[Boatanchors] VT-269

[Carl]

If you do some digging Al you will find that the 717A is really a semi 
remote cut-off tube when used in just about any vintage receiver.
The 717A was developed for 30 and 45 mHz IF's in late WW2 radar receivers 
and also used in some VHF  communications gear.

It is slightly superior to the 6SG7 and 6AB7 at 20M and much better above; 
transconductance isnt the only criteria for performance. The difference is 
more like replacing a 6SG7 with a 6BZ6 or 6DC6. The 6SG7 was used by several 
manufacturers as soon as it was available to design with and was a reliable 
workhorse for a few years.

It should only be used in the 1st RF stage of 2 stage radios such as the 
SX-25. In a single RF radio such as the HQ-129X it really wakes things up as 
I found out in 1955 when I just did what was suggested by an older ham. I 
then discovered 15M and 10 when I got the General.

It can offer some benefit in the first IF but may need some circuit 
tweaking. I wouldnt recommend it unless the mixer was extremely noisy and 
couldnt be upgraded.

I take performance seriously since I use most of my radios to communicate 
with; this includes signal handling ability under "battle conditions". I 
also have the HP and other test equipment to do proper evaluations.

I certainly wouldnt use a 6SG7 in place of the 6AB7 in a SX-28, the front 
end coils are designed for the latters input requirements which are far 
different than the other pentodes. It would be excellent to replace the 2nd 
RF 6SK7 followed by a 6SB7Y mixer.

Carl
KM1H




----- Original Message ----- 
From: "Al Klase" <al at ar88.net>
To: <boatanchors at mailman.qth.net>
Sent: Thursday, February 04, 2010 8:13 PM
Subject: Re: [Boatanchors] VT-269


> 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/
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