[CQ-Contest] Pre tuned Hf amps

David Pruett k8cc at comcast.net
Sun Sep 24 23:46:11 EDT 2006

The K8CC has used such a configuration (separate amps for each band, 
located remotely from the operator) since 1989.  The station has been 
run multi-multi, multi-two, multi-single and even SO2R.  We're learned a 
lot from this experience resulting in the following comments:

    * An amplifier designed with commonly accepted design practice for
      the output network (Q=10-12) will not cover the frequency range
      for both phone and CW on any of the amateur bands from 160-10
      meters without retuning.  This might surprise some people.

    * The operating conditions on many amplifier designs get upset
      switching antennas on the same band (even with similar SWR).  This
      is largely a choice of the tubes used.

    * Rather than using tubes which need elaborate protection circuitry
      (ex: 3CX800s, 8877s, etc.), a better solution in our experience is
      to use tubes which don't need protecting except in cases of the
      grossest faults (ex: 3-500Z pairs, 3-1000Z, 4-1000A, etc.).  These
      tubes are much less sensitive to changes in load impedance when
      you switch antennas.

At K8CC we use a homebrew relay box which had two rig inputs and six 
amp/band outputs.  This box "steers" the RF and keying relay line from 
each radio to the intended band, with the appropriate lockouts.  Such a 
box is economical to build, because all of the RF switching occurs at 
the 100W level.  Simple single-band bandpass filters are used between 
the relay box and each amp, rather than a fancy multi-band Dunestar or ICE.

When we do multi-multi, its a simple matter to take the relay box out of 
line and patch each amp to a single-band operating position.

I agree with one of the respondents in this thread that you can build 
six single-band amplifiers out of robust parts for less than the cost of 
an auto-tune Alpha or ACOM.  Or you can do like my friend K5GO did and 
acquire of six used AL-1500s.  But with six amps it's easy to do multis 
or SO2R; to do these kind of operations with auto-tune amps you need 
two, so the "six cheap amps" approach is a LOT less expensive.



Peter Voelpel wrote:
> I would never want to change monoband amps to a single or two "autotune"
> amps.
> Optimized homebrew monoband amps are the far better solution:
> - much better components, never bandswitch failures
> - better efficieny
> - usually better harmonic reduction
> - no single point of failure
> - more power
> - instant switching of bands, no tune at all
> - much cheaper
> As an alternative, I am in the process of building an additional three band
> amplifier for the low bands, 3 Pi-L output circuits, switched in by vacuum
> switches and a common input.
> No antenna relays necessary since receiving antennas are separate.
> This one is to feed additional antennas and will be driven through a ratio
> adjustable power splitter.
> My amplifiers are not in the shack, except those with radiation cooled
> tubes.
> I just monitor automatically SWR and overcurrent states which drop the
> ptt-line in case of failure, or automatic shutdown of voltages if necessary.
> I think this thread will be better discussed in the amp reflector
> 73
> Peter
> -----Original Message-----
> From: cq-contest-bounces at contesting.com
> [mailto:cq-contest-bounces at contesting.com] On Behalf Of Eric Hilding
>> I'am looking for some information to put an hf amp in an other room I 
>> thinking to build monoband amps who are pre tuned and no need to 
>> retune if you go from ssb part to the cw part of the band.
>> Has any one done ths before?
>> How do you make the protection/feedback to the operator if something 
>> goes wrong?
> Why re-invent the wheel?  Get ***ONE*** "Auto-Tune" amp.  ACOM, ALPHA or
> check out the new Elecraft KPA amps ("coming soon" :-)
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