[Amps] Q: Chimneys for 3-500/4-400??

Rich 2@vc.net
Tue, 3 Dec 2002 13:28:59 -0800


>Bear (?), I've had lots of Henry 3-500Z amps over many years, and they do
>use a pressurized chassis and chimneys, just like Eimac used to say to
>do...but the tubes didn't run cooler as a result (notwithstanding the
>filament voltage problems!), they ran hotter than in similar amps using the
>axial above-and-below deck cooling a la SB-220 and Ameritron.  
>
Amen, Steve.

>I'm guessing:
>
>-The actual air flow around the glass using high-volume, low-pressure
>(axial) cooling is better than using low-volume, high-pressure (blower with
>chimneys) cooling, resulting in more efficient thermal exchange;
>
Not a guess.  The airflow in feet/minute is typically higher with 
transverse cooling.

>-The already-heated air (heated by underchassis components and the filament
>pins) blowing up around the envelopes is substantially hotter to begin with
>than the ambient air blown past the envelopes when high-volume axial cooling
>is used;
>
That, plus the upward airflow is more restricted. 

>-The critical and well-selected spacing of the axial fan to the tube
>envelopes results in about equal cooling of the entire envelopes, including
>the surface opposite the side with the air applied -- kind of clever.
>
and a simple aerodynamic principal.

>While it's really hard to compare "my tubes lasted longer than your tubes"
>stories due to variations in filament voltage, cooling, how hard the tubes
>were really used and also the manufacturer and even manufacturing lot of the
>tubes themselves, I'd have to say I've had 3-500Zs last in the "SB-220
>cooling method" amps every bit as long as, or longer than, the "forced air
>with chimney method" amps like the Henrys, and in most cases that was more
>than 10 years of frequent use -- after which time, the tubes are pretty much
>"free," aren't they?
>
>WB2WIK/6

-  Rich..., 805.386.3734, www.vcnet.com/measures.