> Tom and others, I was always under the impression that the 30L1 grid
> circuit was developed primarily to obtain decent efficiency on the higher
> frequencies.
At Heath and at Ameritron, Orr applied pressure to use the 220 pF
caps from the grid to ground.
The reason does not really matter, the fact the caps are there
is the only important thing. So while you and I may not have the same
view of the reasoning behind the floating grids, the analysis of
what they do remains the same.
> The internal construction of the 811A is such that the grid
> and its lead to the socket has a fair amount of XL at say 28MHz.
Simple enough to measure with my network analyzer. I connected the
source to the anode and used a 1500 ohm impedance, and the
filament to the detector (set at 50 ohms). By grounding the
grid and measuring the transmission loss through the tube,
you can get a good idea of the places where the grid become
ineffective at shielding the anode from the cathode. The larger the
transmission loss value (given in dB) the better the shielding. If
tube gain exceeds loss and feedback is correct, the stage will
oscillate.
For a Svetlana 811A in this order
frequency / no cap ( 220 pF cap short leads)
3 / 51.8 (46.29)
15 / 39.87 (31.65)
30 / 26.88 (25.64)
60 / 26.35 (17.53)
90 / 22.90 (13.65)
120 / 18.96 (9.47)
150 / 15.32 (5.79)
180 / 11.11 (2.02) above was smooth change, here is start of ripples
262 / 4.81 (1.82) minimum isolation
391 / 32.15 (33.14) maxima
445 / 15.61 (18.46) minima
483 / 24.63 (22.91) maxima
578 / 5.47 (4.68) minima
947 / 34.51 (53.85) maxima
>From this you can see why I don't accept the "tunes out the
reactance of the grid lead" stuff, although I've heard it also.
Across the board, over the frequency range where the tube has gain,
a small capacitor makes things worse.
That's why Heathkit, Gonset, and Ameritron neutralized their four
tube 811 amplifiers for ten meters.
> Everything that I was taught and read is that those 220pf grid capacitors
> were strictly to tune out the reactance and permit true RF grounding and
> thereby improve efficiency and stability. Nowhere was there ever any
> mention of some "voltage divider network" to support this latest theory
> from Tom.
> In fact, those original Bill Orr articles about "Super Cathode Driven"
> amps was always referring to tetrodes and most specifically the 4-400A
> and 4-1000A. I fail to see any relationship to the 30L1.
You need to get out your copy of Semi and Super Cathode Driven
Amplifiers, by Orr and Sayer, July 1967 QST, and read it again
Carl, before you state this is "the latest theory from Tom". I got
the "theory" from that text, and conversations with Orr and
engineers at Collins. As a matter of fact, one of Collins'
engineers agreed he could never understand the application of that
circuit in the 30L1.
Orr shows this circuit used with 811A's with a 75 pF capacitor
(from the above you can imagine how poor that is) and I quote
directly from Orr's text..."This feedback technique is used in the
Collins 30L-1 to match the drive requirement of four cathode
driven 811A tubes to the nominal power output of the S-line exciter."
It's my opinion, from measurements of three tone IMD (which more
closely represents voice, I can get good IMD readings from amps that
splatter like crazy in a two tone test) and feedthrough loss that the
grids belong grounded and a resistor belongs in the cathode circuit
to provide negative feedback. Lifting the grid of a triode, and
using a capacitive divider with a PA drawing grid current, is a poor
way of doing things...if negative feedback and stability are the
goals.
This is my last comment on the subject, no sense beating a dead
horse. I thought people might enjoy reading why the capacitors are
there, and trying to understand how it would ever do what was
claimed. If you want a FAX of the Orr article let me know.
73, Tom W8JI
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