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Transformer question

T

T N Nurse

Jan 1, 1970
0
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?
 
P

philo

Jan 1, 1970
0
T said:
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?


there are usually two wires in the center tap...
if they are not touching when you take your reading ...there will be
infinite resistance...
 
J

John Fields

Jan 1, 1970
0
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?

---
The only thing I can think of is that the primary is actually two
windings with an intermittent connection at the center tap and that
pressure from your probe/alligator clip/whatever when it was connected
to the center tap was connecting the two windings so that you got the
dual 69 ohm readings, but then when you disconnected from the center
tap the connection between the windings was broken, resulting in the
infinite resistance reading between them.

Try resoldering the winding ends going to the center tap and see what
happens. Maybe you'll wind up with a good spare!-)
 
R

Ross Herbert

Jan 1, 1970
0
On Sat, 25 Sep 2004 14:35:33 +0100, T N Nurse

|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?


Did you use a digital multimeter?

I have encountered some strange an incomprehensible readings when
taking resistance measurements on highly reactive components such as
transformers when measuring with my Gossen Metrawatt MetraHit 25S
digital meter (a not too cheap instrument), and when this occurs I
always revert back to my trusty old moving coil analogue meter (AVO
model 7) and usually the correct expected measurement is obtained.

Ross H
 
A

Asimov

Jan 1, 1970
0
"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 <[email protected]>
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?
 
A

Art

Jan 1, 1970
0
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.
 
J

John Fields

Jan 1, 1970
0
has been failure within the transformer, be it an open thermal protect
device or actually a break within the actual primary wires.
 
S

Sam Goldwasser

Jan 1, 1970
0
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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Important: Anything sent to the email address in the message header is ignored.
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J

Jamie

Jan 1, 1970
0
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.
 
C

CJT

Jan 1, 1970
0
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.
 
T

T N Nurse

Jan 1, 1970
0
Thanks to everyone who replied to this. The problem has now been
solved - intermittent fault in the probe lead. Grrrrrr!!!!
New probe leads now give the correct readings.
 
T

T N Nurse

Jan 1, 1970
0
Ross Herbert said:
On Sat, 25 Sep 2004 14:35:33 +0100, T N Nurse

|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?


Did you use a digital multimeter?
Yes.


I have encountered some strange an incomprehensible readings when
taking resistance measurements on highly reactive components such as
transformers when measuring with my Gossen Metrawatt MetraHit 25S
digital meter (a not too cheap instrument), and when this occurs I
always revert back to my trusty old moving coil analogue meter (AVO
model 7) and usually the correct expected measurement is obtained.

That's an interesting observation, and perhaps explains the previous
weird measurement. I note that someone else has said that their
expensive Fluke doesn't like transformer windings over 100 ohms.
I tried today with a different multimeter (and leads) and got the
expected readings. I assumed that it was the leads at fault, but
perhaps not.
 
J

John A. Weeks III

Jan 1, 1970
0
T N Nurse said:
Thanks to everyone who replied to this. The problem has now been
solved - intermittent fault in the probe lead. Grrrrrr!!!!
New probe leads now give the correct readings.

At least that is a lot cheaper to fix than the cost of a new
transformer.

-john-
 
J

John Fields

Jan 1, 1970
0
Thanks to everyone who replied to this. The problem has now been
solved - intermittent fault in the probe lead. Grrrrrr!!!!
New probe leads now give the correct readings.
 
T

T N Nurse

Jan 1, 1970
0
{Centre tapped primary of a push-pull output transformer for audio amp}

Looks like it. The two windings came in at 66 and 70 ohms respectively
with combined reading of 135 ohms. Looks like a partial short in
one of the windings. I measured it 3 ways just to be sure - using
the multimeter, using a constant voltage source and checking the
current and using a constant current and checking the voltage. All
3 came in with the same readings. I'm waiting to check with the
manufacturer to check if a 4 ohm difference is out of spec - I
suspect very much that it is.
 
D

David

Jan 1, 1970
0
{Centre tapped primary of a push-pull output transformer for audio amp}

Looks like it. The two windings came in at 66 and 70 ohms respectively
with combined reading of 135 ohms. Looks like a partial short in
one of the windings. I measured it 3 ways just to be sure - using
the multimeter, using a constant voltage source and checking the
current and using a constant current and checking the voltage. All
3 came in with the same readings. I'm waiting to check with the
manufacturer to check if a 4 ohm difference is out of spec - I
suspect very much that it is.

It is not uncommon for the DC resistance of a center tapped winding to differ on each half. For the same number of turns, one half could have more length of wire because that part of the winding is further from the core piece and one turn uses more wire. A better way to test is to put an AC signal on each half separately and see if the secondary gets the same output.
David
..
 
J

John Fields

Jan 1, 1970
0
It is not uncommon for the DC resistance of a center tapped winding to
differ on each half. For the same number of turns, one half could have
more length of wire because that part of the winding is further from
the core piece and one turn uses more wire. A better way to test is to
put an AC signal on each half separately and see if the secondary gets
the same output.

---
I disagree. Using EI laminations and winding the entire transformer on
the center leg, it makes nore sense, if only for the sake of economy,
to wind the CT primary bifilar and avoid using more wire than
necessary. After all, an extra ten feet per transformer is about two
miles of wire, 1000 transformers downstream. Putting down both halves
of the primary at the same time also saves time and tends to make both
halves of the primary look more alike than winding one half on top of
the other.
 
D

David

Jan 1, 1970
0
John Fields said:
It is not uncommon for the DC resistance of a center tapped winding to
differ on each half. For the same number of turns, one half could have
more length of wire because that part of the winding is further from
the core piece and one turn uses more wire. A better way to test is to
put an AC signal on each half separately and see if the secondary gets
the same output.

---
I disagree. Using EI laminations and winding the entire transformer on
the center leg, it makes nore sense, if only for the sake of economy,
to wind the CT primary bifilar and avoid using more wire than
necessary. After all, an extra ten feet per transformer is about two
miles of wire, 1000 transformers downstream. Putting down both halves
of the primary at the same time also saves time and tends to make both
halves of the primary look more alike than winding one half on top of
the other.
John,
That may be true, but this transformer may not have been wound with a bifilar primary. If bifilar I agree, if not, I stand by my original post and it should be tested by either measuring inductance or seeing if there is a difference in a secondary by feeding a signal into each half separately. I know a short on one side will affect both halves but a difference should still be seen.
David
 
J

John Fields

Jan 1, 1970
0
John,
That may be true, but this transformer may not have been wound with a
bifilar primary. If bifilar I agree, if not, I stand by my original
post and it should be tested by either measuring inductance or seeing
if there is a difference in a secondary by feeding a signal into each
half separately. I know a short on one side will affect both halves
but a difference should still be seen.

---
I don't disagree with your measurement strategy except that I'd put a
signal into the secondary and then measure the primary output voltages
on either side of the center tap, the presumption being that if the
winding on one side has shorted turns, the voltage out of that side
will be lower than the voltage out of the other side.
 
J

Jim Adney

Jan 1, 1970
0
I disagree. Using EI laminations and winding the entire transformer on
the center leg, it makes nore sense, if only for the sake of economy,
to wind the CT primary bifilar and avoid using more wire than
necessary. After all, an extra ten feet per transformer is about two
miles of wire, 1000 transformers downstream. Putting down both halves
of the primary at the same time also saves time and tends to make both
halves of the primary look more alike than winding one half on top of
the other.

I think this is only half right. Winding them bifilar would certainly
make them more "balanced" in both turns and impedance, but the total
amount of wire used would have to be the same.

Think of it this way: I still need the same number of turns in each
winding and the same number of total turns. Winding them bifilar
(assuming the same wire gauge) will still occupy the same amount of
the winding window.

So instead of "an extra ten feet" of wire in the outer winding, we end
up using five extra feet in each winding.

I don't know how a real transformer manufacturer would look at it, but
I suspect that the bifilar way has real appeal as long as the voltages
are small.

Once the voltages are large, we may want to spread out the ends and
wind them separately, which most likely means that we're back to the
original method of one on top of (and longer than) the other.

-
 
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