[Boatanchors] Tube reciever design banter

[Carl]

----- Original Message ----- 
From: "Drew P." <drewrailleur807 at yahoo.com>
To: <boatanchors at mailman.qth.net>
Sent: Tuesday, December 22, 2009 11:36 PM
Subject: Re: [Boatanchors] Tube reciever design banter


> On characteristics of the pentode section of the 6MU8 pertaining to AGC 
> control, Carl wrote:
>
> "The full RCA spec sheet has it as Sharp along with the curves. Its dead 
> in
> the water at -9V which doesnt give a good AGC window without getting 
> fancy."
>
> Interesting...using a sharp cutoff pentode as a gain-controlled stage. 
> I've wondered about Heath's choice of the sharp cutoff 6AU6 as RF and IF 
> for the SB/HW transceivers.  At first glance, seems like this shouldn't 
> work very well, but in actuality it does.  You mention an AGC window - 
> this would occur just as the tube is reaching cutoff, where the typical 
> sharp cutoff pentode really isn't so sharp.  There could be a quite 
> substantial AGC voltage which, up until that near-cutoff region of the 
> transconductance curve, would not vary the gain of the controlled stage at 
> all.  Then, within that very small region where the curve bends over, 
> there would be a large change in stage gain for a small change in AGC 
> voltage. Would I be correct in surmising that such application of a sharp 
> cutoff pentode is to obtain AGC delay?
>
> Drew


Im fairly sure that was their reasoning but they were rather after the fact 
in persuing it. Collins and most others gave out on that idea a decade + 
earlier. Many never did go that way. Those Heaths are not known for great 
overload tolerance either.

With tube tolerances and variations between brands it is not easy trying to 
use a circuit in such a narrow window. I tried it with a 6AC7 in a few octal 
based radios with mixed success.

Carl

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