[Boatanchors] Tube reciever design banter

[Carl]

----- Original Message ----- 
From: "Drew P." <drewrailleur807 at yahoo.com>
To: <boatanchors at mailman.qth.net>
Sent: Wednesday, December 23, 2009 12:39 AM
Subject: Re: [Boatanchors] Tube reciever design banter


> Carl,
>
> I note that you express a preference for the 6GM6 as a first (and only) RF 
> amplifier stage. How does the performance (noise figure, gain, intermod 
> susceptibility, AGC control range) of the semiremote cutoff 6GM6 pentode 
> compare to that of a cascode variable mu twin triode such as the 6ES8 and 
> a few other types?

First of all the 6GM6 is a 7 pin tube which makes it an easy replacement for 
the 6BA6, 6DC6, 6BZ6, and others that were used extensively. Inorder to work 
with the existing AGC and hold linearity the voltages and screen dropper 
have to be changed accordingly. Its a balancing act but works well. Once 
double conversion designs eliminated the image problem sufficiently there 
was no reason to continue with multiple RF stages. The requirement now was 
to overcome mixer noise which met with mixed success as the 6BE6 is 
incredibly noisy and on the par with the older 6K8 and 6SA7.

The 6EH7 is viewed as the best of the 9 pin with the exception of 1 or 2 
extremely scarce Amperex tubes that went into a few very expensive 
commercial/marine sets.

Ive had no use for triodes as RF amps since the days of the VHF TV cascode 
6BK7 thru 6DJ8 which were used by everyone in ham converters until the 
6CW4/6DS4 came along. I can imagine the difficulty of neutralizing a BCB to 
10M cascode stage. Around that time I decided SS was the only way to go if 
you wanted to hear the weak ones on 2M and up. I was very thankful for that 
as I certainly didnt want to have to mess around with a parametric 
amplifier.

I dont think Ive ever seen a FM radio or TV that used the 6ES8.


>
> It appears that TV front end design at the end of the tube era seemed to 
> diverge along those two lines.


I dont think Ive ever seen a FM radio or TV that used the 6ES8 or any end of 
the line tubes. I jumped from B&W to a Zenith SS ChromaColor II in one leap 
in the mid 70's and that lasted for over 20 years.

Carl
KM1H



>
> Drew
>
>
>
>
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