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Old 28th March 2011, 04:37 PM   #1
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Default 807 screen voltage help

Im building a 807 amp se triode connected and have run into some problems . My B+ is 435v and my anode voltage is 412v wich makes my screen voltge the same. After reading data sheets on the 807 its seems tha 415v on the screens is a litle too high. Is this the case? Is it fine so long as the screen grid dosent disibate more power that its specs at full output? In pentode mode what is the best way to adjust screen voltage?
Thanx!
Mario.
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Old 28th March 2011, 05:06 PM   #2
Svein_B is offline Svein_B  Norway
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It should work. This has been discussed here and at other "asylums" earlier.

The important parameter is screen dissipation. In triode mode the screen will never be above anode, and the high peak current is avoided.

I recon the screen current at 400V will be below 5 mA giving a screen dissipation well below the specified maximum 3.5W.

Another alternative is to use tetrode mode with local feedback (anode-grid) a'la Schade, and use a voltage divider to run the screen at 250V

Svein.

Last edited by Svein_B; 28th March 2011 at 05:32 PM. Reason: Corrected scrren current estimate
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Old 28th March 2011, 06:20 PM   #3
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Thanx Svein! I was thinking about a voltage devider for tetrode. I calculated the screen disibation in tetrode and it was 1.1 watts with over 400v on the screen so it seems ok. I think i will go with triode though it sounds the way I want it.
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Old 28th March 2011, 09:39 PM   #4
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Quote:
Originally Posted by mario13 View Post
Im building a 807 amp se triode connected and have run into some problems . My B+ is 435v and my anode voltage is 412v wich makes my screen voltge the same. After reading data sheets on the 807 its seems tha 415v on the screens is a litle too high. Is this the case?
That's a definite maybe. Hollow state is a helluvalot more "forgiving" than solid state, where the specs mean what they say, and straying out of the SOA guarantees poofage. With 807s, these have been operated at higher than spec screen voltages, either as pseudotriodes and ultralinear finals, with no apparent problems. Will you get away with it with all 807s? That's where the situation becomes problematic. Weaker 807s might not be able to stand up to higher than spec screen voltages.

Quote:
In pentode mode what is the best way to adjust screen voltage?
Thanx!
Mario.
If running as pentodes, definitely active screen voltage regulation. You can use either a hollow state or solid state voltage regulator. That gets the AC impedance down to minimize stray AC at the screens, and stiffening the screen supply makes for better linearity.
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Old 28th March 2011, 10:56 PM   #5
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Thanx Miles! I'm using NOS Zaerix tubes....don't really know their reputation.
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Old 29th March 2011, 05:07 PM   #6
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I have one Williamson amplifier, 1st I used EL34 6CA7, but after few years I modified it to use 807 instead of 6CA7, B2/G2 voltage is more than 400V, I use that amplifier in my office for 12 hours every day and I have same pairs of tubes for more than a year ... so be cool abt that if you have any NOS 807 ...


Williamson amplifier with 807 / EL34 / 6CA7

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