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Re: [Amps] [BULK] - Re: SB200 loading problem

To: John Parkins <john@g8kvp.com>
Subject: Re: [Amps] [BULK] - Re: SB200 loading problem
From: Steve Thompson <g8gsq@ic24.net>
Date: Tue, 20 Sep 2005 10:02:13 +0100
List-post: <mailto:amps@contesting.com>

John Parkins wrote:
> Hello Steve,
> 
> Tuesday, September 20, 2005, 9:19:04 AM, you wrote:
> 
> 
> ST> You could see the same effect if the tank coil inductance is too low -
> ST> eg if part of it  was shorted out when it should be in circuit.
> 
> ST> Max load cap. gives you a high anode load resistance. The circuit might
> ST> like this if the ht was higher than intended, but I can't see runnning
> ST> 240V into a 220V input, for example, making enough difference. Tired 
> ST> valves that don't have the emission to cope with a lower load resistance
> ST> might also do it.
> 
> ST> What gain and output power are you getting, and how do they compare with
> ST> the spec. for the amp? Does the anode current dip - is the dip at max rf
> ST> output?
> 
> ST> Steve
> 
> I was wondering about the tank coil although I've not noticed a
> problem with it visually, perhaps adding a turn or two??

Can't see a problem with trying it as an experiment - I'd been thinking 
along the lines of whether part of the coil was left shorted by the 
bandswitch.
> 
> The HT runs at 2100 off load and 1950 on load. As I've said in other
> replies I have a spare set of valves and switching them around hasn't
> helped.

I don't know the amp, but from what I read that looks about right.
> 
> Gain and output seems to be OK, although I've seen that some say they
> get more output, on 80 & 40m 80w in gets around 625 - 650w out. As far
> as the anode current, dip it is deepest at max rf out.
> 
I forgot to ask - what's the anode current / efficiency?

In the end, if it's all about right and output power and linearity are 
ok, then you could do worse than leave it alone. In your place I 
couldn't, but I spend much more time fiddling with equipment than 
actually using it!

There's a good looking set of notes at mods.dk if you put in sb200 on 
the article search.

Steve
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