Re: Battery bias directly to grid



Andre Jute wrote:

> Patrick Turner wrote:
> > Why 4 x 220 ohm R in parallel?
> > Wouldn't a single 2k7 or 1k0 be a more appropriate grid stopper?
> >
> > Patrick Turner.
>
> It's just one of those things you have to know if you want to use the
> 417A/5842. There are four grid pins (thanks, Chris, that is indeed what
> I meant, blushing with shame at being caught out in such an obvious
> imprecision) and it is no good just clipping unused pins because the
> internal bit of wire from pin to grid is also an antenna. You have to
> get the resistor right up to the pin, and that is easier with four
> resistors in parallel to four pins than one resistor (somehow attached
> to all four the grid pins? without making extensions that will
> aggravate the problem?).
>
> Next thing, the value of the grid stopper. The most important bit of
> expertise you require with the 417A is that on its grid side you want
> to use only the lowest value resistance for any purpose whatsoever. A
> 50R gridstopper does the biz, so you want to stop there. (Heh-heh. What
> do they say about puns and weakness of mind?) I have in fact
> experimented with big stoppers but found no advantage. 50R 200R max is
> pretty well established practice in the microwatter community. The four
> by 220R I took a long time ago from Steve Bench, who has a lot of
> experience with the 417A on the test bench and is not bamboozled by
> audiophoolery.

Well that all seems clear.
The 417 is a hi gm triode and like other hi gM tubes I have tried these
could be RF unstable.
I have not actually tried the 417A.
But 6J6 and others tend to be a bit oscillatory.

One of the early limitations of triodes was their Miller capacitance.
Combined with long leads which are inductive enough to cause problems at
say 100MHz,
the C&L get together with the high gm and you have an oscillator when you
don't want it and didn't plan it.
Anyone using high gm j-fets such as 2sk147, 2sk369 in cascode with a triode
like a 6DJ8
or a triode strapped high gm pentode can also run into RF problems on a
phono front end.
Short leads are mandatory.
The "antenna" action of the extra grid connections may not actually be
antenna action.
oscillation is more likely to be caused by the queer behaviour at RF due to
stray C and L
at RF.
Adding resistance to the network of stray C&L is often enough to
stop phase shift occuring at RF.

Anything which can oscillate, does oscillate, unless something to stop it
is used.

Patrick Turner.




>
>
> Andre Jute

.

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