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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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8th Jan 2021, 7:29 pm | #1 |
Dekatron
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QQV03-20A double beam tetrodes.
I've been mucking about with some CV2799's (QQV03-20A) I bought years ago after testing them. AFAIK,they're NOS, they're not operating as expected. For those not familiar with these valves they are two beam tetrodes in one glass envelope/bottle they share a common screen grid and a cathode but have separate anodes and control grids.
I have four, I tested them as per the Avo valve tester manual, Va 400v, Vg2 200v, Vg1 -17v - they should have an emission of Ik 20mA - they should have an emission of Ik 20mA, anode voltage applied to one anode at a time, Ik taken off a 1r cathode resistor, which I know isn't Ia but it should be within spitting distance. I got these results - V1 14.3 & 14.3mA V2 8.3 & 8.5mA V3 5.3 & 5.2mA V4 8 & 8.8mA. All under spec, so I got to wondering if I should have been applying anode voltage to both anodes at a time, so did this on V3 & V4 and got odd results, IE 5.7mA & 10mA consecutively. First off how are these valves operated in class B PP RF amps? Are they run in parallel one CV2799 per side or more likely one valve per PP stage? Also looking at the datasheet an a - a Zpri of 10k or 20k is given for a AF power amplifier & class B modulation stage, it's not mentioned what class the AF is, one would presume AB2, a Zpri of 11k and 20k is given, which doesn't make sense as that would put the load line under the 0v grid line; or am I missing something? Does this refer to the modulator? Lastly would an RF amp use an OPT or just a choke? There's not much info out there on these valves, I know they're not intended for AF amplifiers, I can't find any linear amps, trancievers or transmitters schematics for these valves to give me a rough idea of their application. I'm just mucking about at present to see if they'd may make a half decent AB2 amp. not sure if I'll actually build it though, when I bought these they were common and cheap, not so now so much,don't want to use valves that would be put to better use keeping RF going. Thoughts welcome. Andy.
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8th Jan 2021, 7:35 pm | #2 |
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Re: QQV03-20A double beam tetrodes.
Normally used in push-pull class-C at VHF.
Either handling a constant power FM signal, or else being used with high-level AM applied by modulating the HT. Some amateur 'linear' designs used this family for linear amplification of SSB signals, but they usually cut out the middle man and went straight for the QQV06-40. David
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8th Jan 2021, 7:47 pm | #3 |
Dekatron
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Re: QQV03-20A double beam tetrodes.
Yes they're designed so that the two halves are driven in push-pull Class-C.
Usually some significant -ve bias on the grids, enough to achieve cutoff so no anode current flows until drive is applied. They - or their bigger 06/40A brothers - _have_ been used as audio-amps: I vaguely recall seeing one as the modulator in an old trawler-band 'Fishfone' [Woodsons?] driving a TT21 as the RF amplifier. Whatever, they like plenty of -ve bias so the static anode-current is kept low. Class B2 perhaps? They also like high anode-voltages (most designed-for-Class-C-RF-service valves do). |
8th Jan 2021, 7:56 pm | #4 |
Nonode
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Re: QQV03-20A double beam tetrodes.
With a common screen connection applied voltage, surely they need Va applied to both anodes as well to avoid damage?
Rob
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8th Jan 2021, 8:46 pm | #5 |
Dekatron
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Re: QQV03-20A double beam tetrodes.
As valves designed to give good results at VHF and low UHF, they'll eagerly pursue the slightest excuse to misbehave and go into spurious oscillation if the test set-up provides the opportunity, giving misleading results. They might effectively be grid-rectifying and biassing themselves back, or something like that. Reach for the ferrite beads?
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8th Jan 2021, 8:58 pm | #6 | |
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Re: QQV03-20A double beam tetrodes.
Quote:
Lawrence. |
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8th Jan 2021, 8:59 pm | #7 |
Heptode
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Re: QQV03-20A double beam tetrodes.
They were common for vhf transmitter stages in the past - attached is a scan from my old rsgb manual of about 1968...
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8th Jan 2021, 9:02 pm | #8 |
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Re: QQV03-20A double beam tetrodes.
Quite frisky!
They were designed to give plenty of gain at high VHF and getting into UHF. Even if your aim is audio, you need to use good UHF construction practices, and RF grade decoupling... only then do you hit them with beads. David
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9th Jan 2021, 4:19 am | #9 |
Dekatron
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Re: QQV03-20A double beam tetrodes.
I've still got my old Pye Vanguard in the garage. It's just a hulk really, as visible leakage from the many electrolytics, and a mass of ageing composite resistors, meant that all the boards would have needed complete rebuilding.
However, when I stripped it down, I did stop short of removing the PA assembly (03-20), the inverter components, the mod transformer and the associated transistors, fancifully retaining the idea that I might one day "re-purpose it". The chassis offers a lot of space to stuff other circuitry in there. Certainly, for anyone thinking of experimenting with an 03-20 for any purpose, acquiring an old Vanguard hulk would be a good trick if you could find one. B
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9th Jan 2021, 8:54 am | #10 |
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Re: QQV03-20A double beam tetrodes.
Thanks all, had trouble reading those Keith any chance you could send me better resolution scans if I send you my EMA please?. I'll do some more experimenting today and see if I can get em running without sillyness
.It occurred to me last night my 1r "sense" resistors might well be 0.1r, hence low readings, will check today. Still puzzled why Ik doesn't double if both anodes connected, they were only tested very briefly with one anode unpowered, I'm thinking though that Avo testers didn't have provision to test two valves at a time, so only one anode would be tested at a time. Don't know. Lots still unclear about these valves, they're so way out, odd-ish base, odd anode connections, odd envelope, ehmm. Andy.
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9th Jan 2021, 9:20 am | #11 |
Heptode
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Re: QQV03-20A double beam tetrodes.
Sure just PM me, the scans aren't perfect (used an ancient HP 5500) but the originals are higher resolution than the thumbnails.
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9th Jan 2021, 12:14 pm | #12 |
Nonode
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Re: QQV03-20A double beam tetrodes.
Andy, what valve tester are you using ? AVO V/T's & their V/D Manual's codes & settings are designed to give an Ia for each half of a QQV valve(or any double amplifying valve). Not until "TEST" is selected do they apply Va & Vs. Or more accurately V1's Ia then, switch off Test, & select V2, then back to Test to read V2's Ia. Do the same for mA/V. Ik doesn't come into book's spec's of Ia & Gm for valves. As some valves, O/P's for instance, draw several mA of screen current when running at the recommended -ve Vg. However, for accurate graphing or curve tracing of Gm's, folk sometimes show Is. Ik is a summation of Ia & Is, and higher up the curve, just using Ik, one gets an inaccurate Anode related mA/V.
In P - P situations, one requires pretty close Ia's & Gm's for each half-valve. That's why some folk go the extra mile & obtain curves, over & above just a basic "valve test" on an AVO, Mullard or Taylor, etc. Regards, David |
9th Jan 2021, 12:45 pm | #13 |
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Re: QQV03-20A double beam tetrodes.
Andy, common cathode, common screen so for individual Ia you can only get a meaningful result by measuring Ia, not by measuring Ik.
Lawrence. |
10th Jan 2021, 10:01 am | #14 | |
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Re: QQV03-20A double beam tetrodes.
No tester David,just wired upon the bench, thanks for that clarification.
Quote:
Was sorting out my test lash up yesterday, didn't get chance to test further, shortening interconnect wiring and knocking up two cathode followers for driving purposes,hopefully will get chance today. Andy.
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11th Jan 2021, 12:26 pm | #15 |
Nonode
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Re: QQV03-20A double beam tetrodes.
Me thinks, Andy, (can't remember if I've nagged you before), you're an ideal candidate for building your own valve tester. I've banged on enough in the past regarding a decent DC tester could be built for under £200 with 2nd hand &/or re-purposed parts sourced from one's storage boxes/shelves & redundant test equipment.
Regards, David |
11th Jan 2021, 3:02 pm | #16 |
Dekatron
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Re: QQV03-20A double beam tetrodes.
As noted upthread - these are 'frisky' valves - think of them like highly-strung thoroughbred racehorses rather than the more-usual carthorse EL34/6L6 stuff - a lashup, without grid/screeb/anode-stoppers and decent grounding is likely to turn into a VHF oscillator as soon as you turn your back.
Ferrite beads on the control-grid leads along with [ideally wire-wound] stopper-resistors; something like a 200Ohm carbon resistor overwound with enamelled-copper wire to make a 'lossy choke' connected directly in the feed to each anode, ground the cathode directly and use ceramic or silver-mica for the screen-decoupling capacitor. Even though you're only intending to use it at audio frequency, you are still designing/building an RF amplifier! |
11th Jan 2021, 5:05 pm | #17 | |
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Re: QQV03-20A double beam tetrodes.
Quote:
B
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14th Jan 2021, 8:42 am | #18 | |||
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Re: QQV03-20A double beam tetrodes.
Quote:
Quote:
Quote:
Andy.
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14th Jan 2021, 10:06 am | #19 |
Dekatron
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Re: QQV03-20A double beam tetrodes.
Forgot to mention, in this schematic of a QQV03-20A AF amplifier - http://www.r-type.org/articles/art-138.htm you'll notice he's used dissimilar grid stoppers, 2k2 and 33k, anyone have an idea why?
On fig 1 no Vht is indicated,but it is on fig 2 for the QQV07-50 version at 600v, so am puzzled by the use of 20k Zpri OPT,my reading was the lower the HT, the lower Zpri and vice versa. This is especially puzzling as the OP valves are paralleled indicating a ra of 40k?? Andy.
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14th Jan 2021, 2:20 pm | #20 |
Octode
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Re: QQV03-20A double beam tetrodes.
This is only a guess.
Would the different value resistors in the grids to each valve let them see a different impedance to any unwanted RF oscillations and so tend to dampen each other out, if both grids saw the same impedance they would work together in supporting it to oscillate. I would suggest that the 3-20 is 250 volts HT as both that and the other circuit has identical biasing for the first two stages. Adrian |