Hello:

I have a tube radio to repair. It's output valve is an ELL80 (two EL95 in
one tube) and was broken at the base and the socket showed signs of overheat
and cold solder joints. I have the valve on order but when checking the
output transformers (it has 2) onde of them has a primary resistance of
~900ohm and the other ~4000 ohm. I measured the turns ratio using a signal
generator at 1khz and got 45,6 and 46,0 which squared and multiplied by the
speakers output impedance (5ohm) gives ~10kohms which is the expected
impedance for the ELL80. Given all this info how can i check which
transformer is bad? they both drive an speaker just fine using the Sig. Gen.
but i can't believe they can have such different dc resistance.

Thank you in advance

Best Regards

Steve Sousa

On Tue, 23 Nov 2010 18:21:50 +0000, Steve Sousa wrote:

Hello:

I have a tube radio to repair. It's output valve is an ELL80 (two EL95
in one tube) and was broken at the base and the socket showed signs of
overheat and cold solder joints. I have the valve on order but when
checking the output transformers (it has 2) onde of them has a primary
resistance of ~900ohm and the other ~4000 ohm. I measured the turns
ratio using a signal generator at 1khz and got 45,6 and 46,0 which
squared and multiplied by the speakers output impedance (5ohm) gives
~10kohms which is the expected impedance for the ELL80. Given all this
info how can i check which transformer is bad? they both drive an
speaker just fine using the Sig. Gen. but i can't believe they can have
such different dc resistance.

Thank you in advance

Best Regards

Steve Sousa

That's a heck of a difference in DC resistance. If you drive both with a
sig generator and look at the secondaries on a scope is there a big
difference in the signal amplitude between the two? If not I would be
inclined to replace the tube and take some voltages while the circuit is
live while monitoring the plate currents. You can look here for
operational voltages and currents in different classes of operation:

http://www.nj7p.org/Tube4.php?tube=6gu8



--
Live Fast, Die Young and Leave a Pretty Corpse

"Meat Plow" wrote in message
news
On Tue, 23 Nov 2010 18:21:50 +0000, Steve Sousa wrote:

That's a heck of a difference in DC resistance. If you drive both with a
sig generator and look at the secondaries on a scope is there a big
difference in the signal amplitude between the two? If not I would be
inclined to replace the tube and take some voltages while the circuit is
live while monitoring the plate currents. You can look here for
operational voltages and currents in different classes of operation:

http://www.nj7p.org/Tube4.php?tube=6gu8

hello:

there was about half division difference on the scope. I was trying to avoid
testing with the valve because it's expensive and "rare" i don't want to
take much risks....

"Steve Sousa"

I have a tube radio to repair. It's output valve is an ELL80 (two EL95 in
one tube) and was broken at the base and the socket showed signs of
overheat and cold solder joints. I have the valve on order but when
checking the output transformers (it has 2) onde of them has a primary
resistance of ~900ohm and the other ~4000 ohm.


** Assuming the transformers are the same size and do the same job, the one
with the 4000 ohm primary is stuffed.


I measured the turns ratio using a signal generator at 1khz and got 45,6
and 46,0


** Do not use a comma for the decimal point - it is way too confusing.



...... Phil

"Phil Allison" wrote in message
...


** Assuming the transformers are the same size and do the same job, the
one with the 4000 ohm primary is stuffed.


I measured the turns ratio using a signal generator at 1khz and got 45,6
and 46,0


** Do not use a comma for the decimal point - it is way too confusing.



..... Phil


Hello

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?

Best Regards

On 25/11/2010 14:09, Steve Sousa wrote:


"Phil Allison" wrote in message
...


** Assuming the transformers are the same size and do the same job,
the one with the 4000 ohm primary is stuffed.


I measured the turns ratio using a signal generator at 1khz and got
45,6 and 46,0


** Do not use a comma for the decimal point - it is way too confusing.



..... Phil


Hello

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?

Best Regards

Are these of any use:
http://www.vvttransformers.co.uk/vtp135200.htm
http://www.vvttransformers.co.uk/vtp206300.htm

--

Jeff

"Steve Sousa"
"Phil Allison"

** Assuming the transformers are the same size and do the same job, the
one with the 4000 ohm primary is stuffed.


I measured the turns ratio using a signal generator at 1khz and got 45,6
and 46,0


** Do not use a comma for the decimal point - it is way too confusing.


Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?


** No.

The ratio between rated primary Z and winding resistance is never less than
5:1 and is typically 10:1 or more.



..... Phil

On 11/25/2010 12:01 PM Phil Allison spake thus:

"Steve Sousa" wrote:

"Phil Allison" wrote:

** Assuming the transformers are the same size and do the same job, the
one with the 4000 ohm primary is stuffed.

Wouldn't it be possible to have an original 4000ohm shorted to 900ohm too?

** No.

The ratio between rated primary Z and winding resistance is never less than
5:1 and is typically 10:1 or more.

You may have a point there.

OK, let's climb down into N. Cook's pit of bodgery and post mortem
analysis and look at this.

It all depends on the expected failure mode of a transformer, correct?
Two main possibilities he open windings, or shorted windings (or I
suppose a combination). Obviously not an open winding, so probably
shorted ones.

Shorted windings could be within the primary or secondary, or between.
Let's look at the simpler case, shorted within the primary, which should
be 4K ohms.

It seems most likely that any short would be between adjacent layers of
windings. Not likely that there'd be a short from close to one end to
close to the other end. Which should limit the effect of the short. So
if the 4K ohm primary is now 900 ohms, there's something else going on.

But what? Could there be a short from the winding to the iron core?

Has the O.P. checked for a short between primary and secondary? How
about shorts to the core/frame of the xfmr?


--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

"David Nebenzahl = Idiot "


Wouldn't it be possible to have an original 4000ohm shorted to 900ohm
too?

** No.

The ratio between rated primary Z and winding resistance is never less
than 5:1 and is typically 10:1 or more.

You may have a point there.

OK, let's climb down into N. Cook's pit of bodgery and post mortem
analysis and look at this.

It all depends on the expected failure mode of a transformer, correct? Two
main possibilities he open windings, or shorted windings (or I suppose
a combination). Obviously not an open winding, so probably shorted ones.



** Shame about the third and most likely scenario - one that makes the
primary R go way high.

Think " vertigris " .....



...... Phil