DX-100 Low Grid drive after Choke Input

[W3MMR]

So, I wanted to re-visit choke input on my DX-100 and figure out how to increase grid drive. The max voltage on the center terminal of the drive pot is 326vdc when the LV supply is cap input, and 230vdc max under choke input. The voltage is derived from the bias supply cap, through a 6.8k 7w resistor, then into the drive pot.

How can I solve this? Stupid question to some, i know.  But, I want to keep it choke input. The modulator screen voltage is now derived from the LV supply through a relay and mod current is finally rock solid. And for other reasons I would like this to work....

**Also, one thing I noticed was the driver current, according to the panel meter anyway, is the same as it was before switching to choke input. I find that interesting**

Its the last step before it goes back in the case...

Thanks!!

Perry

[N1BCG]

The low voltage supply powers several circuits that rely on having enough voltage. The most sensitive to deviations is the 6AU6 oscillator. The plate supply isn't regulated since the oscillator is electron coupled, but the screen grid voltage is regulated by an OA2. It's important to ensure that the supply voltage remains within the operating current range of the OA2 (5-30mA). Other LVB+ components are the 12BY7 buffer, 5763 driver, the 12AX7 pre-amp, the 12BY7A audio driver, and now the 1625 modulator screens.

Low voltage will have a sweeping effect, but you knew all this...

You can try putting a small value non-electrolytic capacitor in place of the input cap you removed. The exact capacitive value will depend on several factors, but be sure that the voltage rating is sufficient and that the proper 1625 screen voltage isn't exceeded. Same for the OA2 current.

[W3MMR]

Thanks Clark. Book reccomends 5mA of Grid current. I can achieve maybe 2.5. I am still seeing full RF output of 115w, however, the driver current, according according the panel meter, is much higher than it was before only to achieve less grid current. Now, I think what the panel meter is calling "Grid Current" final amplifier grid current. I have grid drive which is driver current. But I can't achieve 5ma of final.amplifier grid current now I guess.

Do I just leave it be?


Perry

[N1BCG]

Thanks Clark. Book reccomends 5mA of Grid current. I can achieve maybe 2.5.

This is 6146 grid current and it will be affected by the RF Drive setting. You might achieve rated output with no mod but positive peaks will be limited by insufficient drive. The driver current is going to read higher because you're trying to get the same power from the 5763 tube with less plate voltage.

Your LVB+ has to come up.

[W3MMR]

It's just kind of odd that Timmy seems dumbfounded that I have low grid current, I don't see any other folks mentioning it, but it seems pretty obvious that there would be an issue...

[Pete, WA2CWA]

You might want to check the meter grid shunt. It should be 5.65 or 5.55 ohms. A sway in resistance will lead to bogus grid readings. Also check the Clamp tube adjustment and the various resistors hanging off the grids of the 6146's. Any of which could affect grid reading.

[KB1VWC]

  Keep it choke input for your screens if that is what you want.  Put a blocking (or steering) diode off the output of the rectifier tube, and go to a capacitor input filter. Anode of the diode to the rectifier tube output and cathode to your new cap. Now attach everything but the 1625 screens to this new cap input supply.  High value 450V caps are both cheap and small.

  Get the best of both worlds. Or you could build up a quick single transistor series regulator and leave the low voltage supply alone?  Either way, you need one or more of those little solid state devices in your old Benton Harbor kilowatt.

  Steve

KB1VWC

[W3MMR]

  So, after chatting with Tim the other morning and going back and listening through the recorded QSO comma he told me to mess around with the value of the 47 puff capacitor to ground off of the grids of the 6146. I didn't have anything smaller at that voltage rating so I just took it out. I was then able to achieve 5 mils of grid current. However, when I went to test my audio, it sounds like it's going into Side Band, and the pattern on the scope did not look normal to me. So I put the 47 puff cap back in,  tested with low grid current, same result with the audio and scope pattern. Took out all the solid state rectifiers, still the same result.. 

   So I switched it back to cap input, and my audio went back to normal!!! it seems like it's something to do with the choke input. I also noticed that I now have a hum with choke input.

  Keep it choke input for your screens if that is what you want.  Put a blocking (or steering) diode off the output of the rectifier tube, and go to a capacitor input filter. Anode of the diode to the rectifier tube output and cathode to your new cap. Now attach everything but the 1625 screens to this new cap input supply.  High value 450V caps are both cheap and small.

  Get the best of both worlds. Or you could build up a quick single transistor series regulator and leave the low voltage supply alone?  Either way, you need one or more of those little solid state devices in your old Benton Harbor kilowatt.

  Steve

KB1VWC

Id love to leave the damn thing alone to be honest. The radio worked so good for a few years. All I did to lower the screen voltage on the modulators was use a dropping resistor in series to get it down to about 300vdc. This was before I solid stated the high voltage supply. If my novice mind knew of a way 2 regulate the voltage at 250 volts for the modulator screens, and leave the low voltage Supply cap input filter, I would period I was thinking about maybe a large zener diode to clamp the voltage period but that will be awfully large.

You might want to check the meter grid shunt. It should be 5.65 or 5.55 ohms. A sway in resistance will lead to bogus grid readings. Also check the Clamp tube adjustment and the various resistors hanging off the grids of the 6146's. Any of which could affect grid reading.

Will look into that... thanks


Perry