----- Original Message -----
From: "Roger" <sub1@rogerhalstead.com>
To: "Rob Stampfli" <rob@cboh.org>
Cc: <amps@contesting.com>
Sent: Friday, January 29, 2010 1:13 AM
Subject: Re: [Amps] Ameritron (et. al.) filament voltages
>
>
> Rob Stampfli wrote:
>> Hi Alek,
>> Thanks for the response. Personally, I agree with you that it is better
>> to keep the filament voltages in check for a number of reasons. However,
>> here's a pointer to a post (from this list) that takes the opposite
>> perspective:
>>
>> http://lists.contesting.com/archives//html/Amps/2007-07/msg00367.html
>>
>> Tom is Tom Rauch, who had a hand in the design the AL-80B, and who
>> certainly knows more about building amps than I ever will. Note he's
>> not disputing the Eimac claim, only counter-claiming that the hazards of
>> ICAS operation will typically kill the tube before an over hot filament
>> will. He may well be right, but I still can't see any advantage to
>> running excessive voltages on the filament. That's why I cut mine back.
For several reasons I take anything from that source with a huge grain of
salt.
There also appear to be several definititions of ICAS with the first going
back to the Class C AM days. Without a running time meter attached to an amp
its a lot of guesswork how emission life was affected. BUT, the typical tube
seemed to be no worse for wear at slightly elevated voltages.
When SSB came along voltages were raised to and sometimes above CW ratings
and robust tubes from the 813 to 4-1000A seemed to flourish without
complaint. Lesser tubes such as the 811A and 572B suffered; especially the
572B which is an 811A with a graphite plate and is run well beyond what its
basics emissions capability would indicate.
The next stage is where some tubes appear to have specs written by the
marketing department, especially the Class C Russian tubes that have been
reinvented for linear SSB service. Toss in the 3CX800 and 8877 pulse
versions with the former being run on CW at 3500V. IMO, that is way beyond
ICAS.
One huge reason for tube failures is that the average ham doesnt have a clue
how to tune an RF stage be it in a DX-100 or a brand new amp. A 6146 is
easily fried by excessive grid drive as well as excessive screen current
which isnt even measured. A modern tetrode amp is no different and its
questionable how good the protective circuits perform.
With fragile triodes its excessive grid current that does the damage.
Ive watched many hams over the past 50+ years tune up and I'll tell you that
grass grows faster. Then they have to do it again every time they do a small
QSY. The ones using antenna tuners are the worse of the bunch.
I do a lot of amp repairs here for customers and its obvious why they
failed. Trying to convince many that their long ingrained tuning procedure
is wrong is a waste of time so after a year or two its back again and they
are cursing the tube or amp manufacturer.
Just my nickels worth.
Carl
KM1H
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