[Hammarlund] Nuvistor Front End of Sp-600

[Carl]

In the HQ-140 the 6BA6 became a noise contributor above 18 Mc.

Its the same with the pair of them in the NC-183D and HRO-60 so I have to 
assume the SP-600 is no different in that respect. A 6GM6 in the 1st RF made 
a huge difference as I wanted decent reception on 10M. The NF at 14 dB is 
still excessive but I havent tackled the mixer yet other than a 6BY6 as I 
dont want to change sockets to a 9 pin. For now I use a 1 dB NF HB SS preamp 
with to bring the system NF down to a point of diminishing returns.

The 6ES8 makes a great low noise mixer, the noise resistance is a miniscule 
180 Ohms, and I have used it in a Pullen circuit in a S-40B test bed.

The 7F8 loctal can also be used as a conventional or Pullen dual triode 
mixer in place of a 6K8, 6SA7, etc without sheet metal changes. Simply swap 
sockets.

The 6BE6, 6K8 and 6SA7 are about the noisiest tubes ever developed as 
mixers.

Carl
KM1H


----- Original Message ----- 
From: "rrkrr" <rrkrr at comcast.net>
To: <hammarlund at mailman.qth.net>
Sent: Thursday, June 26, 2008 4:26 AM
Subject: Re: [Hammarlund] Nuvistor Front End of Sp-600


> The issue with the 6BA6 as the 1st RF amplifier in the SP-600 is that its 
> high noise figure becomes important at the high frequency (above 30 MHz) 
> end of the range.  It really is horrible.  It's bad enough on 10 meters. 
> The circuit that used two nuvistor triodes in cascode ("Nuvistaplug" - 
> Google that and you'll find the circuit) really made a difference when 
> trying to use the SP-600 on 6 meters.
> There is also a dual triode in a standard 9-pin miniature tube base that 
> was designed for connection in cascode circuits and has low noise figure - 
> the 6ES8/ECC189.  These were used in the front end of Racal 17L receivers, 
> which were designed a few years later than the first SP-600s.  The 6ES8 
> American version was used in TV tuners.  These require a larger socket at 
> the first RF stage than the 6BA6 and are not quite as low noise as the 
> nuvistors, however.  If you want a low noise, remote cutoff nuvistor 
> triode for the input stage of the cascode, use the 6DS4.  See: 
> <http://en.wikipedia.org/wiki/Nuvistor>
>
> Bob K4ERR
>
>
> stanleybadams wrote:
>> I think you might want to think what you are going to use the radio for.
>> Casual listening to AM radio?  SSB or AM amateur (for SSB you need the
>> product detector which is a far better mod in my opinion).  Do you want 
>> to
>> hear TV stations on the upper bands as they go away after 2009, unless 
>> they
>> repeal the date?  I do not think that there really is any bang for the 
>> buck,
>> now if you were on Guam copying teletype from the states or ships at sea,
>> that might be a diff thing.   I would suggest the 6BZ6...A very good
>> remote-cutoff tube as a direct replacement for the 6BA6. No other tube 
>> seems
>> to work as wellwithout changing a few other parts around. Or the 6SS
>> whatever the last part is 6SS6 or 5?  That was a lower noise tube for TV
>> tuners.  AM band has gotten noiser with the addition of IBOC and their 
>> extra
>> carriers in the day time anyway.
>>
>> Best to you
>>
>> Stan Adams
>> Memphis
>>
>>
>>
>>
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