[ARC5] SCR-274N Antenna Matching and Mark's transmitters

[Kenneth G. Gordon]

On 21 Aug 2014 at 21:33, Dennis Monticelli wrote:

> Ian,

Hi, Dennis:

That was Mark who was having the very low plate efficiency, not Ian.

> Something is definitely not right.  Assuming you are getting adequate drive to a
> decent set of 1625's, you should be seeing a lot higher plate efficiency.

I agree. Mark's plate efficiency is way too low. I am wondering if his 
wattmeter is reading low for some reason?
 
> On a T-22 I use a series cap of 100pF from the antenna post to a 50 ohm
> antenna and a shunt 390pF from the secondary of the variometer to ground.
>  The L setting is 23.  With a regulated 500V (exact) on the plates and 263V
> regulated to the screens the Ip is exactly 100mA dipped.

That is 50 watts input. Sounds good to me.

>  I measure power out as
> 30W on an accurate power meter.  That's a plate efficiency of 60%, which is what
> I would expect from two parallel 807 types on a 500V rail whose grids are
> properly driven.

Agreed.

I wonder what is going on with Mark's setup?

Mark could connect the transmitter to a 50 ohm dummy load, and then, with 
an accurate oscilloscope, measure the peak-to-peak voltage, then from that, 
calculate his true power output. 

P=E^2/R. 

If I remember correctly, "E" is RMS, which means that he would divide his 
P-P voltage by 2.828. R=50.

Or if he used a VTVM with an RF probe, he would be measuring RMS 
voltage directly.

This would give him a check on his wattmeter, at least.

As you point out, his plate efficiency is extremely low, if his wattmeter is 
reading accurately.

Something is definitely not right.

Ken W7EKB