[Amps] Plate Choke Issue

[mark bitterlich]

I would offer that using an IR temp measurement device (gun), would be safer and easier. I've used them for lots of measurements in amps, power on in fact. You can stay well away from HV and get accurate measurements. My 2 cents.
Wa3jpy
-------- Original message --------From: Manfred Mornhinweg <manfred at ludens.cl> Date: 1/3/18  2:01 PM  (GMT-05:00) To: amps at contesting.com Subject: Re: [Amps] Plate Choke Issue 
Hi AJ,

> What am I missing?

Let's try.

RF heating?

I just did the maths. With ~30µH and no stray effects you should be 
getting a bit more than 1A of RF current in that choke. The RF 
resistance of the wire at 14MHz is around 4Ω. So, maybe 6W of heating 
from RF, in addition to some smaller heating from the DC. Not enough to 
blow up a ceramic insulator, I would say. So it's not this.

You said you "looked for any series". I understand you mean that you 
checked for series resonances and didn't find any in the band. But 
that's not the whole story. Did you actually measure the impedance of 
that choke (magnitude and phase) at 14MHz? If not, then try to do so.

I would not be surprised to find such a choke having its main resonance 
far below 14MHz. Which means that at 14MHz it would have a capacitive 
reactance, and possibly a rather low one. In that case the RF current in 
it would be much larger than 1A, and the heating from RF current would 
be correspondingly higher (I²R). That could well make the insulator 
burst from thermal stress.

I assume the ceramic you are using is fine for RF work. If not, it could 
be so lossy that it heats up from the inside. You can detect this by the 
choke measuring a reasonable inductance but a low Q. Or on an impedance 
meter, a reactance in the area of a few kiloohm in series with a 
resistance of more than 10Ω or so. And when sliding the coil off the 
insulator, the Q gets much better, or the series resistance getting much 
lower. If that's happening, look for a more suitable insulator.

By the way, a longish closewound coil like that one can hardly have a Q 
much higher than 200. Probably less. At 3kV and 1A in it, the reactive 
power in that choke is around 3kW, assuming it's not higher from 
parasitic capacitance. At a Q of 200 that means 15W of RF dissipation, 
and that's pretty much a best-case assumption! If the actual impedance 
is lower due to capacitance, the dissipation will be larger. And if the 
Q is lower than 200, the dissipation will be higher too.

You very likely need a better choke.

Can you measure the choke's temperature after a two minutes transmitting 
or so? Either with a non-contact IR thermometer, or else VERY CAREFULLY 
by quickly shutting down the amp after transmitting, making ABSOLUTELY 
sure all high voltage has been discharged, and then opening the lid and 
touching the choke with your finger. If it smokes and smells like 
barbecue, and you find yourself in dire need of practising your 
vocabulary, you know you have too high loss in that choke. But do this 
finger test ONLY if you have a completely safe discharge system there. 
In the heat of the battle it's just too easy to do something stupid, and 
if you take your time to triple check that the high voltage has drained 
off, the choke will have cooled off too.

The recipe to survival of the choke is very likely a better choke. One 
that has lower RF loss, if possible, and which is large enough to safely 
dissipate that loss. A sharp airflow around the choke would help too.

Manfred

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