"
Nordic Breeds WA4VZQ" wrote in message
... "Kenneth Scharf"
wrote in message
...

The reason that the 6146 tube generally requires neutralization and the
807 does not is because of the higher gain of the 6146. The 6146 will
draw greater plate current at a lower screen voltage (with the same plate
voltage) than the 807. It is the higher 'gain' of the screen grid that
matters here (since a screen grid tube acts as two triodes in cascade as
far as power gain is concerned). The 6146 is actually better shielded
than the 807 and has less lead inductance which helps stability. In
order to avoid neutralization the 807 requires extra shielding to keep
the grid from 'seeing' the plate. Sub mounting the tube socket below the
chassis so the bottom of the plate is even with the level of the chassis
will do the trick. James Millen sold tube shields for the 807 which were
aluminum cans open at both ends with spade lugs to mount to the chassis.
With the 807 sockets mounted to the chassis and the shields mounted on
the chassis, the tops of the cans would be just below the level of the
bottom of the 807 plates.
Recall the AN-ARC5 transmitters? They sub mounted the 1625 tubes so the
plates were just at the level of the chassis, same idea.


Neutralization becomes more of an issue the higher you go in frequency.
Most simple entry level transmitters using a single 6146 in the 1950's and
1960's were not neutralized as the 6146 was only used "straight through"
on 80 or 40 meters. On higher bands it was used as a frequency
multiplier/power stage. The Eico 730, the Elmac AF67, the Heath DX-20,
DX-35, and DX-40, and the Knight T60 used no neutralization. The Eico
723, the Heath DX-60 and the Drake 2NT. however, did. If used on 80 and
40-meters only, I seriously doubt if Frank will need neutralization with
his 5933 as long as the driver tank is located below the chassis and the
final tank circuit is above the chassis. A shield around the base of the
5933 is a good idea, of course.

My experience with 6146's (and one time with a 5933) was that from 40
meters on up, they could be easily be made to self oscillate if they were
lightly loaded. (talking final amplifier stages), and this is where a poor
man's neutralization procedure took place)
When lightly loaded, or better yet with no drive at all, swinging both
the input and output tuning condensers (sometimes referred to as
"capacitors") would induce an oscillation on the
intended band of operation. A single piece of wire used as a negative
feedback condenser could be wiggled about to find a place that tamed things
down.
Only problem with this was that the rough neutralization achieved was
frequency (or band) dependent. Usually it would be "good enough" if
neutralized on the highest band to be used.
Parasitic oscillation was a whole different problem, and didn't seem to
care much if the stage was neutralized or not!!!!!!

Lynn, W7LTQ