"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.

In the 1936 issue of QST, an article entitled "Operating Notes on the
Transmitting-Type Beam Power Tube" noted:
"Careful tests show that with shielding of this nature the tube has
no tendency to oscillate at frequencies up to and including the 7-Mc.
band; it functions as a true screen-grid amplifier. On 14 Mc. it
will self-oscillate after a fashion, but usually with negligible power
output.
When separately excited, however, it settles down nicely and behaves
like any normal amplifier, showing no tendency to go off on its
own — this, too, when driven by the fourth harmonic of a Tri-tet
oscillator operating at a very low power level. On the whole, therefore,
we have found no occasion for attempting to neutralize the tube,
which is fortunate, because it is always a rather messy job to neutralize
a tube with such low grid-plate capacity."

73, Barry WA4VZQ