[Amps] Drake L-4B fan - too small and too slow?

[R.Measures]

On Dec 28, 2005, at 1:05 PM, Alan Ibbetson wrote:

> Having replaced the 0.82 ohm fuse with a proper glitch resistor in the
> HT supply of my L4-B, like y'all advised, I've been continuing to abuse
> my Drake L-4B amplifier on CW.
>
> As I said a while ago, doesn't everyone run their old amps on the "SSB"
> higher voltage setting? Well, maybe not with an L-4B! Mine runs too hot
> to touch the top of the case after one or two of my rambling
> transmissions.

The L4-B needs a few V of cathode bias to reduce the ZSAC to something 
reasonable.  A series string og fwd biased Si diodes can be used to 
achieve this.
>
> I've upset myself by putting thermochromic labels on the sides of each
> 3-500Z envelope. They've both gone black up to the 232 degree mark. 
> This
> is above the 225 limit on the plate seals and way above the 200 degree
> limit on the base seals.
>
> First question: am I getting an accurate temperature measurement this
> way?

yes

> Does the radiant heat from the anode hitting the back of the label
> affect their accuracy?

putting the detector adjacent to the anode stem glass-metal seal would 
minimize this problem.

> Even if my measurements are suspect, did Drake
> really expect the top of the case to run so hot?

The cooling system is not as well thought out as it is in the SB-220.
>
> Second question: why did Drake only fit a 2.5 inch 1550 rpm blower, 
> when
> Eimac suggest a 3 inch 3100 rpm unit? The Drake unit is made even more
> puny by our 50Hz mains supply, which just compounds the problem.

A brushless 12v DC motor would solve the problem.  It could be powered 
by a FWD operating from the 5.5v filament winding.
>
> I could disable the factory blower motor, remove the fan blades, fit a
> flange on the chassis rear, and plumb in an external blower I happen to
> have here. But it's mechanically messy.
>
> Anyone know of a drop-in faster running motor that would save me 
> burning
> the paint off the top of the cabinet? Or is there a better way?
> -- 
>
> Cheers,
>
> Alan G3XAQ
> alan at g3xaq.demon.co.uk
>
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org