"T N Nurse" bravely wrote to "All" (25 Sep 04 14:35:33)
--- on the heady topic of "Transformer question"

TNN From: T N Nurse
TNN Subject: Transformer question
TNN Organization: Malathion Church of Holy Redemption
TNN Xref: aeinews sci.electronics.repair:39778 sci.electronics.misc:18316

TNN I have a 50 watt valve amp which seems to have blown its
TNN output transformer. A new one is on order, but I was puzzled
TNN by some of the reading I got off it and wondered if someone
TNN could give an explanation. All the measurements were made
TNN with the transformer *_completely out of circuit_*, both primary
TNN and secondary.

TNN The transformer is from a push-pull standard guitar amp (Trace
TNN Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing
TNN red hot before the fuse popped. Checking the bias voltages, I
TNN had around -46v on the grid of each of the El34s, so that was
TNN a reasonable figure. I then removed the output transformer
TNN and did some resistance measurements on it. When measure from
TNN the centre tap to either of the primary outer connections, the
TNN reading was 69 ohms for each. But when I measured across the
TNN primary outer connections, instead of the expected 138 ohms,
TNN I got an open circuit. I rechecked it numerous times but with
TNN the same result. I even removed the cables from their connector
TNN and measure across the bare wires, but still the same result,
TNN 69 ohms from the centre tap to the outers, but open circuit
TNN between the outers.

TNN On the basis of these bizarre reading, I assumed the transformer
TNN is faulty and ordered a new one, but can anyone offer an
TNN explanation as to why I got such resistance readings? I have a
TNN vague recollection of similar results on a small 15 watt amp I
TNN repaired many years ago and replacing the transformer fixed it
TNN and it went on to give good service, but I would like to know
TNN what is actually going on. Anyone?

The only thing I can think of is that (burnt) oxidized metal to metal
contacts can form a rudimentary semiconductor and will not allow current
to flow in reverse just like a diode. Try swapping your ohmmeter wires.

A*s*i*m*o*v

.... Are part-time band-leaders semi-conductors?

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" wrote in message
...
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?

On Sun, 26 Sep 2004 09:03:32 -0400, "Art"
wrote:

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.

---
You obviously missed the point, which was that a finite resistance
could be measured from the center tap to either end of the primary but
that the end-to-end resistance was infinite.

--
John Fields

John Fields writes:

On Sun, 26 Sep 2004 09:03:32 -0400, "Art"
wrote:

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.

---
You obviously missed the point, which was that a finite resistance
could be measured from the center tap to either end of the primary but
that the end-to-end resistance was infinite.

I liked the two separate windings with two wires not making contact
for the centertap. Has the OP retested or checked for this possibility?

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its most likely your digital meter.
my fluke i use at work will not properly
read a field winding above a 100 ohms due
to inductive reactions. it just sits there
blinking now and then giving me an OL reading.
try putting it on DIODE mode.


John Fields wrote:

On Sun, 26 Sep 2004 09:03:32 -0400, "Art"
wrote:


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.


---
You obviously missed the point, which was that a finite resistance
could be measured from the center tap to either end of the primary but
that the end-to-end resistance was infinite.

John Fields wrote:
On Sun, 26 Sep 2004 09:03:32 -0400, "Art"
wrote:


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.


---
You obviously missed the point, which was that a finite resistance
could be measured from the center tap to either end of the primary but
that the end-to-end resistance was infinite.

Any chance there's more in there than a transformer? Like a couple
of protective diodes (perhaps as snubbers) that are back-to-back when
viewed from end-to-end? It would be odd, but not completely out of
the question.

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