[ARC5] BC-375 Re-Tube Project- Scratch Three

[Ian Wilson]

The 'gain' of a triode isn't a particularly simple quantity, since gm,
rp and mu are all interrelated.

How well a particular triode functions in a particular circuit will
depend both on the tube and the circuit.

The 211 is a pretty low mu device (12) with moderate gm (2.75mA/V
in A1 service* with 750V on the plate and -46V on the grid). I would
not expect an 811 to do particularly well in a circuit designed for a
211, since it has enormously higher mu (160) and will work at zero
bias.

An 812 might be a better fit with its mu of 29. I can't find any A1
figures to compare, but in Class B it wants -40V in the grid at
1250V on the plate, so this is not perhaps out of the ballpark.

I was expecting 812A's to be much cheaper on eBay than 811A's
but it's all over the place. So there must, I suppose, be some
audio amplifier that uses 812's.

* Note: I am not suggesting that the 211 is really operating in A1
when used as an oscillator. But seeing the gm/rp figures is often
helpful when looking for subs.

73, ian K3IMW





On Mon, Nov 18, 2013 at 10:22 AM, David Stinson <arc5 at ix.netcom.com> wrote:

>
> ----- Original Message ----- From: "Kenneth G. Gordon" <
> kgordon2006 at frontier.com>
>
>
>  David: Have you yet tried connecting the screen to the GRID (G1). As I
>> said
>> earlier, this results in a HIGH-MU triode, which SHOULD work much better
>> as an oscillator.
>>
>
> Did indeed.  No joy.  In fact, my experience would indicate
> that such a connection actually lowers tube gain.
> Even tried tying all three of the grids of the 837 as "G1."
> While it worked better than connecting just the screen
> to G1, it was still unusable.
> I may end-up being forced to go to the next $$ tier and
> try 811 and 572B.   813 and 814 will not physically fit.
> 805, 810, 838, 845 etc. are just as expensive as the 211
> so are not options.
> I'm convinced there's an answer.   Just gotta keep trying ;-)
>
> 73 Dave S.
>
>
>
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