[BC] Are there AM stations still using Tube Rigs?

[Cowboy]

On Sunday 22 March 2009 01:34 pm, Ronald J. Dot'o Sr. wrote:

>  The output network is like adding tuning and loading controls to the 
>  AM-1A and is NOT a substitute for an ATU.

 True.

>  I have a simple method for adjusting the ATU used with an AM-1A.  I 
>  start out at low power, say 50 watts with no modulation and adjust the 
>  coil taps one turn at a time for the lowest reflected power.

 Not at all applicable to a directional. 

>  It's important to do the adjustments without modulation so that the 
>  tuning is centered on the carrier frequency.
>  When you add modulation the sidebands will raise the reflected power 
>  slightly.  So far the sidebands seem centered during our annual proof 
>  for occupied bandwidth with this method.

 Without a sweep, you don't know that.
 ( but it's a good guess )

>  In the past I've adjusted the ATU with a bridge to a 50 ohm, 0J match 
>  but the TX didn't like it when I applied power, as I guess things 
>  change under full power, so I adopted this method which doesn't 
>  require the test equipment and works for me.

 No, things don't change.
 You do have a tolerance in bridge calibration, and you *may* have
 some Z rotation in the line if the bridge cal doesn't exactly match the
 manufactured tolerance in the line.
 If things change under power, then something is heating and the whole
 thing should be revisited. 

>  I adjusted the ATU for the best match for the AM-1A and then switched 
>  to the Gates 1 and adjusted the tuning and loading for the best match 
>  into the ATU according to the tuning instructions in the Gates manual.

 If the AM-1A is a real 50+j0 output Z, then this is probably going to get
 a better adjustment on the Gates than most any other way.

>  The SS TX's do NOT tune and load the same way as the tube rigs so read 
>  the tuning procedure in the manual and stick to it.

 This much is very, very true, unless the manufacturer says otherwise.

-- 
Cowboy