Wow, sure wish I caught this thread earlier.
I suspect that this amplifier was built without the 'safety' choke in parallel with the Load control. This is needed to prevent a portion of the power supply's DC voltage from building up across the plates of the Load control. The amplifier's RF voltage alone should not make it arc. But a few hundred Volts DC PLUS that RF voltage will do it. D&A included that choke on some units, left it off of others. Go figure.
A RF choke with at least 22 uH should go from that control's hot terminal to ground. We usually place it alongside the antenna relay, from the center wire of the output coax where it attaches to the relay, to chassis ground on the rim of the antenna socket. The physical location is less important where it's connected. And if your amplifier has this part in it already, you may simply need a new Load control.
Any time the plates begin to arc, the surfaces of the plates become rough. A rough surface will now break down at a lower voltage than a smooth surface. Becomes a sort of downhill snowball. Pulling out a badly scorched plate or two can fix this if the damage is isolated. Only the plates on the rotor shaft can be removed without damage. The fixed plates really can't be messed with.
You REALLY have to tune it up for max modulated power to get the coil right. Simply doing this with the carrier alone will NEVER get you where you want to go.
The exact setting of the coil in an amplifier with six tubes gets pretty critical.
The reason is the built-in capacitor found under the plate cap of each tube. The more tubes you put in parallel, the bigger this accidental capacitor becomes. If each tube has on 15 pf to ground on that plate cap, six of them becomes the equivalent of a 90pf capacitor. Parallel capacitors add in value.
In this circuit, that capacitor is directly in parallel with the front-panel Plate Tune control. This means that when the plates are farthest apart, and the control is set to minimum, you still have a big capacitor in the circuit. The coil has to be shrunk down below what a normal design would call the optimum inductance value. Shrinking the coil so that it tunes 27 MHz makes its exact inductance value way too critical.
Simple rule to get this sorted out. If the capacitor is maxed, with the plates all the way meshed, you need MORE. A maxed-out setting means you don't have enough.
Same deal when the plates are all the way apart. You have too much.
Doesn't sound helpful. You can't make the capacitor's value smaller than the minimum setting.
What you CAN reduce (or increase) is the coil. Since the circuit's minimum capacitance is determined by all those plate caps in parallel, you will always have more than the optimum capacitance in the circuit. That makes the coil's inductance setting way too critical.
Not enough capacitance (maxed out), needs more coil.
Too much capacitance (plates all the way apart), too much coil.
The 'squeeze and stretch' method is the best place to start. You'll find that it's like the artillery officer trying to get the elevation of the barrel right to hit the target. The spotter says the last shot was long, so you raise the barrel. Next shot is short. So you lower the barrel half as much as you raised it. Spotter says you overshot. So now you raise the barrel, but only half as much as you lowered it.
It's called "bracketing" the target. Sooner or later you'll zero in on it.
Stretch the coil, and now the plates on the Tune control are all the way meshed. Squeeze it and now they are at the other extreme, all the way apart. Stretch it half that much next try.
Sometimes shorting two adjacent turns is needed to reduce the coil's inductance more than stretching it can achieve. ANY TIME YOU SHORT A TURN, YOU SHOULD SQUEEZE THE REMAINING TURNS BACK TOGETHER. Same rule for removing a short between turns, but the other way around. Stretch a coil that has had a shorted turn unshorted.
Frequently results in an overshoot, with the Tune control now at full mesh. Shorting across only one-half of a turn on the coil is a valid thing to try when this happens.
Enough repetitions with the same model, and you'll get a feel for how much 'stretch' is the equivalent of shorting one whole turn of the coil.
The pic shows one tube resting against the rear of the Load control. Once this tube gets hot, the cold spot caused where the glass rests against the metal of the control will fracture the glass. ALWAYS make sure you have a gap between the tube's glass envelope and the rear surface of the Load control.
A better fix would be to move that tube socket to the rear a quarter of an inch or more. Way more work that I could justify, compared to simply leaning the tube safely away from the control.
73
