On 12/18/2010 1:33 PM, Pete Smith wrote:
> Just one minor quibble - the first two inches of the top rear portion of
> my SB-220's RF cage is not open, presumably to make sure that the air
> from the fan at least starts out in the right direction.
Due to the shrouded fan it probably makes little if any difference.
> The only
> thermal problems I have had with my amp was the melting of the solder in
> the filament pins of the tubes. The explanation I have heard for this
> has nothing to do with cooling air or the lack,
It does.
> but rather to resistive
> heating due to a slightly resistive connection between the pins and the
> socket clips.
These pins depend on airflow for cooling. If there is a resistive
heating problem the cooling becomes even more important. I understand
this is a common problem these amps which appear to have marginal
cooling on the pins.
> After re-soldering the pins, I polished their outer
> surface and then swapped the clips for those on two of the three grid
> pins. No problems since.
As for the separator being black or shiny it is a "passive" element. It
directs airflow to reduce turbulence. In this case I'd not expect black
to make it more efficient, BUT being highly reflective might increase
the tube *envelope* temp a bit. I doubt it'd have little if any effect
on seal temp. A black cabinet will radiate the heat more efficiently.
The separator's primary function is to keep the air flow linear.
Most high power tubes require cooling of the seals and cross flow from
the edge of a typical muffin fan is not the ideal way to do it. OTOH it
only needs be sufficient. (marginal is pushing your luck) The 2K4 I
owned had an excellent cooling system for the tube seals and you could
get plenty of light from the anodes with out a seal problem. I don't
know if any of them ever suffered melting of the pin solder. Carl
probably knows the answer to that.
It seems most hams prefer the self contained desk top amp. Generally
these amps have two strikes against them to start. The small size means
limited space which restricts cooling under the best of circumstances.
You need free flowing air in and OUT. IOW no restrictions in the airflow
path.
Then there is the cost. Today's legal limit amps are in general,
larger and use air system sockets or a facsimile of those sockets. These
amps are larger, heavier, and generally *louder* with multi-speed
fans/blowers. Some put the final(s) in a sealed compartment blowing air
in and have an exhaust fan as well in a push pull cooling arrangement.
The air coming out of one running digital can get downright toasty. But
these "desk top" amps are real hernia builders that push the definition
of "desk top" and require a *sturdy* desk!
Compare the weight and size for many of the "old" legal limit amps (1KW
DC or 2KW PEP INPUT) like the 220 to today's legal limit 1500 Watts
output (any mode). The transformers weigh as much as the entire "older"
amps with the Alpha's and Emtron's being considerably heavier. I am
not talking about the older console amps like the 30S1 and Henry amps.
The Hallicrafter's HT33 at 113# was an exception to this weight
difference. Unfortunately they ran on 110 VAC only and the finals for
the 33A and B models cost a small fortune IF you can find a good one.
73
Roger (K8RI)
> 73, Pete N4ZR
>
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>
>
>
> On 12/18/2010 1:20 PM, Mitch Cox wrote:
>> The cooling on a standard SB-220 is a hit or miss proposition at best. You
>> have a large fan blowing at 2 tubes without anything directing or channeling
>> airflow with a completely open top letting the purposed airflow from the fan
>> escape right through the roof of the amplifier before it even reaches the
>> tubes.
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