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WE91A 6C6
WE91A 6C6
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Old 3rd January 2019, 08:39 PM   #1
jdrouin is offline jdrouin  United States
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Default WE91A 6C6

I've got parts and some solid mahogany that have been asking to be made into something, and my imagination keeps returning to the idea of a "statement" WE91A using 6C6 as input (original to the 1938 model) and running the 300B at the WE-sanctioned op point of 400Vak/60mA into a 3K load (11.5W output). It would be respectful of the original yet kind of excessive all at the same time. The design goals would be for a retro artpiece (P2P wired, no silicone or chips) that features the characteristics I've come to like after breadboarding a stereo version of Joe Roberts' abridged 91A from Sound Practices (using 6SJ7).

I like the idea of an imposing appearance, with big transformers and an all-ST tube complement, and using the Hammond 378CX PT that I have (800VCT, 534mA, DCR= 21mA) for a dual mono build. That suggests the following in each channel: 5U4G, 0C3, 6C6, 300B, for a grand total of eight glorious ST tubes.

I’d need a B+ of 496V to pull it off. Playing with LTSpice and PSUDII, just to get a sense of ballpark voltages and feasibility, it can be done safely with a 5U4G rectifier (ST-shaped) in mono, but not when the power supply has the current draw from both channels.

Of course, PSUDII does not allow for modeling two rectifiers and one PT, and I'm not sure how to calculate that situation manually (I usually use the procedure in this article). The current draw from both channels would hit the PT secondary, but each rectifier would be carrying the current of only one channel, so I'm not sure how to calculate the behavior of the PT and what kinds of voltages it would put out. Does anyone know how that would work, or can you point me in the right direction?

I've attached the original 1938 schematic with 6C6, as well as screenshots of my quick-and-dirty LTSpice and PSUDII experiments.
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File Type: png we91a-6c6-beta-01.png (885.3 KB, 869 views)
File Type: png Screen Shot 2019-01-03 at 2.57.10 PM.png (79.5 KB, 908 views)
File Type: jpg Screen Shot 2019-01-03 at 3.37.14 PM.jpg (520.8 KB, 830 views)
File Type: png Screen Shot 2019-01-03 at 3.36.26 PM.png (347.5 KB, 801 views)

Last edited by jdrouin; 3rd January 2019 at 08:42 PM.
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Old 3rd January 2019, 09:33 PM   #2
Eli Duttman is offline Eli Duttman  United States
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Russian and Chinese 5U4G "equivalents" are highly problematic. Can you say arcing? OS U.S. made 5U4Gs are (IMO) too expensive. FWIW, my attitude is cosmetics be damned and the cylindrical bottle 5U4GB gets my nod. However, there is a pretty good chance that a Chinese or Russian ST bottle rectifier will be OK, into a choke I/P filter.

I don't read Hanji, so could easily be misconstruing things. That 4 μF. on the Chinese datasheet draws my eye. Compare that with the 40 μF. found on the Sylvania datasheet.

Perhaps the best solution to the rectifier issue is the UX4 based/ST bottle 5Z3, which is electrically equivalent to the 5U4G. RES shows a suspiciously low $11 price, which suggests their stock (if any) is the Chinese Octal tube. Definitely inquire, before ordering. AES shows a $24.90 price, for what is clearly the item in question.
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Last edited by Eli Duttman; 3rd January 2019 at 09:35 PM. Reason: added 40 μF.
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Old 4th January 2019, 12:00 AM   #3
PRR is offline PRR  United States
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Quote:
Originally Posted by jdrouin View Post
......]Of course, PSUDII does not allow for modeling two rectifiers and one PT, and I'm not sure how to calculate that situation manually .......
1) double the transformer resistance, sim for one channel

2) PSUD *does* have "multiple rectifiers" (at least the current version). Use "5U4-GX2" which is a double 5U4-G. (Not all bottles available in multiples, so #1 trick still handy.)

3) You can plot it out on the 5U4 curves. G.E. often has good info. Interestingly you *can* run one 5U4 for both channels-- as I thought, because it is in-sight of many happy amps I have known. To hit your goal you want a little more AC voltage, and more DCR......

There is not a "Maximum cap". Any practical size can be used IF there is sufficient winding or added resistance. The "40u" on the Sylvania is clearly "typical", not "Rated". They also give typical "effective plate supply impedance", but the G.E. sheet gives values for the various AC voltages. For 400VAC they suggest 52r; you should add a 33r to your transformer. (With dual rectifiers I would put 32r in each plate, to also tend to current-balance.)

Yes, reading Chinese might be interesting. However the *oldest* datasheets for 5U4 or its 4-pin father did cite 4uFd, because that was a "big" capacitor at the time. Note also the "2K ohms".... I wonder what that means?
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File Type: gif jdrouin-5U4-Plot.gif (39.5 KB, 92 views)
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Old 4th January 2019, 12:51 AM   #4
jdrouin is offline jdrouin  United States
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Thanks, Eli and PRR. 5U4GB is in my stash and an incredible workhorse, so that's totally possible. And it's what I used to breadboard the 91A last year as well as a number of other 300B circuits.

Using PRR's "first trick" of doubling the PT secondary DCR, using 5U4G, and keeping all else as if mono, I get 491V B+. So, I just need to find a way to squeeze ~4.5 more volts out of it.

Replacing the 4H/65R choke (Hammond 159S) with another 2.5H/43R choke (Hammond 159T), brings it to 493V but the ripple increases to 14.9mV.

Playing with the input cap value can bring it even closer to 496V with lower ripple. A 50uF input cap gets a ~494.6V B+ with ~4.5mV ripple, but that's higher than the "usual" 40uF input cap as per the Tung-Sol datasheet.

EDIT: I'd been using the mfr. published specs for the chokes in LTSpice. I just measured my two 159T chokes at 39R (not 43R), which raises the B+ in PSUD2 to just under 494V. So, who knows -- the real thing could be spot on (or further off) when breadboarded. The 378CX DCR of 21R was measured, btw.
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Last edited by jdrouin; 4th January 2019 at 01:15 AM.
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Old 4th January 2019, 03:26 AM   #5
jdrouin is offline jdrouin  United States
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Does anybody know what size grid cap to use for the 6C6? The datasheet says "small metal," and I measure the cap at about 3/8" diameter. Online retailers don't seem to use a common nomenclature and 6C6 is never listed as one of the "compatible" tubes in the lists.


I see 310A listed as using 9mm plate caps, which is just about 3/8".
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Old 4th January 2019, 05:31 AM   #6
Eli Duttman is offline Eli Duttman  United States
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The (IMO) easy way to squeeze some extra "height" out of the B+ rail is by employing 5AR4s. Up to 60 μF. as the 1st filter cap. is safe and the type exhibits the smallest forward drop among commonly used 5 VAC vacuum rectifiers.

OS 5AR4/GZ34 stock is costly. Affordable current production Sovtek 5AR4s are highly satisfactory, provided the series SS diode tweak is implemented. FWIW, I suggest using UF4007s, instead of the 1N4007s shown in the graphic. The idea is that vacuum rectifier noise suppression or not, less switching noise, from the outset, is better.
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Old 4th January 2019, 05:31 AM   #7
PRR is offline PRR  United States
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Quote:
Originally Posted by jdrouin View Post
...So, I just need to find a way to squeeze ~4.5 more volts out of it.....
Seriously? 1% shy is a problem??

Wall voltages vary +/-5% all day long, many places; worse in other places.

Hammond's good-old designs are wound for lower wall voltage than most outlets give today.

1% shy on voltage is <0.1dB reduction of maximum output which is quite insignificant. Especially on an amplifier which does not have a sharp "CLIP!".

Use two 5U4 for visual symmetry and because you have them (I don't think the few volt difference matters). Use 33r 2W in each plate feed.
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Old 4th January 2019, 06:48 AM   #8
Diabolical Artificer is offline Diabolical Artificer  United Kingdom
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This may be a bit controversial but why use a valve rectifier in the first place, surely SS rectification and a big cap is better for SE PSU?

Back to the schematic, why the decoupling on U1's grid resistor - C5? And also what is the purpose of R11/12 and C8 please?

Andy.
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Old 4th January 2019, 10:08 AM   #9
Ketje is offline Ketje  Belgium
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Quote:
Originally Posted by Diabolical Artificer View Post
Back to the schematic, why the decoupling on U1's grid resistor - C5? And also what is the purpose of R11/12 and C8 please?
Andy.
The schematic is a mess, like that no purpose at all
Cleaned it up a little, but not my idea
Mona
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Old 4th January 2019, 12:07 PM   #10
Diabolical Artificer is offline Diabolical Artificer  United Kingdom
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Thinking about it, I suppose R11/12 are bleeder resistors, C8 there to couple AC artifacts to ground.

Andy.
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