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 Post subject: 811A Triodes
PostPosted: 22 Oct 2018, 15:07 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi Everyone, I was looking though some types of tubes and ran across the 811A. Do they really have an amplification factor of 160? I would have guessed 16 maybe 20 or so. Also has anyone used them as SETs at modest voltages say 400-500. If so what current and load? I might come up with some sort of amp using them as I can get some at a reasonable cost. Same for the 812A. It does have a low amplification factor about like I would expect.

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Bruce

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 Post subject: Re: 811A Triodes
PostPosted: 22 Oct 2018, 15:41 
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A very quick answer. Got in touch with them back in mid 90's but was advised not to use them. Anyway there were several version with various gain and other data. I think, but I am not sure that the SV572 may have been close equivalents too.

Here's something I found among my 811-files. It gives a few leads.

Attachment:
811Aiter1.jpg


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 Post subject: Re: 811A Triodes
PostPosted: 22 Oct 2018, 16:14 
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I have a pair of brand new output transformers as specified in that schematic. If you're interested let me know.

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 Post subject: Re: 811A Triodes
PostPosted: 23 Oct 2018, 02:06 
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The "problem" is the need for grid current. In the simple schematic above, a 6V6 is used as driver, but I have designs with the use of EL34/6CA7 or 6EM8 or, more frequently, the use of transformers. I did spot a design where a 2A3 is used as driver.
Anode voltage is not too suicidal, but power is pretty low. I read from some diagrams that 8-10W, which is what a KT88-SE could give. But a KT88 isn't a DHT ...

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 Post subject: Re: 811A Triodes
PostPosted: 23 Oct 2018, 17:31 
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Hi, 8 watts is not so bad, I only need about 1/2 that now. I have to ponder if I want to play with these. I can get pretty much any B+ from 100 to 1000 without an issue although I prefer to stay in the sub 500 ranges for cost reasons. Any thoughts on how these guys sound?

Good listening
Bruce

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 Post subject: Re: 811A Triodes
PostPosted: 23 Oct 2018, 18:34 
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I did a design for a 211A RF triode which is relatively similar to this one. It was an SE design putting out about 20W peak power into the output transformer. Call it about 18W at the speaker. Only complicating factor was the 1350V B+ power supply for the output stage; needs to be a relatively carefully designed beast. Used a 6SL7 driver with about 220 Vp.

I suspect the same could be done for this tube as well. Although I suspect the 211 would be better behaved.

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 Post subject: Re: 811A Triodes
PostPosted: 24 Oct 2018, 02:16 
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gofar99 wrote:
Hi, 8 watts is not so bad, I only need about 1/2 that now. I have to ponder if I want to play with these. I can get pretty much any B+ from 100 to 1000 without an issue although I prefer to stay in the sub 500 ranges for cost reasons. Any thoughts on how these guys sound?


The 811 is a triode that won't kill as many cats as an 211 or 845 will do. And using anything from the shelf - 6L6, KT88 ... will result in the required 5-10W. So I guess the 811 has a factor of coolness that is pretty high.

I have just restarted a project, using triodes 5998A, a sibling to 6AS7G, but with a higher gain. On my shelf is a two-tube amp based on this dual triode, but I will aim for an amp with two tubes and the two triodes in each tube are paralleled. That way I will end up at 7-8W - triode power! :up:

When it comes to 811 I am so confused as there seems to be (or have been) a number of versions -A, -B, -3, -10 all with various gain, and add to theat Svetlanas 572.

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 Post subject: Re: 811A Triodes
PostPosted: 25 Oct 2018, 21:57 
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I finally remembered what led me to choosing the 211A as an RF DHT power tube candidate over the 811 for a SET design. The 811 has a massive Rp. (>15K). In a SET configuration it requires either a massive output transformer of significant negative feedback (or likely both) to achieve reasonable low end performance.

The Rp of the 211A was 3250Ω at my design point. Which is still relatively large by SET standards, but something tolerable with most transformers available today.

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 Post subject: Re: 811A Triodes
PostPosted: 12 Nov 2018, 18:39 
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Here is a driver circuit I am using with my 211 amp. It could more than likely drive a 811A, but the output transformer is the real challenge as previously said.

The 126B will drive 100VRMS. This is about it's limit, but excellent in my opinion.


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