jeremy-ca wrote:
>
> ----- Original Message ----- From: "Tom Rauch" <w8ji@contesting.com>
> To: "Steve Thompson" <g8gsq@eltac.co.uk>; <amps@contesting.com>
> Sent: Saturday, September 29, 2007 2:02 PM
> Subject: Re: [Amps] 4CX250 IMD
>
>
>>>> They are simply showing the degradation when the
>>>> tube is run at its TYPICAL rating of 2KV.
>>>
>>> and at much higher output power. The IMD goes bad because
>>> the tube
>>> is being pushed to deliver too much power, not because the
>>> HV is
>>> higher.
>>
>> That is correct. People are comparing an entirely different
>> set of parameters, not just a voltage increase alone.
>>
>> It is the output power increase that is largely responsible
>> for higher IM, although a loose screen supply could
>> contribute if the higher voltage starts to produce a more
>> dynamically varying load on the screen supply.
>
> What both of you continuously fail to understand is that the tube is
> being run within its manufacturers published ratings. No amount of side
> stepping can change that fact.
>
> It is also an established fact that in commercial or home brew ham
> amplifiers the tuning procedures emphasize tuning for maximum output
> while keeping within the parameters established by the tube
> manufacturer. A 1000W DC Input 2X 4CX250 amp is operated at 2000V@500ma
> yielding a single tone output of approximately 600W or a PEP of your
> choice. Nowhere do they say to reduce the power output to conform to
> some unknown or unestablished IMD.
>
> Accept that simple fact and lets not draw this into some long drawn out
> meaningless drivel on semantics or nit picking as some have a habit of
> doing.
I hadn't ignored the ratings - it's just that a published
performance of -23dB IMD at 470W pep doesn't contribute much when
you're considering designing for -30 or better. It tells you that
you're going to be running less than 470W, but at that level of
distortion you can't rely on the IMD reducing 3x faster than the
power in predicting what power you might be able to run.
All I was trying to do was point out that 4CX250s can and do make
good amplifiers with reasonable linearity. Yes, it takes a little
care and understanding to get the best out of them and I'd taken
it for granted that would be forthcoming. If you're in an
environment where the user can't give it then maybe 4CX250s are
not a good choice - but that doesn't mean they're universally no good.
I don't accept your 'simple fact'. I think that homebrewing will
represent pretty much all use with them these days - anyone
searching for information on using 4CX250s will quickly encounter
the work by G3SEK, GW4FRX, G4AJW before them and many more, and
will hopefully learn how to use them to best effect.
Steve
PS If we're going to pull the plug on devices and equipment which
can cause interference if not used with a bit of care and
understanding, I think many of our shacks would look rather bare.
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