Date: Mon, 4 Mar 2013 12:28:34 +0100
From: "Ulf (SM0NOR)" <ulf@sm0nor.com>
To: amps@contesting.com
Subject: [Amps] Transformer choice
Message-ID: <18ED311A-6956-4387-9638-7860C470F500@sm0nor.com>
Content-Type: text/plain; charset=windows-1252
Hi Guys,
I'm looking for some feedback on two transformer offers I have. I'm intending
to build a table top amp, so I'm accepting some compromises regarding the
transformer. I have made inquiries to a couple of winders and I have boiled
them down to 2 offers. I was asking for 2000 Volts @1A =2000VA (+ G1 and G2
windings). I'm targeting to get 2200-2400 watts output in PEP service and about
1K in continuos service.
Both companies claim 2000VA but at different core weights
Company A:
17 Kilo toroid, D=200 mm, H=110. About 3% voltage drop at full load. Max temp
rise = 105C over 40C ambient. DC resistance, about 25 ohms.
Price: ?480
Company B:
11 kilo toroid, D=180 mm, H=85mm. About 2,5% voltage drop at full load. Can't,
or don't want to specify other data.
Price: ?195
So what are your reflections? The A is obviously a the better choice. They are
serious and they offer detailed specs, exact delivery times etc. But all comes
at a premium price. Company B is smaller and they obviously compromises with
their specifications, but the price is a lot lower and so is the weight.
Now, I don't expect the cheaper and smaller core to do wonders. The bigger one
will for sure be more rugged and closer to "true 2000VA", especially if I run
it in true continuous mode. I'm inclined to go for the the bigger one, because
I want the best. But OTOH, I don't want to waste money because of a "feeling".
Perhaps the smaller one will be enough given the table-top size/PEP service I'm
targeting? After all, it matches what's inside most commercial amps in this
category.
I'm happy for any feedback...
### 105 C temp rise over a 40 C ambient is going to get really HOT. A 65 C
rise over a 40 C ambient is as much as I would want to go. Company B doesn’t
even quote a temp rise. Nor do they spec the
all important sec DC resistance. To get 2.4 kw pep out at 60% eff, it will
require 4 kw pep input. You do the maths. I would not go with co B.
Usually when a xfmr maker doesn’t quote a temp rise
nor the sec DC resistance, you always end up with a substandard xfmr.
## I would suggest asking co A again, but this time for a much lower temp
rise spec. 18-25 ohms for a 2 kva xfmr is good. Ask for detailed specs for
co B. If they don’t provide, don’t buy it.
later... Jim VE7RF
73's
Ulf (SM0NOR)
ulf@sm0nor.com
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