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 capacitance.
Since the tube is not operated anywhere near its limits, the fact that the maximum plate dissipation is slightly different does not enter into the equation.
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 more.
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.
Glen, K9STH"
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 capacitance.
Since the tube is not operated anywhere near its limits, the fact that the maximum plate dissipation is slightly different does not enter into the equation.
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 more.
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.
Glen, K9STH"