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Hammond Transformers - VA means what?

1/26/1998 4:49 PM
dave chun
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
Mike B

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.  
Mike B
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
Mike B

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.  
Mike B

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