SB-102 VFO

[Kenneth G. Gordon]

On 7 Feb 2008 at 18:53, Dwight Smith wrote:

> Sorry, I was fumble fingered typing, I was referring to the 6HS6 Tube.
>  My question is, was it standard in all SB-102's or was it added as a
> revision later in production?

Heath's choice of the 6HS6 as the receiver RF amplifier and 
the 1st receiver mixer has an interesting story attached, but 
I won't to into that here, since I can't prove it.

However, the 6HS6, 6AU6, 6BA6, 6CG6, 6HR6, and 6AH6 
all have the exact same socket connections, but differ in 
some other respects such as plate dissipation ratings and 
transconductance.

The transconductance is the important factor where 
sensitivity and low noise are concerned, since lowest noise 
and highest sensitivity occur where the transconductance is 
highest.

The transconductance of the 6HS6 is 9500 micromhos, 
whereas that of the 6AU6 is less than 1/2 that at 4450 
micromhos, and replacing a 6AU6 with a 6HS6 in both the 
RF and 1st receiver mixer results in a very noticeable 
increase in apparent sensitivity and lower noise.

However, the maximum voltage and current ratings of the 
two tubes are enough different that the 6HS6 tends to not 
last as long as the 6AU6 in that service. Besides, there were 
a lot fewer 6HS6s made than 6AU6s.

Under most operating conditions, the 6AU6 is perfectly 
suitable and lasts a lot longer than the 6HS6.

As I remember it, Heathkit went BACK to the 6AU6 
sometime during the run of the SB-102 because availability 
of the 6HS6 was very much lower than the 6AU6.

Now, that being said, there is another tube that I have found 
to be very suitable as the RF and mixer tubes, and that is 
the 6AH6.

It has a transconductance of 9000 micromhos, is a lot 
tougher than the 6HS6, and is VERY much easier to find.

I use those in all my HW/SB transceivers.

Ken Gordon W7EKB

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