The AM Forum

Anyone have a fav schematic for a low-power AM TX using an 837 final or two? ;)

73DG

The old VHF aircraft transmitter designs using push pull from WW2 designed by the British, were widely copied and integrated here in the states. The SCR-522, VHF T-89, ARC-5, and T67 ARC-3 and ARC-4 are examples that all used the RCA 832 for around 8 Watts out up to 150 MHz. Typically these used a single ended crystal or VFO oscillator and and an 832 tripler into an 832 Final.   

With a tapped heater you can light it on 6 or 12 V.  So let's say you want to use it on HF instead of VHF. Push Pull is really not the easiest topology. I did this with a 10 Meter transmitter with an 829 and just put the two sections in parallel.  It acted like two 6L6's in parallel or maybe a single 6146 but it needed a lot more drive. Other wise it worked fine. With both 837 sections in parallel I assume you might get 25 Watts out on 10M. Ether way, a pair of 6V6's in P-P will modulate an 837. It would make a cool mobile rig.

The two old schematics I attached show how the tube was driven. You can drive it with a single ended stage, even if it is P-P input and output on the 837.

Ummm...the 837 is a 7-pin based single pentode.  Maybe cornfused with the 832?

https://frank.pocnet.net/sheets/049/8/837.pdf

73DG

Wasn't that used in an Army set? IIRC there is a green colored metal box of spares, that had one of those 837s in it.

The pre and early war TBX family of radios used the 837. The TBX-6, my favorite used one 837 as a high-powered oscillator and just for fun allowe3d you to suppressor modulate it for AM. They used a special high output carbon microphone. I operated one at Dayton a couple times on the WW2 3885 AM Net.
The tube it 12 volt heater and the only other voltage required for the transmitter was 500 volts for the plate and screen. Beyond that the only problem was it used a huge heavy crystal that fit in a round holder so rocks were not easy to come by but the VFO was stable enough that you can set it at home, lock the knob and it will stay on channel when you are using it in the field.
The transmitter section of the TBX was powered by a crank generator and the receiver used dry batteries for the 3 volts for the filaments and 90 volts for the plates. They use mostly 34 tubes in the receiver and the whole receiver only drew under 10 Ma under normal operation and that always amazed me that the old battery tubes were that efficient.

Well Ya! The 837 not the 832.

Hey the 837 was abused and it was the basis for early linear amplifiers among other things. Hey and its really a fine VFO - the ART-13 uses it in the oscillator as do some of those VHF rigs I mstakenly posted. Think of it as a man's 6AG7. With it's loose supressor grid, you can modulate it with almost nuthin...

Might be better to consider a low power rig based around 807 tubes.  Or 1625s which are 12.6 v versions of the 807.

Pretty low cost tubes, and there are lots of articles about rigs built around them.

The ARC-5 aircraft radios used in WW2 , for example.

Thanks for the advice.

I have plenty of tubes for my projects.

The 837 was a neat, early design.

73DG

Hey Dennis,

Here's what I've come up with for that Navy transmitter we talked about. The printer scan cut off part of the image, but what's missing is the 4 - 100vdc zeners to regulate voltage on the 837's and the 803 screen.

No modifications whatsoever to the transmitter itself. I'm allowed to be a purist doody-pants. HV, LV, VFO spot and keying are all done external to the transmitter along with a high/low power output switch for the B+ on the final.

I'm running out of space in the shack for new stuff without risking upsetting the feng shui, so I'm going to build the power supply into the bottom 3 shelves of my bookcase. It'll blend nicely.

I'm ready to make the pharmaceutical trade in Yuma sometime Friday or later.

Just let me know if you or I am bringing the Thai friends.

Jon

829B Transmitter Exciter For Ten and Six, pp492-494, and  832 Transmitter for 144 Mc, pp441-444, The Radio Handbook, R. L. Dawley, Editors and Engineers, Thirteenth Edition 1951

Sorry guys, my mistake of cornfusiing the 837 with the 832 has messed up the threead to the point of folks playing with both tubes!

Mike WU2D

No harm, no foul  ;)

73DG