Saw this on the Heathkit reflector, and thought it would be something good to share.
From Glen, K9STH
"Recently, I have been asked as to how I came up with the idea to replace
the 6AU6 in the Heath VF-1 VFO with a 6AH6 and for the other changes I
recommend which are to replace the 0A2 screen voltage regulator with an
0B2 and run the VFO from a regulated 150 VDC source. As such, I thought
that others might be interested in this. The following is the answer
that I gave:
The idea for replacing the 6AU6 in the Heath VF-1 actually came from the
Heath Company. The very first TX-1 Apache transmitters used the 6AU6
in the VFO. However, the stability was just not satisfactory for use
with the SB-10 SSB adapter. After some experimentation, it was found
that when the 6AH6 was used in the VFO without any changes to the
circuitry, the stability improved immensely and the only difference was
that the trimmer capacitors for calibration on each band had to be
adjusted slightly to compensate for the difference in inter-electrode
Since the tube is not operated anywhere near its limits, the fact that
the maximum plate dissipation is slightly different does not enter into
I grew up 35 miles from the Heath plant (LaPorte, Indiana) in St.
Joseph, Michigan. The mailing address was Benton Harbor but the actual
location of the plant was on Hilltop Road in St. Joseph. After getting
my driver's license, I would occasionally drive up to the Heath plant
just to "drool" over the latest equipment. Heath had a display room,
where one could actually purchase and take home equipment, that was
located to the left as one entered the main entrance to the plant. If
you had any questions about any equipment, the person at the order desk
would call back into the plant and someone, often the lead engineer on
the kit design, would come out and answer your questions.
The engineer would often suggest changes to the equipment to improve the
operation and, usually, hand you a paper bag with some parts and a
schematic, sometimes "hand drawn". There was never a charge for this.
One time, I had a question about frequency stability on my DX-100
transmitter and the engineer suggested replacing the 6AU6 with a 6AH6
and changing the screen regulator to an 0B2 instead of the 0A2. He also
commented that this worked great on the VF-1. Although he didn't
comment on using regulated 150 volts for a separate power supply for the
VF-1, I tried that, using the 0A2 removed from the screen of the tube
to regulate the external power supply. That improved the stability even
Ever since then, I have recommended changing the VF-1 tubes to a 6AH6 and OB2.
I never knew the name of the particular engineer who suggested the
changes. However, if I remember correctly, he had been involved with
the design of the TX-1 Apache as well as the DX-100 and VF-1.
There were a number of Heath employees that were amateur radio operators
who were very active. I used to work them on 6-meter and 2-meter AM.