811a SET amp.

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Nick
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#271 Re: 811a SET amp.

Post by Nick »

What I mean (and didn’t say clearly) was a simple CCS would probably do just as well as a cascaded one.
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#272 Re: 811a SET amp.

Post by pre65 »

According to the data sheet for a 76 max current is 5ma.
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#273 Re: 811a SET amp.

Post by Nick »

pre65 wrote: Fri Nov 17, 2017 1:49 pm According to the data sheet for a 76 max current is 5ma.
I can't find that limit on any data sheet I have seen. Though TBH, 5ma looks like a good op point. Maybe a little less to preserve the thing a bit more.

and I dont think a 76 needs a grid stopper. Its a relatively docile valve.
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#274 Re: 811a SET amp.

Post by pre65 »

Nick wrote: Fri Nov 17, 2017 2:04 pm

Though TBH, 5ma looks like a good op point. Maybe a little less to preserve the thing a bit more.

and I dont think a 76 needs a grid stopper. Its a relatively docile valve.
I'll try a 680R instead of a 600R as the 10M45s setting resistor.

Thanks for all the useful tips, much appreciated.
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#275 Re: 811a SET amp.

Post by IslandPink »

I used one as the first stage of the Aurora amps as advised by Lynn, at something near 10ma where it sounded very pure & clean driving the phase-splitter transformer. I must have had something less than 150v on the anode, as the dissipation limit is 1.4W according to the data sheet.

"What I mean (and didn’t say clearly) was a simple CCS would probably do just as well as a cascaded one"
I assume you meant cascode ? :D
Wasn't the aim of the cascode was to avoid it sounding shite, by reducing the shunt capacitance ?
Maybe that isn't so important for this sort of device ?
I still remember the graph with all the different CCS's and tests data from Gary Pimm.
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#276 Re: 811a SET amp.

Post by pre65 »

The coupling cap is listed at 0.22uf @ 400v.

I've nothing apart from a World designs SAX (from Clarity cap ?) and I assume that there are better ones available.

Any suggestions ? (as long as not silly expensive :lol: )
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#277 Re: 811a SET amp.

Post by IslandPink »

Did I ever mention that I really like Mundorf Zn caps ? :D ( tinfoil/polyprop )
https://www.hificollective.co.uk/compon ... ap_zn.html
https://www.hificollective.co.uk/catalo ... 8_168.html
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#278 Re: 811a SET amp.

Post by pre65 »

IslandPink wrote: Sun Nov 19, 2017 12:43 pm Did I ever mention that I really like Mundorf Zn caps ? :D ( tinfoil/polyprop )
https://www.hificollective.co.uk/compon ... ap_zn.html
https://www.hificollective.co.uk/catalo ... 8_168.html
You may have done Mark. :lol:

Hardly expensive either. :D
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#279 Re: 811a SET amp.

Post by pre65 »

As said before, grid bias like this circuit is new to me.

The circuit diagram states the bias as +24v. Does this voltage need to be variable ?
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#280 Re: 811a SET amp.

Post by Cressy Snr »

Yes it does.
You adjust your bias in terms of its current.
Lets say the bias requirement was 100mA, you'd twiddle your bias voltage adjuster until you got 100mV across the 1ohm sense resistor you see at the bottom of the filament on the cct diagram.

That current may or may not correspond to +24V at the grid. It depends on the tolerance of the valve itself.
The 1ohm sense resistor needs to be a close tolerance part.
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#281 Re: 811a SET amp.

Post by pre65 »

Thanks Steve, that all makes good sense. :)
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#282 Re: 811a SET amp.

Post by Nick »

The 1ohm sense resistor needs to be a close tolerance part.
Why? 1% should be more than good enough I would have thought. Unless you have a calibrated meter you are not going to know for certain much more accurately that that, and I wouldn't think you would want to be that close to the dissipation limit of your valve.
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#283 Re: 811a SET amp.

Post by Cressy Snr »

1% resistors ARE close tolerance to me.

Come on Nick! Do I have to be so godamned precise with everything.
I was just trying to help Phil.

Might be best all round if I just STFU.

Sorry Phil.
Last edited by Cressy Snr on Tue Nov 21, 2017 6:58 pm, edited 2 times in total.
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#284 Re: 811a SET amp.

Post by IslandPink »

Have you been buying a lot of bulk-foils recently, Nick ?
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#285 Re: 811a SET amp.

Post by Nick »

IslandPink wrote: Tue Nov 21, 2017 6:50 pm Have you been buying a lot of bulk-foils recently, Nick ?
No but:

1. The lowest tolerance I can find on Rapid (first place I looked) was 5%, and that was for 2W types, the standard metal film is 1%, and that’s basically what I would expect everyone to use by default.

2. for me (and I would bet just about anyone who does this stuff, close tolerance would make them think of 0.1%, not 1%

3. I don’t see why even 5% would be a problem, again, I would not expect you would want to run that close to the valve limit

4. Whatever.
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