Viking II, no grid drive on 20, 15 or 10

[AB2EZ]

The rf choke, L6, between the grids of the 6146's and the remainder of the grid biasing circuitry plays a key role here.

If the reactance of the choke is too low on 20m - 10m, then the driver may be looking into too low an impedance (see the comment regarding C56 below)

Make sure this choke isn't damaged, and consider replacing it. Recall that, above its self resonant frequency, this RF choke will behave as a capacitor.

Also, make sure that C56, the 0.1uF bypass capacitor, is connected to the correct side of R30 (the clamp tube circuit adjustment pot). If this capacitor is connected to the wrong side of the pot, it would provide a low impedance path from the bottom of L6 to ground.

Stu

[w4bfs]

   Thinking about modulating a class C RF stage with drive "just" to the threshold of grid current. As the Rf B+ varies at an audio rate, at some point going upward the output will not follow the formula P=E^2/R where at 100% modulation the plate voltage at the peak of the audio cycle doubles. Here the PEP power is 4X the carrier level. I contend that without grid current we hit a point where the tube emission reaches a limit based upon the peak grid voltage available. When this happens, the R value in the formula rises, and upward modulation flat tops. Every plate modulated rig I've had does this; reduce grid drive, and modulation peaks flatten prematurely.

  Jim
Wd5JKO

there is an 'ER' issue from several years back with an article called the 'grid currentless wonder' ... I have been in qso with this rig and its tx audio is magnificent .... its a 15W exciter that is a grid modulated 6DQ5 (mod tube and entire audio string is one 12AX7) .... the sweepie is class C of course but is not driven hard enough to draw grid current ...recommended ...73  John

[WD5JKO]

there is an 'ER' issue from several years back with an article called the 'grid currentless wonder' ... I have been in qso with this rig and its tx audio is magnificent .... its a 15W exciter that is a grid modulated 6DQ5 (mod tube and entire audio string is one 12AX7) .... the sweepie is class C of course but is not driven hard enough to draw grid current ...recommended ...73  John

  A grid modulated amplifier is a form of "efficiency" modulation. The assumption of grid, screen, or suppressor modulation is that the tube is driven hard into class c. In order to develop 100% modulation, the RF output is reduced by biasing back the modulated grid till the carrier output is about 1/4th the normal value. This drops the efficiency into the low 30% region when unmodulated. This value increases with modulation. In this example, a 6DQ5 cannot put out a 15 watt carrier unless:

A.) The tube is overloaded beyond device plate dissipation ratings
B.) The modulation is restricted to something less than 100%, and therefore the drive is increased allowing higher resting carrier efficiency.

A strong signal modulated to only 50% can sound extremely nice. At a distance though, and after ionospheric propagation takes its toll, that signal will be just a blip on a pan adapter with no recoverable audio.

If we reduce the drive further such that the driven grid has no grid current, doesn't the efficiency degrade even further?

Jim
Wd5JKO

[WA2ROC]

I found the problem and fixed it!

Not the vfo cable, not the band switch, not the tubes.

I started replacing bypass caps finding none were bad.

Then I got the multimeter out and checked resistances in the 6AQ5  circuit. None were bad so on to the oscillator resistors. The screen dropping resistor had increased in value.

TO INFINITY!!!

Replaced it and tuned up on 40 meters. I have more grid drive than Bellvue has bed pans.

20 meters tunes up great, so does 15. 10 meters Ok but final tuning is limited.

HAPPY HAPPY HAPPY!

[AB2EZ]

Dick

Congratulations!

A good lesson for all of us in troubleshooting vintage electronic equipment.


Stu

[w4bfs]

nice going Dick ... enjoy

Jim .... read the article .... adjustment procedure has adjusting until modulated pep audio is 4x  carrier power .... over dissipation I cannot speak to since I don't have the exciter .... it has been mentioned in some literature (unknown) that a 6DQ5 is underrated .... it probably can do what a 6146 can do or slightly more .... it has comparable transconductance ...73 ...John

[w4bfs]

I apologize if anyone has taken umbrage with my comments ....no offense meant .... this ham radio stuff is way too cool to get bent

the grid currentless wonder is in Electric Radio issue 118 by Robert Dillworth W4LQE .... this is worthy material to digest ...73