[Boatanchors] 3-500Z chimneys

[Jim Wiley]

Well, I won't disagree with David's point, but let me say this:  I had 
at one time 2 separate Drake L-4B amplifiers.  The older unit came with 
curved-top chimneys, the newer unit had straight-sided chimneys, both 
the new and the old style chimneys were Drake original equipment, and 
both new and old chimneys had Eimac logos and part numbers on them.  
Unfortunately, I sold the newer unit a while back, and don't have it to 
read the numbers from. 


A point to consider may be that both amps had rather large (about 1.5" 
diameter) finned heat-radiating plate caps that attached to a parasitic 
suppressor made from fairly robust copper strap wrapped around three 
2-watt resistors - the combination of which would have added 
substantially to the heat radiating capabilities of the system.  In any 
case, both amps had / have been used extensively in high duty cycle 
service (FSK and AM linear modes) running 900 watts or so on FSK or 
10-meters FM, and 400 watts AM output power (this was before the current 
375 watt resting carrier / 1500 watt PEP  output limit for AM came to 
pass).  Both amps have the original 3-500Z tubes, still in service for 
going on 35 years now, with no signs of giving up.   The heat radiating 
caps are cast aluminum and are of  the "star" configuration, not the 
"stacked disk" design.  The anodes of the tubes run a nice dull red or 
even a bit orange after a few minutes key down time. 


So while getting adequate cooling to the plate connection is indeed 
important, an often overlooked point is that it is equally important to 
have adequate cooling for the base and sides of the envelope as well.  
Either style of chimney would have accomplished this.  The Drake design 
was well thought out, with the tube bases sub-mounted in a pressurized 
plenum, fed by a low-velocity squirrel-cage blower, with the air exiting 
past the bases, via chimneys,  and out the top of the cabinet.   The 
result was  a almost perfectly silent cooling system that worked very 
well.  In fact, the operator had to look at the amp to see if it was on 
or not - you cannot hear it run in a normal household environment. 


By way of comparison, listen to a Heath SB-220 some time.   Sounds like 
a junior hurricane got loose inside, and makes using headphones  not an 
option for hearing the other guy.  The SB-220 simply used a large fan 
blowing over the sides of the tubes, and while the tubes probably 
(maybe?) got adequate cooling, it sure wasn't as elegant a design as the 
Drake, and the SB-220 absolutely cannot handle the repeated 10 minute or 
more  key down times that the Drake can.  To be fair, however, the Drake 
has a much more robust power supply than does the Heath, and the new 
price of the Drake was on the order of twice that of the Heath. .   


- Jim, KL7CC



David Knepper wrote:
>  
> -------Original Message------- 
>  
> From: David Knepper 
> Date: 10/18/2009 1:42:33 PM 
> To: RAY FRIESS; Boatanchors Mail List 
> Subject: Re: [Boatanchors] 3-500Z chimneys 
>  
> Caution: It may seem expedient to use Coleman light fixtures as a substitute
> chimney for 3-500Z, for example, but the truth of the matter is that there
> is little air flowing to the top of the tube or around the plate seal which
> is critical. 
>  
> That is why the original chimney glass curves at the top so that air can
> sweep across the plate seal. 
>  
> They are definitely cheaper though and more readily available. 
>
>