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Re: [Amps] Checking Plate Choke resonance

To: "'Carl'" <>
Subject: Re: [Amps] Checking Plate Choke resonance
From: "DF3KV" <>
Date: Mon, 12 Oct 2009 19:10:27 +0200
List-post: <">>
Hi Carl,

Yes, in the late 50s I still used large choke inductances like 2.5mH to feed
the parallel resonance output circuit of my 40/80 transmitters and
There at the cold end any choke resonance does not matter.
Parallel feed of course is something else.
I started with parallel feed in the beginning of the seventies with much
smaller tubes when I used many parallel sweep tubes which of course worked
into very low output impedances.
That was the only way to run qro as we were limited to 150W of total

The first choke fire happened to me when I built with a pair of QB3/300
(4-125A) all band at 3.5KV.

If component specs allows it I try to use parallel feed at the antenna end
of the pi-circuitry which is usually no problem with low voltage tubes like
3CX800 or GU74.

And right with the bypass capacitors, they must be made pretty large to be


-----Original Message-----
From: Carl [] 
Sent: Montag, 12. Oktober 2009 16:35
To: DF3KV;
Subject: Re: [Amps] Checking Plate Choke resonance

The RF Parts choke is purchased from Ameritron.

The 10X rule goes back to the 30's when everything was series fed plug in 
coils with link coupling.

When shunt feed started gaining popularity in the 40's the choke inductance 
dropped dramatically because now series resonances could not be tolerated. 
The old 1 to 2.5 mH chokes werent good beyond 40M if that. Shunt feed was 
used on the old plug in coils and then to the pi network which started 
gaining acceptance in the 50's.

The famous National R-175 choke of the 40's was only 225uh as was the 
modified R-175A that removed a 15M resonance.

Heath used 90uh down to 80M as did the B&W 800. The B&W 801 and 802 were 
only 110uH and advertised for 160-10M.  The Alpha 77 used the 801 and later 
the 802 when WARC bands came along.

With such low impedances the choke bypass cap became critical to keep 160M 
in particular RF out of the PS electrolytics. Most companies use(d) an 
inadequate 1000pf and Dentron only 500pf. Its no wonder there was and still 
is premature filter cap failures.

With .01 @ 6KV  caps only costing $2-3 new these days there is no excuse to 
not add them to your amp.


----- Original Message ----- 
From: "DF3KV" <>
To: <>
Sent: Monday, October 12, 2009 9:04 AM
Subject: Re: [Amps] Checking Plate Choke resonance

I have never seen such high impedances at amplifier chokes.
Most of the chokes in ham radio amps are not even twice the plate load.
The famous Rfparts 225µH transmitting choke is just 2,5k ohms on 160m.
Much smaller chokes are used in commercial amplifiers.


-----Original Message-----
From: [] On

As a general rule the impedance
of the choke should be about ten times the plate load impedance or more.

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