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Old 11th May 2011, 03:25 AM   #1
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Default Video pentodes as outputs

Like the title says has anyone done any work using video pentodes like the 12GN7 as PP output tubes? Some tubes in that family are rated for 10 watts Pd so a pair should give some modest power. The part that seems odd to me is only needing a few volts of drive.
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Old 11th May 2011, 04:07 AM   #2
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The non-alignment of the g1 grid wires with the g2 grid wires makes for fairly substantial screen grid current compared with typical grid aligned beam/pentode output tubes. Output power efficiency gets lowered, and g2 overheating may be an issue. Note the 8417, which used a frame grid like the video tubes, also had reliability problems. Very close g1 spacing to the cathode for the high gm. Overheating makes grids warp. And any positive g1 grid current is likely to melt the tiny g1 framed grid wires. In a TV, the video amp has a nice stable signal, while an external sourced audio signal could overload the g1 and fry the tube.

Should make a great driver tube though, if run at reduced power (below rating) for safety.
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Old 11th May 2011, 05:28 AM   #3
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I am thinking seriously about paralleling of 5-10 tubes 6J52P in triode mode, for 20-50W of total dissipation. But I would not consider to run them on 10W per tube, it is too hard, though in specs. They are similar to 12GN7, and are specified for 10W dissipation as well, but used to live short lives in Soviet TV sets in such regimes.
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Old 11th May 2011, 06:11 AM   #4
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Originally Posted by astouffer View Post
Like the title says has anyone done any work using video pentodes like the 12GN7 as PP output tubes? Some tubes in that family are rated for 10 watts Pd so a pair should give some modest power. The part that seems odd to me is only needing a few volts of drive.
It is very odd. They made the same claim for the 6AG7 right there in the spec sheet: that it could be used as a Class A1 audio final producing 3W at 7% THD. They also give an rp= 130K. That's way, way, way too much plate resistance for an audio final. The Zo will be enormous. These won't sound good, but I suppose the manufacturer wanted to sell as many as possible, and TV manufacturers wanted to use as few types as possible.

Types like this have such large plate dissipation ratings since the purpose is to produce decent gains while keeping the Rp under 10K for wideband operation, and to slam current into CRT cathodes to charge up internal device capacitance at 4.0MHz rates.

You can find much more suitable audio finals in the same power class.
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Old 11th May 2011, 07:08 AM   #5
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Well, I would look at taking a studly compactron like the 6AG9 (has a mid-mu triode plus hi-gm 10W pentode), dropping a jfet under the triode for a high output impedance front end, and indulging in some feedback from the pentode plate to that of the triode. Hybrid Schade-spud, anyone?
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Old 11th May 2011, 11:36 AM   #6
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Rp is plate resistance, not load resistance. Any pentode, audio or video, will have high Rp. A video pentode has high gm and current, so can work into a low load resistance for wideband operation. It is true that a video pentode will give high Zout, but so will any audio pentode. That is why feedback is used, to reduce Zout.
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Old 11th May 2011, 11:44 AM   #7
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As long as we're on the subject of video pentodes....I've seen some mention of using them as 1st and 2nd stages of gain. Has anyone any experience in doing this? There is a schematic for a 6CL6 as a phase inverter floating around too.

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Ray
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Old 11th May 2011, 08:03 PM   #8
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Originally Posted by rsumperl View Post
As long as we're on the subject of video pentodes....I've seen some mention of using them as 1st and 2nd stages of gain. Has anyone any experience in doing this? There is a schematic for a 6CL6 as a phase inverter floating around too.

Thanks,
Ray
Yup...One thing for sure, the 12BY7 & 12GN7 configured as normal signal pentodes are unscreened and expect microphony. There is alot of "twang" when tapped. These tubes were designed for the mass TV market when large screen tube colour was fashionable before SS stole the show.. My favourite gape for the 12GN7, 12BY7 and host of others is configured as triodes for Williamson drivers with very respectable 25dB gain and is a very versatile tube. Configured as triodes, makes'em them far more usable.
For conventional use the low Vg-K makes it biassable using a couple of red series LED's as CCS's but drive these tubes at least 7-15mA anode current which have B/W into MHz. As the curves are pretty linear I use'm as the ultimate, cheap audio tube workhorses. Biassing read Morgan Jone Valve amps 3rd edit page 126,175.
With the fanstastic MHz b/w coupled with very high gm; beware to avoid lanky wiring and NOT using a CCS as the anode load as the tube can self oscillate.
Otherwise a darned good tube, like the 12BY7 which is a bit tamer but just as equally ideal. strangely, my GE versions of these tubes have same sized anodes and yet have different dissipation figures.
The 6CL6......this tube troubles me somewhat. Some vendors quote it as an EL84 substitute...NO!....it has a different spec...I find it flighty and snappy at anything approaching Vg-K cutoff and will oscillate when given half the chance. It's quoted also as a MiL RF pentode which it is.....probably due to it's robust construction. Good luck !

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Old 11th May 2011, 09:29 PM   #9
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Thanks for the information Richy! Have you experimented with the 5763's or 7551's and their 12 volt equivalents? I have lots of these tubes and would like to do something with them.

http://tubedata.milbert.com/sheets/049/7/7551.pdf
http://tubedata.milbert.com/sheets/049/5/5763.pdf



Thanks,
Ray
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Old 11th May 2011, 09:43 PM   #10
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5763 makes a fine Class C amplifier or frequency multiplier. In Class A it has a reputation for preferring oscillation to amplification.
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