The AM Forum

I found this schematic folded up in an old handbook that was so worn it was headed to the trash. It is a copy of a hand drawn schematic of a 100-T, a transmitter made by Harvey, a radio company located in Cambridge Mass. I found the ad which shows the set in a 1947 Radio Amateurs Handbook. This is a nice balanced rig and a good snapshot of pre-war commercial ham radio design.

Mike WU2D

Here is the rest.

Not to complain, but could you take a scan of the same type down the center of each schematic page? There is a little bit missing for example the buffer V2 in the RF section, and in the audio section there is something missing from there in the middle.

I scanned the middle. Time for a bigger scanner!

There are some interesting circuits being used here and many things to take note of.

The way they do the grid biasing for the 814 final by tapping off the B+ return is interesting. I guess you could think of the 814 as the "pair of 6146's" of the day. Note that the Pi network is already being used but the 6AG7 is absent with a the 6V6 doing oscillator duty. The added grid to plate feed back on the 6V6 oscillator is interesting. The band-switching is clever and practical. There is no lack of metering and each unit has its own power supply on board.

In the modulator unit, the 6SJ7 high gain microphone stage is followed by the 6SF5 high gain class A stage right into a triode connected 6F6 running in Class A or B. Again the Bias is for the 807's cleverly tapped off the B+ supply return. Also note the 0.01 ultrasonic suppression caps on the driver transformer secondary. 

It is interesting to compare a manufacturer's design with a "handbook" design.

Mike WU2D

If no one minds, I spliced the schematics together. It's more fun to examine these interesting points that way.  I like a 6F6 in class A. Some people disparage pentodes in favor of beam tubes but both have much merit.

I am looking for bandswitching, but see only the doubler in/out. Does it use plug-in coils or have a wide band tuning scheme?