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Old 27th February 2009, 06:20 PM   #1
nhuwar is offline nhuwar  United States
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Default 811a rating question

I have been looking on spec sheets for a 811a trying to find the transconductance but haven't had any luck. Could someone point me in the right direction. I have tried TDSL but those spec sheets don't have it.

Any help would be greatly appreciated.


Nick
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Old 27th February 2009, 09:21 PM   #2
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I looked in all my industrial strength tube data sheet binders (GE, Wstgh. etc.) plus some old reference's and it's not mentioned anywhere. But we do know the amplification factor or mu () is 160.

Gm = /Rp Therefor, if you figure the plate resistance for the circuit parameters used, you can compute the Gm.
To use round numbers, an Rp of 10K ohms would give a Gm 16,000 micromhos.
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Old 27th February 2009, 10:07 PM   #3
nhuwar is offline nhuwar  United States
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Cool thanks, I need to remember that formula. I was thinking about using it as a pass tube in a regulator and now I'm pretty certain it will do a good job.

Thanks Hallow


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Old 27th February 2009, 10:15 PM   #4
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Quote:
Originally posted by nhuwar
I was thinking about using it as a pass tube in a regulator...
OMG, why would you want to use (waste?) such a nice tube in that service? Oh well, I just happen to like this tube and it's relatives the 812A, 809, 1623 & 8005.
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Old 27th February 2009, 10:43 PM   #5
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Quote:
OMG, why would you want to use (waste?) such a nice tube in that service? Oh well, I just happen to like this tube and it's relatives the 812A, 809, 1623 & 8005.
The Chinese 811a's are cheap and plentiful and handle 65 watt Pd intermittently. I would rather use them than a 6550 or 6L6.
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Old 27th February 2009, 11:17 PM   #6
nhuwar is offline nhuwar  United States
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Hollow state astouffer has it right I can get a matched quartet of Chinese 811a or jans for cheap.

Funny you mentioned the 812a, that bad boys on my too do list of amps to build. But as for the application there isn't many other tubes that are affordable that will handle the current.

Originally I was plaining on using as 4-400 as the pass tube but I would be too close to the max dissipation to not forced air cooling, plus the filament transformer. The transconductance on the 4-400a isn't that high either.

Trust me I don't hold the 811a in low regard, it's just it fit's the application too well to pass it up.

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Old 27th February 2009, 11:47 PM   #7
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Ah yes, I forgot about imported Chinese tubes. I never use or buy them so my head went right to NOS American stuff. Now I agree with you and astouffer.

Victor
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Old 28th February 2009, 12:38 AM   #8
nhuwar is offline nhuwar  United States
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So Hollowstate have you used the 812a in a amp? I want to use this tuber to make a killer push pull amp but don't know anyone who has used it.

Nick
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Old 28th February 2009, 04:15 PM   #9
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hey-Hey!!!,
811A will need high plate voltage or grid current to pass reasonable amounts of current. Tubes designed for pass element serivce have minimum plate resistance( and usually mu ). The error amp takes care of the low mu factor.
cheers,
Douglas
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Old 1st March 2009, 02:54 AM   #10
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Quote:
Originally posted by nhuwar
So Hollowstate have you used the 812a in a amp?
No Nick, I have not...not yet anyway. I have a good collection of 812s and a new pair of Electra Print 10K PP transformers that I got off eBay a couple of years ago. They are big and probably in the 50 to 60 watt range. In fact I have everything I need except time. Unfortunately earning a living gets in the way.
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