The PDX-400 is a late version of the "Phantom" model. It has a transistorized keying circuit in place of the tiny 6AQ5 tube older versions used to key the relay. It will not chatter the relay when the radio's carrier is turned down, the way the tube-keyed models did.
Umm, you might want to keep an eyeball on the four (three?) driver tubes. Those models had no fixed bias voltage on them. The radio's AM carrier is what regulates the current on those tubes. As a result it takes more AM carrier than you really want, to keep them from getting too hot. ONLY the VERY earliest PDX had four tubes in the driver stage. All later versions had three drivers and one tube in its own socket nearer the chassis center. That fourth tube MUST be in place, even if it has no connection to the "linear" hookup. Since this model splits 12.6 Volts AC across pairs of 6.3-Volt heaters, that one, single tube has to be plugged in. Or, one of the three drivers will have a dark heater. That's all it does, to make one of the driver heaters light up. Trying to eliminate this tube, putting a resistor in to drop 6.3 Volts will probably not work right. IF that 50-50 voltage split isn't nearly perfect, the "odd" driver tube may try to pull more power than the other two. When that happens, it will "cherry" and fail. Using another tube in that single socket as the "dropping resistor" just works better.
But back to the lack of fixed bias on the driver tubes. As a side effect, using it on SSB tends to cherry the drivers, since there is NO carrier on that mode.
Until some fixed bias (like what's already wired to pins 2 aqnd 6 on each final socket) is added to the driver tubes, you risk overheating (and popping) driver tubes.
The most popular fix is to unsolder pins 2 and 6 from each of the (three or four) driver-tube sockets. A bypass disc capacitor gets added to ground on each, and a wire is run to join all (3) or 4 sockets on those two pins. A 1k 5 Watt resistor goes to ground, with a rectifier and filter like the one on the final tubes' grid pins. This will put a negative 8 Volts DC more or less onto the driver tubes' control grids, and reduce the heat on those tubes when the drive level is low.
An even easier and cheaper method will accomplish the same thing, but with some loss of overall power. The driver tubes have their own circuit on the high-voltage relay. It is fed from the high-low switch, which feeds either from the "high" B+, around 950 Volts, or from the "low" side, around 475. If the driver's ' B+ input' wire to the driver's circuit of the HV relay gets moved over to the 'low' B+, the high-low switch will only change the final tubes from high to low. The drivers will always be on low. Most of all, they will not draw the high current when drive power is low. Drawback is, they won't drive the finals as hard on "High" side. Simpler, cheaper, not too popular with "wattmeter watchers".
The "PDX" model evolved radically as the FCC tightened the noose around D&A Manufacturing the last year or two. It started out as a straight 10-tube "update" of the Phantom models. Had four tubes driving six, with a band switch so they could pretend it was a ham linear. The FCC tightened the rules against "CB" amplifiers, eliminating the "multi-band" loophole they had been using for years.
Then it morphed into a "15-meter CW transmitter", with a morse-code-key and crystal socket on the front panel. The coils inside were tuned for 21 MHz (NOT 27) and one of the driver tubes got moved towards the center. That one had the crystal socket and morse key hooked to it. It was meant to drive the three driver tubes. With a carrier ONLY, mind you.
Sure. Yeah. We all believed that. In the mail a day or two after the PDX had arrived, were instructions to change it back to a 27 MHz linear, like the buyer had wanted all along. Your "standby" switch went in place of the "Key" jack, and a SSB-delay switch into the hole where the crystal socket had been.
The familiar "Grid Tune" control that was on the rear panel of the Phantom was moved inside, and hooked to the "CW" tube. I've seen plenty of PDX specimens where this circuit was not hooked up. They would run a wire directly from the 'radio' side of the antenna relay to the cathode (input) pins on the three driver tubes. This made the input match around 3 to 1, and limited the power you could get. Copying the input circuit from the Phantom model wasn't all that tricky, and made a big improvement in performance.
The enourmous bulky coils they installed for 21 MHz (and on some of them 14 MHz) would get "jumped" down to the the few turns needed to tune 27 MHz. Performance can often be improved by simply cutting these coils down to size, removing the exess turns. Again, copying what was alread built into the older (pre-bandswitch) Phantom would tend to work pretty well.
Some maniacs would hook up the single "crystal" tube as a "PRE"- driver, but that's a silly idea. Unless your radio only has a half Watt AM carrier, and no more than 3 Watts PEP, this "One driving Three driving Six" setup will be grief. Just too much of a good thing, as they say. Driving the three driver tubes right from the radio-side of the antenna relay is fine for a "normal" CB-type radio with only one final transistor. Bigger "two-final" radios may very well overdrive the three-tube stage.
Seems to me I have a pic here somewhere of a burned, exploded driver-tube plate choke from a PDX. When one of the drivers gets hot enough to break down, that Z-14 choke takes it on the chin, serving as a high-voltage "fuse" for the drivers. Seen it lots of times.
The shenanigans that D&A had to pull, trying to ship the PDX throught the loopholes in FCC regs was a running cat-and-mouse game, until it finally caught up with them. Their tube manufacturing closed down for good, some time around 1980 or so.
Just which "version" of the PDX you have covers a fair bit of ground. And if you see the plates of the driver tubes starting to glow dull red, then orange, you're standing at the edge of the cliff with ten toes over the edge.
73
