[Drake] RF tubes ...test data

[windy10605 at juno.com]

The ongoing story about looking for a better RF amp tube than the 6BZ6 in
the Drake 2B. The article in this month's Electric Radio was on the same
subject but Ray Osterwald was looking for a better tube than the 6DC6
used in the SX-101A restoration.

I have a test fixture which is actually more flexible than a TV-2 for
setting up circuits and testing ...the thing even includes meters from a
TV-2 which I found at some swapmeet (thanks to whomever had those in the
box under the table at Belton). 

The first thing which comes to mind is that tube specifications are great
for "testing" tubes and what the tubes "will" do, the second is that the 
operational data parameters are good for ONLY that given environment
(plate voltage, screen voltage, cathode resistor, etc). What happens in
the real world with the circuit design you have can be completely
different. For example: the 6BZ6 circuit used in the Drake 2B has a plate
voltage of 135V, a screen voltage of 60V, and a 150 ohm cathode resistor.
As someone pointed out in a previous note "the gain of the 2B RF amp is
very low and is meant to largely overcome L/C losses". Absolutely correct
! If you look at the Gm of a typical recommended RF amp tube, they range
from 8,000 to 14,000 ....but with this 2B circuit, with the lower
voltages and cathode resistor, you are looking at an actual use Gm of
2,000 or less. The resultant curve is different.

That's one of the things about ER's article which stumped me initially
....the tube charts don't look like they reflect the measured results.
Ufta!! the tube charts are based on what the tubes "can" do, the measured
results are based on the "actual user circuit" specifics.  

Here is some data off my tester for new tubes in a "Drake 2B circuit
environment" (plate current in ma):

Plate = 135VDC, Screen = 60VDC, 150 ohm Cathode resistor, Supressor Grid
to ground, Grid bias variable. 

Grid V   -0.5  -1.0  -1.5  -2.0  -3.0  -4.0  -5.0          
 
6BZ6       2.4   1.6   1.0    0.8   0.5   0.3    0.2    Gm 1400
6DC6      3.8   2.9   1.8    1.3   0.3   0.1   OFF   Gm 2000
6GM6     3.0   2.0   1.2    0.9   0.6   0.4    0.2     Gm 1800
6EW6     2.8   1.6    0.6   0.2  OFF OFF OFF (needs different bias)

With cathode to ground
6BZ6      4.0   2.3   1.3    0.8   0.5    0.3   0.2      Gm 2700
6DC6     7.3   5.5   3.1    1.7    0.4    0.1  OFF    Gm 4200 
6GM6     6.5   3.2  1.5    0.9    0.5    0.3   0.2      Gm 5000

6BZ6 Cgp 0.015pf, 6DC6 Cgp 0.02pf, 6GM6 Cgp 0.026pf

My conclusion from the above is that potentiometer in the cathode for
"tweaking" would be useful ....or at least "interesting". ..........and
I'm sticking with the 6GM6.

73 Kees K5BCQ