductor wires intact?

Thanks for the help

Yep its the same idea - you leave the heater circuit connected but disconnect everything else going to that tube

[Enzo]

The red and blue wires don't matter, I'd clip them. But cutting hte pins off het socket will disconnect the red and vlue wires as surely as cutting them, right?

This is not as complex as it seems. Grounding the grid of that tube makes it not do anything - the grid controls the tube after all. SInce the cathode is already grounded, it seemed a convenient place to use. But any ground will work fine. CLipping the red and blue wires will disconnect the tube from the high voltage power supply. That also will stop any function the tube might have had. The resistors and caps don't care if they have anything to do or not. That section of the dual pot now becomes irrelevant, so it can sit there or go away or whatever.

[atmars]

Thanks for the clarification. You guys have been a great help. I think I felt a little bit of the stupid leave my body!