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Old 30th November 2005, 07:29 PM   #61
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Default Hi-frequency AC filament supply

Quote:
Originally posted by tubelab.com
I have taken a different route, AC heating using 50 KHz filament current. I did some experimenting with this about a year ago and I can tell you that the filament power must be a clean sine wave if this is going to work. Otherwise you will get all sorts of intermodulation products that cloud up the FFT scope, and the sound.

In the old Hell & Bowel 16mm filmsound projectors that I used to run in high school there were three 6V6's on the amplifier chassis. Two perform the P-P audio output function while the third tube is used as a HF power oscillator to provide energizing current for the optical sound systems' exciter lamp. It was never clear to me why they didn't just use pure filtered DC for this light bulb. The reason for this care and trouble is that any intensity modulation would be amplified as an audio signal. This power oscillator ran at a pretty high frequency (several hundred KHz?) as I recall the transformer was an air cored coil reminiscent of an oscillator coil in an old console type tube radio.

This idea would not be hard to implement in a DIY amp employing DHTubes. One could wind the filament xfmer easily DIY and one power oscillator could handle all the filaments, each getting it's own isolated winding as needed.

BTW, these amps make wonderful practice/jamming amplifiers for guitar and harps. Just go in from the Mic input on the front control panel. I had one complete working projector some twenty years ago and used it as an electric guitar amp. It got left behind during a major long distance move back to civilization from the wilderness of the Western Arctic with appreciative musician friends where there aren't many things that will work as a guitar amp. I found a complete 16mm projector at a thrift store in Buffalo about 4 years ago and scopped it for five bucks remembering how sweet that little amp in there is.
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Old 30th November 2005, 09:54 PM   #62
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I used a toroid core for the transformer, and a solid state push pull amplifier to drive it. That was driven with an HP audio oscillator. The problem comes from regulating the gain to keep the filament voltage constant, without letting the amplifier clip. Each 811A requires about 25 watts of filament power. The resistance of the filament changes drastically as the tube warms up. Most bipolar transistors tend to melt at this power and frequency. Mosfets work fine.
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Old 1st December 2005, 04:22 AM   #63
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Quote:
Originally posted by tubelab.com
I used a toroid core for the transformer, and a solid state push pull amplifier to drive it. That was driven with an HP audio oscillator. The problem comes from regulating the gain to keep the filament voltage constant, without letting the amplifier clip. Each 811A requires about 25 watts of filament power. The resistance of the filament changes drastically as the tube warms up. Most bipolar transistors tend to melt at this power and frequency. Mosfets work fine.

I would be taking the vacuum tube power oscillator route I think if I were to experiment in this direction. It would be an excellent opportunity to add more impressive looking tubes to a chassis and would keep the purity issue of sand-free better served for anyone to whom that might be a philosophical or religious issue. The low cold resistance of the driven filaments in the load would not be a problem for a tube based power supply. True, a mosfet based HF filament supply would result in a higher efficiency amplifier, but efficiency is apparently not a concern in tube audio hi-fi. If it were, it would almost not exist.
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Old 1st December 2005, 02:58 PM   #64
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When I have the time to explore this further I will probably resume the sand state experiments, since I have that engineering mentality about choosing the best component for the job.

For those that want to follow the pure tube path, we need to find the schematic for the old Tektronix 504 scope. It used a power supply that derived all of the operating voltages off of a power oscillator operating at an ultrasonic frequency. If my fuzzy memory is correct it used a 6DQ6A. It's been 25 years since I worked on these.

Anyone out there got the diagram?
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Old 17th February 2006, 03:27 AM   #65
BOYTUBE is offline BOYTUBE  Indonesia
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Hi....
I am New member
we have same problem, because I need Schema Pushpull 811A


regards
BOYTUBE
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Old 29th August 2007, 10:56 PM   #66
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Default 811 Amp

Hello,
I was looking for an 811 SET circuit to build and it looks like I've found one. Thank you Astouffer.
I also saw the second amp with the transistor (nasty things) in the circuit. But with my heavy towards thermionic emission (an age thing),that one is not for me.

Has anyone got an update or mods to the circuit since 2005?
I noticed some chat about pentodes and bias.... any comments on that??

I did see the "Ganti 811" amp but not keen on the transformer couplings between each stage.

I'm reading that the Svetlana 811 valve is the one to go
for. So I'm looking for a pair if anyone has a vendor.

Thank you for any help.

John
Aug-29-07
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Old 30th August 2008, 12:57 AM   #67
Mike L. is offline Mike L.  United States
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I have a couple of 811A's and have been anxious to build something with them. I have a copy of the schematic attached to post #5 of this thread but am having difficulty understanding several things.

If some kind soul would help me, what is the device labeled E202? Also, what voltage/wattage are the zener's?

I just noticed something else, it looks like there is a 100 ohm resistor attached to the paralleled 33 ohm resistors on the cathode of the 811A, but then there is an arrow with 200 ohm/3W in parentheses. Is this a 200 ohm potentiometer?

And finally, the 2SC3783 transistor is hard to source in the U.S.. Can anyone recommend a suitable substitute?

Thanks.
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Old 30th August 2008, 03:58 AM   #68
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Quote:
what is the device labeled E202? Also, what voltage/wattage are the zener's?
E202 looks like a constant current source diode. The zeners are a good question. HZ36L is a valid part and the datasheet says 400mw at 36 and 24 volts. http://www.datasheetcatalog.com/data.../6/HZ36L.shtml

Quote:
I just noticed something else, it looks like there is a 100 ohm resistor attached to the paralleled 33 ohm resistors on the cathode of the 811A, but then there is an arrow with 200 ohm/3W in parentheses. Is this a 200 ohm potentiometer?
Its probably two 100 ohm 3 watt resistors wired in series for 200 ohms at 6 watts.

That transistor regulator isn't really necessary. You could get by with a suitable dropping resistor and bypass cap. If you want to give it a try MCM electronics lists the 2SC3783 as in stock for $5.25
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Old 1st September 2008, 03:18 AM   #69
Mike L. is offline Mike L.  United States
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Thanks very much for the info. Would you have any idea what the appropriate current level from E202 would be?
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Old 1st September 2008, 05:46 AM   #70
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www.datasheetarchive.com is your friend
Attached Files
File Type: pdf datasheet-1.pdf (99.2 KB, 69 views)
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