[AMRadio] Re: Need list of good AM radios to start lookingfor


John E. Coleman (ARS WA5BXO) wa5bxo2005 at pctechref.com
Wed Nov 16 15:47:28 EST 2005


	I don't abide by the switching of the tube to a tube that has less
plate dissipation and lower current ratings than the original.  To me it is
like saying that I am only going to need to have a 50 watt lamp in the
ceiling so I will put a 0.5 amp switch in place of the 10 amp that is there.

	
	The power that you run is determined by the load resistance that the
plate circuit is working into, and the voltage.  In the case of
pentode/tetrodes raising the plate load resistance by unloading the final
tank, will generally increase the screen current beyond what you want.  So
the better way is to keep the load controls where they are and reduce the
plate/screen supply voltage.  Reducing the voltage on the final plate/screen
supply will maintain the same load that the modulator sees and the plate and
screen current will both drop by the same percentage.

	If you maintain the same voltage and loading position on the knobs,
therefore reflecting the same Z to the plate and then plug in a smaller tube
you will just over heat the smaller tube.  If you then unload the tank and
re-dip the plate current you will find that the screen current will go
higher than is proper for the smaller tube.  You would be better to have
left the larger tube in the socket and followed the same procedure.  One way
of unloading more than the stock rig is capable of is to put more
capacitance on the output connector are in parallel to the loading
capacitor.  But remember that the lower you go in plate current the high the
screen current will go.  Most of these rigs have a meter in the cathode
circuit therefore monitoring both screen and plate current at the same time.
It has been found that this meter will never go below a certain level.  The
lowest it is possible for it to go is when the plate cap is removed.  The
meter will still show screen current.

	The best way is still to reduce the supply voltage at the
plate/screen modulation point.  A large resistor that has a bypass cap
across it works well.

John,
WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim candela
Sent: Wednesday, November 16, 2005 12:53 PM
To: Discussion of AM Radio
Subject: RE: RE: [AMRadio] Re: Need list of good AM radios to start
lookingfor .



Dan,

    I like your suggestion. I never tried it, so I am no authority on this
modification. In general, this would change the modulator load impedance,
and the RF pi network formulas would suggest juggling the L/C ratio to
maintain the same 'Q'. I might be tempted to use some sort of negative cycle
loading to contain the modulator, and minimize splatter by keeping the
negative modulation below 100%, and letting the positive peaks fly upward
beyond 100%. Doing this however may raise the PEP drive power too high for
the linear, so that's another issue to contend with. Running both modulator
and final RF stage at reduced B+ may still be the best approach when driving
an amplifier with low drive requirements. Another option is to use the 2e26
and low B+ for really low drive amplifiers.

Regards,
Jim

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of dw73454 at alltel.net
Sent: Wednesday, November 16, 2005 10:57 AM
To: Discussion of AM Radio
Subject: Re: RE: [AMRadio] Re: Need list of good AM radios to start
lookingfor .




> You do usually need to drop the
> power a bit to keep from over driving most linears. One >modification is
to
> run the RF Final 6146 and modulator off the low B+.
>
> Jim WD5JKO

I have also heard that you can just replace the
6146 with a 2E26 and do nothing else. Is this true?
And....what are the tradeoffs, if any?

73 de Dan -- WAØJRD ..


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