[flipper]

On Wed, 27 Jun 2012 17:30:35 -0700 (PDT), Patrick Turner
wrote:

On Jun 27, 12:15*pm, flipper wrote:
On Tue, 26 Jun 2012 15:36:48 -0500, John Byrns
wrote:

In article ,
flipper wrote:

snip.

I'm also going to try the DC pentode because, like you said, it seems
to me that ought to work about the same.

Actually that wasn't what I was trying to suggest, that was either a "typo" on
my part or I was having a "senior moment". *What I was actually talking about
was a circuit using a heptode such as a 6BE6, 6CS6, or 6BY6, I'm not sure how
the term "dual control pentode" managed to escape my keyboard.

You're quite right. You said the "6SA7/6BE6/6SC6 family of heptodes"
and it was I who 'converted' it to dual control heptodes because
'total cathode current appears to be largely independent of the
voltage on G3' on those too.

An interesting point about your 6ME8 transmitter is that it appears to be a
vacuum state analog to the solid state designs using a three transistor "cell",
which seem to be the designs of choice with many old radio fans. *In this
circuit two of the transistors are connected as a long tailed differential pair
with RF drive applied to the base of one transistor and with the modulated RF
output taken from one of the collectors. The third transistor operates as a
current source in the emitter "tail" circuit of the first two transistors, with
audio applied to its base to modulate the RF output of the differential pair.

Yes, I know the topology and was 'that close' to building one but am
not sure which was the cart and which the horse here. I originally
started thinking of the 6ME8 back when Patrick was tinkering with
Gilbert Cells because I noticed a 'similarity' between it and the 3
transistor 'cell', of which the Gilbert has two on top a current
source.


Yeah, I was going to say the talk about 3 transistors was actually
about a Gilbert Cell. I couldn't spend another 3 months perfecting
what I did with 3 transistors because I always have other things to
do.

I thought you were trying to build a Gilbert cell, back then. That
takes more than 3 transistors.

Using a gilbert cell chip probably would have worked better then
me trying to fiddle with discretes. But could a Gilbert cell be made
using 3 triodes?

Well, no, because a Gilbert Cell isn't '3 transistors'.

For the '3 transistor' cell I think pentodes might be more appropriate
because the bipolar collector current vs collector-emitter voltage
looks just like a pentode. Although, I'm not sure what the effect of
screen current mixed in might be. Or a single 6ME8 because that's
essentially how it works with the deflector plates acting as the two
upper transistors and G1 the bottom.

That's why I used a 6ME8: it's 'conceptually pure' with equivalent
'ins and outs'. As used, however, one plate is simply tied to B+ so,
if screen current and plate current 'see saw' in a heptode, which the
curves indicate is the case, then it might work similarly. I.E. what
goes into the 'unused' second plate externally connected to B+ is
'internally attached' screen current in the heptode. At least it seems
to be, in theory, but I didn't know what 'subtleties' might crop up
and, especially when doing something 'new', like to cut down the
unknowns as much as possible.


Another tube with strong g3 action is the 7 pin 6DT6 commonly used for
quadrature FM detection. Another one I was intersted in was "gated
beam pentode" 6BN6, which some said never worked well as 6DT6. Maybe
these have apps at BC band F.
Patrick Turner.

Gee, I don't know about the 6BN6. That thing is a gated beam
discriminator made for FM demod and has a 'limiter' built in, which is
sort of the antithesis of AM. Of course, one doesn't have to use it
'as intended' but it's beyond me.