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Post subject: Primary Impedance of Super Twin Output Transformer
Posted: Fri Dec 16, 2016 7:37 am
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I'm new to the forum.

Have a 70's model super twin with weak output. Amp is labeled by Fender as 185 watt RMS output. I suspect the output transformer may be bad and am looking for information about it.

Does anyone know the primary impedance of the audio output transformer ? I've looked all over but have not been able to find it yet.

Best Regards

CE


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 2:03 am
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Newdyne wrote:
I'm new to the forum.

Have a 70's model super twin with weak output. Amp is labeled by Fender as 185 watt RMS output. I suspect the output transformer may be bad and am looking for information about it.

Does anyone know the primary impedance of the audio output transformer ? I've looked all over but have not been able to find it yet.

Best Regards

CE


How did you'll read primary impedance ? Or you are talking about DC resistance ? it is another story and you can't test transformer with this method.

Primary impedance must be close to 1900 ohms
To test any transformer I use signal generator and voltmeter.

Hammond replacement transformer show 0.240 ohms DC resistance at primary, each side. About half with the center tap.
https://www.hammfg.com/files/parts/pdf/1750WR.pdf


For a low output there are many things to check before looking at output transformer .
Tubes first.
Voltages ; B+, heater supply
What is your experience to fix tubes amps ?
Any electronic skill ?


Last edited by stratele52 on Sat Dec 17, 2016 4:36 am, edited 1 time in total.

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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 2:35 am
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stratele52 wrote:
Hammond replacement transformer show 0.240 ohms DC resistance at primary, each side. About half with the center tap.
https://www.hammfg.com/files/parts/pdf/1750WR.pdf


That's the wrong tranny for this amp......as usual.

:roll:

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 2:59 am
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Retroverbial wrote:
stratele52 wrote:
Hammond replacement transformer show 0.240 ohms DC resistance at primary, each side. About half with the center tap.
https://www.hammfg.com/files/parts/pdf/1750WR.pdf


That's the wrong tranny for this amp......as usual.

:roll:


Arjay


As usual ...

Original tranny may not be far to read DC ohms IMO.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 4:14 am
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Newdyne wrote:
I'm new to the forum.

Have a 70's model super twin with weak output. Amp is labeled by Fender as 185 watt RMS output. I suspect the output transformer may be bad and am looking for information about it.

Does anyone know the primary impedance of the audio output transformer ? I've looked all over but have not been able to find it yet.

Best Regards

CE


My general WAG for primary impedance would be 1/3rd of 6000-ohm (impedance of one push-pair of 6L6GCs). Since there are three push-pull pairs of 6L6GC which are arranged in parallel with each other. Equals around 2000 ohms.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 7:22 am
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First, thanks for all the replies!

My experience level is moderate. I was trained as a electronics tech "back in the day". I've done quite a lot of building various types of equipment over the years. Also, general trouble shooting of various types of equipment. But, not much experience with guitar amps.

I have a Techtronix scope, HP 200AB signal generator, AC variac, Fluke meters, Tube tester, a collection of small parts, etc.

I have checked the B+, looked the amp over for obvious damaged components, etc. I traced through the signal path with the scope and don't see much signal on the load side of the output transformer. So, I'm looking to test the transformer.

I've done some basic DC resistance checks on the transformer. Primary resistance measurements are: Blue to Red = 20.7 ohm, Brown to Red = 19.6 ohm, Red to White / Blue = 2.5 ohm, Red to White / Brown = 2.9 ohm. Secondary is .4 ohm. No shorts between windings or to ground.

Plan to use the variac and small step down transformer (or audio generator and scope) to find the turns ratio then calculate the impedance using the formula (the impedance ratio is the square of the turns ratio). Then calculate the the transformer impedance using 4 ohms as the know secondary impedance.

Problem is, I don't know the specified impedance. That's why I'm asking.

CE


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 7:30 am
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Forgot to mention, I've also tested all the tubes with my Eico 667 tube tester. The tubes all look good. Also, I have not measured the screen resistors but they don't look overheated.

When I got the amp it was running with only 4 output tubes plugged in. I suspect it was probably abused. I'm trying to rule out the output transformer as I know they are expensive and hard to find in good condition.


CE


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 7:41 am
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Newdyne wrote:
Forgot to mention, I've also tested all the tubes with my Eico 667 tube tester. The tubes all look good. Also, I have not measured the screen resistors but they don't look overheated.

When I got the amp it was running with only 4 output tubes plugged in. I suspect it was probably abused. I'm trying to rule out the output transformer as I know they are expensive and hard to find in good condition.


CE


Tubes tester are not reliable, don't trust it if you want to know if tubes are good or not.
Tubes may be very weak and tubes tester show good.

Tubes tester will say truth if tube is dead or short


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 8:04 am
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How I test transformer,
This test always find good or bad transformer.



1- Signal generator set to 1000 hertz AC 0.5 volts.

2- Send this signal to OT secondary ,re- check with accurate meter to have 0.5 VAC on OT tap. Or close.

3- Read AC voltage at OT primary

4- For exemple if you read 14.35 VAC you divide by 0.5 ( or what you read at 1- )

14.35 / 0.5 = 28.7

5- Square the result ; 28.7 X 28.7 = 829.67

6- Multiply by secondary output impedance where you are connected , say 8 ohms
829.69 X 8 = 6631

7 - This reading 6631 is the primary input impedance

If you use two 6L6 in push pull , look on Tube Book for Plate Load resistance . You’ll see you that you need 6000 to 3800 ohms for two 6L6 in PP class AB

The reading we got 6631 is on spec
_______________________________________
If transformer is no good your reading will be very far from what we should get .

By experience these test do not work on Super Reverb Transformer or other with 2 ohms Output Transformer impedance

You can do this test on unknow OT to see how / where you can use it


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 10:19 am
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There is another way that I test impedance for resistive loads. Not sure how it works for inductive loads. But it should.
Hook a signal generator to the load in series with a potentiometer.
Use a value similar to the expected impedance.
Hook your scope across the load.
Set the pot to zero ohms
Choose a frequency and raise the signal level to full scale on the scope. Impedance changes with frequency.
Rotate the control until the scope shows 1/2 full scale.
At that point the impedance of the pot equals the impedance of the load.
Measure the DC resistance of the pot to obtain impedance.
I use this for speakers and input stages. Never tried it on a tranny.


Last edited by TimsAudio on Sat Dec 17, 2016 10:26 am, edited 1 time in total.

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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 10:24 am
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TimsAudio wrote:
There is another way that I test impedance for resistive loads. Not sure how it works for inductive loads. But it should.
Hook a signal generator to the load in series with a potentiometer.
Use a value similar to the expected impedance.
Hook your scope across the divider of the pot.
Set the pot to zero ohms
Choose a frequency and raise the signal level to full scale on the scope. Impedance changes with frequency.
Rotate the control until the scope shows 1/2 full scale.
At that point the impedance of the pot equals the impedance of the load.
Measure the DC resistance of the pot to obtain impedance.
I use this for speakers and input stages. Never tried it on a tranny.



I will try it , thank's TimAudio


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sat Dec 17, 2016 6:12 pm
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I should add one critical note to be clear .
The source impedance of the signal generator needs to be lower than the measured impedance. My generator is 600 Ohm
So to measure speakers and OT secondaries, I hook the signal generator to a SS amplifier to obtain the low impedance output.
For speaker measurements, I use a 25ohm WW 2 watt pot. I put a nameplate on it with ohms written around the dial so I don't have to disconnect and measure each frequency step.


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sun Dec 18, 2016 4:31 am
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TimsAudio wrote:
I should add one critical note to be clear .
The source impedance of the signal generator needs to be lower than the measured impedance. My generator is 600 Ohm
So to measure speakers and OT secondaries, I hook the signal generator to a SS amplifier to obtain the low impedance output.
For speaker measurements, I use a 25ohm WW 2 watt pot. I put a nameplate on it with ohms written around the dial so I don't have to disconnect and measure each frequency step.


So you do that for all speaker measurements. Speaker impedance are always lower than your generator impedance .
I'm right ?

My way to test impedance do not work on Super Reverb Output xfrs. IMO it is because their impedance are lower than my audio generator.
Did you agree ?
How can I know my generator impedance ?
A Marconi Instruments TF2103. A 1970 generator


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sun Dec 18, 2016 6:29 am
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Thanks everyone for all the good information.

I will try these methods this week to see what I get.

Does anyone know the primary impedance spec (from the factory) of this transformer? I know the secondary is 4 ohms. One of the answers in this forum was 1900 ohms. I suspect this is about right, but not sure.

I'm trying to make an educated guess of whether the output transformer is good or bad and would be very interested to know if anyone has knowledge about this spec.



Thanks

CE


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Post subject: Re: Primary Impedance of Super Twin Output Transformer
Posted: Sun Dec 18, 2016 6:57 am
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Newdyne wrote:
Thanks everyone for all the good information.

I will try these methods this week to see what I get.

Does anyone know the primary impedance spec (from the factory) of this transformer? I know the secondary is 4 ohms. One of the answers in this forum was 1900 ohms. I suspect this is about right, but not sure.

I'm trying to make an educated guess of whether the output transformer is good or bad and would be very interested to know if anyone has knowledge about this spec.






Thanks

CE


For about same power and output impedance, transformer primary impedance are always very close. You can trust what we write, me and
BMW2002Ti ; close to 1900 ohms

Impedance don't need to be accurate


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