[Amps] RF voltage on VHF strip line

[N1BUG]

On 11/01/2016 07:06 AM, Steve wrote:
> What effects/damage are you seeing?

Arcing somewhere which kills the zener diodes in the screen supply.

> Does it always happen in the same place?

I don't know where it happens. That's the problem. I am asking 
questions in an effort to improve my understanding of relevant 
theory in hope that may lead to a reasonable guess as to what the 
problem is. Really, I am grasping for straws here.

This amp has the screen grid connected directly to chassis, with the 
cathode 'floating' for DC but heavily bypassed for RF. It is a grid 
driven amp. The screen voltage is stabilized by a shunt zener diode 
regulator.

When the arc occurs, it is a frying sound. This only happens with RF 
present. It kills the zeners within a second or so. I'm quite 
certain it is an arc from plate circuit to chassis, which means 
plate to screen. The fact that I hear a frying sound puzzles me 
somewhat. My experience in the past has been when B+ is involved 
there is a loud snap. In this amp, virtually everywhere there is HV 
RF there is also B+. Yet, the arc is a frying sound with no big 
snap. This has happened twice. The first was at a power level of 
around 1500 watts, the second less than 900.

I have hi-pot tested the tube to 10,000 volts, at which potential it 
shows just about one microamp leakage plate to screen. I think 
that's OK.

Unfortunately I took the plate line apart to look for damage before 
I had the hi-pot tester. I found some slight darkening of one screw 
and the edge of the hole where the teflon shoulder washer was. Not a 
lot. Not anywhere near what I would have expected to find since this 
thing had arced twice, destroying the 50 watt zeners both times. 
But, I know all the material was impeccably clean when it was 
assembled, so I believe something must have happened at this 
shoulder washer.

I should add the teflon is .010" and is coated both sides with Dow 
Corning 4 Electrical Insulating Compound. In theory this excludes 
practically all air from getting into the plate line sandwich. In 
reality, small pockets of air can and do get trapped during 
assembly. *Maybe* the fact there is very little air accounts for 
very little visible evidence post-arc?

> Can you run the amp with reduced plate voltage?

Unfortunately, no.

> 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.

The distance from edge of hole in top brass plate to screw which is 
in contact with lower plate is no more than 0.15". In theory the DC 
4 compound is supposed to fill any tiny voids here (under the 
shoulder washer) and prevent an arc. This is a proven amplifier that 
did work at one time - for several years. After not using it for a 
long time I recently rebuilt and cleaned up the whole thing. But it 
has this arcing issue now. Unfortunately I have no access to a lathe 
and my teflon shoulder washers are far from perfect. Maybe when I 
reassembled it I didn't get enough DC4 compound in this one spot to 
fill all the tiny voids.

> Teflon is very soft

I'll say. I hate working with the stuff. It's almost a semi-solid 
material which will easily distort under pressure.

> 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.

That's something to think about.

Thanks for the reply.

At this point my 'blind' plan is to rebuild the plate line again, 
using .015" teflon instead of .010" (shouldn't make any difference 
in performance), making sure everything is very clean (again), using 
fresh DC 4 compound (I used old stock before), and new shoulder 
washers (such as they are). This time I will hi-pot test the line to 
10 kV prior to trying the amp. It should be able to pass that test. 
If it doesn't, I will know I have a problem somewhere. I don't know 
what else I can do.

73,
Paul N1BUG