[AMRadio] Grounded grid amp

[D. Chester]

> The article is titled "Receiving Tubes in Grounded Grid SSB Finals", by
> Norman McLaughlin W6GEG and was published in the Sept. 1956 issue of CQ
> magazine.  He tested it with 6AG7s, 6L6s, and 6CL6s but settled on 6AG7s
> because they were available cheap as surplus.
>
> 73, Bob W9RAN
>

> I recall some insane amps being run with 6AG7s, how about six or more 
> upside
> down in a pan of water with 1KV on them?  life must have been short.  also
> from that time were a bunch of 803s in GG configeration.    Bernie

There is a better reason than being cheap surplus to use 6AG7s in a GG amp. 
The 6AG7 is a pentode with the suppressor grid connection coming out of the 
envelope as a separate lead to a dedicated pin at the base.  For the same 
reason, the 803 would be a good g-g tube.  Many tubes, including he 6L6, 807 
and 6CL6, have the suppressor grid or beam forming plates internally 
connected to the cathode inside the glass envelope.  You can ground the 
control and screen grids, but the suppressor/bf plates remain connected to 
the cathode, allowing a capacitive path directly from the input (cathode) of 
the tube and the plate  with the grounded-grid circuit.  This causes the 
same type of  feed-through as normally occurs with an un-neutralised triode 
tube in the conventional grounded cathode circuit, and defeats the whole 
purpose of the grounded grid configuration, which is to provide isolation 
between the input and output of the stage by allowing the grounded control, 
screen and suppressor grids to serve as an rf shield between cathode and 
plate.  This capacitive feed-through may cause instability and 
self-oscillation in the grounded grid stage, unless the stage is 
neutralised, just as in the case of the triode in conventional grounded 
cathode service.

I recall in the mid-50s a linear was described in the ARRL handbook using 
modified 1625 tubes.  Certain brands of tubes had the beam-forming plate 
connection brought out of the glass via a separate wire lead, but that lead 
was connected to the cathode inside the bakelite base of the tube. The 
modification consisted of drilling a hole in the side of the base to gain 
access to the wire leads, unsoldering the cathode pin and moving the bf 
plate lead to bring it out through an usused pin on the tube base.  The 
cathode pin was then re-soldered.  A commercial version of this same amp 
used 837 pentodes, which are manufactured with a dedicated suppressor pin, 
instead of 1625s.  I believe they also sold units with factory modified 
1625s, and modified tubes were available from the company for use as 
replacements.

Other pentodes with a separate suppressor lead include the 802 transmitting 
pentode, and the type 59 audio power amp pentode.  The 59 was used in the 
final audio stage of broadcast receivers and is otherwise similar to the 47, 
42 and 6F6.  I use a 59 in the first rf stage of one of my homebrew 
transmitters, and with a simple switching arrangement can change it over 
from a pentode crystal oscillator to a grounded grid buffer stage for my 
external VFO.  It is very stable, without a  hint of self-oscillation when 
the external VFO is used. The 802 is better shielded than the 807 and has 
less tendency to be squirrelly in the conventional grounded cathode 
amplifier circuit, but it doesn't quite have the plate dissipation and power 
output capability of the 807 even though both tubes are the same physical 
size. Likewise, the 6AG7 (and its miniature equivalent 5763) is extremely 
well shielded, much better than a tube like the 6V6 or 6AQ5,  and is usually 
stable in rf amplifier service without neutralisation.

Don k4kyv


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