[Amps] Alternative cooling strategies for SB-220?

[Carl]

The SB-220 fan draws air in from the rear and the output is split by the 
chassis to pass over the socket pins and the glass envelope. It exhausts 
thru the top and side perforations.

Remoting a fan is impratical due to the friction loss and a big blower would 
be needed.

Nice to talk about for 3 days but not very practical.

As I said before, add a resistor or as another said, go back to the stock 
fan. However graphite anode tubes need the extra air if those are being 
used.

Carl
KM1H



----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: <amps at contesting.com>
Sent: Thursday, December 17, 2009 2:09 PM
Subject: Re: [Amps] Alternative cooling strategies for SB-220?


> Not sure what mode you want to operate (i.e. what duty cycle) but if
> high d/c the duct trick with the blower remoted may have this problem:
> the 220 fan is normally placed in the cabinet in such a way that air
> is drawn in through the p.s. from what I remember of it.  If you run
> duct direct to the rear hole next to the RF deck, you loose that p.s.
> cooling through the v.doubler and B+ transformer.  But I have no idea
> how hot the p.s. gets under normal condx or if it gets hot at all.
>
> Another thing you can try is a tx keyed relay switch on the power line
> to the fan that bypasses a resistor on tx but is n.o. so the fan power
> goes through a voltage dropping resistor to slow it down so it is only
> full speed and noisy when you are transmitting.   You have to wire it
> so the fan is not powered in series with one side of the B+ primary
> (if that is the stock SB220 arrangement).   If you make a long RTTY or
> SSTV tx and the tubes get real hot you will need to use a timer relay
> with a set delay time so the fan speed drops around 30 seconds after
> you switch to rx.
>
> Just some ideas.
>
> 73
>
> Rob
> K5UJ
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