The AM Forum

I am building up a portable system to use at the Dayton (Now Xenia) Hamvention on the local 3885 AM net on Saturday. It’s a low power net that’s mainly WW2 field radios and the like. My system consist of a RCA receiver and a ARC-5 T-19 transmitter that’s being powered by a Narco solid state power supply modulator that was designed to work with vacuum tube VHF radios back in the fifties and sixties. Being the whole system has to be able to run from a car battery did not want to use the original dynamotors because of the current drain, weight and just don’t have them anyway.
I started with a gutted ARC-5 transmitter chassis and have put the MO and two 1625 tubes back in it using the original design from back in the old days. The thing is that I don’t want to run that much power being that I only have about +330 for the plate supply and maybe five watts of modulation from the Narco T-12 power supply – Modulator.
Have the receiver up and working with no issues and just started playing around with the transmitter last night and the MO of the transmitter is working great with no chirp or drift with +165 on its plate along with using the same +165 volt supply to feed the screen grids of the two parallel 1625 tubes for the PA along with the modulated output of the narco providing +330 on the plates. This was giving me about 8.5 watts but did notice the modulation was a little low maybe running sixty percent or so. I have not played around with the modulators drive level yet and may have more available but first a couple questions come to mind.
1- How important is screen regulation on the 1625 tubes? Did see that the screens were drawing between ten to twenty mills but not seeing any significant change in the voltage when watching it. One thing I have seen is that it appears a lot of effort is put into screen regulation on tetrodes. Do I need to think about this?
2- If I want to reduce power what’s the lowest screen voltage that I can run on the 1625 with +330 on the plates? Was thinking that maybe I can put a dropping resistor in line with the screens and knock them down to +100 volts? Is there a point where the screen can be too low and cause the 1625 tubes to become an issue?
3- How will a short antenna affect modulation? Have been using a dummy load so far but in the field this will be working with a short vertical antenna. The transmitter has a loading coil that can be used to lengthen the antenna and know that I will more than likely be adding an additional coil to the base of the antenna so is this one of those thing where you set the modulation level into the antenna you are using to set it properly?
The best thing about this project is that unlike almost everything else power is not a big factor. If I can pump out five watts that will be plenty for the event. Will attach a schematic of what I have for the transmitter and maybe a picture of the progress so far.

Ray F/KA3EKH

Picture of the receiver and transmitter so far.

Nice setup!

You should be able to modulate the screens through a dropping resistor off the 330 volts from the modulator. You can pick the dropping resistor to suit whatever voltage/power level you want. I would think that would also raise your modulation percentage.

There should be quite a few of those power supply/modulators left over from avionics upgrades over the years. I've got an old Narco Superhomer around here somewhere but it doesn't have the power supply.

Greg

like Greg said use a dropping resistor for the screen ... right now screen is tied to your B+ which is low impedance (ac)... using the dropping resistor raises that impedance at the screen and allows the class C plate (and screen) modulation to work .... screen voltage regulation is only important for linear operation to achieve low distortion

Modulation in phase with the plate on the screen is a good thing? I have access to the +330 volt supply prior to the modulation transformer if I just wanted to run more voltage to the screen but somehow thought there was some relationship between grid and plate to the extent that you did not want the same signal on both?

Similar setup to yours, note the screens...

http://www.amwindow.org/tech/htm/50c5/50c5.htm

Varying the screen voltage in step with the modulation on the plates increases both efficiency and linearity.


Phil

Good info on screen modulation in concert with plate modulation at the link below.

http://www.amwindow.org/tech/htm/scrnmod.htm

That was a good article except for one caveat:

C2 should be = 1/(6.28*10k*XC1)

http://amfone.net/Amforum/index.php?topic=21251.50


Phil - AC0OB

A 20K 5 watt resistor between the plate and screen grid did the trick. Increased the modulation and simplified wiring. This means that the +165 is only used for the MO so I put a switch upstream that will allow the MO to be powered without keying the transmitter so I can “spot” the transmitter on the receiver. The idea being I can hear the other traffic on 3.885 at the event and set the receiver and then set the transmitter by spotting it on the receiver.
I did notice that the output loading has big effect on modulation and distortion. Transmitter sounds best loaded at half power around five watts but has noticeable distortion at nine watts. Best audio is with loading at 35 Ma for 5.5 watts, that’s an indirect power of about 11.5 watts and getting a little over 5.5 out gives me an efficiency of around 50%. Not bad for something built out of seventy year old technology.
The whole system consumes just under 6 Amps @ 12.0 volts so that’s about 72 watts to produce 5 watts of RF power or 2 watts of audio from the receiver by today’s standard of Yahoos and Kenwoods that’s a tremendous waste of power but there is no substitute for the joy that’s derived from a transmitter and receiver that’s using twelve tubes and designs from 1938, next week that will be 80 years ago!

The VHT-3 and other Superhomers, had the power supply on the rear deck, the Mark I, Mark II, and mark 12 had separate power supply/modulators.   

Where did you find the connector for that supply?   I don't recall EVER seeing one of those except on a Mk12.

Mike

If wanting to run screens from the non modulated 165V source, a choke might be good. A physically small and possibly inexpensive one, since you probably have only 20mA there. With 165V the resistor in your original diagram (what value is it?) could be a lower value if needed. It might also be a wirewound pot allowing some adjustment of power because screen voltage controls gain.  Maybe 8-16H for that screen which would be 10-20K Ohms at 200Hz?

This is the final schematic for right now. May be subject to change when I get to trying short antenna and how things progress at that point.  In regards to the connector for the power supply modulator that’s the advantage to posting what you’re doing on line. Also have information about this going on over at MRCA/Millist email reflectors and you would be surprised just how much assistance you can get in terms of the missing part or assistance in design and layout. I would never think of using the plate supply with modulation on the screen but it has huge benefits!