How to measure primary impedence of a unknown output tranny

[johndandry]

I have a old Output transformer from a Acetone Amp. The amp has 2x EL84.
I would like to find out what the input impedance is. How do I go about measuring that? Thanks!!
The tranny is in my new Rocket build.

[martin manning]

Not trying to be a smart-ass or anything but... (well, maybe just a little ;^)

[sixstringer]

Probably 7-8k (correct for 2-EL84 push pull), here is a link on how to determine am unknown primary value:
http://www.regiscoyne.com/tech/unknownOT/

[ampgeek]

A two hole Rocket, by the "book", would be looking for a 10.4 K primary OT.

Double check the BOM here for confirmation. Looks like 5.2K for the four hole standard design.

https://tubeamparchive.com/viewtopic.php?t=5684&start=0

Many of the well experienced and fine sages here have said that this primary "mismatch" compared with the more garden-variety plans is a significant part of the Trainwreck charm. I have only built the Express with the specified OT.

Cheers,
Dave O.

[johndandry]

Thanks Sixstringer

[loctal]

If you are building a Trainwreck style amp, you can insure your results by using any of the excellent transformer sets that are available from the people at The Amp Garage. However, I build from tranformers that I have on hand.

Sometimes you can find a schematic for the device that used your transformer which may disclose the impedance.

Most of my transformer recycles are just unknown chunks of iron. I keep notes. I have a small 1 amp 6.3 volt transformer which is applied to the primary of the unknown. Measure the output AC voltage from the secondarys. You will have to experiment to make sure that you have found the correct wires for the primary. Do not trust the color of the wires. My 6.3 volt transformer usually puts out 7.45 volts.

From past experience, I know that a 9/10K primary will yield 1.75V from a 500 ohm secondary, .313V from a 15 ohm secondary and .231V from an 8 ohm secondary.
6K primary yields 2.20V from 500 ohm, .392V from 15 ohm, and .291V from 8 ohm secondary.
3K primary yields 3.22V from 500 ohm, .570V from 15 ohm, and .420V from 8 ohm secondary. YMMV.

Using this cheesy method, you can measure a transformer in about a minute. 60 Hz is kind of low to be testing transformers and a signal generator would provide better results. Take into account the size of the transformer. An AC15 has huge transformers compared to a Fender Deluxe.

[tubeswell]

You need a low VAC source to hook up to the secondary, and you (carefully) measure the resulting VAC across the primary (being careful not to short any windings or zap yourself).

You need two VAC meters so you can simultaneously measure the exact VAC you are inputting into the secondary (at the same time as you read the VAC on the primary.)

The resulting Pr:Sec VAC ratio is equal to the square root of the Pri:Sec impedance ratio (so you need to square the VAC ratio, and then multiply the secondary impedance by the nominal chosen speaker load in order to get your answer)

i.e.: say you input 12VAC onto the secondary and get a reading of 379.5VAC across the primary. The pri:sec voltage ratio is 379.5:12 (or 31.62:1). The impedance ratio will be the square of this voltage ratio, i.e.: 1000:1. Then you multiply the impedance ratio by (say) an 8R speaker load on the secondary i.e.; [1000/1] x 8, and you get 8k on the primary.

[tubelectron]

johndandry,

Here is the right way, checked by experience on countless known and unknown output transformers :

1 - connect an 8 ohm resistor load to the secondary of the transformer.
2 - connect an audio generator to the secondary of the transformer.
3 - connect a AC DVM to the secondary of the transformer.
4 - connect a AC DVM to the primary of the transformer.
5 - set your audio generator at 400Hz and obtain circa 0.25V to 0.5V read on the secondary DVM, depending on the output abilities of your instrument.
6 - read the voltage on the primary DVM.
7 - calculate : Vprim / Vsec = R.
8 - calculate : (R)2 * 8 = Z primary.

Note :

A - if the transformer isn't 8 ohm secondary but say 4 ohm and you used a 8 ohm resistor, the measurement is still valid : in the 8 - formula, just replace 8 by 4. The error is negligible.

B - if the transformer is still mounted and connected in the tube amp, turn it off and remove the power tubes, and then do the same measurement process.

A+!

[lumox0013]

I really like this page expains it well for me. cheers
http://www.harpamps.com/transformers.html

[tubelectron]

I really like this page expains it well for me. cheers
http://www.harpamps.com/transformers.html

Well, you will only have approximative results doing that, because 50 or 60hz is often a too low test frequency for audio, and the transformer is not loaded at the secondary by a 8 ohms load resistor for the measurement. I checked it, trust me...

A+!

[rdjones]

Well, you will only have approximative results doing that, because 50 or 60hz is often a too low test frequency for audio, and the transformer is not loaded at the secondary by a 8 ohms load resistor for the measurement. I checked it, trust me...

A+!

As long as you use a low voltage to stay well away from saturation the reading at 60Hz should be valid, no ?

rd

[tubelectron]

As long as you use a low voltage to stay well away from saturation the reading at 60Hz should be valid, no ?

It should, but you often find differences with the 400Hz measurement, due to the transformer's laminations and windings quality. These differences are not huge, but it could represent as far as 10% for little old radio or guitar amp output transformers, on which the bandwith to the basses is weak. That's the reason why I prefer 400Hz.

A+!