[Boatanchors] VT-269

[Al Klase]

Well, the word "boondoggle" comes to mind.  Yes, you might be able to 
eek out a little noise figure improvement on 10 meters by installing a 
sharp-cutoff pentode in the first RF amp, but you're screwing up the 
gain structure and AVC action of the receiver, resulting in distortion 
on strong signals.

Putting sharp-cutoff tubes in the IF is an even worse idea.

A properly restored SX-28 qualifies is a high-performance receiver in 
stock trim.  If I were looking for a plug-in tweak to the front end I'd 
try 6SG7's.  These were the next generation RF (remote-cutoff) pentodes, 
and were used in the AR-88.  Their performance is equivalent to the 
post-war 6BA6's.  Higher transcondance means not just more gain, but 
better noise figure. However. the importance of being able to control 
all the available gain cannot be overemphasized.

There are parallels between the development of vacuum tubes in the 
1930's and '40's and today's progress in CPU's and memory chips.  
Significant improvements were made every 12-18 months.

Al

On 2/4/2010 5:19 PM, Mr. and Mrs. Magoo wrote:
> An SX-25 came to me with a VT-269 plugged into a socket where a 6SK7 should
> be.  I can find references to the VT-269 and a cross to a WE-717A, but I
> have no data for testing nor do I have a base diagram.  Does anyone know if
> it is, in fact, compatible with a 6SK7?  One source suggested a 6SJ7 "with
> wiring changes"!
>
> Bill VE3NH
>
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-- 
Al Klase - N3FRQ Jersey City, NJ http://www.skywaves.ar88.net/