The MW-5 was one of the first generation Harris PDM transmitters, no modulation transformer. The PA tube was simple enough being just a triode but its cathode was connected in series thru the PDM filter to the plate of the modulator tube. So the PDM signal that controlled the modulator tube, a tetrode or pentode don’t remember what it was also controlled the output power. When you started the transmitter there was some sort of feedback loop with the PDM driver that sensed the output and if everything was not right the transmitter would not start. And that happened often. Being the RF output amplifier floated above the modulator all of the components of the output stage were isolated above ground including a strange looking filament transformer for the PA tube.
The high voltage power supply was a lot larger than you would normally encounter in a radio transmitter being something like 10 or 12 Kv because the two tubes were in series. Think normal plate voltage on the PA was around four or five thousand volts but remember that it floated above the modulator that had another five or so thousand applied. On a couple of the ones I had they were good transmitters usually only requiring new tubes once a year and the occasional fan belt and light bulbs, they had lights in the PA cavity so you can peak in at the grid and plate current meters but the transmitter at WJDY that got squashed had no end to issues. Things like transistors in the PDM driver that ran in class D use to short out on occasion and were a real pain to get to, the HV shorting contactors would die and drop down and short out the HV supply with colorful results and also had to replace the HV transformer on that transmitter and whoever installed the transmitter originally did it with about a little under two foot clearance behind the transmitter so you were not able to fully open the back door and had to remove it from its hinges and pass it around to the front of the transmitter then to outside because the building was so small. When the HV transformer failed had to disassemble the front of the transmitter and drag it out that way. Another fun thing about that site was that once the CP current meter failed, being that station went on the air back at the beginning of time they used antenna current meters that were directly in line instead of the Delta loops that everyone uses today. The CP current meter at the input of the phasor failed and it was mounted panel of the phasing network isolated by some phenolic block or something like that just behind a window on the cabinet. When the meter failed somehow the RF started arcing from the meter to the cabinet and after burning up the meter and block it was installed on the plasma from the RF set about burning a hole thru the front of and down about six or eight inches of the steel rack plate that was part of the phasing network. This was not the thin steel that we see in modern rack panels but steel plate that was maybe just a little bit thinner then quarter inch. Of course the whole time the transmitter was burning traces up and down the cabinet it had no issues staying up and doing the destruction. After knocking the building and the MW-5 over we installed a new prefab building at that site but the old phasor cabinet is still in operation to this day, considering its been in continuous operation from 1949 until today it may be one of the oldest items in broadcast radio that’s still in constant use. Think that’s about sixty eight years of service! You can see a picture of the site and phasor on one of my web pages at:
http://staff.salisbury.edu/~rafantini/transmitter_site_tour.htm
