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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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2nd Jul 2017, 3:02 pm | #1 |
Pentode
Join Date: Jul 2015
Location: Newbury, Berkshire, UK.
Posts: 243
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Audio amp on CV2799 ?
During my search for unmarked tetrode I found a nice looking CV2799.
Some people claim the amps built with them sound well. I am planning to build my own valve amp, and probably would use 2 valves (= 4 tetrodes) per channel. So my question is : Has anybody enough experience to comment the idea ? For help with design and development I would give 4 CV2799 FOC. Thank you, Jacek |
2nd Jul 2017, 3:35 pm | #2 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
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Re: Audio amp on CV2799 ?
Better known as the QQV03-20: http://www.r-type.org/exhib/aai0249.htm
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2nd Jul 2017, 6:29 pm | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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Re: Audio amp on CV2799 ?
I'm aware of at least one commercial AM transmitter design that used a 3-20 as the modulator and its big brother the QQV06-40a as the low-band-VHF output stage.
It seems a real shame to use these VHF/UHF valves in mundane audio applications. |
2nd Jul 2017, 8:04 pm | #5 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: Audio amp on CV2799 ?
You can't do UL P-P with only one- the common screens preclude this.
You could use two in parallel P-P. The linked circuit is SE parallel arrangement.
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2nd Jul 2017, 8:43 pm | #6 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
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Re: Audio amp on CV2799 ?
Shhh... don't give them ideas!
We need ideas for discouraging them. David
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2nd Jul 2017, 10:56 pm | #7 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: Audio amp on CV2799 ?
They'll probably be EXTREMELY difficult to tame due to wanting to take off at VHF as intended......
OK, David? Much better idea (?) 20 EF80s or EF91s in p-p parallel. Series heaters on 120Vand about 200V HT should do it
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....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O Last edited by Herald1360; 2nd Jul 2017 at 11:04 pm. |
3rd Jul 2017, 6:06 am | #8 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
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Re: Audio amp on CV2799 ?
Mr Carlson over on his patreon coarse made a simple amp using these.
I've had a play with them too, they work well. Andy.
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3rd Jul 2017, 11:11 am | #9 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: Audio amp on CV2799 ?
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3rd Jul 2017, 11:15 am | #10 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: Audio amp on CV2799 ?
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3rd Jul 2017, 11:19 am | #11 |
Rest in Peace
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
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Re: Audio amp on CV2799 ?
Should anyone need QQV03-20 valves, please let me know via a P.M. I have many: some used; some brand new & unused. Regretfully, no corresponding valve sockets. Ditto QQV03-10. (But they are B9A).
Al. |
3rd Jul 2017, 1:53 pm | #12 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Re: Audio amp on CV2799 ?
Quote:
It's presumably because of its interesting looks, rather than any inherent merit in this application, which just can't be native to the valve. However, there is likely to be a following who absolutely believe that it sounds fantastic in an audio amp. Everyone is of course welcome to believe what they wish, but it does sound a bit Emperor's new clothes syndrome. The circuit posted up there earlier has two in parallel, but with noted problems in audio amp service related to uneven current sharing within the envelope of a single valve (it has a split control grid and two anodes) As I say, good-looking valve, even striking, so at that level it's unsurprising that someone might like a pair to adorn the polished surface of an audio amp's chassis.
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Al Last edited by Al (astral highway); 3rd Jul 2017 at 2:02 pm. |
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3rd Jul 2017, 2:04 pm | #13 |
Banned
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
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Re: Audio amp on CV2799 ?
I have sold loads of QQV03-10s and a few 20s for little money, had them for ages and nobody was very interested till I put " be good in an audio amp" in the sales spiel, then they went.
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3rd Jul 2017, 2:08 pm | #14 |
Pentode
Join Date: Jul 2015
Location: Newbury, Berkshire, UK.
Posts: 243
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Re: Audio amp on CV2799 ?
I was planning to buy 9 of them for 13.5 (you know wherE) , mainly due to its interesting look and size. My investigation confirms what Astral_Highway said. In my opinion there is a lot of them in far East, at a good price and these people are trying to find a way to utilize the valves in homemade equipment that can be sold well.
Thank you, Jacek |
3rd Jul 2017, 2:23 pm | #15 |
Guest
Posts: n/a
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Re: Audio amp on CV2799 ?
And to quote Flanders and Swann (from a song of reproduction)...
"The ear can't hear as high as that Still I ought to please any passing bat" |
3rd Jul 2017, 3:40 pm | #16 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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Re: Audio amp on CV2799 ?
It is only specified for +ve grid current and I just wonder how linear it is with -ve grid current. It's also only 2.5mA/V so it will need an extra stage of amplification.
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3rd Jul 2017, 3:54 pm | #17 |
Pentode
Join Date: Jul 2015
Location: Newbury, Berkshire, UK.
Posts: 243
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Re: Audio amp on CV2799 ?
Correct !
Valve circuit is like combining single lenses. No one is linear, every one has aberrations, chroma, distortions, but when combined properly we are getting Zeiss. |
3rd Jul 2017, 6:13 pm | #18 | |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: Audio amp on CV2799 ?
Quote:
-ve grid current? Don't think you'd get much of that. g-k is a diode. With 300V Va and 200V Vg2, you'll need about -14V to keep Pa down to 10W. With 2.5mA/V everything looks OK for class A1. Individual g1 bias supplies might be needed to balance up the standing currents or you could downrate the total dissipation a bit.
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....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O Last edited by Herald1360; 3rd Jul 2017 at 6:26 pm. |
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3rd Jul 2017, 6:25 pm | #19 | |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
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Re: Audio amp on CV2799 ?
Quote:
As they have worked their way through stocks of real audio types the prices have obeyed free-market laws and are now so high that they are now looking for anything thermionic that can possibly be used to amplify audio. It seems that any old stock valve of any type must be preferable to anything new or (gulp) any transistor. It's a bit like tigers and rhinoceros facing extinction because of strong beliefs in folk-medicine. With powdered rhino horn becoming too expensive other species are coming under threat. Perhaps people ought to learn how to design good amplifiers using current devices? Perhaps they already have. But this is fashion, and fashion can be terribly illogical. I'd rather see horns on rhinoceros, tusks on elephants and old valves in period equipment. David
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3rd Jul 2017, 6:57 pm | #20 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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Re: Audio amp on CV2799 ?
The 3-20 and 6-40 are well known for their use in Class AB1/AB2 RF SSB linear service up to a couple of hundred MHz, as a trawl through the various RSGB handbooks of the 1950s and 1960s will reveal.
To get best linearity you need a *regulated* screen-supply [250V] and a low-impedance grid-bias schematic that can deal with having to sink a few milliamps of rectified grid-current without the voltage changing that much. One alternative is to put a big Zener [or a small-Zener with an aiding transistor] in the cathode connection so the cathode sits at a constant voltage [27V comes to mind] above ground. Decouple it well, and add grid/anode-stoppers everywhere! These valves like high anode-voltages for optimum linearity: 750V on the anodes is fine, I've seen designs using up to 1KV! |