Discussion in 'Amplitude Modulation' started by N2DTS, Nov 23, 2021.

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  1. N2DTS

    N2DTS Ham Member QRZ Page

    One of these turned up on ebay, looked brand new inside and out, with free shipping!
    So I bought it.
    Its an ssb cw rig that does AM, and looking at the manual, its both sidebands and the filter is bypassed on AM....
    6dq5 output tube, so about 12 to 15 watts carrier, silly, but ok for into an amplifier I guess.
    I suspect it can sound good if its not going through a filter....
  2. KP4SX

    KP4SX Premium Subscriber QRZ Page

    FB! The HX-50A was my first SSB xmtr. The SSB filter was wider than 3 KHz :) The freqs are stamped on a label on the filter. I don't recall about the AM on it.
    I was fortunate to find the coil set and xtal for 16om at a surplus shop in Charlotte.
    Have fun with it!
  3. KE0ZU

    KE0ZU Ham Member QRZ Page

    Over the years I've had 2 straight 50's and two 50A's. The A's were nicer than the earlier versions. I've had a very nice 50A, with the 160 band installed, for at least a couple years and like it very much.

    whats silly?

    Many SSB/AM transmitters were low level modulated, which meant all the following stages had to operate in the linear mode. Thus, 15 Watts of carrier, translates to about 60 Watts or so PEP in AM mode, very similar to the SSB PEP output. it sounds fine and the AM audio response is determined by the filtering in the mic amp stages.

    Last edited: Nov 25, 2021
    N2EY and KP4SX like this.
  4. N2DTS

    N2DTS Ham Member QRZ Page

    Got it today.


    The seller in California put it in a box with 1/2 inch styrofoam insulation so some tubes were loose inside and the tuning
    knob mashed in and its shaft bent, but everything was fixable.
    It worked with a new 12BY7 driver tube!
    Lower sideband mostly, some upper sideband, and the carrier.
    Audio was bad, unsure what microphone was supposed to be used with the HX50, NOT a D104 with a 100k ohm grid resistor!
    Also, audio gain is way over the top.
    I put in a 2.2 meg and removed one .005 to ground and it sounds ok.
    I used a .001 around the filter (3 MHz) and it does good AM, good CW and fine DSB .
    The filter was removing the other sideband, despite the manual saying its bypassed in AM mode.....
    VFO seems VERY stable, CW power output is about 60 watts, 15 watts carrier on AM with plenty of modulation.
    I had a number of contacts using it under less then ideal conditions.

    As a receiver, it seems like it was a nice build, but its a transmitter so the rf output part looks like they had no clue.
    But it works, and its interesting you can adjust things to give as much audio on the carrier as you want as drive level and audio are adjustable
    along with loading of the final.

    I have no use for this rig, but its so clean I had to check it out.

    Whats silly? The 15 watts.
    But there seems to be no shortage of super clean versions, they held up well I guess.
    Last edited: Dec 6, 2021 at 2:55 AM
    KP4SX likes this.
  5. N2DTS

    N2DTS Ham Member QRZ Page

    I had to bypass the first microphone stage, way too much gain for a D104!
    With a 4 meg grid resistor and one coupling cap change, the HX-50A seems to sound great!
    15 to 20 watts out seems good.

    Bypassing the 3 MHz filter gives nice double sideband AM out to 6 or 7 KHz of audio limited by the D104 I think.
    Easy tune up, stable and accurate VFO.
  6. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Wow! That's prettier than mine was in 1971!
  7. N2DTS

    N2DTS Ham Member QRZ Page

    I tried a dynamic mic and put both preamp stages back in circuit.
    Sounds really good, goes really wide.
    Can sound fantastic with some audio processing....
  8. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Hmm. I had to bypass the first microphone stage, way too much gain for a D104!
    With a 4 meg grid resistor and one coupling cap change, the HX-50A seems to sound great!
    15 to 20 watts out seems good.

    Just trying to follow what you did but the first 6C10 transmitter audio stage has a low impedance grid resistor R102 that would be suitable for low impedance mics, but too low a value for unamplified D-104's.

    A >= 3.3M resistor for R102 would be appropriate for an unamplified D-104 if one also changes R103 to a 820 ohm resistor, and a 0.01uF cap for C103.

    Additionally, a 0.005 uF should be inserted between J101 and R208 to avoid shorting the grid bias on V101a's grid.

    It doesn't make a lot of sense to increase the first stage's grid resistance and then have a high impedance dynamic mic or an amplified D-104 shunt that grid resistance and alter that stage's bias, hence the need for a 0.005 uF between J101 and R208 if R102 is >= 3.3M.


    Last edited: Dec 7, 2021 at 2:55 AM
  9. N2DTS

    N2DTS Ham Member QRZ Page

    I fooled around with the HX-50-a all day today, trying to get the noise level down.
    I replaced all the bias electrolytic caps (mostly 20UF 250 volts) with 22 Uf 450 volt caps.
    I tried various design changes to take the dynamic mike input and I can get the audio to sound good but always
    had a lot of background noise that sounds like a high impedance mic plugged into a 600 ohm input, that open mic cord hum.

    It was not the power supply hum as I added caps (temp setup), tried bypassing stages for RF, different cathode bias resistors,
    I could change the gain all over the place but could not get rid of the noise.
    As a test, I put line level audio into the audio gain pot and the transmitted audio sounded fantastic.

    The mic preamp tube is a 6C10 which is a triple triode compactron tube, and one triode is amplifying the carrier RF before
    it goes into the unbalanced modulator.
    I guess that is where the noise is coming from.
    That seems like a bad design to put the 3 MHz amplifier in the same tube as the high gain mic preamp!

    With outboard audio the rig sounds really fantastic, nothing to limit the response....
  10. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    It sounds as if the outboard audio connection is the way to go.


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