[AMPS] SB 220 vice SB221

Sun, 23 Nov 1997 08:10:16 +0000

Hi Bob and all,

You wrote:
\
> Late versions of the SB-220 and, I suppose all of the SB-221 runs, had a
> different capacitof for C1 which had just a little bit wider plate
> spacing in the design. This was an attempt to stop arcing and spitting
> when the parasitics might occur.
That's utter nonsense.

The spitting and sputtering is not due to parasitics at all.

The plate variable/ tank arcing problem most often is incorrect 
tuning, and secondarily incorrect relay sequencing in the relay 
contacts.

What happen in ANY less than 360 degree plate current conduction PA 
is the tube "tugs" on the tank (remember the tube is a varying 
resistance, not a source). The tank flywheels (or more correctly acts 
like a pendulum) and swings past zero volts negative from it's Q.

If you underload the PA, or remove the load through ANY type of 
problem (it can be an output  relay contact that fails to close, or 
an antenna or tuner that arcs or faults), the PA has nothing to 
dampen the swing of the tank voltage. In a few cycles, the voltage 
can build up to several times the anode voltage. This voltage will 
continue to build up, until the weakest point in the system fails or 
until the voltage becomes so high power is transferred to the 
load.  

Normal fully loaded PEAK voltage in the tank of an AB class amplifier 
is about equal to (only slightly less than) the supply voltage. look 
at the dc voltage, and you can expect about TWICE that dc voltage on 
the tube anode during peaks and about the same PEAK voltage as the dc 
supply voltage on the tank cap (in the case of the tank it swings + 
and - because of the blocking cap).       

But if you underload the tank for ANY reason, the voltage can easily 
reach three times the dc voltage.

The same is true if your exciter has a power transient (common with 
many rigs), because overdrive is  like underloading so far as the 
anode system is concerned. 

All of this is well documented in engineering texts, and not that 
difficult to follow through with logical thought. Choke type fly-back
or resonant switching supplies work on the same principle.

73, Tom W8JI

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