[Drake] Re: GB> Drake 2B RF amplifier

[N2EY at aol.com]

In a message dated 7/30/05 11:35:44 PM Eastern Daylight Time, 
windy10605 at juno.com writes:


> 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." 

That's one of the formulas. It's either from the Radiotron Designer's 
Handbook or MIT. 

There's a QST article about 1964 that gives a lot of good info on evaluating 
RF tubes. It's called "Improving Your Receiver with a Frame Grid Tube" or some 
such. Only QST article I know of with "Frame Grid" in the title. Works up a 
figure of merit for small pentodes based on a number of things like gM, Rp, 
Cgp, etc. Champions the 6EH7/6EJ7, but the concepts are valid for all/

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

The real problem with triodes as RF amplifiers in receivers is they'll 
oscillate unless you either neutralize, run 'em in grounded grid, or cascode. All of 
which present circuit and particularly bandswitch difficulties.

G2DAF, in his later design, used a pushpull triode RF amp with no 
neutralization. The circuit looks "wrong" but it works because the total stage gain is 
about unity with the gain at maximum!

What G2DAF did was to use a high-gain low-noise mixer. The RF stage which 
precedes it has just enough max gain to make up for the losses in the tuned 
circuits. It can also become an attenuator if that proves to be useful. Very clever 
design! 

> 
> 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.

But is it? How about Rp and Cgp? 

The 6GM6 may be an improvement - I think that tube may be newer than the 2-B 
design. Recall that the 2-B is basically a 2-A with 500 Hz selectivity added 
to the IF, so the basic design is even earlier than the 2-B's appearance.

The "Frame Grid" article deals with all of this. 6GM6 is certainly worth a 
try.

But does the front end of a 2-B really need hopping up?


73 de Jim, N2EY