On Sat, 07 Nov 1998 00:37:29 GMT kf4kl@ipass.net (Doug Hall) writes:
>
>Hi amp folks,
>I'm currently searching for a 100% duty cycle amp for RTTY contesting
>use. I'd also like it cover 160m and run legal limit. Obviously the
>easy answer is to buy an Alpha or Henry or whatever the amp d'jour is.
>But given that I can't (or won't) justify the cost of such an amp,
>what are the most reasonable alternatives? I'm willing to relax the
>specs a bit, ie. I could live with 1200w or so instead of 1500w as
>long as it would hold up in a 48 hr RTTY contest. As far as 160m
>coverage goes, I seldom use that band at present except for occasional
>stints on AM with an old Johnson Ranger, but one of these days I'd
>like to chase some DX on 160m. Here's what I'm considering:
The following will allow a reliable 1200-1500W or so on RTTY and cover
160M.
Ameritron AL1200 and AL1500. Newer models have better cabinet airflow but
older ones can be updated. Also adding a silicone rubber chimney right to
the top of the cabinet will reduce heat stress.
Any of the older 3 8874 Alphas
Ten Tec Titan....reportedly holds up well at 1500W
B&W PT-2500A Has RTTY switch position.
Many of the above are available on the used market at attractive prices.
>
>1. Build an amp using a somewhat reasonably priced tube or pair of
>tubes, ie 4CX1600B, 4-1000A, 3-500Z, etc. I have some limited
>experience with amp and power supply building from a previous job
>where we produced a commercial amp using an 8877. I've chased
>parasitics and plate choke resonances and such, so I think I could
>build such an amp and get it working. But I just finished pricing the
>4CX1600B amp project shown in the latest ARRL Handbook, and the only
>way I can justify such a project is if I can scrounge components at
>upcoming hamfests and off the net. Otherwise I'd come out cheaper
>buying an amp. Building quantity 1 of a legal limit amp is not cheap
>if you buy all new components, and I don't have many amp parts laying
>around here. I envy some of you your junkboxes.
I would not choose the first two tubes. The 1600B is too fragile and
requires adding complex protection circuits. The 4X1 is past its prime
and not an easy tube to tame at high voltages.
The 3CX3000A7 is one tube that is pretty much indestructable. At 100W of
drive it will run 1800+ W out key down all weekend. The 225W grid
structure makes it immune to glitches, tuning errors, etc. Good used
pulls show up in the $200-300 range and RF Parts has an excellent socket
assembly.....their chimney is no good but some silicone rubber from
McMaster-Carr will cure that.
Another choice would be three 3CX800A7's. Tubes show up as medical pulls
in the $100-150 range and sockets are cheap if you scrounge old GE Master
Pros. Another BIG plus is that the tank circuit can use air variables and
a flea market JV-9000 series switch. There is a lot to be said for low
voltage tubes when adding up PS and RF costs.
Three 3-500Z's or GG 4-400's are OK if you have the facilities to modify
the tune cap to a unequal value split stator and have sufficient drive.
At 3KV they will loaf at 1500W RTTY with a chimney style air system.
There are many other possibilities...I just highlited a few.
73 Carl KM1H
>
>2. Take a reasonably priced used amp (ie. SB-220, L-4B, etc) and add
>160m, additional cooling, and a stiffer power supply.
The L4B would make an excellent choice with PS upgrade. The RF deck will
easily handle 2KW.
The SB-220 is both PS and bandswitch limited.
>
>3. Buy two SB-220's and run them into a combiner (or separate antennas
>if I want) at 600-700 watts each. Don't laugh - two SB-220's, a
>splitter and combiner can be had for a little over $1K, and this gives
>me a backup amp in a contest if I lose one, albeit at reduced power.
>This doesn't address the 160m requirement, however.
An acceptably precise broadband combiner at that power level is not
simple to build.
>
>Anyway, those are some things I'm thinking about. I recently picked up
>an SB-220, and it works very well, but I don't know how hard I could
>push it during an RTTY contest. I got kinda longwinded on 20m the
>other night while ragchewing on RTTY with AP2TJ, and while the plates
>were glowing nicely nothing seemed excessively hot at 700-800 watts
>out. SB-220 owners - how hard can you push this amp without bringing
>on an early demise? And what's the limiting factor? Power supply,
>cooling, or tank components? All of the above?
All of the above! The PS will limit you to around 1100-1200W PEP for a
48hr SSB contest. Many contesters blew xfmrs by trying to push higher
with 140W drive, etc. An outboard xfmr will help but the bandswitch is
the next weak link.
And while we're on the
>subject, how much power should you get out of an SB-220? My amp has
>the original tubes in it, and I get around 1100 watts out on 20m,
Limited duty and 140W of drive will get you in the 1300-1500W range...but
to what purpose? That is a CB mentality.
>although I'm embarassed to admit that my only wattmeter is the one
>built into my MFJ tuner. (OK, so I need a real wattmeter, too.)
>
>As I said, I know that the easy way is to buy a 100% duty cycle legal
>limit amp. For those of you who have them, more power to you,
>(literally and figuratively!) but I'm looking for the Next Best Thing.
>
>Your comments are welcomed. I've been on this reflector for a little
>over a month now and have really enjoyed it.
>73,
>Doug, KF4KL
>===========
>
>"People don't care how much you know until they know how much you
>care."
>
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>
>
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