[AMPS] splatter

[Rich Measures]

>Rich:
>
>>Does it make sense that the IMD products generated by a grid-driven
>>4-250A would DECREASE as the grid is driven from the AB1 region of zero
>>grid current into the AB2 region of grid current.?  If this were true,
>>then it would be "EXACTLY correct" that the G2DAF circuit is pristine
>>clean.  :)
>
>Have a look in The care and Feeding of Power Grid tubes: there's a set
>of curves of IMD against input as you get into grid current.

page number?

>Provided that the grid (and screen, if applicable) 

If applicable?   Have you ever seen a triode Class AB1 HF amplifier?  

>stay at constant
>volts, and that the increased loading caused by grid current doesn't
>screw up the cleanliness of the driver, why should grid current cause
>distortion? 

Study the constant current curves for grid driven configuration and the 
answer will  be revealed to you.  

>....... there have been lots of  Class AB AF amplifiers built
>where the tubes ran grid current............they needed a low impedance
>supply so that the drive volts didn't sag, if distortion was to be
>avoided.

This is true for Class AB2 push-pull AF amplifiers.  Do you believe that 
Class AB2 push-pull amplifiers and Class AB1 single-ended amplifiers have 
identical rules and characteristics? 
>
>After all, 3-500Z s in grounded grid draw grid current, without being
>disastrous on linearity. 

Are we talking about cathode-driven Class AB2 or are we talking about 
grid-driven Class AB1?

>I suspect that the G2DAF problems came from
>poor drive regulation, insufficient screen volts, and being touchy -
>Dick  Thornley certainly produced some good looking pictures of two tone
>tests, 

Yea, verily.  During a 2-tone test the screen V was constant.  During 
voice modulation the screen V varied widely.  

>and I seem to remember some guys at the Marconi Company getting
>good IMD on the spectrum analyser using that circuit. Somehow, I was
>never too impressed  - seemed to me to rely too heavily on getting the
>ratios of voltages, drive etc just right.

Good IMD relied on using only constant level 2-tone modulation.  
With voice modulation the total IMD was around 21db down..  
cheers, Peter
Rich---

R. L. Measures, 805-386-3734, AG6K   


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