[Elecraft] Ameritron 240V inrush limiter: any benefit on a KPA500?

[Roger]

Life sometimes is not as easy as it seems at first glance...............

The heater inrush current topic is a topic by itself and more complex than
what can be read and heard in most of the ham communities. There used to be
a very good treatment of heater management at the BURLE tube company
homepage. Whether the page is still alive or not I don´t know. If alive you
can learn a lot about the Miller-Larson effect, correct soft start for
heating etc.

What is sure is that especially in the case of the TL-922 the heater inrush
current is up to 10!! times the nominal value just like it is the case with
a cold filament bulb. This value has been measured - the only valid way to
make a technical statement anyway. EIMAC requires the inrush current to be
limited to not more than 3 x nominal value therefore inrush current limiting
is a good idea in case you want maximum tube life. Burned powerswitches as
stated elsewhere in this forum are not uncommon with TL-922s without current
inrush limiting. Another very important factor is heater voltage however
this is another story.

Not only the heater benefits from a smart soft start but also the high
voltage powersupply - especially the diodes and after all the filter
capacitor/s could go bad. It may work for a long time without soft start but
it is no good engineering practice unless very "soft" transformers are used.
This would be a disadvantage at least for the high voltage powersupply.



But coming back to the original question: 

With switching mode powersupplies (as in the KPA500 and 1500 and most of the
solid state amplifiers with ps included) usually no further measures are
necessary or would help in that respect as most  SMPS have soft start
circuits included.



Somebody requested more information on transformers and the magnetizing
mechanism - a good source is the website of any transformer manufacturer.
They usually have pretty good theoretical explanations of this and other
interesting parameters relevant in the world of transformers.

As with many things in ham radio the best thing is to perform measurements
and decide based on the results rather than "feelings" or  "I have heard
from someone who have heard from someone........"

Roger, DL5RBW


-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Victor Rosenthal
Sent: Sunday, January 19, 2020 1:11 PM
To: David Wilcox
Cc: Elecraft at mailman.qth.net
Subject: Re: [Elecraft] Ameritron 240V inrush limiter: any benefit on a
KPA500?

The simplest circuit, which I added to my TL922, is a relay operated from a
DC supply with a capacitor across the coil. The circuit can be set up to
provide from a fraction of a second to a much longer delay. 
When the relay is de-energized, a small resistance is inserted in series
with the line to the plate and/or filament transformer. When the relay pulls
in, the resistor is shorted out and the line is connected directly to the
transformers.

I set mine up for about half a second, mostly to increase the life of the
amplifier's on-off switch, and also to provide a more gentle start-up for
the tube filaments. It has a side benefit of preventing the "thump" which
sometimes occurs.

The cold resistance of the filaments is very, very low, and the inrush
current can be very high. As K4TAX said, in a properly designed amplifier,
the transformer itself limits the inrush current to the filaments and the
initial charging current of the capacitors in the plate supply to a safe
value for the tubes, rectifier diodes, and other components.

But I have had to replace on-off switches in the similar SB220, which means
finding the right switch or it looks ugly. And I believe that the life of
the filament will be longer if it heats up more gradually. In the big WE
transmitter I mentioned in my previous message, there was a five-step switch
to turn on the filaments, and I was told to wait about a second between
steps.

It might be unnecessary, but it was just a few junkbox parts.

73,
Victor, 4X6GP
Rehovot, Israel
Formerly K2VCO
CWops no. 5
http://www.qsl.net/k2vco/
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