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Johnson Valiant grid current with plate switch off.

Discussion in 'Amplitude Modulation' started by AG5CK, Jan 25, 2021.

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  1. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    The first thing I would try would be to try adjusting the driver plate/final grid tuning capacitor to see if that affects the rogue grid drive. If it does, that shows some kind of self-oscillation in one of the earlier stages, or else the VFO and driver stages are somehow still delivering RF. If none of the tuning controls affect it, then it's probably a DC voltage issue; something is not cut completely off or there is voltage somewhere it's not supposed to be. That grid current has to come from somewhere.

    That reminds me of the time the 1st stage in one of my transmitters was self oscillating with ALL the DC voltages cut off by opening the primary windings of all the plate transformers. Here's the story:

    Several decades ago, right after I had just got my transmitter working on 160m (a single 304-TL modulated by triode connected 813s), I started hearing an annoying unmodulated carrier popping up near my operating frequency. It was a very stable, solid, clean carrier with no QSB and would appear day and night, so I assumed it had to be someone tuning up near-by, but they never ID'ed or attempted to transmit code, and I never heard any modulation. Just a continuous carrier out of nowhere that would sometimes stay on frequency for hours. This had me totally baffled for a couple of weeks, as I thought it it had to be someone local, but I knew of no nearby hams in the vicinity. Was someone playing a joke on me, or maybe it was a new ham close by, trying to get a transmitter on the air.

    Then, one time I just happened to bump one of the tuning knobs on the rf exciter, and the carrier went away. Upon further investigation, I discovered that rotating the plate tuning knob on the first buffer stage following the VFO changed the frequency of the carrier! It turned out to be an inexplicable self-oscillation in the first stage of my exciter that occurred on 160m only; it was a very stable self-oscillation that would have made an excellent VFO. But the strangest thing was, this was happening with nothing on but the tube filaments, with the transmitter in standby mode with the HV and LV plate supply voltages completely turned off by cutting the a.c. voltage feeding the transformer primaries. But how could an rf stage go into self-oscillation with no DC voltages on any element of the tube?

    The offending stage used a 6V6 as a buffer stage following the external VFO. The power supply used a 5Z3 rectifier tube. I began measuring voltages with my multi-meter, and discovered that the plate of the 6V6 stage registered about +15 volts. I could short out the B+ line to the stage, and the oscillation would stop. I pulled out the 5Z3 rectifier, and the oscillation stopped, but shorting out the LV plate transformer secondary winding with a clip lead did not kill the DC voltage or the oscillation. Therefore, this was not the plate transformer picking up stray 60~ a.c. by magnetic coupling to another transformer. After some head-scratching, it finally occurred to me what was happening. For stand-by operation during receive, I turned off the DC voltage from all the plate and screen supplies by opening a relay in the a.c. line to the primary of each transformer, leaving the secondary winding connected but un-energised. The 5Z3 stayed lit all the time, since all tubes were fed with a separate filament transformer. With the transformer supplying no a.c. voltage to the plates of the rectifier but maintaining DC continuity to ground, there were enough stray electrons emitted by the heated filament with high enough velocity to randomly hit the rectifier tube plates, causing enough of a charge to accumulate and generate about +15 volts DC output from the power supply, and sustain enough steady current through the transformer winding and the 6V6 to allow self-oscillation in that stage.

    My solution to the problem was to change the 6V6 in the 1st buffer stage to a 6AG7, which is a far better shielded tube at rf. That completely killed the oscillation. That 6V6 with its internal grid-plate capacitance must have been extremely prone to self-oscillation, which occurred only on 160m because it operated straight through on that band but worked as a frequency multiplier on all others. I still use that rf exciter unit to drive my 8005s modulated by 838s homebrew transmitter that I normally use on 40m.

    That was a typical example of the kind of technical problems I have always had with my radio equipment. Very rarely do I ever see a textbook malfunction with a textbook solution, like you read about in the handbooks. It's nearly always either a frustrating intermittent, something that works but not quite right, or else some weird problem like the one described above, something I wouldn't have thought of in a thousand years.
    AG5CK likes this.
  2. AG5CK

    AG5CK Premium Subscriber QRZ Page

    Thanks Don. So far the rogue grid current issue hasn't happened again.

    I put a few hours of talk time on it late last night into the early morning. So far everything has been working as it should. I don't know what was going on.

    I would recommend a Valiant to anyone wanting to learn about old transmitters. They seem to break down a lot.
    W1BR and K0OKS like this.
  3. W3SLK

    W3SLK Ham Member QRZ Page

    The one I have has been stipped of its speech amp. I'm currently rebuilding it from scratch using W1HZK's mods. I even have 6146B's in for modulators. But it is tough trying to maintain ambition on this project!
  4. W1BR

    W1BR Premium Subscriber QRZ Page

    A friend offered to put up a Valiant for me next summer when he visits friends in the Midwest. I'm tempted to take him up on the offer, although it means the BW-5100 loses its spot. I'd rather work on a Valiant after spending a few weeks in a crowded Ranger II--which reminds me of a small toaster oven.

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