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HT-37 Solid State Mod Info Needed

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

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  1. KG5GCC

    KG5GCC Ham Member QRZ Page

    I just got a Halicrafters HT-37 that has sat in storage for many years. I have read about the transformer shorting issue and want to go ahead and go solid-state with the HV and LV power supplies before I do anything else with it. I have some tube bases to build the diodes and resistors into and will disconnect the filament windings underneath the socket and then just plug these new solid state rectifiers into them.

    Now the trouble I am having is finding just exactly what value and wattage resistor is needed in both the HV and LV sections.
    And a question or two on the diodes. Most what I read people used 1N4007 diodes and ran 2 in series for the LV and 3 in series for the HV. Why is this? Is there a better HV diode I could get from Mouser and just use 1 of it in each string?
    And finally should I add a soft-start inrush limiter to them and what value should it be?
    Anybody who has done this mod to their rig and remembers what values you used, please let me know.
    Thanks in advance,
  2. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    -edited. researching other info.
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Here are all the HT-37 articles from Electric Radio Magazine. You can purchase back copies of each issue. Please support ER by not pirating.

    HT-37 ER Mag.png
    KG5GCC likes this.
  4. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Articles from ARRL QST magazine archives. Members can download for free. Non-member can purchase article copies.

    KG5GCC likes this.
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

  6. W1BR

    W1BR Premium Subscriber QRZ Page

    The diodes have to withstand the charge voltage on the filter caps and also the PIV voltage on the transformer secondary windings. And, with the HV section being choke
    input the diodes PRV rating becomes even more critical. I did the modification on my HT-32 eons ago, and I don't remember what I used for resistor values... but I can
    tell you with solid state rectifiers the HV can quickly approach the rating for the oil filled filter cap unless precautions are taken.
    KG5GCC likes this.
  7. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    The B++ (6146 plate voltage) will increase around 60 VDC (that is the typical voltage drop in the 5R4) to around 800 VDC to 820 VDC. The 6146 tubes have no problems handling that voltage and it is under the 1000 VDC rating of the oil filled filter capacitor. Other transmitters, like the Collins S-Line, typically have between 800 VDC and 825 VDC on the final amplifier tubes.

    You can replace the 10 mfd oil filled capacitor with 3-each 33 mfd 450 VDC electrolytic capacitors in series to give 1350 VDC capability. Put 2-watt 100 K-ohm resistors across each of the capacitors. I have a supply of 33 mfd 400 VDC capacitors that I can give you. That would make 1200 VDC capability. You will still have to come up with the resistors. These capacitors are much smaller than the 10 mfd existing capacitor.

    For the 5U4 B+ rectifier, the voltage drop is around 40 VDC. Typically, in the Collins 516F-2, when the tube rectifiers are replaced with solid-state, a resistor of around 150-ohms at 20-watts is inserted right after the rectifiers to drop the B+. This value resistor should work fine with the HT-32.

    Glen, K9STH
    KG5GCC likes this.
  8. KG5GCC

    KG5GCC Ham Member QRZ Page

    Thanks everyone for all the good info. just got the chassis cleaned up and the SSB modulator out and ready to do some more prelimanary checks on it. Pretty sure it still has a good power transformer from the ohms readings I have.
  9. W1BR

    W1BR Premium Subscriber QRZ Page

    My HT-32 high B+ was well over 900 volts before the 6146 tubes warmed up after switching to diodes. Your mileage may vary, but measure it. Especially before the 6146 tubes begin to draw idle current!
  10. KE0ZU

    KE0ZU Ham Member QRZ Page

    I just look at the tube data sheet FOUND HERE and look at the Voltage drop at maximum current value.

    Then use Ohm's law to determine what series resistance to use, and minimum power handling capability resistor to put in series with the diodes.
    KA0HCP likes this.

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