Driving a linear with only 15 watts?

[Darwin, Keith]

Since a linear amp is supposed to be "linear", is there any reason that
you have to run 65 to 100 watts into one?  What if you used a linear
designed for 65 watt drive and only gave it 15 watts?  Does the spectral
purity change?  Does the efficiency drop?  Does the linearity degrade?
 
- Keith KD1E -
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[Nigel A. Gunn G8IFF/KC8NHF]

Should work fine with proportiannally lower output. However, 15 watts
might not be enough to hit the RF PTT in the amp so you ought to hard
switch it.


Darwin, Keith wrote:
> Since a linear amp is supposed to be "linear", is there any reason that
> you have to run 65 to 100 watts into one?  What if you used a linear
> designed for 65 watt drive and only gave it 15 watts?  Does the spectral
> purity change?  Does the efficiency drop?  Does the linearity degrade?
>  
> - Keith KD1E -
>
>  


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[Don Wilhelm-3]

Kieth,

If you have a true linear amplifier, you can always feed it with less power
than the power required for maximum output - if it produces 1000 watts with
100 watts in, it will produce 100 watts with 10 watts in (i.e. the power
gain should remain the same).
The spectral putity should either be unchanged or be better.

If the linearity degrades, it is not a real linear amplifier and it will
distort your SSB waveform when used - on CW there is only the carrier, and
any non-linear distortion would only show up if you graphed the input power
vs. output power.

Yes, do it if you have a linear amp.  The output efficiency should remain
the same, but if you consider the power pulled from the AC line vs. output
power (not usually calculated) the efficiency will be reduced.

73,
Don W3FPR

> -----Original Message-----
>
> Since a linear amp is supposed to be "linear", is there any reason that
> you have to run 65 to 100 watts into one?  What if you used a linear
> designed for 65 watt drive and only gave it 15 watts?  Does the spectral
> purity change?  Does the efficiency drop?  Does the linearity degrade?
>
> - Keith KD1E -
>

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[Charles Greene-2]

Darwin,

I have a linear that has about 1000 watts output when driven with
approx. 60 watts in class AB2 for CW and a little more when operating
class AB1 for SSB.  It's an old Hallicrafter HT33A with a PL172
pentode amplifier tube.  I tried it using my K2/100 on PSK31 with 20
watts average power from the K2/100.  The amp put out 400 watts ave
power.  The IMD coming out of the amp was identical to the IMD of the
K2/100 without the amp, indicating no more distortion was added by
the amp.  I don't expect you to hunt for a HT33A, but the principles
are the same.  The efficiency is lower because of the idling power,
but since it is all coming out of the power line, who cares?

73,  Chas W1CG



At 11:13 AM 3/8/2006, Darwin, Keith wrote:

>Since a linear amp is supposed to be "linear", is there any reason that
>you have to run 65 to 100 watts into one?  What if you used a linear
>designed for 65 watt drive and only gave it 15 watts?  Does the spectral
>purity change?  Does the efficiency drop?  Does the linearity degrade?
>
>- Keith KD1E -
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[w2bvh]

Very interesting.

What IMD values would you consider "bad". "ok" and  "excellent"? As
examples, QST July '03 rates the ICOM 703 transmit 3rd order IMD at  24
db below PEP when running 10 watts. The March '06 QST shows the YAESU
FTDX9000 3rd order IMD at  35 db below PEP when running 200 watts.

24 db below 10 watts is 40 mW +/-, a small but still respectable qrp
signal that's rf "junk".
24 db below 200 watts would be 1 watt +/- !

Is there an FCC mandated 3rd order IMD requirement?

Inquiring minds (at least mine) want to know ;-)

73,
Lenny W2BVH

Charles Greene wrote:

> Darwin,
>
> I have a linear that has about 1000 watts output when driven with
> approx. 60 watts in class AB2 for CW and a little more when operating
> class AB1 for SSB.  It's an old Hallicrafter HT33A with a PL172
> pentode amplifier tube.  I tried it using my K2/100 on PSK31 with 20
> watts average power from the K2/100.  The amp put out 400 watts ave
> power.  The IMD coming out of the amp was identical to the IMD of the
> K2/100 without the amp, indicating no more distortion was added by the
> amp.  I don't expect you to hunt for a HT33A, but the principles are
> the same.  The efficiency is lower because of the idling power, but
> since it is all coming out of the power line, who cares?
>
> 73,  Chas W1CG
>
>
>
> At 11:13 AM 3/8/2006, Darwin, Keith wrote:
>
>> Since a linear amp is supposed to be "linear", is there any reason that
>> you have to run 65 to 100 watts into one?  What if you used a linear
>> designed for 65 watt drive and only gave it 15 watts?  Does the spectral
>> purity change?  Does the efficiency drop?  Does the linearity degrade?
>>
>> - Keith KD1E -
>> _______________________________________________
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>
>
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[Charles Greene-2]

Lenny,

My k2 is one of the better ones when it comes to IMD on PSK31.  It
also depends on how you operate.  Any otherwise good rig will have
excessive IMD if you overdrive the audio, use ALC, compression or
companding.  I don't recall the exact figures when I made this test,
but it was a little over 30 dB out of the K2 and also out of the
linear amp.  That was with 20 watts out of the K2/100 and 400 watts
out of the linear.  That's 13 dB gain of the linear, and most linears
only do about 11 dB.  I don't do that much because 20-30 watts is as
much power as you need on PSK31.  I see a lot of excessive IMD on the
bands.  In my opinion, anything over 25 dB is excessive, and I have
seen signals with an IMD of -10 dB, that take up 200 or more Hz
bandwidth.  I don't think there is a FCC spec per se, except for the
requirement to use minimum bandwidth.


73,  Chas, W1CG

At 10:32 PM 3/8/2006, w2bvh wrote:

>Very interesting.
>
>What IMD values would you consider "bad". "ok" and  "excellent"? As
>examples, QST July '03 rates the ICOM 703 transmit 3rd order IMD
>at  24 db below PEP when running 10 watts. The March '06 QST shows
>the YAESU FTDX9000 3rd order IMD at  35 db below PEP when running 200 watts.
>
>24 db below 10 watts is 40 mW +/-, a small but still respectable qrp
>signal that's rf "junk".
>24 db below 200 watts would be 1 watt +/- !
>
>Is there an FCC mandated 3rd order IMD requirement?
>
>Inquiring minds (at least mine) want to know ;-)
>
>73,
>Lenny W2BVH
>
>Charles Greene wrote:
>
>>Darwin,
>>
>>I have a linear that has about 1000 watts output when driven with
>>approx. 60 watts in class AB2 for CW and a little more when
>>operating class AB1 for SSB.  It's an old Hallicrafter HT33A with a
>>PL172 pentode amplifier tube.  I tried it using my K2/100 on PSK31
>>with 20 watts average power from the K2/100.  The amp put out 400
>>watts ave power.  The IMD coming out of the amp was identical to
>>the IMD of the K2/100 without the amp, indicating no more
>>distortion was added by the amp.  I don't expect you to hunt for a
>>HT33A, but the principles are the same.  The efficiency is lower
>>because of the idling power, but since it is all coming out of the
>>power line, who cares?
>>
>>73,  Chas W1CG
>>
>>
>>
>>At 11:13 AM 3/8/2006, Darwin, Keith wrote:
>>
>>>Since a linear amp is supposed to be "linear", is there any reason that
>>>you have to run 65 to 100 watts into one?  What if you used a linear
>>>designed for 65 watt drive and only gave it 15 watts?  Does the spectral
>>>purity change?  Does the efficiency drop?  Does the linearity degrade?
>>>
>>>- Keith KD1E -
>>>_______________________________________________
>>>Elecraft mailing list
>>>Post to: [hidden email]
>>>You must be a subscriber to post to the list.
>>>Subscriber Info (Addr. Change, sub, unsub etc.):
>>>  http://mailman.qth.net/mailman/listinfo/elecraft
>>>
>>>Help: http://mailman.qth.net/subscribers.htm
>>>Elecraft web page: http://www.elecraft.com
>>
>>
>>_______________________________________________
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>>

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