[AMRadio] Stock or modify? BC rig "value"

Jim Wilhite w5jo at brightok.net
Wed May 31 11:28:56 EDT 2006

>> I personally prefer to keep my broadcast rigs stock, using them on 160. 
>> The
>> only mods that I have ever made in these transmitters are slight changes 
>> to
>> tuned circuits to get them to hit 1885

Some considerations about moving a broadcast transmitter to 75 or 40 that 
one should make is the design of the RF deck.  Keep in mind that lead length 
from the tube to the blocking capacitor, from there to the tune cap and on 
to the coil will exhibit additional inductance beyond the coil properties.. 
Usually these leads are very long which will not affect the circuit near the 
design frequency.

This inductance plays a part in tuning the transmitter on higher frequencies 
and must be compensated.  It seems easy just to retap the coil and retune 
the prior circuits.  There is an optimum L to C ratio for any given set of 
frequencies.  That balance should be maintained and if it isn't, the tuning 
controls become very critical making the tuning very demanding.

Also the coils in these rigs are very large in diameter so you have more 
inductance for a given number of turns.   When an RF circuit is designed for 
a upper and lower frequency then tapped below that some transmitters leave 
the unused portion of the coil floating which will affect the circuit.  Some 
transmitters will ground the unused portion causing another problem.

When you move one of these things to 160 you are not too far from the design 
parameters, but going to 75 presents challenges.  40 meters another set of 
troubles in the design.

Remember also that if you drive a transmitter with 1 KW output, designed for 
optimum performance at one frequency, with a VFO you should retune the 
circuits when you change frequency or you can generate spurs and trash that 
the final will just amplify.  That is why some people use them as crystal 
control or leave them on one frequency.  Give this more than passing thought 
because if you change 5 Kc then the circuits are out of resonance.  The 
final will not transfer maximum power to the load efficiently and heat is 
generated in the tuned circuits and the tubes.

73  Jim

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