Tube Amps / Music Electronics
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|1/26/1998 4:49 PM|
||Hammond Transformers - VA means what?|
Voltage/Amps? (1.4 times the Watts rating?)
If so, what kind of value could I get away with in an AC15/Matchless Spitfire type of circuit?
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|1/27/1998 1:49 PM|
The "VA" rating is used for power transformers. Without getting technical here, (as there are iron, inductive, capacitive, and resistive losses in the exact solution) you may consider it equal to the total power that can be drawn through the unit. It's the product of RMS volts x RMS amps, which is wattage. It's the basic size, sort of like quoting an engine's horsepower rating.
The hammond transformer catalog lists the safe working VA for each trans. they offer. It's the total power draw of the primary, and should be equal to the sum of all the VA's for each secondary (plus inefficiencies).
For a Spitfire circuit, you may be able to sqeak by with a Hammond 270DX, but I would recommend a little more reserve power for effortless reproduction of transients, and would use a 270EX, or even a 270FX. My selections are based on using a 275-0-275 volt secondary, @90ma, 125ma, or 150ma respectivrly. All three units have the necessary current capabilities for the heaters used in that circuit.
|1/30/1998 9:57 PM|
I used a Hammond 270EX in my
Spitfire/Lightning clone. It gets
a bit hot after the amp has been
on a while, but nothing to get
too concerned about. Next time,
I would use the 270FX for a little
more thermal reserve. However, if
the high voltage secondary is not
loaded to capacity, the B+ will
be higher than expected. In my
case, with 275-0-275 volt secondary
and a 5ar4 rectifier tube, I
expected about 340-345 volts on
the B+ line. Instead I got about
360V. I would expect that with
the 270FX xformer, the B+ would
climb higher still - not desirable
in the AC15/Matchless circuit.
I also called an apps engineer at
Hammond to find out if the amount
of heat generated by the 270EX
in my application was OK. He did
not seem concerned at all - he
told me that the transformer was
able to deliver rated output
current up to 85 degrees celsius
ambiant temperature. FYI.
|1/30/1998 10:10 PM|
Doc and Mike, you suggest the larger transformer for the extra headroom and thermal reserve, but is there not a contrary argument for getting the smallest transformer that will work so it will saturate more easily to enhance distortion? Obviously I'm a novice, just asking for my own edification. Thanks.
|1/31/1998 9:21 PM|
The larger transformer suggested in
this post was the power tranny.
Using a larger transformer (rated
for more output current) allows
it to run cooler and provides a
greater reserve of current during
loud transients. The tranny you
are referring to is the output
transformer. I guess the idea of
undersizing this component is that
the core will begin to saturate
at higher volume levels. The result
is a degradation in frequency
response that might create a
"browner" sound by rolling off the
higher frequency distortion
components. I've never actually
tried this because I think it
would compromise reliability. The
core losses are manifested as
heat and there is potential for
blowing an output transformer.
Interestingly, some companies
such as Matchless use a different
design philosophy and use output
trannies that are much larger
than required. As Doc stated in
a previous post, this is probably
part of the reason their amps
have such a crystalline high end.
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