HT-37 Solid State Mod Info Needed

Discussion in '"Boat Anchor" & Classic Equipment' started by KG5GCC, Dec 8, 2019.

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  1. KK6IYM

    KK6IYM Ham Member QRZ Page

    I used a "bucking" transformer to drop the voltage when I restored my HT 37. It is a filament transformer and mounts above the choke.


    KP4SX likes this.
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Using the information on the maximum current each tube can draw will result in having a series resistor too low in resistance because the tubes, in the HT-32, are not drawing the maximum. The B++ feeds only the plates of the final amplifier 6146 tubes but the B+ feeds all of the rest of the tubes in the transmitter plus the screens of the 6146 tubes.

    To determine the total current draw from the 5V4 one needs to insert a current meter between Pin-2 of the 5V4 and the junction of L-25 and C-79B and put the transmitter in transmit. The voltage, with the tube rectifier installed, needs to be measured and then the rectifier replaced with the solid-state devices. The new B+ voltage is then measured. Subtract the old B+ reading from the new B+ reading. Then using the current measurement and use Ohm's law to determine the value of the series resistor.

    Glen, K9STH
  3. W1BR

    W1BR Premium Subscriber QRZ Page

    View attachment 609853
    Hi Norm

    An excellent solution that solves several problems at once. Best solution yet.

    Here's a shot of my rectifiers, both are the same and are interchangeable, since the
    diodes have more than ample PRV ratings. Used old octal tube bases with painted
    PVC pipe caps for covers.


    HT-32 rectifiers.JPG
    KP4SX likes this.
  4. KE0ZU

    KE0ZU Ham Member QRZ Page

    If the only issue you were wanting to address was a "to the Volt" B+ solution, then your approach would be fine, however, thats not the only issue at hand.

    Together with Norm's bucking transformer suggestion, which addresses the issue of over all excessively high input voltage to the radio's transformers and tube filaments, my suggestion would certainly get the voltages within the +/- 10% region noted in the manual, and require a lot less time at the bench.
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The bucking transformer definitely helps with things like heater / filament voltage. However, when the tube rectifiers are replaced with solid-state devices, especially the lower B+ will still be too high. To drop the AC mains voltage to the point where the B+ is back to where it was with the tube rectifiers, the heater / filament voltage will be too low. A dropping resistor is still required.

    The high B+ (or B++) doesn't really matter because the 6146 final amplifier tubes can handle the higher voltage without any problems.

    Glen, K9STH
  6. W1BR

    W1BR Premium Subscriber QRZ Page

    As I mentioned earlier, the high voltage B plus can get close to the oil filled capacitor's voltage rating UNTIL the 6146 tubes begin to draw current. At least in the HT-32
    they are constantly biased into linear operation, which is required for a minimum load. Adding the bucking transformer reduces the surge voltage concerns. It also results in cooler transformer operation.
    Last edited: Dec 10, 2019
  7. KG5GCC

    KG5GCC Ham Member QRZ Page

    Thanks for all the good advice and info. I have some parts on the way and will tear into it this weekend. It also seems I may be needing the VOX relay; the coil is open. Should read 9K but I get high Megohm readings.

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