Heathkit SB310 Antenna switch

Discussion in '"Boat Anchor" & Classic Equipment' started by KB2IDY, Apr 13, 2019.

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  1. KB2IDY

    KB2IDY Ham Member QRZ Page


    My SB 310 is my swl setup and uses a longwire with balun. Since it covers the 40 meters band and has an accurate dial, I have been using it as a cw rcvr for the one tube (6AQ5) crystal xmitter I recently built. I have a separate dipole set up for the latter, which pushes maybe 4 watts. Since I am using both antennas, and they are relatively close, I am simply turning down the RF pot on the SB310 when I xmit.

    My question, aside from the inconvenience of manipulating the pot during QSOs, is whether there is enough RF to damage the receiver. I was thinking of investing in a decent quality a b switch, though I am sure there are better alternatives to that as well.


  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The tube type front end in the SB-310 can handle a lot more power that what your 4-watt transmitter is going to run!

    Yes, I do have an SB-310 as well as the more featured SB-301. The SB-310 is the SWL version of the older SB-300.

    Glen, K9STH
  3. KB2IDY

    KB2IDY Ham Member QRZ Page

    Thanks Glen. Just thought I would ask as the blocking/howl can be bad when I forget to roll the pot. It's great fun rolling tubes and grabbing QSOs with junk stuff...the best part of the hobby in imho.


  4. G3YRO

    G3YRO Ham Member QRZ Page

    The normal procedure in Valve type receivers is to lift the bottom of the RF Gain from ground on transmit, using a Relay. (or a spare contact on your main Tx/Rx relay in your transmitter)

    If you then wire in an external pot across the relay contacts, that becomes your "Monitor Level" control.

    Roger G3YRO
  5. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    The SB-310 already has a mute jack on the rear panel.

    To use this, the function switch is put in the standby position and then a relay is connected to the RCA jack, on the back of the receiver, marked "mute". The relay is wired so that the normally closed contact connects the center conductor to ground (the outer ring of the jack). When transmitting, the relay is activated and the contacts are open and the ground is removed from the center conductor. This, in turn, "mutes" the receiver.

    Glen, K9STH
  6. K1LKP

    K1LKP Premium Subscriber QRZ Page

    Keep your eyes open for
    something like this

    73 - k1lkp
  7. K7MEM

    K7MEM Ham Member QRZ Page

    If you are going to build something, why not build a QSK interface. You can make a simple one like QSK T/R Switch for your Vintage Boat-Anchor Station. Or you can go with diodes. With only 4 watts, you should be able to find a nice simple circuit.

    I have a Heathkit HR-1680/HX-1681 Rx/Tx CW Only combo. I built it around 1981, while I was living in Germany and operating as DA2EU. The transmitter has a Full QSK interface. The interface is solid state, but handles the output of the pair of 6146s just fine. It's so nice to just hit the key and go.
  8. G3YRO

    G3YRO Ham Member QRZ Page

    Yes a lot of Amateur valve receivers already have this facility . . . I know my Drake R4A did.

    Roger G3YRO
  9. KB2IDY

    KB2IDY Ham Member QRZ Page

    Thanks to everyone for the good advice. I did not appreciate the value of the mute jack, and am now using a hand held push button switch to mute the rcvr when I key. I may try to build an electronic switch down range. Although I did not say so in my original post, I had also tried transmitting back through the rcv antenna, though with not much success.
  10. G3YRO

    G3YRO Ham Member QRZ Page

    You mean you fed your Transmitter output straight into the Receiver Antenna socket?

    Not a good idea . . . you're likely to burn out the fron end coils . . . quite apart from the fact that you would not be feeding all your Tx power into the antenna!

    You just need a simple 2-pole Tx/Rx Switch or Relay . . . that changes over the Antenna between Rx and Tx . . . and also Mutes/un-mutes the receiver.

    If the keying line of your Tx just goes to ground, you can easily achieve semi-break-in (ie automatic switching) . . . just put a diode in the keying line and in the relay ground line . . . and put an electrolytic capacitor across the relay coil to achieve the desired delay.

    Roger G3YRO

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