[Amps] Transformer choice

[Jim Thomson]

Date: Mon, 4 Mar 2013 12:28:34 +0100
From: "Ulf (SM0NOR)" <ulf at sm0nor.com>
To: amps at contesting.com
Subject: [Amps] Transformer choice
Message-ID: <18ED311A-6956-4387-9638-7860C470F500 at 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 at sm0nor.com