[Patrick Turner]

On Jan 18, 7:44*am, "Alex" wrote:
"Ian Iveson" wrote in message

...

Is it useless knowledge I wonder. How many datasheets give anode mu for a
pentode? How many give both anode mu and screen mu?

Useless. Pentode mu is not usually given in datasheets and, unlike triode
mu, greatly depends on the operating point.

Muxh more practical would be discussing the following question:
How to estimate mug1g2 in a pentode if it is not given in a datasheet?

Someone alredady suggested finding a plate curve close to cutoff and then
mug1g2 = Vg2/Vg1.
And what if the curves are not given in a datasheet?
Then I use the following estimation:
mug1g2 = Vg2 / (|Vg1| + (Va / S)) .
The second term in the denominator virtually brings the tube to cutoff from
a specified class "A" operation point, then the propblem reduced to the
above principle. This estimation is very rough.

What would you do?

Later I might present similar derivations for screen mu defined as -- *
dVg2 / dVg1 @ dVa = 0; dVa = 0, as originally suggested by "Flipper".

That's not what he suggested, and what he suggested was wrong anyway, so
what's the point?

Not much point, but I found I forgot another condition which can
mathematically make another assumption redundant. This condition is the
follows. Suppressor grid does not steal electrons. So if plate current
increases while triode mu measurement, screen current has to reduce by the
same amount.

Thus: triode mu = dVg2 / dVg1 @ dIg2 + dIa = 0 and dIg2 = --dIa.
I will try to review calculations with the later condition. It will throw
another equation in a system of concurrent equations, and one of the
unknowns will be eliminated.

Alex

Hi All, I have come a little late to this thread but some time ago I
gave a lot of thought to making up formulas which one could apply to
the design of output stages using power pentodes or beam tetrodes
where there is cathode FB tertiary windings.

Nothing in RDH4 adequately discusses the issues about screen Gm in
regard to Ultralinear or Acoustical (CFB from OPT) operations.

I did post extensively a few months ago on my findings and with
formulas but mainly everyone ignored me, and the shots of information
went over everyone's head it seemed to me. So I ain't gonna re-post;
go search the archives for postings under my name.

I extensively tested 6550 to measure the Gm and Ra of both normal
tetrode operation with triode connection using G2 connected to anode
and with G1 taken to a fixed bias and using G2 as the control grid.

As everyone should know, the G1 Gm varies hugely with Ia and Ea so to
know how to design or understand a 6550, you must completely ignore
the data sheets where the G1 Gm and Ra is stated for Ea and Ia
conditions that nobody in their right mind would ever use.
Typically data sheets give G1 Gm = 11mA/V, and Ra = 18k, so this gives
µ = 198. This is with Ea at about 250V and Ia at 140mA. But modern
6550 samples may have lower Gm, and to find outabout YOU MUST MEASURE
the tubes at the exact operating conditions, and forget the data
sheets which at best are a vague guide only.

I found that with Ia = 50mA and Ea = 450V and Eg2 at about 300V, G1 Gm
5.5mA/V, and Ra was 33k, so µ = 181, showing yet again that µ is the
most constant parameter. G2 gm is simply measured with Eg2 at say
300V, but with no RL in the anode circuit except a 100 ohm resistor
and with G1 at 0V with cathode biasing. A signal is applied to G2, and
the anode current measured in the 100r and I got G2 Gm = approx 0.83mA/
V. Ra was found to be slightly lower than the Ra when the tube is in
beam tetrode, at about 30k I recall, so the µG2 becomes 0.00083 x
30,000 = 25 approx.

From this info one may draw up an equivalent circuit of the beam
tetrode (or pentode) with a current source producing current between
cathode and anode terminals = Vg1 x G1 Gm and with a shunt resistance
equal to Ra also between a and k.
There is also a current source of VG2 x G2Gm also strapped across a
and k but in beam tetrode mode the G2 voltage is constant so this
current source has infinite resistance.
But where you have some voltage from the anode fed back in Ulralinear
or triode or CFB then the screen grid current source becomes active
and the fed back voltage changes the total Ia outcome. I won't
elaborate by providing the formulas I have derived before. You can all
get busy and do it yourselves like I did, but I found that I could
predict what the Ra would be for UL, Triode, or CFB and what gain I
would get armed only with the measured G1 Gm and Ra which gave me beam/
pentode µ, and the triode µ when the tube was tested in triode for
gain with with a CCS load.

After all this investigation I concluded that the Acoustical
connection was the most effective way to set up any beam/pentode in an
OP stage to get reasonably good gain at least as high as using a 300B,
and to get Ra lower than triode connection and lower even than the Ra
of a 300B. Quad used 10% of the P winding as a CFB tertiary winding
but 15% to 20% is far better if you employ methods to keep the driver
stage distortion nice and low by never ever using Quad-II's gutless
wonder EF86 driver stage.

One could follow one's nose of basic logic and apply it to signal
pentodes after one has taken appropriate measurements and applied the
relevant equivalent modelling.

There was also a paper titled "Amplifiers and Superlatives" published
in Wireless World in about 1955 which described the operation of beam
and pentodes with varying amounts of FB from the anode winding or
simply by means of having a CFB winding with fixed Eg2, like Quad-II.
The paper is now online somewhere and can be googled. Unfortunately, I
defy anyone to be able to understand the maths because there is a
mystery factor "m" included in the accompanying maths, and it isn't
clearly explained or why it is there. So I derived my own formulas
which work AFAIAC.

I now rarely need to explore further; I just set up the tubes for
acoustical CFB and adjust the fixed Eg2 for lowest THD and that's
that.
When I first used CFB in a large 8585 amp in 1996 I had Ea only 400V,
and the screens were fed from taps from the anode coils, about 25% of
anode to cathode total voltage. So Eg2 = Ea, which is fine for where
Ea is low. Then I had 12.5% of FB in cathode winding coils and at 50W
in class A from 4 x 6550 I had THD = 0.5% without any global loop FB
and the amp sounded magnificent.
with some Eg2. Rout was much lower than using straight ordinary 37.5%
UL taps.
One only needs 10dB of GFB to reduce the Rout of the CFB of about 1.5
ohms down to 0.5 ohms. Distortion spectra is better than pure class A
triode.

Patrick Turner.