I echo what KH6AQ advised. In addition an increasing load SWR could be a factor, I saw that happen once on a smaller triode. I would assume its being monitored closely though, so focus on the plate tank, especially a blocking cap that might be heating up. Do replace the grid sensing though and continue to report. All hail the mighty YC156! take good care of my baby! 73 de Reid Brandon W6MTF
Is this THE "Reid", friend & mentor to Bruce Gary (SK)?
N0UN
Bruce Gary was a very smart guy. He flew over to the middle east, and fixed all those grid driven Asamoto amplifiers, that all oscillated when fed into a resonant ant..( and blew up the xcvr to boot)....but worked just fine into a DL.
I just went through a similar scenario with yet another Asamoto grid driven tetrode. Worked fine on 75m into an ant, or DL on 75m....( worked on 75m but real high gain, like 22.5 db)... but would osc on all other bands, whether into a resonant ant, or DL...and that was with amp keyed, and NO drive applied, go figure. Would osc with a DL on both the input and the output.
The fix was to completely rebuild the PI tuned input. The mile long path was eliminated. Terminating resistance reduced from 1000 ohms, down to 500 ohms. On the advice of Robert, W0VMC, a small parasitic suppressor was inserted right at the grid input of the socket. The new coupling cap was wired directly between the 500 ohm terminating resistor ( 2 x 1k flat packs in parallel, and heatsinked to rear chassis) and the new grid parasitic suppressor. Virtually zero lead length anywhere. Bypass caps installed on both sides of the cathode. (oem config only had bypass caps on left side of cathode).
Only one end of both C1 + C2 caps used to terminate the stator.
Ok, now it's 100% rock stable on all bands 80-10m. Gain reduced to 20 db. Neutralization not required. (Oem config didn't have neutralization either). Input SWR is 1:1 from 80-17m. On 15m, it shows a 1.5:1 swr, which is caused by the huge 100 pf of input C of the 4x10 tube...( and 160 pf on a 4x15). That C can't be absorbed by the C2 air variable on the PI tuned input. The fix is a relay switched small coil, to cancel out the XC of the tube. Only used on 15-12-10m.
...
The original oem config had current traveling through a mile of coax,from input side of TR relay, then up to one end of the hotside of the C1 air variable cap, then through the entire length of the stator of the C1 cap, then through the roller coil. Roller wired wrong, with current flowing the entire length of the central shorting bar, when on upper HF bands, then through the entire length of the hotside of the C2 air variable, then a mile of strap to the hotside of the 1000 ohm globar. Cold end of same (really long) globar bonded to chassis via a tall, solid aluminum standoff. Return current path is then through the aluminum standoff to chassis, then through a mile of chassis..and back to bypass caps at only one side of the cathode..at the socket. No grid suppressor used. It's no wonder it oscillated. It was a dog's breakfast from day 1.
On another grid driven amp, this time a hb 4x5 grid driven affair, a 450 ohm globar is used in conjunction with a 1:9 un-un. No pi tuned input. "no tuned input required". All sounds wonderful, until you realize the tube's 100 pf of input C is directly in parallel with the 450 ohm globar. That just screwed up the input swr, on all bands. An equal amount of XL has to be inserted in parallel with the 450 ohm globar....and that's a lot of coil required for the lower bands, since the Xc of 100 pf increases... as freq decreases. EG: 100 pf = 418 ohms of XC @ 3.8 mhz. 418 ohms of XL = 17.5 uh. A small 20 uh roller coil had to be used, and tweaked one each band. On 160m, it's worse, with 35 uh required.
Multiband, grid driven tetrodes, using large tetrodes is not as simple as they appear. If done correctly, they don't have to be neutralized. If they do require neutralization, that opens up another can of worms. The typ, small value vac cap used, can, and will develop 'whiskers' with B+ applied to one side of the vac cap. A special, inert gas type is typ used, or the conventional vac cap has to be over rated. Depending on input config used, the neutralization cap will interact with the tuned input, throwing both off. Then you also have the screen and bias supplies to deal with, plus screen and plate over current protection. Ok, rant over.
