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Old 23rd May 2013, 03:36 AM   #1
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Default 6CB5 in triode PP?

I have 8 of these, and two 250V power transformers.

So, assuming ~325-350VDC, can the 6CB5 run in triode at this voltage? What would the ideal plate load be for a PPP amp using 4 of these.

Has anyone tested output power of a PP triode strapped 6CB5?

Thanks!

Blair
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Old 23rd May 2013, 04:00 AM   #2
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Quote:
Originally Posted by deicide67 View Post
I have 8 of these, and two 250V power transformers.

So, assuming ~325-350VDC, can the 6CB5 run in triode at this voltage? What would the ideal plate load be for a PPP amp using 4 of these.

Has anyone tested output power of a PP triode strapped 6CB5?

Thanks!

Blair
Due to the limited screen voltage rating, these make lousy pseudotriodes. Why would you want to? Run 'em as pents, with screen regulation, and you'll get more output from one pair than from four as pseudotriodes. These TV HD finals also have some excellent sonic performance as pents. Since these were made for pulling bigamps through horizontal deflection coils, you don't need much more than ~350Vdc anyway, the load resistance is a good deal lower than for most audio finals, making for easier to design and build OPTs with good high and low frequency performance.

Anyway, that's how I'd use 'em. I did a project using 6BQ6s instead since I had all the iron and a good deal of the other parts in the junk box already, so that kept the cost way down. A PP pair proved capable of doing 37W, and was more than enough to saturate this OPT that was rated for 30W (probably for PP Class A 6L6s -- it also included what look to be screen tertiary windings for UL operation, unused since 6BQ6s don't need any extra help from lNFB) and a 4K4 (P-2-P) primary.

If you insist on pseudotriode, you might want to do a search on "Tubelab". I think he's done some investigation on these as pseudotriodes. Depending on the type and condition, sometimes you can get away with busting the screen voltage spec. YMMV.
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Old 23rd May 2013, 04:06 AM   #3
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Thanks Miles!

Your posts are always insightful. Pentode is fine really. Just a toy to play with while I buy parts for my bigger amps.

Maybe 3.3-4K / 60W Edcors for a pair in pentode?
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Old 23rd May 2013, 04:06 AM   #4
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Don't forget each 6CB5 is going to require 2.5 amps of heater current.

jeff
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Old 23rd May 2013, 04:08 AM   #5
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No sweat. I have ~15A of 6.3V to okay with per transformer.
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Old 23rd May 2013, 04:12 AM   #6
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Holy cow, did you raid a big old organ amp or something?

jeff
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Old 23rd May 2013, 04:16 AM   #7
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Nah,

Two Bogen MO-200As I keep accumulating tubes and iron on whims.

ADHD I guess....LOL!
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Old 23rd May 2013, 04:40 AM   #8
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Wow, eight 8417's. That must have been interesting.

jeff
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Old 23rd May 2013, 04:47 AM   #9
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Quote:
Originally Posted by deicide67 View Post
Thanks Miles!

Your posts are always insightful. Pentode is fine really. Just a toy to play with while I buy parts for my bigger amps.

Maybe 3.3-4K / 60W Edcors for a pair in pentode?
Forch, I saved all the original notes. Loadline (attached) Do keep in mind that this particular project never got built, so it's very much a preliminary design.

With an Rl= 635R/phase, you'll need ~2.54K (P-2-P) A 3K3, 60W Edcor should be just fine here, and the somewhat lighter plate load will drop the output a bit, and maybe the distortion as well (though it doesn't look like you're gonna get very much of that anyway).

Even though these H3 estimates are rough, and high-side errors, they do correlate somewhat to sonic performance. However, in the final analysis, you still have to spend some time listening to the open loop performance to see how that distortion sounds, and what's likely to be necessary to correct for those sonic defects. If the 6CB5 is like the 6BQ6, all it'll need is enough gNFB to take the "edge" off. There shouldn't be any of that "pentode nastiness".
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Old 23rd May 2013, 06:58 AM   #10
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Hi!

Quote:
Originally Posted by deicide67 View Post
So, assuming ~325-350VDC, can the 6CB5 run in triode at this voltage? What would the ideal plate load be for a PPP amp using 4 of these.
Yes you can use them this way. I have built plenty 6CB5A amps like that and even at higher voltages. There are many articles about this tube, amps and circuits on my blog:

VinylSavor: 6CB5A

I mostly use them SE but I have built Class A PP amps with the 6CB5A too.
With for of them triode strapped in Class A you could get up to 25W. Much more in Class AB

Best regards

Thomas
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