Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dwinstonwood on March 21, 2022, 01:57:34 pm
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Hi all,
I'm all confused about how to wire a 9-pin socket for 6.3V in parallel. Specifically, how I would wire up a socket for a 6922. The Valve Wizard states that the 6922 has the 9AJ pin-out (as opposed to the 9A for 12A_7 tubes). But, I can't figure it out from his diagram. In his diagram it looks like pins 4, 5, and 9 are all wired together for 9AJ. :w2:
I'd really appreciate a simple drawing showing how the two wires are connected, and what is done with pin 9.
Thanks!
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IAW RCA the 6922 can only be wired for 6.3v operation. Connect one heater lead to pin 4 and the other heater lead to pin 5. PIN 9 IS NOT PART OF THE HEATER! Pin 9 is an internal shield between the two triodes. Show us a link to the Wizard's diagram, or if from a book, tell us which one and which page number.
http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/6922.PDF
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Do you need the 9A/9AJ switch? Then why look at that?
6V goes to pins 4 and 5. Just like a 6V6 takes 6V on pins 2 and 7, or a pilot light takes it on tip and shell.
Pin 9 is an internal shield because these tube will sing 100MHz and leak signal from one triode to the other. We would normally ground pin 9. I'm not sure why he connects it to a live heater pin. Maybe to simplify the switching.
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Thanks sluckey and PRR!
I was wrongly assuming the pin 9 was part of the heater elements. Thanks to both of you for pointing out that it's an internal shield. So, 9 should be grounded... kind of the same way that we ground pins 2 and 7 - the internal shields - in an EF86.
Here is the document. I should have included it above: http://www.valvewizard.co.uk/heater.html
I have a meager stash of tubes on hand. But, I have some Philips JAN 6922's; some GE 5654W's; and some EL84's. I'm trying to think about what I can do with them.
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Here is the document. I should have included it above: http://www.valvewizard.co.uk/heater.html
Read the writing. Disregard that pic.
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Thank you.
So, here is another question I'm embarrassed to ask.
I have never been completely clear on how to interpret the numbers in datasheets like this one:
http://www.tubebooks.org/tubedata/HB-3/Receiving-Type_Industrial_Tubes/6922.PDF
It lists "Characteristics," which has two columns of numbers (?); "Maximum ratings," and "Design Center Values."
If I was to design a conservative preamp stage using this tube, Which column of numbers should I use to guide my choice of voltages and bias resistor, etc.?
Thanks! And, I hope there's a somewhat basic answer to this question! :icon_biggrin:
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This datasheet appears to have a table showing various plate voltages, plate load resistors, and cathode resistors: https://www.tubeampdoctor.com/media/pdf/24/19/71/tad-datenblatt_rt-6922_190912_version_1_0.pdf
I suppose I could go by those examples.
Or, make an attempt to draw load lines myself.
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... It lists "Characteristics," which has two columns of numbers (?); "Maximum ratings," and "Design Center Values."
If I was to design a conservative preamp stage using this tube, Which column of numbers should I use to guide my choice of voltages ...
The Maximum Ratings are but a single column. However, that data sheet is not super-friendly.
The "Design Center" system second-guessed tube variation, and also wall voltage variation (in the old days). It was pretty conservative and left a large safety margin.
When you see other data sheets for the same tube with the "Design Maximum" system, the manufacturer no longer leaves extra margin for wall voltage variation. It's up to the user/builder to verify the tube won't exceed any rating under conditions that might occur. So the exact same tube shows higher rated limits, though generally the tube did not become more-capable.
"Absolute Maximum" ratings are just that: don't ever exceed it, no matter what happens. No manufacturer publishes the true limit, so the tube won't die 1 volt over the rated absolute max, but no one is sending you replacement tubes when they do give out.
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The ECC88 data sheet (https://frank.pocnet.net/sheets/030/e/ECC88.pdf) is more helpful, having curves, etc. That's what you will need if you have to pick values for a design. The "Characteristics" in the RCA data sheet probably won't be too useful for you, as you probably won't be idling at 12-15mA per triode.
The 6DJ8/ECC88 is intentionally an Odd Duck. If you intend on building with it, it is wise to copy a known-good plan from another amp. That's because typical 6DJ8/ECC88 circuits are not easy for a newcomer to design.
This datasheet appears to have a table showing various plate voltages, plate load resistors, and cathode resistors: https://www.tubeampdoctor.com/media/pdf/24/19/71/tad-datenblatt_rt-6922_190912_version_1_0.pdf
I suppose I could go by those examples.
Yes, but almost no amp design using 6DJ8 simply uses a single-triode like they assume in their chart. Cascode is more typical, and you're best copying a known-good plan there.
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> I suppose I could go by those examples.
Was good enough for Leo Fender.
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Thanks a bunch HBP and PRR.
I probably won't actually try to build anything with those tubes, it's more of a learning exercise just to try my hand at picking a tube not usually seen in classic guitar amps (probably for good reason) and attempting to put together a working circuit around it by being able to read a datasheet and draw load lines. It's good to know that the 6922 isn't an easy tube for beginners to work with.
But, I am learning.
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probably won't actually try to build anything with those tubes
since you're learn'n, maybe build something for those tubes??
Bread-board up a single tube, hook cool test equipment up, take measurements.
the lightbulb moments might come easier with breadboard data over calculators, datasheets, and crayons
although, once the lights start twinkling, you'll still need calculators, datasheets and crayons :icon_biggrin:
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Thanks for the nudge shooter. I just bought a Hammond 270EX PT from Hawk for $75.80 plus shipping.
https://www.hammfg.com/files/parts/pdf/270EX.pdf
This PT is a good choice for my EL84's using a 5Y3 for just over 300V, or with 6V6's and a solid-state rectifier for around 390V.
I guess I'll start experimenting.
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That was pretty quick. I ordered it from Hawk around 11pm Tuesday night. It got here Friday morning. 3 days.
I have a spare Hammond 1750 20W 6,600 OPT and an extra 194A choke. Maybe it's time to put together a breadboard.