[Amps] Fixed Transmitting Capacitors: REVISITED

[Robert B. Bonner]

This is a great question and the answer is pretty simple but not too
obvious.  How can the same part be trusted in an amp?

It isn't that the capacitor is of low value or quality.  It's the current
rating the unit has to support.

LETS talk blocking caps (like a 100pf 5KV) keeping the HV off the tank
circuit.

I'm not going to do math right here but I could be.  Just apply ohms law and
the power laws to get an idea.

1500 ohm plate impedance and 1500 watts output.  How much current is flowing
through the cap passing the RF to the tank?

Its Current flow not voltage that causes heating. Remember voltage is just
electrical potential not actually doing work until you draw current.

There is low current flow at this device compared to voltage separation.
However its breakdown could cause a death and it is a very important device.

In this application you would more likely see a VOLTAGE BREAKDOWN as it is
blocking 2000-7000 v from killing you.

Now let's look at the same cap used as a loading cap padder...

50 ohm output impedance, ANTENNA CURRENT and those same 1500 watts.

The voltage is now LOW, the current very high because the impedance is very
low.  The tank is just an impedance matching device to match high impedance
tube to low impedance antenna...

Now with several AMPS of current flowing through this same 1000pf 5KV padder
you have device heating and value drift.  Your chance of failure is current
based.

The good news is this devices failure just blows it apart and the amp goes
down instead of the operator dying.

The chance of failure in a padder instead of a blocker is many times higher
in my experience.  

Its all component limits, values and math.

I hope that helps.

BOB DD



-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Ed Swynar
Sent: Tuesday, October 17, 2006 4:36 PM
To: amps at contesting.com
Subject: [Amps] Fixed Transmitting Capacitors: REVISITED

Good Day All,

You may recall a small thread that I helped kick-off here a few weeks
back,in the matter of using high-voltage fixed ceramic / door knob / mica
capacitors as padders in the 160-meter plate tuning circuitry of my
homebrewed 2 x 813 linear amplifier...

Based upon my own personal experience --- coupled with the comments I
received from others --- I permanently removed the padders in my set-up, &
successfully replaced them a paralleled fixed vacuum / air variable
combination.

In the meantime, I've been thinking about just what specific application(s)
such capacitors might have available for them, since they're potentially so
meddlesome in the way I had used them here: are they, in essence, good only
for BYPASSING purposes...? And if so, then why would manufacturers even
bother to make variants of these things, like the "N750" units that I tried
here...? They're obviously over-rated for the likes of padding in
low-powered VFO circuits --- so what good are they, specifically...?

Now, here's the question that REALLY begs an answer: if capacitors of this
ilk are SO inappropriate for use in QRO-type applications due to internal
heating, their ity to handle excessive RF current, etc. etc., then just how
appropriate are they for use as output coupling capacitors --- i.e. the
blocking cap that's placed between the final tube(s) and the pi-network
tuning circuitry...? I know that components placed here must handle quite a
load: years ago, when I built my first amplifier (4 x 811A) I naively
inserted a simple 2.5 kv (if I recall correctly) 0.001-ufd. disk ceramic
capacitor in that position --- whilst undergoing tests with the linear, the
little cap literally exploded like a small grenade, shooting ceramic
"shrapnel" all over the place! 

What specific guidelines does the enterprising young (or old, for that
matter) homebrewer follow to ensure safety and longevity, both for himself
AND his creation...?

Is the door knob coupling capacitor in my amplifier yet another candidate
for some future reoplacement with a vacuum cap...?

Any & all comments based upon personal experience are hereby solicited &
welcomed...after all, "...Inquiring minds need to know!" Hi Hi

Tnx & vy

~73!~ Eddy VE3CUI - VE3XZ
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