>There are a few incorrect statements here.....
>
>> > Consider that additional cooling air is going to be needed to realize
>> >100w extra dissipation--except perhaps in the SB-220, which has an
>> >over-supply of cooling air.
>
>Being involved with Heath, ...
Involved? As I understand it, Mr. Rauch had nothing to do with the
design of the Heath SB-220.
>....and reviewing the design of the 220, I can
>assure you the air supply of the SB-220 is not "over-supply".
Is Mr. Rauch telling it like it is, or just blowing smoke? To check out
Mr. Rauch's statement, take a look at the tubes in an old SB-220, whose
fan still turns freely. The red Eimac ink on the 3-500Z envelopes is a
fair indication of temperature-history. The brighter the red, the lower
the temperature. If the red color has changed to brown, high
temperatures are indicated. Missing or faded brown ink indicates even
higher temperatures.
>The SB-220 was designed to be used a 1 kW DC input power on CW,
>dissipating about 500 watts for TWO tubes total. The cooling system
>is inadequate for 1000 watts dissipation, since the anode and base
>seals will go over temperature. The problem is so bad the base pins
>often unsolder themselves.
The most common reason that 3-500Z pins unsolder in a SB-220 is that the
oil wicks for the two, porous bronze bearings on the fan motor shaft were
not replenished with oil. Without oil, the bearings sieze and the fan
stops fanning. Without cooling air, the 3-500Z's 75w
filament-dissipation alone melts the solder in pins 1 and 5.
In early SB-220s, there were no oiling holes in the fan motor. In
later production, a motor with oil holes was used. [see "Circuit
Improvements for the Heath SB-220 Amplifier", QST magazine, 11/90 & 12/90.
Rich---
R. L. Measures, 805-386-3734, AG6K
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