SB-102 VFO

[Kenneth G. Gordon]

On 9 Feb 2008 at 7:33, Guy Giacopuzzi wrote:

> 
> So you're saying the 6AH6 is a good sub for the 6HS6? And you'll get
> decent sensitivity with it?

Yes. Absolutely. I use them in both my HW-101 and my SB-
102.

The sensitivity of the Heathkit HW/SB transceivers is 
actually unusually high, even with the 6AU6, measuring, on 
average, below 1 microvolt for MDS when everything is 
aligned properly.

Personally, I feel the much lower noise level (higher signal-
to-noise ratio) of the Heathkit rigs vs some others is more 
important than the sensitivity.

Be that as it may, I have measured the sensitivity of my 
HW-101 at 0.1 microvolt for a Minimum Discernible Signal 
(MDS) with the 6HS6 OR the 6AH6 installed in place of the 
6AU6.

Now, let me hasten to add that this level of sensitivity is very 
seldom usable or valuable at frequencies below 10 Mhz, and 
only really usable at frequencies above 10 Mhz when band 
noise is very low.

In addition, using tubes with very high transconductances, 
which are usually sharp-cutoff types, makes the receiver 
more prone to overload on strong adjancent channel 
signals. I.e., the dynamic range is often reduced by using 
such tubes.

However, in areas where the nearest ham running a full KW 
is at least several miles away, I have found the sub to be 
quite useful.

A short story will illustrate what I mean: back in the 1960s, I 
and a friend, Bob Preston, W7DPG, in Missoula, Montana 
did quite a lot of 'phone patching into Vietnam and other 
SEA military bases for Air Force MARS. The frequencies 
used were above the 20 meter band. I used my modified 
SB-100, with "hot" RF and mixer tubes, as the driver for a 
modified BC-610 linear amp (a pair of 304TLs) into a Hygain 
TH-6 at 100 feet. The combo ran about 2600 watts input.

Bob, W7DPG, used, first, an FTDX-560, then, when it 
proved to be very unreliable, a Collins KWM-2A to, again, 
drive a similarly modified BC-610 linear amp.

We lived only a few miles apart and would work together to 
provide as many patches as we could when the bands were 
open.

We noticed that I could regularly continue making patches 
up to an hour after Bob could no longer hear the SEA 
stations well enough for patching.

At first, we put this difference down to the differences in our 
antennas. He was using a home-built full-sized 3 element 
quad, while I was using the TH-6. However, we got 
suspicious when we realized that other users of quads were 
experiencing the opposite effects to those we were.

So, I took my modified SB-100 (I called it an SB-101.5) over 
to his shack and we switched back and forth between the 
two rigs during an operating session.

We discovered very quickly that the Heathkit was more 
sensitive and far quieter than the KWM-2A and could, thus, 
hear signals that the other rig couldn't. The lower noise level 
from the "hotter" tubes was the biggest factor, in fact.

Bob went on to become a "gateway" station in AFMARS, 
eventually had two stations, both based on a pair of KWM-
2As, in operation, and racked up an incredible total of 
completed phone patches. He told me recently that it was 
well over 10,000 patches. 

In my case, a guy-anchor snapped during a wind storm and 
dumped my antenna and tower into the back yard. I was 
"financially embarrassed" at the time and was never able to 
get back into operation. This happened about a year after 
we started operations. 

I DID receive a commendation from General Creighton 
Abrams for my services, though, which I will always 
treasure.

In any case, yes, the 6AH6 makes an excellent sub for 
either the 6AU6 or the 6HS6.

Ken Gordon W7EKB

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