[Milsurplus] BC-375

[W7QHO at aol.com]

Ron,

To add to the comments you've already received.....

Great find!  Unusual to get one "in the box."  Let me echo the plea of DON'T MODIFY IT!  Good news is you don't have to but the set does have some characteristics all it's own you need to know about.

First, you'll need a power supply, 1000 - 1200 vdc and 24 vdc as others have already mentioned.  The 24 volts must be DC.  For the high voltage you can run on less or somewhat more (more on this later).  There were military AC supplies but these are very hard to find.  You can also run it off the original dynamotor but then you'll need a 24 volt supply capable of handling a lot more current.

A  lot of the bad reputation these sets have stems from the way the military ran them. If you load the 375 on phone according to the book (i.e., 220 ma PA current, about 100 watts out)  you'll get serious flat-topping on positive modulation peaks and a lot of FMing.  The resultant signal will be fully intelligible only on a receiver with a wide bandwidth like the BC-348 with which the 375 was originally paired.  The flat-topping can be overcome by reducing the loading.  With a 1100 volt supply, 50 watts of carrier is about right for 100%, (relatively) distortion free modulation.

The FMing varies by band, very slight on 160M, objectionable on 75 and worse on 40M.  Can be virtually eliminated, though, by slightly over neutralizing the PA (the neutralizing control is located under the chart on the front of the TUs).  Easiest way to do this is to tune in a modern rig with FM capability (TS-50, etc.).  The FM component comes through clearly and sounds quite good.  Talk into the mic and adjust the neutralizing control until the audio disappears.  Only a small adjustment should be required.  One note here, make this adjustment with the transmitter feeding the load that will be used on the air.  Looking into a 50 ohm load, for example, the adjustment for a 1:1 swr will be different than for a 2:1 swr.

KEY-DOWN DRIFT:  The oscillator  tube in the 375 has a huge plate and a lot of plate-to-cathode capacitance.  On key down, the plate heats up and the capacitance changes and so does the frequency (of course).   On forty meters this can be as much as 2-3 KHz in the first 15-30 seconds.  Much less and tolerable on 75 and even less on 160.   I don't use my 375 on 40M.

CW:  The amount of chirp with these sets is directly related to regulation of the HV supply.   They sound reasonably good running off a dynamotor.  My home-brew supply is only good for 10-15% and on CW the thing doesn't just chirp, it WHOOPS!  

PLATE VOLTAGE;  At 1000 volts one can run about 30 watts of carrier with good modulation characteristics in my experience.  At 1100 volts this increases to 50 watts and 60 watts at 1200 volts.  At 900 volts the permissible carrier level drops to 25 watts.  

Hope this helps.

Dennis D.  W7QHO
Glendale, CA