[AMPS] SB 220 vice SB221

w8jitom@postoffice.worldnet.att.net w8jitom@postoffice.worldnet.att.net
Mon, 24 Nov 1997 07:39:42 +0000


> From:          Rich Measures <measures@vc.net>

> Three times the anode supply potential of 2900v would be 8700v.  It makes 
> little sense to me that the anode voltage could possibly soar to such 
> heights when a tank is essentially out of resonance. 

If you read what I wrote carefully, you'll see I specified the tank 
IS resonant, the load is simply removed.

How this happens is simple.
 
A tube appears like a diode in series with a time varying 
resistance loading the choke/blocking cap junction.

The tube pulls the junction towards the chassis. That "tug" causes 
the end of the blocking cap towards the tank to go more negative than 
the chassis reference.

If the tank is underloaded, the tank  swings too far negative. If it 
does that the tube simply cuts off, a tube with negative anode 
voltage does not conduct. No amount of instantaneous grid voltage 
will cause the tube to conduct so the tank "free-wheels". The tube 
does NOT pull the tank back up positive, it simply can't.

By the time the tank starts to swing up near the peak positive 
voltage positive (remember it changes slowly because of the Q, so the 
exciter is already cutting the tube off. Because of this the tank is 
free to overshoot positive. The RF choke's high impedance allows the 
voltage at the anode to exceed the supply voltage.

The limit to this excursion is ONLY the Q of the tank, the mismatch 
presented, and voltage breakdown someplace in the system. Most 
ignition systems use this very same effect to fire spark plugs, and 
many switching supplies operate this way. So will your PA, if you 
overdrive it for a given amount of coupling to the load.

Motorola describes this effect in solid state PA's, since it is well 
known to cause the destruction of high power solid state devices 
during operation into high VSWR loads. You can find it mentioned on 
page 254 of Radio, RF, and Video Applications (DL413D rev 1)  as
they discuss reliability of solid state FM broadcast modules in 
high power transmitters. They say ""the amplifier must be designed 
with the capability of supporting a VSWR of 3:1 as a minimum. This 
point can be very determinant when considering that on a high 
efficiency circuit (non-class A) the collector voltage swing can be 
close to three times the collector supply voltage."

With single ended vacuum tubes and larger mismatches it can be even 
more. I've measured PA's that produce TEN times the dc supply voltage 
on the tube when the load is removed!

It's pretty well known among RF engineers that the peak anode (or 
collector) voltage can easily exceed the supply voltage by more than 
two times, and as many as several times, depending on tank loading. 
Of course the truth doesn't sell kit$, so guys after your money 
ignore it!

> In an SB-220, saturation begins to occur at roughly 150W drive.  
> Increasing the drive beyond 180w does essentially nothing to the output 
> level or to the peak voltage across the Tune C.    

You call yourself a liar.

50 watts can overdrive the SB-220, if the loading control is 
closed too far. Anyone who has ever watched a grid current meter, or 
listened to a splattering signal from an underloaded amp, will know 
even a few watts can overdrive and saturate an amp (causing it to 
switch hard) when it is underloaded.  

Rich expects everyone to believe the tube suddenly "saturates" 
at 150 watts in this case, but when it's a parasitic he suddenly 
whistles a different tune. In one case the tube can't saturate... 
in another it can draw so much current it bends tungsten wires and 
explodes high fusing amperage chokes.

Just like the tank arcing... Rich says a parasitic causes and arc 
because it can produce many times the rated voltage and arc a part 
or switch over...but at the same time he claims fundamental 
RF drive can NEVER do the same thing. He argues with himself.

Which way is it Rich? Does the tube saturate at 150 watts, and that's 
it. Or does the current behave like you argued a week ago with 
Peter Chadwick?

Can the tank generate voltages much greater than the supply system 
and arc, or is it always limited to two times the dc?

Make up your mind.

73, Tom W8JI

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