[ARC5] Smart People: Triode Connecting Pentodes

[Kenneth G. Gordon]

On 15 Nov 2013 at 0:59, Fuqua, Bill L wrote:

>   There was a lengthy discussion on AMPS years ago about supercathode drive amplifiers.
> And sometime during that I found some information on the screen to plate amplification factor.
> But, I can't recall where.  So, I could be wrong about that. 

OK. Thanks.

>    But low mu is the reason for its low gain performance in grounded grid applications. 
> The mu for a 304TL is 12. Which has similar performance in grounded grid applications per
> the Radio Handbook. But their example is class C if I recall. 200Watts in for around 900 Watts out.
> Have to look it up. I remember toying around with the idea of a grounded grid 304TL once.

I have specifications from Eimac for 304TLs in AB2 service, grid 
driven: a pair would require 90 watts drive for 1500 watts output.

HOWEVER.... the 304TL is, apparently, not well suited for linear 
service as the resulting IMD is too high.

I don't understand anything at all about that, and especially not why 
that is the case, but that is what I have been told.

I have an old article on a 304TL in GG service. It worked well enough 
for the fellow who built it, but, again, I have been told that such 
an amp would not meet modern specifications for IMD.

I and Bob Preston, W7DPG, used first one, then two 304TLs in modified 
BC-610s to run phone patches for AFMARS back in the 1960s.

We used regulated grid bias, and set the bias for 50 mA idle current 
for one tube. I used "projected cutoff" bias from the RCA 
Transmitting Tube manual. At the time, I thought it was Class B, but 
thinking about it since, I now suspect it was Class AB2 or even 
perhaps AB1.

Output was 2600 watts DC. The plates turned yellow-hot on voice 
peaks.

>From what I have learned about this setup in the last few years, we 
must have been outputting a lot of IMD, but since we were well away 
from any ham band, no one ever complained.

And the guys in SEA sure liked us.

I would STILL like to learn WHY the 304TL does not make a good linear 
amp...

I have enough NIB ones and sockets to build Push-Pull AB2 amps for 
several bands.

Oh. One more thing: I learned some time ago that the 304TL has a bad 
resonance right around 40 MHz, no matter what circuit it is used in. 
The solution is to install a 40 MHz trap in each grid lead. That 
makes them unconditionally stable if everything else is as it should 
be.

Ken W7EKB