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Old 21st May 2008, 11:59 AM   #1
atmars is offline atmars  United States
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Default idiot needs help making sense of funny squigles - Filmosound

I am somewhat new to tube amps, so any help would be appreciated.

I got a hold of a transformerless Filmosound amplifier and I don't have enough skill and knowledge to get a handle on how the circuit works (link at bottom - works I hope). I've gotten it up and running as a guitar amp and sounds quite good considering I haven't done much in the way of tweaking and I know the filter caps are in need of replacement.

So I have few questions before I go forward.

1. Safety. I got an isolation transformer for this thing so as to not get killed. I am about to add a three prong plug to the chassis. Will this cause any problems I am not anticipating? Also, the switch seems to cut both the hot and neutral sides of the mains. Does this make the amp a floating chassis as opposed to a hot chassis?

2. Before I start to replace parts, i would like to know what parts of the circuit can be discarded, but I don't know enough about amplification circuits to make heads or tails of a lot of this. So far i have removed the old three prong input and the photo tube. All good. However removing the adjacent small tube #2 (5873?) kills the circuit. I am under the impression this section of the circuit is largely superfluous? Any ideas on how to remove what is not needed?

I am also under the impression that the 2 25L6GTs on the right are the power tubes. Half of the 6SL7GT is the phase inverter and the other half is the preamp tube. What the heck does the other 25L6GT in the left hand corner do? And why the dual pot?

I have many more questions - but any insight that you could lend would be helpful.

Thanks

http://i294.photobucket.com/albums/m...mosoundL-2.jpg

http://s294.photobucket.com/albums/m...mosoundL-2.jpg
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Old 21st May 2008, 12:04 PM   #2
SY is offline SY  United States
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Bless you- that isolation transformer is crucial! The other crucial part is to attach your power line's ground (green) securely to the chassis. You want any exposed metal bit to be well-grounded. That may or may not cause a hum loop with other equipment. If it does, you have two choices- either use an input transformer for galvanic isolation (see the excellent white papers on this at www.jensentransformers.com), or isolate the signal ground from the safety (chassis) ground through a groundbreaker, usually a smallish resistor, like 20 ohms or so, that is bypassed at RF.
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Old 22nd May 2008, 01:45 AM   #3
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Quote:
I am also under the impression that the 2 25L6GTs on the right are the power tubes. Half of the 6SL7GT is the phase inverter and the other half is the preamp tube. What the heck does the other 25L6GT in the left hand corner do? And why the dual pot?
It appears to be a power oscillator. It generates a high frequency signal to light the bulb that illuminates the films sound track, which is read by the photo detector tube. AC can not be used to light the bulb or there would be loud hum. A low voltage, clean DC is hard to get without a power transformer.

The dual pot is used to lower the intensity of this bulb when the volume is low to extend its life.
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Old 22nd May 2008, 04:10 AM   #4
atmars is offline atmars  United States
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Thanks for the replies.

Should I try and take this tube out of the circuit?

I have a few reasons for thinking it might be a good idea.

1. The chassis is packed with stuff. it looks like someone emptied a shoe box full of odds and ends into it. Any extra space would be useful for mods.

2. I'm guessing less stuff means less noise and more power.

3. Simpler circuits are easier for me to understand.

Should I try and get rid of this stuff - if so, any ideas on how and/or what must be preserved? Or should I just leave well enough alone?

Thanks
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Old 22nd May 2008, 05:05 PM   #5
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Quote:
Should I try and get rid of this stuff - if so, any ideas on how and/or what must be preserved? Or should I just leave well enough alone
It looks like all 4 25L6's share the same cathode bias connection, which includes a light bulb. The light bulb offers a non linear resistance curve that helps prevent runaway. Removing or upsetting the bias in the 3rd 25L6 could upset things in the other two tubes.
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Old 23rd May 2008, 01:01 AM   #6
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The voltage gain pentode is not a 5873, as that type is a "flying" lead subminiature twin triode. OTOH, the 5879 is a small signal pentode. Look at the data sheet.

The 300 mA. series heater string limits flexibility. If the O/P tubes are replaced by 12L6s, and the 6SL7 replaced by a 12SL7, that portion of the signal topology could remain unchanged, while gaining flexibility from a parallel connected 12.6 VAC heater supply. Radio Shack Catalog # 273-1511 is a 3 A./12.6 VCT trafo that will supply all the heater power you could need. The UF86 is a nice 12.6 V. heater small signal pentode that would work well in the voltage gain position. UF86 data sheet here.

Some work is needed in replacing the light bulb in the 'L6 bias network with a resistor, but it's a worthwhile thing to do. All the extraneous (for guitar) stuff can be removed.
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Old 23rd May 2008, 01:37 PM   #7
atmars is offline atmars  United States
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Thanks for the info --

Quote:
The voltage gain pentode is not a 5873, as that type is a "flying" lead subminiature twin triode. OTOH, the 5879 is a small signal pentode.
This is correct. I didn't have the amp in front of me, only the hard to read schematic. The tube is indeed a 5879.

Quote:
Some work is needed in replacing the light bulb in the 'L6 bias network with a resistor, but it's a worthwhile thing to do.
Quote:
It looks like all 4 25L6's share the same cathode bias connection, which includes a light bulb. The light bulb offers a non linear resistance curve that helps prevent runaway.
Sorry to be dense, but where is the light bulb that is mentioned? I don't see it on the schematic (unless it is on the projector section, of which it don't possess the actual item). I know there is no light bulb on or in the actual chassis that I have in front of me.

Also, what is the function of the little transformer following the mic input at the top left?
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Old 24th May 2008, 12:58 AM   #8
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Sorry to be dense, but where is the light bulb that is mentioned?
Your not dense. Repeat examination of the schematic shows that the 5879 heater, not a light bulb is in the 25L6 bias circuitry. It's pretty obvious that the OEM design is highly cost conscious. Placing the 5879 heater in the power tube bias circuitry is a CHEAP way of energizing it with DC. DC on that tube's heater significantly lowers the amp's residual hum level.

Quote:
Also, what is the function of the little transformer following the mic input at the top left?
Isolating the mike from the AC mains and stepping the I/P voltage up are 2 obvious possibilities. Examine the phone jack mounting. I'd expect to find some sort of insulation between the jack and the metal chassis.
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Old 24th May 2008, 04:53 AM   #9
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Interesting thread -- I'll be watching, as I just got the very same amp off eBay earlier this week. I'll be getting an isolation transformer before attempting to plug it in.
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Old 24th May 2008, 06:00 PM   #10
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Guys,

Look at the O/P tube data sheet. If you place the voltages shown on the schematic onto the plate curves, the "idle" current per O/P tube is approx. 20 mA.

Ohm's Law tells us that 40 mA. passing thru 300 Ohms yields 12 V., which is close enough to the schematic. Bypass a 3 W. Vishay-Dale 300 Ohm wirewound resistor with a 150 muF./25 WVDC 'lytic and you can eliminate the sound head lamp oscillator. If you retain the 5879 voltage gain pentode, a different source of DC is needed, for its heater.

Notice the tertiary winding on the O/P trafo. That winding appears to be driving tone control circuitry in a NFB loop. If I was converting a pair of these "puppies" into HIFI monoblocks, the tertiary winding would "sit" between the PP pair and the RC bias network mentioned above. For guitar service, simply tieing those leads off seems right.
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