What happens to the output power as the drive is increased? It still
sounds like a bandswitch or one of the fixed caps used for 80M.
Remove the cover and bypass the cover switch with a short jumper.
Then you can see what is going on inside.
As far as fixing the input VSWR why not simply copy Alpha? They use a
9:1 trifilar ferrite balun driving a network of 15 220 Ohm 2W carbon
resistors in 3 banks of 5 for the two tube amps. You can see the turns
detail in the photo. Mounting close to the socket is a good idea. VSWR
is near perfect and the tubes cant be overdriven using the high drive
network.
http://www.alpharadioproducts.com/low_drive_mod.asp
I scrap the ALC board, reroute bias wiring and mount RFC 4, C-24 and
F-2 to a terminal strip underneath near the sockets.
Carl
KM1H
----- Original Message -----
From: "Adrian Rees" <rees.a@btconnect.com>
To: <amps@contesting.com>
Sent: Wednesday, August 06, 2008 5:49 PM
Subject: [Amps] Dentron MLA2500
> Hi all
>
> Take a look at this PDF File from the late Kris Lund.
>
> http://hamradio.shacknet.nu/shack/amp_mla2500/dentron_mla-2500b_cooling.pdf
>
> My MLA2500 has a crazy problem on 3.5MHz. I can tune it up with, say
> 10
> watts. It'll have a input SWR of 1.5:1. Once tuned (into a dummy load)
> I
> lift up the drive power, and the input SWR goes through the roof. Like
> 20+
> :1.
> The individual Band Capacitors are all OK.
>
> On all other bands its fine.
>
> So I am going down the route described in the article by Kris Lund.
>
> Adrian MW1LCR
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
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