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From: Larry Benko <>
Date: Wed, 04 Dec 2013 20:00:49 -0700
List-post: <">>

In your argument the output is not matched to the new open circuit and as such can not produce the voltage I calculated as being the maximum possible. You are correct in this case. However if the amp had been tuned on 10m and you changed the the exciter to 80m but did not change the amp, the output capacitance would now be approximately 8 times less changing your analysis considerably.

I only bounded the problem which seems to have been ignored by several folks. As usual folks gravitate to certain parts of a discussion and can't or don't want to look beyond them.

Several people contacted me directly saying an amplifier that produces 274Vrms (1500W into 50 ohms) could never produce 500V or 1000V output, no matter what. However the tube/FETs etc. don't know anything about the load. The only effect the tube sees is through the output matching network and if matched to a 200 ohm load with a 200 ohm load present the tube performs exactly as in the 50 ohm load with the match set to 50 ohms.

Most of us have seen that mis-tuning, wrong band selection, no antenna, etc. can cause arcing in different places in different amps. Some amp makers are more conservative in different areas than others. No surprise here! A good engineer tries to find a reasonable tradeoff between bulletproof, cost, size, etc. I think this discussion started with a question about the output voltage needed for padder capacitors.

This discussion certainly has been interesting and hasn't been boring to too many!

Larry, W0QE

On 12/4/2013 7:03 PM, Peter Voelpel wrote:
It´s quite simple without measurements.

Lets assume the amp is tuned for full legal output into a low SWR on 80m
which needed 1000pf of load capacitance.
Then suddenly the SWR goes sky high because the coax connector went open.

How can the voltage across the load capacitor become 5KV when the reactance
of the load capacitor is just 45 ohms on 3.5MHz?
That would require about 110 Amperes of RF current through that capacitor.
And where is that big current coming from in a 1.5KW amp, not in a 100KW?

-----Original Message-----
From: Amps [] On Behalf Of Larry Benko

I'm not sure where this is going.  The original statement was about the
output capacitor voltage maximums.  Obviously other things in the
matching network can arc over first but that is a different issue.  For
that to be valid we need to talk about a specific circuit which is fine
but it is a different that the original statement about the output
capacitor voltage vs SWR.

I have made countless measurements on matching networks over the years
and at the lower frequencies where the circuit is easy to measure the
measurements match the simulation very well.

You or anyone else can believe what you want but I have made
measurements which match the theory.  Anyone care to make measurements
showing what I posted is incorrect?  Do it in a methodical and
scientific manner and post for all to see.  The measurements can be made
at the levels available from a signal generator easily.

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