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Old 21st February 2004, 05:15 PM   #1
exurbia is offline exurbia  Australia
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Question 811A in P.S.E or P/P

I have just come across a box of RCA 811A's . Seeing as i have so many can i put them to use in multiples ? I've also got a few DA42 triodes if anyone is interested.
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Old 21st February 2004, 11:00 PM   #2
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Hi,

Quote:
Seeing as i have so many can i put them to use in multiples ?
Technically, yes, that's possible.
But why would you want to unless perhaps you intend to run them in PP class A ?

Both are quite impressive bottles, beware of the 1kV B+ voltages.

Cheers,
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Old 22nd February 2004, 03:58 AM   #3
billr is offline billr  New Zealand
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if you are interested i have quite a few diagrams with 811A's in se mode.

they are tricky sods to drive, generally in A2, bias about +25, voltage swing about 40v on the grid, can't remember the anode current and voltage at the moment, somewhere inthe 600v region with aboiut 60~70ma, as i say, cannot remember exactly.

they were also used in class B amps, whith interstage tranformers, as they are transmitting triodes, they need power in the grid. there is an ALtec cinema amp that uses a pair of these things with 866 mercury vapour rectifier. pushes out about 250 watts, eek. high ht tho, 1000v+

I will be playing with an 807 cathode follower into the 811. don't need much gain from the first stage, approx 50 or so, so i will probably use a 6dj8 or the old 6sn7 in an srpp, need to have feedback on this amp, o/p z is high, damping will be low, also distortion will be higher than desired, the curves are quite 'gappy' if you get my drift.

pleased to be of help.
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Old 22nd February 2004, 04:36 AM   #4
exurbia is offline exurbia  Australia
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Hi Frank,

Thanks for the idea of PP class A. Do you have any suggestions as to a suitable cicuit. HT supplies are no problem, my 845's are running 1250. I think i will get rid of the DA42's as they appear to be a bit hard to work with.

Thanks,
Peter
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Old 22nd February 2004, 04:49 AM   #5
exurbia is offline exurbia  Australia
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Perhaps i should have said.
" I wonder how many 811A's it would take in s.e to produce 200 watts " That is if it is not too hard in regard to power supplies and contribution to global warming
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Old 22nd February 2004, 09:16 AM   #6
exurbia is offline exurbia  Australia
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Bill,

This sounds really interesting, I also have plenty of 807's, 12e1 and 11e3. do you know if it is possible to drive the 811 in class A push pull using any of these as drivers? If you are on the lookout for high power Tx valves just ask, I may have what you want.
The idea of PSE is really intriguing, I think it would make for a visually stimulating amp. Three valves per chassis could never be mistaken for anything but SE.
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Old 22nd February 2004, 09:46 AM   #7
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Generally any triode operating in Class A2, normally avoided in audio, will draw grid current to around 25% of the plate current.

Since at full on you'll be pulling around 150mA through the plate, this means almost 40mA. At maybe 40V positive on the grid at full throttle (I don't have the curves at hand), this is a lowish input impedance around 1K. A bridging drive ratio of 10 to 1 is wise, and probably lower, since when the signal is at maximum negative excursion, the grid is negative wrt cathode and NO grid current flows at all, so impedance at this point rises to hundreds of kilohms.

The abrupt transition from positive to zero grid current means that source impedance from the driver stage needs to be very low, on the order of perhaps 50R. Higher source impedances in the driver will cause considerable H2 and H3 as it is weighed down by grid current at the transition point around zero volts bias.

Effectively, this requires an audio amplifier to drive the grid. In Class A1 triodes, grids should be shaken, not stirred; in Class A2 the grid should be grasped with homicidal intent. This makes it very difficult, and forces you to use a large, direct coupled cathode follower OR an IT transformer with very considerable step down. Both solutions are expensive.

Something you might consider is the original RF drive mode; cathode drive. Ground the grid, and drive the cathode with a very low impedance drive. This can be a tube, but many designers use SS, a mosfet being very suitable. A very real advantage is that the drive impedancea required, although low, is much more even, with no transitions, and the power required to drive the cathode appears in the plate circuit and adds to output power. Have a gander at Bob Danielak's site, where he explores just about all possibilities: http://www.geocities.com/TimesSquare...music_etc.html

Cheers,

Hugh
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Old 22nd February 2004, 09:16 PM   #8
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Hi,

Quote:
Thanks for the idea of PP class A. Do you have any suggestions as to a suitable cicuit.
Not all that many around but that datasheet shows one for class B operation already.
Not that I'd recommend it but it gives you a good idea on what it takes to operate this kind of bottle.

811A in Class A2.

811A SE

Cheers,
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Old 22nd February 2004, 11:37 PM   #9
billr is offline billr  New Zealand
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as someone else in this thread has said, as the 811a is a transmitting triode you need to put power into the grid, the diagams that i have seen use a 6F something, I plan to use a 807 as i have plenty lying around, and i just love those anode/plate caps.

The grid current is around 10ma for the application that I am using, from the curves, the grid current can get up to 40ma, thing is that is at 1200 volts or so, I don't plan to run the valve at that level, basically because you don't get a useful load line.

hope that this helps

kind regards
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