Re: Question about grid behavior with triodes


"Wessel Dirksen" <wdirksen@xxxxxxxxx> wrote in message news:1191138439.933007.272280@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
: On Sep 30, 5:07 am, Patrick Turner <i...@xxxxxxxxxxxxxxxxxx> 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.
: >
: > Patrick Turner.
:
: Thanks for the input everyone. I would like an opinion about the
: interstage connection for my 3 stage amp using 6CG7's and KT88's all
: in triode:
:
: The first 6CG7 stage is a CCS unbypassed grounded cathode stage
: running at about 7mA with -1.9V bias. With a 2VRMS max input. The grid
: leak resistor is 1M. I calculate the Zin at 100K, and Zout at 12K.

I assume you have a 100 K input pot, with a 1M from wiper to ground
feeding V1's grid, then, Zi is between 100K and about 90K, depending
on the wiper setting in that case. Zo is the dynamic plate resistance,
from the published 6CG7 curves, at -2V/7mA 'wiggle' the plate voltage
-/+ 10 V, you get -1, + 1.2 mA, so Rpdyn is 10/1.1 or 9K there. so
Zo=9 K. Your cathode R is 270 Ohm. With a 300K load, the loadline
drawn in that graph is just about horizontal, so a 4Vtt net input would
give 85 Vtt output, Av=21. 85Vtt/300K *270 Ohm gives some 85 mVtt
cathode degeneration, nfb, still leaving your gain at about 21 :-)

Rudy

: It has a gain of around 17. This is DC coupled to the next stage at
: about 100V.
:
: The second 6CG7 stage is a positive PS long tail phase splitter and
: driver for the power tubes. The Rl's are adjusted for balance. The
: total current through Rk is about 17.5mA. with about -6V bias. Zin is
: 300K. Zout is 5K. The gain I calculate to be about 6. This is cap
: coupled to the power tubes which are cathode biased now at about 70mA.
:
: This sounds good to about 1/2 power. Then it sounds a wee bit thin,
: not bad per se. So I'm thinking. The first stage would be +/- OK then.
: But would there be alot of grid current driving the second stage? 1V x
: 17= 17V RMS with a tube biased at -6V? Have I created a monster? If so
: what are my options?
:
: Thanks so much,
:
: Wessel
:

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