[Amps] question for you experts...

[Gary Schafer]

While I agree that a "safety choke" on the output of the amp would be a good
idea,, it is obvious that the chokes used in most ham amplifiers are not
there as safety devices as they would be much larger in current carrying
capacity regardless of what any manufacturer may claim.

If you will remember back when SSB amplifiers were introduced to the market
and well into the 80's most manufacturers claimed that the PEP was 2X the CW
power of the amplifier. Not many really understood what PEP was. It made for
good marketing hype though. So much for relying on amplifier manufacturers.

If the plate coupling capacitor does short and the choke does open the load
capacitor will most often arc and short the B+ supply to ground at a few
hundred volts as most load capacitors are rather close spaced devices, often
broadcast receiver type variables. That will provide more protection than a
small choke that is rated at 100 to 300 ma. 

If anyone has a Bill Orr handbook you can see what he says about the choke
in the output. Here is a direct quote on "Tetrode Pi-network amplifiers"
from the 17th edition of his Radio Handbook page 659 paragraph 28-4:
 "The Pi circuit is usually shunt-fed to remove the d-c plate voltage from
the coils and capacitors. The components are held at ground potential by
completing the circuit through the choke (RFC1)." 
RFC1 in this case is the choke at the output end of the circuit. He does not
mention it as being a safety choke.

If you look at some of the amplifier circuits described you will see that
chokes rated at 100 ma are often used. Sometimes 300 ma. Hardly enough to be
considered a "safety choke"

Yes in many of the ARRL handbooks they do describe the choke as a "safety
choke" but still only show a 100 to 300 ma choke in most amplifier circuits.
Maybe this is a carryover from lower power transmitters when that may have
been an adequate current rating. But common sense would tell you that it
won't do for today's high power amplifiers as a safety choke. 

A choke of some kind is necessary across the output Pi-network circuit to
hold the DC at ground level as the tune capacitors can charge thru the plate
coupling capacitor if there is no path to ground.
Also serving as a safety choke would be nice but is obviously not the prime
concern of most builders or a larger choke would be used.

The plate tune capacitor and tank coil will also charge to a high DC level
without the choke at the output but the load capacitor with the close
spacing will be the first to arc in the circuit.

73
Gary  K4FMX

> -----Original Message-----
> From: Lennart M [mailto:lennart.michaelsson at telia.com]
> Sent: Wednesday, March 28, 2012 10:35 AM
> To: 'Carl'; garyschafer at comcast.net; 'Kathy Bookmiller';
> Amps at contesting.com
> Subject: SV: [Amps] question for you experts...
> 
> 
> Being far from an expert but a ham for more than 50 yrs and having spent
> my
> entire life working with RF I would say:
> A DC short circuit at the output of an amplifier (with lethal plate
> voltage)
> is a must.
> Many transmitters operates in an on/off mode. At no circumstances there
> should be a chance for plate high voltage to appear at the antenna
> connection. Why? Obvious!
> In some cases DC build up at the antenna port may ruin the out/input of
> a
> TRX.
> This DC can come from large antennas acting as antennas for thunder
> storms
> passing by. Always keep antennas at DC ground - all the way from the
> antenna
> switch to the transceiver.
> Period.
> 73
> Len
> SM7BIC
> -----Ursprungligt meddelande-----
> Från: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> För
> Carl
> Skickat: den 28 mars 2012 16:13
> Till: garyschafer at comcast.net; 'Kathy Bookmiller'; Amps at contesting.com
> Ämne: Re: [Amps] question for you experts...
> 
> Im far from waffling Gary if you actually read what I said.
> Believe what you want but understanding a bit of the history of its use
> and
> general acceptance might help temper your comments.
> 
> 
> ----- Original Message -----
> From: "Gary Schafer" <garyschafer at comcast.net>
> To: "'Carl'" <km1h at jeremy.mv.com>; "'Kathy Bookmiller'"
> <wb2aio at yahoo.com>;
> <Amps at contesting.com>
> Sent: Tuesday, March 27, 2012 10:20 PM
> Subject: RE: [Amps] question for you experts...
> 
> 
> > Well Carl, which is it? You seem to be waffling both ways.
> >
> > The only "safety benefit" that the choke offers is it keeps the DC
> voltage
> > at zero on the output terminal of the amplifier so you won't get
> zapped if
> > you should touch the antenna connector or an antenna connected that
> does
> > not
> > have DC continuity in normal operation.
> >
> > 73
> > Gary  K4FMX