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Message-ID: <XXqlICAeg+N4EwSb@dennison.demon.co.uk>
Date: Sun, 21 Nov 1999 12:39:58 +0000
To: rsgb_lf_group@blacksheep.org
From: "Mike Dennison" <mike@dennison.demon.co.uk>
Subject: Re: LF: B&K Audio Amps
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In message <000701bf340f$e90c1460$d2a0883e@lvm>, LAWRENCE MAYHEAD
<LAURIE@g3aqc.freeserve.co.uk> writes
>I am using a B&K Audio amp as a power amp on 136.But it runs out of steam 
>rapidly beyond 100Khz.I can only get about 100W! Are there any mods etc to get 
>more from this amp,or should I start again and build from scratch?  73 Laurie. 

The first thing to check is whether you ordered the Normal or the
Professional version. You would think that the Professional would be
better but it actually has a lower cut-off frequency. I don't know how
to distinguish between them, but you may find it written on your
receipt.

Next, cut the capacitor that is effectively in parallel with the input.
The location depends on which amp you are using, but it on my 200W and
300W amps it is just the heatsink side of the electolytic near the
input. You may have to replace this with a smaller cap (400pF?) if there
is any instability.

Then make sure you have a large capacitor (few microfarads) in series
with the output. This helps reduce the chances of the amp dying.

That's really all that's necessary, except to check that you have enough
drive, and the 50-100 ohms of your antenna is matched correctly to the
few ohms output impedance (link or tap on your coil, or a toroidal
transformer).

If you want to try more, Lech, G3KAU, has explained some mods in the LF
Experimenters Source Book, available from the RSGB (www.rsgb.org/books/g
entec.htm). This also includes a circuit diagram.

I get about 200W from my 200W amp, and it is very reliable. I got 250W
out of the 300W one, but blew it up eventually (my fault, not the amp).

-- 
Mike, G3XDV
IO91VT
http://www.dennison.demon.co.uk/activity.htm