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Gonset G76

Discussion in 'Amplitude Modulation' started by N2DTS, Jun 4, 2019.

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  1. AC0OB

    AC0OB Subscriber QRZ Page

    The G76 is an interesting design and it uses one of my favorite tubes in the final, the 6DQ5.

    I would recommend as a replacement tube the Sylvania 6DQ5 because in my experience with various brands, the Sylvania has the highest power sensitivity and is the most reliable.

    The only pique I have with the G76 is with the Clamp tube circuit. It does not allow the owner to
    adjust the final's screen voltage for optimum power. I would limit the Final's screen voltage to 150V with the circuit below.


    Attached Files:

    Last edited: Jun 17, 2019
  2. N2DTS

    N2DTS Ham Member QRZ Page

    True, but the ones I had in the past would do 70 watts out without issue, and others report the same.
    I have loads of spare tubes, so no worry there.
    I am not worried about 10 or 20 watts so much as why the power is limited in this G76.
    I tried a different 6DQ5 tube with the same results.
    On past G76's I had normal amounts of grid drive, on this one its high and has to be de tuned to be in range which indicates a problem, grid drive is too high.

    I had other stuff to do last night but managed to remove the 25uf cathode bypass cap in the driver tube (audio drive and audio output on RX) to add some neg feedback and lower distortion.
    Made little difference (or none) in audio output.
    The 470k resistor from grid to ground on the mic input is next, to a 1 or 2 meg and the .001 bypass grid to ground will be changed to a 220pf cap.

    I need to check all the tubes, check values in the grid drive circuit, do an alignment, etc.
    I ordered a jones plug for the power hookup (10 pin) and need to add a 12 volt DC setup for the relays.
    Kind of hard and unsafe to work on the radio with JS clips with HV on them all over the place...

    I worked someone a long time ago who was getting 100 watts carrier out with good audio and negative cycle loading, but I have no idea how he did it.
    800 volts or more maybe, but I see no reason to push things that hard.
    Some people just like to try and do such things, like getting a Johnson rig to sound like a broadcast transmitter...
    Its all for fun I suppose..

  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page


    Check the following resistors:

    R-11 1,000-ohms

    R-12 27,000-ohms

    R-70 220,000-ohms

    R-71 100-ohms

    If R-11 has gone higher in value, the meter will also read higher than the actual current. R-12 will have a lesser effect but it still can cause problems. R-70 and R-71 can also have an effect as well as V-17 a 6AQ5. However, R-11 and R-12 are going to have the greatest effect on having a wrong reading.

    If any resistor is very far from the correct reading, replace it with a 1% tolerance version which are inexpensive.

    Glen, K9STH
  4. N2DTS

    N2DTS Ham Member QRZ Page

    Well, the radio will not put out any more then 50 watts on 80 meters, regardless of the meter reading.
    You can get more current to show on the meter, but no more power output.

    I was wondering about it, what would allow more final current but no more power output?
    Excessive bias= low current,
    More grid drive seems to lower power output,
    Tuning and loading controls work normal,
    Low screen voltage=low plate current,
    High screen voltage=more power output...
    Maybe something in the pie net....

    Will check components....
  5. N2DTS

    N2DTS Ham Member QRZ Page

    I may try these changes, but space is tight for new parts...

  6. AC0OB

    AC0OB Subscriber QRZ Page

    Q1) What is the value of R11 and the voltage across the sampling resistor? This will give you an indication of grid current.

    Q2) What is the frequency of the waveform across the 1k sampling resistor? Is it the correct frequency?

    Q3) What is the screen voltage at Pin 8 of the 6DQ5?

    Last edited: Jun 17, 2019
  7. N2DTS

    N2DTS Ham Member QRZ Page

    I need to get a power plug before doing much more with this thing as I got a touch of 720 volts last night.
    I did the 2 meg d104 grid resistor and removed some bypass caps, added some coupling caps, and that lets the hum through.
    Ran the filaments off DC and got rid of most of the hum (as a test), and tweaked the selectivity by changing some resistors on the IF cans.
    I also tried a number of 6DQ5 tubes and had a wide range of results, but no more power out then 50 watts.
    The tube I have in now seems to act normal like the old radios I had in the past, grid drive is in the ballpark.

    I had the D104 in one had and reached over the radio and brushed the HV and smoke came out of my finger.
    I have not done something like that for a LOOOONG time but it was 720 volts across the chest, not the best way to get a shock.

    But I should be good if I have a heart attack as I have been pre jump started...

    While waiting for the connector, I will test all the tubes and measure some resistors, fix the meter glass/plastic and other non voltage type things....
  8. K4KYV

    K4KYV Subscriber QRZ Page

    Why only 2 megs? Per the D-104 data sheet, Astatic recommended at least 5 megohms.

    I used to have a hum problem with my single-ended pre-amp with a 12AX7, and I had to run the filament(s) on DC to get rid of the hum. My present pre-amp uses a pair of 6F5s in push-pull (each one is equivalent to half a 12AX7), and I get acceptable hum level with a.c. on the filaments. The push-pull circuit with tubes selectively matched for the best balance and hum, allows the D-104 to look into a 20-megohm load.
    WZ5Q likes this.
  9. N2DTS

    N2DTS Ham Member QRZ Page

    On this radio (50 watt carrier) I don't see the need for a lot of low end or close to hifi audio.
    I want it to sound a little bit punchy but smooth and clean.

    I have found, with tube amps for the D104, you don't need very filtered DC on the filaments to get rid of the hum.
    You can use just a diode and cap at the tube socket, the cap value sets the voltage to the filament, in the 32V I used something like 220uf and got 6 volts and
    eliminated the hum from the unbalanced filament setup.

    And, with my hearing, I find I like a good amount of mid range punch, and since many guys are getting old (er) a lot of low end might just sound stuffy.
  10. AC0OB

    AC0OB Subscriber QRZ Page

    I feel the same way. I prefer a flat mid-range with some HF pre-emphasis for bit of punch.

    Below is a suggested set of mods for the G-76 which "flattens-out" the audio bandwidth and gives a bit of boost to the 3.5 kHz area:

    One of the obvious things needed is to get DC off the mic gain potentiometer which is one of the mods.



    Attached Files:

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