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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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15th Jan 2021, 7:51 am | #21 | ||
Dekatron
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Re: QQV03-20A double beam tetrodes.
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Andy.
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15th Jan 2021, 8:18 am | #22 |
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Re: QQV03-20A double beam tetrodes.
I'd be quite wary of something like the QQV03-20 in an ultra-linear connection. those valves are designed around the assumption of especially well decoupled g2 supplies. They're part way to the group of RF tetrodes which are called high-perveance. To find more on those you'll need to hunt down a copy of "The care and feeding of power grid tubes" by Eimac inc.
Valves in this class frequently exhibit negative resistance g2 characteristics at RF, can self rectify and manifest as positive g2 current coming OUT of the bottle. Regulated screen supplies have had to use shunt regulators to dump the current. THe QQV03-20 bunch aren't quite as bad tempered as the 4CX250B, but these things can get into trouble with just a short bit of wire, nevermind an LF transformer. The 4CX250B was so cantankerous that they made a series of special valveholders for with ceramic decouplers integrated into the ceramic insulation, and made them in a couple of different values to suit different frequency ranges of operation. In general, care is needed with UHF tetrodes. They are not as tame as KT66s and EL84s. David
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15th Jan 2021, 5:08 pm | #23 |
Dekatron
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Re: QQV03-20A double beam tetrodes.
Yes, given the two halves of the 3-20 and 6-40 have a common screen-grid connection you can't run the two halves in 'ultra-linear' push-pull. Good stabilisation of the screen-supply is necessary to get the most out of these valves [glow-regulator valves are good here, since they can help 'sink' current of the screen suddenly turns into a current-source at some part of the cycle].
The Eimac "Care and feeding" notes are still available here: https://www.cpii.com/library.cfm/1#22 CPII are the successor company to Varian/Eimac - it's great that they still keep their 1960s tech notes available online! |
15th Jan 2021, 6:40 pm | #24 |
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Re: QQV03-20A double beam tetrodes.
The ultra-linear circuit, some posts back has one strapped together QQV03-20 on each side of the push-pull see-saw, just to get around the common g2.
I can see a time when almost all valves have been sucked into the valve audio business and only the diodes are left.... David
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15th Jan 2021, 7:11 pm | #25 | |
Octode
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Re: QQV03-20A double beam tetrodes.
Quote:
If you have the valves and want to do something with them why not, it could be a fun build that gives good audio. Andy if you have not already thought of it, may I suggest mounting the valves so the cathode is level with the metal chassis, this would mean sinking the holders underneath the chassis. It could help keep the sproggies at bay. Adrian |
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16th Jan 2021, 7:17 am | #26 | |
Dekatron
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Re: QQV03-20A double beam tetrodes.
To be clear I wasn't using them in UL, with 420v HT that's out. I was aware of the negative resistance phenomenon, I remember it being pointed out before.
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Back to the amp, experiment didn't work, OPT not suited, but the valves behaved like lamb's, not a peep, despite no screen grid stoppers and a long length of wire connecting the drivers to the control grids. *I'm pulling your leg David, I know diodes strapped together won't make a transistor, you have to drill a hole through the valve and insert a bit of tin foil to get a diode to act as a triode. Leg pulling over if you search online for audio amplifiers that use "exotic" valves, you'll struggle to find any or a very small number of actual builds. Theres one chap on DIYaudio - Tubelab, who's put a lot of work into using TV line output valves, Pete Millet uses the odd video pentode as drivers, but that's about it. It takes a lot of work and time to iron out all the wrinkles and often you end up with poor results, why bother when you can use an EL34 or whatever, all the data is there as is lots of info and examples of successful builds using bog standard audio OP tooobs. In this case I have a box of odd valves like the QQV03-20A/CV2799, 6BG6GA & 6146 I bought years ago before I knew better and thought I'd have a play. Further to this I swopped out the CV2799's and replaced them with 6146's; they sang like larks in aspic, mainly down to poor layout. I've built an AB2 driver section, the distance from cathode follower driver to OP section is too great, hence the oscillation. I'll plug on experimenting but for now the CV2799's go back in the box, if anyone has a need for them for their intended use drop me a line, your diodes and other non audio valves are in no danger from me. Andy.
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16th Jan 2021, 2:11 pm | #27 |
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Re: QQV03-20A double beam tetrodes.
I did once build myself an amplifier with TV line output valves - an awful lot of them, it was transformerless. The total heater power would have run a class-A thing like a Krell of the same audio output power capability. Daft!
David
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16th Jan 2021, 6:50 pm | #28 |
Dekatron
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Re: QQV03-20A double beam tetrodes.
TV line-output valves have been extensively used as audio-amplifiers and also as AM modulators in amateur transmitters, and linear-amplifiers for SSB, where they work rather well [SSB linear-amplifiers and line-output-stages share a common interest in being able to handle high peak-currents - some of the later US line-output valves were rated for a peak cathode-current of 1.4 Amps! ].
Not just amateurs who've used line-output-valves [personally I prefer the US term "Sweep Tubes"] - the stage-amplification for the Woodstock Festival used loads of such valves, as did the amps used by the Grateful Dead's "Wall of Sound" - see https://audiophilestyle.com/ca/bits-...onic-duo-r828/ To me, that provides perfect 'heritage' justification for using line-output valves in audio amps! |
18th Jan 2021, 4:12 pm | #29 |
Octode
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Re: QQV03-20A double beam tetrodes.
I guess that Andy's posts about the QQV03-20 may have come to an end, but thought this may be of general interest to any that like playing with unusual valves. It is a .jp website and I see posts from the person from time to time on fb as new valves are bought/tried.
http://nerd.jp/tubeamp/ Someone who seems to enjoy playing with oddities to the audio world, mainly as pre-amps. Adrian |
19th Jan 2021, 8:02 am | #30 |
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Re: QQV03-20A double beam tetrodes.
Thanks for that link Adrian.
A
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