750W is key down. Youre forgetting that many still use CW for chasing DX on
6M. When operating on SSB I always suggest loading to the max to establish a
point and then reduce power with the mike control and follow the Ip and Ig
in the Tuning Section of the SB-200 manual. An operating and test result
procedure is shipped with every amp.
Some hams will always over process and overdrive an amp, it is not limited
to a 6M conversion.
And yes, when it is operated properly the IMD is excellent.
Carl
KM1H
----- Original Message -----
From: "Nils Petter Pedersen" <la7sl@online.no>
To: <amps@contesting.com>
Sent: Tuesday, September 25, 2007 9:49 AM
Subject: Re: [Amps] 6 mtrs:
> Hi all !
> I have so far not contributed to this discussion, but as a long time ham
> (1968) as well as a professional, I would like to say a few words:
> I think it is possible to make a pair of 572B's work relatively well on 6
> meters by careful design taking the tubes internal construction into
> consideration.
> In the early days of VHF there were not too many tubes available designed
> for these frequencies. So manufacturers often operated tubes well above
> max
> frequency ratings. I remember an RCA Nera TV transmitter operating near
> 200Mhz using a pair of tubes rated at a max freq of 50 Mhz for full
> ratings.
> The construction was unconditionally stable and several units were
> produced,
> indicating a good repeatability. The transmitter ran at abt 60% of max
> power
> ratings given at 50Mhz. And this is what worries me most about the
> conversions discussed here. They say they get 750 watts or more from those
> units.
> Manufacturers max rating for two tubes ICAS is 600 watts below 30Mhz ! It
> is obvious to me that 750 watts ICAS at 50 Mhz is way out of all good
> engineering and operating practices ! Say 350 watts and I would probably
> accept it, but I would NOT recommend it. Why should we push all
> limits when we have suitable tubes at reasonable prices that will give a
> much more reliable unit ? If it is just to prove that it can be done, I
> understand it,
> but to make a bussiness out of it ? It is at best very questionable
> engineering practices in a unit that is promoted for sale. But can it be
> done ? Certainly !
> Is it repeatable ? Most certainly ! Is it to be recommended ? Most
> certainly
> not ! I have myself built a 2 meter FM transmitter using EL-95 as a PA.
> That is an AF amplifier tube ! No freq of max ratings is given. It is
> strictly AF amp. And it was run well above its max power ratings ! So
> sure,
> it can be done,-
> but would I recommend it ? NO !! And I would love to have one of those
> converted SB200's on my desk, tune it to 750w, apply a two tone test
> signal at 750watts PEP , and watch my spec. analyzer ! I would also love
> to
> see it at 500w pep ! IMD -32dB ?? Hardly :-)
>
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>
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