[AMRadio] Testing tubes

[DOXEMF at aol.com]

In a message dated 10/2/03 7:45:41 AM Eastern Daylight Time, 
jcandela at prodigy.net writes:


>     I can see how changing a tube can alter the DC operating point, and 
> this
> will have an effect on the distortion products of an amplifier, especially
> if the tube is driven such that the available dynamic range is being used.
> Hmmmm. By a changing to a Chinese 12A_7, shift the point, gain compress on
> one side, and then start writing stories on how wonderful the thing sounds!
> 
>     BTW gain compression on one side can be a valuable tool in a super
> modulated AM rig......
> 
> Regards,
> Jim
> 

Yep,that's exactly the desired effect in guitar amps which yields
increased note sustain and harmonic enrichment.
       I can see how the asymetrical soft clipping could be used in the 
proper phase for supermod. with little circuit modification.
      That's kind of the point of the whole discussion here. It's not so much 
how a tube or circuit behaves when within it's textbook operating
conditions, it's when they are exceeded that the different characteristics
show up. Most evident in power output ratings and clip points being dependent 
on the power supplys dynamic range. Max out the supply
with your final current and there will be less available for the modulator.
      Usually the preceding audio and osc stages have a seperate supply
so are not effected. I have heard FMing on quite a few of the smaller 
transmitters when pushed hard on their own.
      In Hi Fi tube gear with barely adequate common supplies that point is 
extremely critical since the linear operating point will be changed in several 
places in the circuit with supply sag and recovery.

Enjoy,
Bill KB3DKS


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