Re: Question about grid behavior with triodes

On Sep 30, 7:38 am, Patrick Turner <i...@xxxxxxxxxxxxxxxxxx> wrote:
Eiron wrote:

Patrick Turner wrote:

Eiron wrote:
Patrick Turner wrote:
Wessel Dirksen wrote:
Hello RAT friends,

......snip...

So putting 2V rms on the grid of a triode with a fixed bias of 2.4v
may not be such a good idea. Here's a circuit to experiment with -
http://members.lycos.co.uk/fiultra/BatteryT68bis.jpg

--
Eiron.

2Vrms = +/-2.8pkV, and grid current would flow in a tube with fixed bias
= -2.4V.

However, as its done in the above fiultra preamp stage, is OK because
the gain attenuater is before the
417A and probably quite low level output would be wanted, and therefore
applied input voltage
probably will be below 0.2Vrms. The bias of 2.4V suits having a healthy
huge 20mA bias current in the 417A
which is supposed to sound very well.

Yes, we all know that you wouldn't turn the volume control up enough
to put a positive voltage on the grid but Andre's circuit is still
wrong. It simply needs a resistor at the top of the volume control
or 'Input 2Vrms' correcting.

There isn't a single website or book which has no mistakes, or which has
what
looks like a schematic which doesn't work properly.

Andre was trying to illustrate a biasing method using a battery,

Absolutely. It is part of a chapter in The KISS Amp devoted to
discussion of places *not* to put the battery! Anyone who cares can
read "Using batteries for negative grid bias" at
http://members.lycos.co.uk/fiultra/KISS%20126%20by%20Andre%20Jute.htm
and will also see there a schematic showing batteries in every
possible position, all on the same drawing that is clearly not
intended as a circuit to be built. I don't know where this little man
Eiron thinks he's going with his nasty little nitpicks.

and
with no
actual proposed use in mind shown on the schematic. Perhaps accompanying
text says different.

It doesn't.

Dummies could be led astray, if they saw the schematic and slavishly
built it for a preamp and stupidly
insisted it have 2.4Vrms input applied. What on earth would one do with
the huge output voltage?

As an aside, a diversion: If only there were a high-mu tube that would
handle 24mA like the 417A *and* the full output of a good CD player
(and still be as linear as the 417A), you could use the resulting
"huge output voltage" to drive an 845, God's own tube, without
bothering about Dr Miller's social disease. It's a sort of two-tube
pipedream for those of us who like transmitting tubes. A major part of
the reason for using the 211 in A2 rather than the superior 845 in A1
is that the 645 drive requirement is so much more onerous. A suitable
"huge output voltage" driver would solve that problem instantly.

Providing the source resistance was low, 2.4Vrms could be applied; but
as I said, full volume would NEVER
be used, lets assume 2.4Vrms covers what comes from a CD player.

Sure, when I show a complete circuit (rather than a discussion
document such as Eiron searched days for on the 10k pages of my
netsite), it is always designed for a named speaker and intended to be
used with a QUAD 66/67 CD player, so I show 2Vrms input available. If
a dummy wants to feed all of that to his tubes, maybe he should also
pick another hobby suitable for deaf people.

Perhaps Eiron doesn't understand the difference between DC and AC
conditions in an amp.

Andre's schematic isn't wrong, and you appear to be a nit picker.

Ah, I see, you read Eiron the same way I do.

The added resistor isn't needed. Put it there if it makes you happy
though,
such things are OK to use.

Spend the money, buy the correct DACT ladder attenuator I specify
where I use similar circuits for real, and leave your own
miscegenations off my circuits, Eiron. With the correct components I
specify where I use this circuit for real, the resistor you're trying
to add will feature big in the sonic signature. And that handful of
caps you want to add, stick them the same place as the superfluous
resistor.

Patrick Turner.

Eiron.

I've been straightening out Eiron on electronics and cycling for years
now. He is a slow learner.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review


.

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