[CQ-Contest] Contest amps under $1000

Dave Hachadorian k6ll at juno.com
Fri Aug 20 12:30:00 EDT 2004

On Fri, 20 Aug 2004 10:16:59 -0400 "Alfred J. Frugoli (KE1FO)"
<frugoli at worldlinkisp.com> writes:
> What amps are out there on the used market fur under $1000 that are 
> sufficient for contest service (i.e. occational high swr due to tx 
> into wrong antenna at 3am, easy to tune etc.)
> I've investigated several amps already, and here are my thoughts.  
> SB-220
> If driven hard can get close to 1.5kw output but it was only 
> designed for 1kw input cw and 2kw input ssb.  Tubes are easy to come 
> by.  Possible VHF parasitic issues, although if operated within 
> specs these issues are deminished.  No 160

I have an SB-220, and it works pretty well. The power
transformer is a weak link for contesting, but I have
modified the amp for electronic bias switching, which
quickly cuts off plate current in the absence of rf drive,
such as between each dit in the letter "H." My EBS is
homebrew, but Ameritron has a nice circuit board that you
can buy for about $50. If not implemented properly, EBS
can generate some funny-sounding artifacts, but the
Ameritron board, which I have in another amp, is clean.

You can also buy an external transformer from Peter Dahl,
but then other components in the amp (tuning cap, bandswitch,
etc.) will start to fail. 

> TL-922
> Similar to SB-220 except cost is higher - worth the extra 300-400$?

If 160 is important, maybe. I think parts are still available from
Kenwood? If so, that would be a major advantage. I don't know what
you would do if you blow up an SB-220 bandswitch.

> MLA2500
> Good solid amp.  Again, claims of 1.3kw out in some places, but 
> designed for 1kw input, not output rating.  8875 tubes very 
> expensive if they can be found at all.  Can convert to 4cx400 or 
> 3cx800 but I'm not an experienced builder and the directions for 
> these conversions make my head spin.

I personally blew one of these up in a contest at a multiop
station. It cost us about $450 to replace the tubes, after
extensive scrounging. Based on that experience, I wouldn't
want one. I was tuning up on 10m at the time, and everything
seemed to be tuning up ok, but it was a false resonance or
something, and it blew.

> Alpha 374
> No 160 (unless it's an A model).  Not much out there on this amp, 
> except that those who have them seem to like them.  Bandpass tuning 
> is a nice feature and could come in handy in a so2r situation.  8874 
> tube is still available.

I've used the Alpha 78, which is similar, and it seems to be a
very solid amp. The bandpass tuning is a little quirky though
(it works differently on 10 and 160), especially in a multimode
contest, but that is a relatively minor issue.

Dave Hachadorian, K6LL
Big Bear Lake, CA

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