[AMRadio] Valiant carrier

[D. Chester]

----- Original Message ----- > Rick, Johnson put ~600 volts on the plate of 
the Valiant and Viking
> series radios which is for Continuous Carrier operation.  Then with 330
> Ma. you get input of 198 watts and the best output I have ever seen from
> a Valiant is ~120 watts, which is about 60%.
>
> The DX 100 radio has just two 6146s and gave the same power output.
> They put about 800 volts on the final which is Intermittent service.  I
> believe you will find that 70% figure is under certain circumstance you
> may not have in the Johnson, or for that matter in the Heath.
>
> Jim/W5JO


I suspect a lot of hams would be disappointed if they measured their rf 
output on any band with a real power measuring instrument that was 
accurately calibrated, regardless of the rig or the tubes. My HF-300 
transmitter, when fully loaded to a KW DC input class-C, generates only 
about 3.5 amps through a thermocouple rf ammeter working into a 50? dummy 
load... a little over 60% efficiency. A far cry from the 75-80% you are 
supposed to get with class C.  And that's with about 50% additional grid 
bias voltage, achieved by driving to full grid current using 150% the 
recommended grid leak resistance, well beyond the point where additional 
grid drive results in any increase in rf power output. I have tried various 
L-C ratios in the tank circuit, and that didn't change anything. Also tried 
several different thermocouple meters just to make sure it was not a meter 
error. After letting the thing run for several minutes and then turning 
everything off, and feeling all the components in the rf output and ATU 
circuitry, the only thing that appears warm to the touch is the tubes and 
the dummy load. That eliminates the likelihood of any obvious loose or 
high-resistance connection, or excessive loss in one component.

But that's not too surprising, even for 160 and 80, let alone 10m, 
considering that neither the tank circuit nor the matching circuitry should 
be expected to be 100% efficient. When running "leen-yar" mode, the 
efficiency can be expected to be even less, particularly with slopbucket, 
since full amplifier efficiency occurs only at the crest of voice peaks. A 
little insignificant  loss here and another little insignificant loss there, 
all add up.

My Hammy Hambone Mirage "wattmeter" indicates more rf power output (into a 
flat 50? dummy load) than I am running DC input! I should now be richer than 
Bill Gates, since I have apparently run across a solution to the world's 
energy crisis. :) I wouldn't put a tremendous lot of confidence even in the 
accuracy of a Bird 43, particularly when feeding a typical ham antenna.

Once you consider tank circuit losses, ATU losses and transmission line 
losses, rf power efficiency calculated by comparing DC input to the final to 
RF input to the radiating part of the antenna, is likely to be substantially 
less than the figures given in the transmitting tube manuals, which 
represent ideal conditions with no consideration of  losses external to the 
tube.

I don't know how the 6146s in a DX-100 hold up as long as they do, with 
close to 800 volts on the plate and modulated 100%. As I recall, the maximum 
recommended plate voltage for class C plate modulated service is about 600 
volts, which is what the Ranger and Eico 720 run, to give 40 watts carrier 
output with 65 watts input, just a hair over 60% efficiency. I suspect that 
is pretty typical. Using those same figures, the Valiant should run about 
120 watts out. Back in the 60s I  remember a novice friend of mine upgraded 
to General, and loaded his Eico 720 up to 75 watts input as opposed to the 
recommended 65 watts for AM, and his 6146 went south in just a few weeks. 
He replaced the tube with a new one, and decreased the loading back down to 
the recommended 65w, and never had any more trouble with tube life.

In the text of one of their near-monthly rulemaking dockets of the mid 
1970s, the FCC estimated the typical carrier output efficiency to the 
antenna to be about 50% for a plate modulated class-C final.

Don k4kyv

_______________________________________________________________

This message was typed using the DVORAK keyboard layout.

http://www.mwbrooks.com/dvorak/
http://gigliwood.com/abcd/