How to define a quality OT?

[airboss]

No this is not a query about finding out the primary impedance of the odd unmarked OT.

I got in the habit awhile back of grabbing up any kind of interesting output transformer I could find on Eflea. I got a new in box Peerless 20 watt with 5K and 3K primary taps and 500, 125, 32, 16,12,8, and 4 ohm secondaries. I got a new Thordarson 20 watt 5k to a similar whole bunch of secondaries. It also has 10% screen taps. I guess it pre-dates David Hafler's claims about UL. I got Leslie 8K to 16ohm ot and bunches of various 6600 to 8 ohm Hammond Organ iron. There are a few 7200 to 8 ohm Hammond organ ot designed for 12 watts of cathode biased 6V6 or 6BQ5. Same part number. Some others. Unfortunately I have very few pairs. I think I got four of the Hammond 6V6 jobs. I also have a OEM transformer that I had Phil Heyboer make for me by tearing apart and reverse engineering a particularly nice 6600 ohm Hammond Organ OT. I make it with 8800 and 5000 primaries to 4,8,and,16 on about 35 watt iron. I'd like to know if it's any good for Hi Fi. It sure will knock your socks off with a pair of EL-84s in fixed bias in Ken Fisher's Trainwreck topology. That's a guitar amp.

Anyway, what defines a quality OT? If you were to put together a test rig to check out quality how would you do it and what would you look for? What makes a James or Lundahl better than (say) an Edcor or a Hammond? I've got a couple of pretty good scopes, (tek 453 and 2465) an audio signal generator, an HP frequency counter and lots of good volt meters. I got variacs. It seems maybe I need something that will generate a square wave maybe. Some sort of pulse maker; like a clean DC supply and a SCR trigger to wack the windings with.

Anyway, think about how you might approach that. I'd like to take a whirl at it.

[TubeManiac]

square sine on a transformer is not a good idea i think. I thought it wil make high induction voltage spikes. Correct me if i'm wrong

[MassMat]

Hi,

You can always measure primary inductance of the OT. Only thing you need is a sine wave generator, an AC volt meter, and a 10K~300K resistor. There is a post at DIYTUBE that lists inductance measurements of some common OTs:
diytube.com :: View topic - ST-35 with Tango Output Transformers

Matt

[SY]

Side-to-side primary balance and leakage inductance would be the first two things I'd measure and use to judge a transformer. If bass is a concern (I don't use tubes below 100 Hz), then primary inductance. These are all standard and well-documented measurements.

[kstagger]

A few years back, John Atwood did some tests... luckily One Electron hosts this information.
Push-Pull Transformer Test

On a side note, I used the _big_ Hammond 1627SEA in my last amp build. I was surprised by the sound quality...

[jlsem]

My own informal listening tests have shown that the higher the quality of the laminations, the less distorted the transformer sounds. Permalloy is the best in this regard and this is due to its more linear magnetization curve. These were single-ended amps but I'm sure the same would hold true for push-pull.

John

[ChrisA]

Originally Posted by Danny Hall

Anyway, what defines a quality OT? If you were to put together a test rig to check out quality how would you do it and what would you look for?

I've got the same problem. Several old OPTs. Some Hammond organ some from broken guitar amps.

Here is my plan..... I've going to use a good quality solid state amp. and use it to drive the 8 ohm secondary. I'll load the primary with a high value power resister to simulate the tubes.

I have a computer with a good studio grade 24-bit audio interface. I figure use software to put a sine wave test signal into the amp. I'll use a dummy load and an about 10000:1 voltage divider on the secondary and run that signal back to my computer.

I should be able to directly measure the frequency response at several different power setting. yes I know I'll need 20, 50 or even 100 watt power resistors for the dummy load.

This plan assumes that the backwards test is valid, using an 8 ohm amp to drive a high impedance load should work. The computer should make direct measurement of harmonics and distortion possible. I can input a sine wave and look at a spectrogram of the output.

[airboss]

Originally Posted by TubeManiac

square sine on a transformer is not a good idea i think. I thought it wil make high induction voltage spikes. Correct me if i'm wrong

See KStagger's post. I had seen that test somewhere in my misspent youth. The trick to eliminating the danger of runaway transformer is to make sure it is terminated into a load.

[airboss]

Originally Posted by SY

Side-to-side primary balance and leakage inductance would be the first two things I'd measure and use to judge a transformer. If bass is a concern (I don't use tubes below 100 Hz), then primary inductance. These are all standard and well-documented measurements.

It's interesting Sy that side to side simple DC resistance testing shows that there is seldom very good balance. Usually it is off by over 10% IMS.

Leakage inductance testing is a good one to add to Attwoods procedure to predict bass response but he seems to use a simple sweep and watch for the -3db point.

One thing I've noticed as a the tear apart kid is that some transformers are wax filled some are not. Just intuitively it's got to make a difference. The old transformer building texts out of the 1930's and current practice in motor winding shops all emphasise glueing everything together well.

[kevinkr]

Originally Posted by ChrisA

I've got the same problem. Several old OPTs. Some Hammond organ some from broken guitar amps.

Here is my plan..... I've going to use a good quality solid state amp. and use it to drive the 8 ohm secondary. I'll load the primary with a high value power resister to simulate the tubes.

I have a computer with a good studio grade 24-bit audio interface. I figure use software to put a sine wave test signal into the amp. I'll use a dummy load and an about 10000:1 voltage divider on the secondary and run that signal back to my computer.

I should be able to directly measure the frequency response at several different power setting. yes I know I'll need 20, 50 or even 100 watt power resistors for the dummy load.

This plan assumes that the backwards test is valid, using an 8 ohm amp to drive a high impedance load should work. The computer should make direct measurement of harmonics and distortion possible. I can input a sine wave and look at a spectrogram of the output.

Actually this approach is very possibly going to give you an optimistic result due to the low source impedance of the solid state amplifier. Sticking an 8 ohm resistor in series with the amplifier will give you something more representative particularly on the bottom end. Leakage inductance and capacitance in the primary are a pretty significant issue, and using a pair of resistors to ground to represent rp of each tube with some shunt capacitance (miller capacitance + some strays) across them should give you a better sense of the transformer performance particularly if you contemplate triode output tubes. Ground the primary center tap.

This at best is an approximation, performance with real output tubes is likely to differ significantly unless you are very careful. The approximation will be close for PP transformers only when the test signal current in the primary matches that of the application at a given power level pretty closely. In the case of SE transformers it is harder still as you need to add the effects of the idle current to be anywhere close.

I've had some problems doing this in the past.. Never totally satisfactory. Now I usually drive the primary if possible and often use a real output stage to do it..