[AMRadio] 75TH vs. 75TL (was Class C stuff)

[Donald Chester]

>From: Brett Gazdzinski <brett.gazdzinski at wcom.com>
>
>The 30k-1 is easy to change the bias on.
>The original setup ran 75th's, and when one of mine went open filament, I
>put 100th's in, and gave the good one to
>Charlie (n3bqw, now k3ya).
>
>100th's take a lower bias voltage, the 100tl's much higher.
>Same with the 75th and 75tl.
>From memory, the 100th is around 50 volts, and the 100tl
>is like 180 volts....

Don't overlook the fact that not only is the bias requirement  different, 
but the audio voltage swing requirement at the grid is also different.  The 
TL reqires a much higher audio voltage at the grid than the TH does, while 
the grid current will be less.  In other words, the grid impedance of the 
low mu version is much higher than that of the high-mu version.  For the 
same class-B driver stage you will need a different turns ratio on the 
driver transformer.  The 100TH's probably have a step-down driver 
transformer.  You will likely need a step-up driver xfmr for the TL's.

The audio driver transformer is the weak link in most conventional 
transformer-driven class-B audio amplifiers.  It has the most severe service 
requirements and a good one is complex to build and expensive to 
manufacture.

Don K4KYV

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