simple modulator

Discussion in 'Amplitude Modulation' started by KC2ZFA, Jul 16, 2020.

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  1. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    please feel free to criticize what's shown in the attachment.

    I do have the parts for a solid-state audio driver (and most of the parts for a tube driver), but to begin with I'll just use a reverse connected output xfmr, an OT20PP, to feed the grids.

    I believe R2 is not needed when using the reverse xfmr, but is needed when using an audio driver.

    R1 has been mentioned as 100 ohms for triode connected 813's, but not for the listed triodes.

    Only the secondary of the filament xfmr is shown.

    The 50K should allow the HV to stay on during receive (?).

    The amplifier driving the OT has a mute that will be part of the T/R sequence.


    Attached Files:

    WA3JVJ likes this.
  2. N2DTS

    N2DTS Ham Member QRZ Page

    Looks ok to me, although I do not leave the voltage on.
    For 811/572b tubes a string of diodes with a rotary switch will enable bias so you could take it up to 1750 or even 2000 volts.
    Each diode drops .6 volts.

    What is the RF deck?
    Same power supply?
  3. W3RSW

    W3RSW Ham Member QRZ Page

    This is very similar to the modulator I’d built for my twin 813 rig modulated by 572B’s.
    I used a Fair Radio surplus Gramer xfmr. Corp. 30 watt transformer backwards as you’ve shown fed by Radio Shack 35 watt PA amp. The Gramer has 30, 60, 125, 250 and 500 ohm inputs. I used the 30 ohm fed by the R.S. 8 ohm tap for best, but not over kill matching. Using the 30 ohm tap yielded 1.19k for each grid instead of 4.5 k.*
    So you can see many Audio transformers don’t have to be exact spec’d if your aware of voltage vs Z matching, etc. I did not need any resistors to ground feeding the 572 grids, but did put 250 pf to ground followed by 1,5 ohm series to each grid. Oh also I put 2200 pf across the 30 ohm input as it was too bright and didn’t want the full 10 k Hz freq. swing. Try stuff pleasing to your ears. Fun.

    One side of the Tied filaments go directly to the filament transformer, the other side individually to 30 ohm /10 w resistors, as I didn’t have a ct. fil xfmr., their other sides commonly tied to a 10 watt 6.8 volt zener bypassed with a 500 uf/50 volt electrolytic. If you have a CT fil xfmr., you won’t need these two 30 ohm resistors of course.
    Between the zener and the 2ea. 30 ohm tie pt. is a normally closed jack in which I can plug in a current meter. I don’t think you’ll need grid metering.
    (all the 30’s numbers are coincidental, not a typo. ;)

    I normally just use a MPF-102 FET source follower after an Astatic crystal D104 mike with 9meg input into The RS amp Which has three Chan. Sort of parametric EQ, preamp, etc, so isn’t super hi if but still sounds very nice.

    *Interestingly the Gramer intended application Secondary is rated at 9kCT, 100 ma, 30 watts, 750 WVRMS max., but 535 RMS +/- 1 dB, 50 to 10k cps. It’s a hefty, potted little thing.

    Also you may wish to install In the cathode circuit just after the zener a 50k 10 w resistor, the bypass of which is controlled, to short out the 50 k in ‘transmit.’ This cuts modulator noise in your receiver by cutting off cathode current in ‘receive.’
    I installed it in mine but simply jumper around it since I couldn’t see much difference. ;)
  4. W3RSW

    W3RSW Ham Member QRZ Page

    The 9meg is in the FET follower’s input, about 4.7 k from source to gnd. , With low z from the top of it of course feeding the RS amp.
  5. N2DTS

    N2DTS Ham Member QRZ Page

    This is the one I built/use.
    Its the deck with the lit tubes.
    That uses a Hammond hifi transformer driven by a symetrix 20 watt per channel audio amp.



    From bottom, power supply, exciter/vfo, modulator, rf control deck.
    Seems to work great.

    You can do it this way with the tubes behind glass by mounting the tube sockewts on a sub chassis:

    Or go big time!

    If you have the room, do the glass...
    WA3JVJ, NQ8J and KD2ACO like this.
  6. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    You build the good stuff, Brett!
  7. W3RSW

    W3RSW Ham Member QRZ Page

    Meant to add that your circuit already shows the T/R coupled relay in cathodeS line. Another reason I left mine jumpered was that the modulator idling current is left on during receive, this acting as a another partial regulating load on the common HV power supply which feed both my Class C RF amp 813’s and the mod tubes. I keep remembering design details from the dregs of a last decade build. Mike’s FET audio cut off and grounded during receive w PPT stalk on D104. So far so good.

    and Brett, Gotta agree, yeah VERY nice stchuff!
    Last edited: Jul 17, 2020
  8. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    yes, ‘DTS homebrew stuff is top of the line.

    What kind of zeners for the switched setup ?

    I have a 6ag7-807 mopa I built and a dx-40 I repaired as exciters. Other stuff too, in various construction or repair stages.

    The PA is a classic 812A push-pull stage with JVL grid and TVL plate coils. Been fixing and testing this thing for years. I think it’s ready to play. It has its own supply.
    WA3JVJ likes this.
  9. N2DTS

    N2DTS Ham Member QRZ Page

    If I was building the stuff today, I might leave the power supply on all the time, but at the time I went fast step start.
    Now days I just removed the step start relay and since its choke input, I just start it up.
  10. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    thanks for the details ‘RSW. I want to stuff the audio chain and modulator into the box shown below. The blank panel will be the mod’s panel.

    Mod xfmr and supply external to this box. BBAEB331-DCE7-4139-B302-0CF32CA7BB29.jpeg DF661865-1DC9-4846-84BB-8336D29F80BC.jpeg F109600E-0676-45D4-A4AE-2994CD415284.jpeg
    WA3JVJ likes this.

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