[CQ-Contest] amps in series

[k8cc]

Dave,

K5GO and I tried this setup back in the early 80s at his dad's station (the 
original N5DX).  We had no problems at all when daisy-chaining combinations 
of SB-220s and L-4Bs.  Most of this work was on CW at about 1KW output, 
even though we were driving the amps with Drake C-Lines.

One important characteristic of SB-220s and L-4Bs is that they use a single 
relay for T/R switching.  For example, the SB-220 uses a single 3PDT Potter 
& Brumfield style 10A power relay for input switching, output switching and 
cutoff bias control.  The 1000W RF from the first amp passing through the 
second amp is carried around the input on the same rated contacts that 
switch the output, so the relay is not likely to fail.  Your point about 
the RG-58 on the input is valid, this being the weak link in the 
system.  Oh well, this is AMATEUR radio - what would life be like without 
living in the design margins?  :-)

All of the MFJ/Ameritron amps I've seen share this same "single T/R relay" 
approach so these might be more candidates for daisy-chaining.  However, 
amps which use lower power reed relays and such for the input switching are 
definite no-no's for daisy-chaining.

Before I would daisy-chain six SB-220s as a poor man's Alpha 87a, I would 
instead look at minimizing how many I would need in series.  With your 
setup using a tribander, you might have to have three amps daisy-chained 
for 20/15/10, but it would be better to have a RF coax relay to select amps 
in separate paths to the 160, 80, and 40 antennas.  By plugging the low 
band antennas directly into amps dedicated to those bands, you would also 
eliminate having a high power coax switch after the amp chain to switch to 
those antennas.

73,

Dave/K8CC

At 07:34 PM 10/25/01 +0000, David O Hachadorian wrote:

>I have a spare amp, and this evening I had the bright idea to configure 
>the amps so they could be tuned to different frequencies, and easily 
>switched in and out for changing bands rapidly.
>
>I daisy-chained two amps, with the rf output of one connected to the rf 
>input of the other.
>
>The two relay control lines are connected to a spdt switch, so both amp 
>relays can never be on at once.
>
>By using this one DC spdt switch, the need for two spdt rf switches is 
>eliminated. The only slightly weak link is the short piece of RG-58, 
>inside the second amp, that goes from the rf input connector to the relay. 
>It now carries 1500W when amp number one is in use, but no problem so far.
>
>I suppose I could line up six SB-220's this way, with a sp6t switch and 
>have an el cheapo Alpha 87a.
>
>Maybe someone has tried this and reported on it already, but I had never 
>heard of it, and it seems to work fb.


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