[Drake] Drake 2B RF amplifier

[windy10605 at juno.com]

I think I have this right .........

I received the info on ENR (Equivalent Noise Resistance) from Chris from
an old RSGB Handbook (apparently a better source than the old ARRL
Handbooks) "For a pentode used as an RF amplifier: ENR = Ia (2.5/gm + 20
Ig2/gm)/(Ia + Ig2) kilohm. where Ia is the plate current in mA and Ig2 is
the screen grid current in mA." 

RF Triodes, by their design, have much lower ENR than Pentodes ....like
about 1/10th, but don't have the amplification of a Pentode. 

Since the 6BZ6 RF tube used in the Drake 2B is a Semi-Remote Cutoff
Pentode, which means smooth transition cutoff occurs at 10-20% of screen
voltage (which is the 2B design point) as opposed to 20%++ for Remote
Cutoff, or too fast by using a Sharp Cutoff Pentode ...looks like it
"should" be another Semi-Remote Cutoff Pentode. Like a 6GM6. Turns out,
the 6GM6 under the same conditions has roughly half the ENR (Equivalent
Noise Resistance) and would be a better choice for the Drake 2B RF
amplifier and it's just a plug-in.


73 Kees K5BCQ