Re: Transformer question
From: Art (plotsligt_at_comcast.net)
Date: 09/26/04
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Date: Sun, 26 Sep 2004 09:03:32 -0400
>Having an open resistance reading on the primary in indicative there
has been failure within the transformer, be it an open thermal protect
device or actually a break within the actual primary wires. At that point it
is not necessary to determine the condition of the secondary unless you have
a specific reason for doing so, such as, determining what the shorted output
valve may have damages along with the transformer.
>Specifically overheated or damaged resistors, capacitors, etc in that
particular circuit. You may be able to compare the values with those on the
other-side of the push-pull circuit to ascertain if indeed there is
additional damages.
>FYIW the bias voltages sound normal for the operational
characteristics of those particular valves. I would be interented also in
the cathode current and plate voltages on eachof the devices. May be best to
replace both of them since one obviously has failed, trying to get a matched
pair, if available an reasonable cost. Cheers: Good luck in rectifying the
initial failure and remedies.
"T N Nurse" <tnnurseNOUCE99@hotmail.com> wrote in message
news:tnnurseNOUCE99-C4311E.14353225092004@singer.cent.gla.ac.uk...
>I have a 50 watt valve amp which seems to have blown its
> output transformer. A new one is on order, but I was puzzled
> by some of the reading I got off it and wondered if someone
> could give an explanation. All the measurements were made
> with the transformer *_completely out of circuit_*, both primary
> and secondary.
>
> The transformer is from a push-pull standard guitar amp (Trace
> Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing
> red hot before the fuse popped. Checking the bias voltages, I
> had around -46v on the grid of each of the El34s, so that was
> a reasonable figure. I then removed the output transformer
> and did some resistance measurements on it. When measure from
> the centre tap to either of the primary outer connections, the
> reading was 69 ohms for each. But when I measured across the
> primary outer connections, instead of the expected 138 ohms,
> I got an open circuit. I rechecked it numerous times but with
> the same result. I even removed the cables from their connector
> and measure across the bare wires, but still the same result,
> 69 ohms from the centre tap to the outers, but open circuit
> between the outers.
>
> On the basis of these bizarre reading, I assumed the transformer
> is faulty and ordered a new one, but can anyone offer an
> explanation as to why I got such resistance readings? I have a
> vague recollection of similar results on a small 15 watt amp I
> repaired many years ago and replacing the transformer fixed it
> and it went on to give good service, but I would like to know
> what is actually going on. Anyone?
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