[Amps] RF voltage on VHF strip line

[Steve]

What effects/damage are you seeing? Does it always happen in the same place?

Can you run the amp with reduced plate voltage? That might show up 
whether it's a marginal effect where rf on top of dc is just enough to 
cause breakdown or if it's something purely related to the rf.

A couple of thoughts to add to John's comments:

With a few kV of dc+rf, you need air leakage paths >0.2"/5mm to avoid 
breakdown. If there's anywhere in the assembly where the teflon doesn't 
overlap the brass far enough it might be a weak point.

Teflon is very soft and tiny bits of swarf can become embedded in it 
compromising the insulation.

Air gaps can be a weak point too. For example, consider if the teflon is 
.01"/.25mm thick and the brass is slightly out of flat so there's a 
.002"/.05mm air gap somewhere when it's clamped together. In that area 
the air and teflon are like two capacitors in series and the voltage 
divides between them inversely with capacitance. In this example, 
roughly 30% of the (rf + dc) voltage appears across the air part which 
is likely to cause breakdown and lead to damage to the teflon.

Steve

> Suppose you have a 4CX1500B sitting in the middle of a half wave
> plate strip line for 144 MHz. Some may prefer to think of it has two
> quarter wave lines with the tube between them. This is a "teflon
> sandwich" line where the upper brass plate is connected to the tube
> anode, the lower is not. The lower brass plates are grounded at the
> far end from the tube. Between the upper and lower plates we have a
> thin sheet of teflon that forms the dielectric for the DC blocking
> capacitor.
>
>
> Can someone help me understand whether (and if so why) there would be
> significant RF voltage across the blocking cap? Obviously RF voltage
> is going to vary along the *length* of the line, being high at the
> tube and near zero as we get toward the grounded end. But what about
> voltage across the teflon sheet between the upper and lower plate *at
> any given point along the line*?
>
>
> I am trying to understand WHY this thing would want to arc or break
> down only when RF is present. My mind wants to believe that the RF
> voltage across the capacitor at the point where the teflon shoulder
> washers insulate the screws holding it together would not be
> significant, and that the voltage would be just B+ regardless of
> whether RF is present or not. However I see evidence this is not the
> case and want to know what it is that I don't understand.
>