[AMPS] splatter

Tue, 10 Jun 1997 21:27:58 +0000

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

Rich wrote:

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

You ask if anyone has ever seen AB1 triode PA's. Yes, I 
have seen dozens of them. There is no law of circuit behavior that 
prevents operating a triode in AB1.

> >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.  

Look at the curves all you like, and it won't tell you much about 
IMD. IMD is rooted in two causes. Phase shift as amplitude level is 
varied and gain change with varying amplitude.

Grid current has nothing to do with it, unless it affects the above 
two things. Ian's suggestion would reduce or prevent gain 
compression. Rich's suggestion would INCREASE gain compression. 
Since gain compression is the leading cause of high level IMD 
products, Rich's suggestion would increase the likelihood of IMD 
while Ian's would decrease it.

In fairness though, Rich's suggestion does have some usefulness. 
A high resistance control grid return would make the tube 
harder to damage. and as a secondary benefit, it would eliminate 
the need for a CW SSB switch.

Just bump the tube with a little extra drive, and you would have 
instant class C operation. 

While Rich's idea might be fine for an automatically switched class C 
CW--class AB1 SSB amplifier, I prefer the people up and 
down the band from me NOT use his automatic class-C PA.

> >....... 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? 

There is only one difference in tube operation between RF and audio 
frequencies. The only change is in the rate of current and 
voltage change. Audio is not a different kind of electricity, it just 
changes level slower. 

That sounds like a smoke screen.

> >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?

A 3-500 can be run AB1. If I build a AB1 3-500, and I run it AB2, 
would it be dirtier than an AB2 3-500?

The answer is no, unless I use Rich's large grid resistor! Using a 
large grid leak resistor would guarantee the tube would move into 
class C with any amount of grid current. The large grid resistance 
would increase gain compression and increase splatter.

73, Tom W8JI 

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