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How to define a quality OT?
How to define a quality OT?
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Old 3rd June 2010, 05:21 PM   #11
airboss is offline airboss  United States
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Quote:
Originally Posted by kstagger View Post
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...
This is terrific K. Thanks
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Old 3rd June 2010, 05:25 PM   #12
airboss is offline airboss  United States
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Originally Posted by ChrisA View Post
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.
We do think alike Chris. That was my planned method for the square wave test for sure. I've got a 250 Watt wirewound 7.5 ohm Rheostat. Drat, I wish it went to 16, but in Atwood's procedure I think that 8 ohm response is often a good predictor of the response at other frequencies. Just depends on the coupling of the coils on the core.

With regard to the primary side "tube simulating" resistor, it has always bothered me that music is not a sine wave. It's a wall of wildly changing frequencies and impedances. To make matters worse, the speaker is not really at it's nominal impedance except in a very precise spot. So, what is going on in that transformer is immensly complex magnetically. To my mind, the measurements that Atwood made, only allow us to predict performance based on "known" good transformers like the Dynaco Z-565. So, anyway, we'll plug in the load resistors knowing that it really doesn't mean much in the big scheme of things. He alludes to that in his remarks about why use 6550 to drive an 8K primary? It's good reading.

I wonder if Mitchell Feigenbaum is an audio buff? He could do the math probably.

<edit> Very good points KevinKR! Probably Atwoods procedure eliminate some of that problem.

Last edited by airboss; 3rd June 2010 at 05:42 PM.
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Old 3rd June 2010, 06:48 PM   #13
EC8010 is offline EC8010  United Kingdom
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I've never been convinced that any leakage measurements I've made on output transformers were plausible. However, a good indicator is to drive the transformer with a 10kHz square wave from the source impedance it will see in use (half a-a if triode, rather more than a-a if pentode), look at the secondary and measure the HF ringing frequency. The higher the better. If it's as low as 30kHz consider using it as a heater transformer. If it's >50kHz offer money, and if it's >100kHz (Radford claimed 300kHz) tear the seller's arm off and beat him over the head with the sticky end until he allows you to give him money for it.

Oh, and kitchen scales are good for the low frequency measurement.
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Old 3rd June 2010, 07:56 PM   #14
ChrisA is offline ChrisA  United States
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Originally Posted by kevinkr View Post
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.
Yes of course testing in a real tube amp is the best. But it gets expensive if you have a large OPT. So I assume we are talking about test rigs that cost 5% or less of the cost of a full on tube amp.

So assume I do place the 8 ohm resistor in series with my amp and build a better tube model that include the Miller effect and only test PP OPTs. What kind of accuracy do you think I could get?
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Old 3rd June 2010, 11:28 PM   #15
kmtang is offline kmtang  Canada
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Hi Dear,

PLITRON - audio transformers - toroidal transformers - toroids - output transformers - current transformers - power transformers - medical isolation transformers - power toroids

This web site includes few very informative articles about the quality of output transformer for audio application. I was impressed with them and bought a pair of their toroid OPT for my 300B couple years ago. You could probably see that in the picture under my name on the left.


Johnny
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Old 4th June 2010, 05:54 PM   #16
LinuksGuru is offline LinuksGuru  Europe
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Quote:
Originally Posted by Danny Hall View Post
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.
Since layers on bobbin are wound on top of each other, mean turn length of outer layers is larger, and therefore, their DC resistance is higher. BTW, 10 - 20 Ohm difference is nothing compared to several KOhm of output tube impedance. 10% discrepancy means nothing bad.

Quote:
Originally Posted by Danny Hall View Post
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.
Varnishing transformers makes them immune to air humidity, prevents vibration of windings, but can somewhat increase stray capacitance.
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Old 4th June 2010, 06:06 PM   #17
airboss is offline airboss  United States
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Quote:
Originally Posted by LinuksGuru View Post
Since layers on bobbin are wound on top of each other, mean turn length of outer layers is larger, and therefore, their DC resistance is higher. BTW, 10 - 20 Ohm difference is nothing compared to several KOhm of output tube impedance. 10% discrepancy means nothing bad.
Thanks, I had wondered what sort of ballpark we are playing in.



Quote:
Originally Posted by LinuksGuru View Post
Varnishing transformers makes them immune to air humidity, prevents vibration of windings, but can somewhat increase stray capacitance.
The thought crossed my mind last night. I would use a wax of some sort. They are available in different temperature melting points. I guess the only way to know for sure is to make some measurements with poorly potted trafos, vacuum pot them, then re-check. I have some candidates for that.
Thanks
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Old 4th June 2010, 07:01 PM   #18
indianajo is offline indianajo  United States
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Default good hifi amps

One thing I am sure of, Hammond Clock company (organ) transformers were never meant to do high frequencies. They might be great for electric guitar work, probably not are so good for ST70 clones. My Hammond H100, one of the most expensive organs they made with a 3 channel output, has no feedback on the power amps. I bet if you do square wave tests, or batter HF intermodulation tests, that the Dynaco transformer (which sounds good on piano) will outperform a Hammond organ (which is only designed for organ at low to middle frequencies). No relation to the present Hammond transformer company. Hammond organ iron is labeled AO-number. My H100 organ is the donor of many an E-bay listing part, because the tube sockets can't handle the 20 mv reverb signals, and some cheap volume pots on the preamps sometimes drop out. Other than that, I think it is the best organ Hammond ever built.
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Last edited by indianajo; 4th June 2010 at 07:02 PM. Reason: grammer
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Old 4th June 2010, 08:23 PM   #19
airboss is offline airboss  United States
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Only thing I can argue with is the last sentence JO. You may well be the only H series lover in the world. I have an M3. Great spinet. But real organists prefer the RT3. 25 pedal B3 Based. Sometimes they go cheaper than B3 or even A100 because folks don't know what they are.

No, I don't expect much from those Hammond transformers. The leslie stuff though was rebranded Stancor so it might just be ok.
Quote:
Originally Posted by indianajo View Post
One thing I am sure of, Hammond Clock company (organ) transformers were never meant to do high frequencies. They might be great for electric guitar work, probably not are so good for ST70 clones. My Hammond H100, one of the most expensive organs they made with a 3 channel output, has no feedback on the power amps. I bet if you do square wave tests, or batter HF intermodulation tests, that the Dynaco transformer (which sounds good on piano) will outperform a Hammond organ (which is only designed for organ at low to middle frequencies). No relation to the present Hammond transformer company. Hammond organ iron is labeled AO-number. My H100 organ is the donor of many an E-bay listing part, because the tube sockets can't handle the 20 mv reverb signals, and some cheap volume pots on the preamps sometimes drop out. Other than that, I think it is the best organ Hammond ever built.
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Old 5th June 2010, 06:09 AM   #20
jechentau is offline jechentau  Netherlands
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ah, cool
mentioning big OPTs; i to much satisfaction run a plitron/vanderveen/amplimo
PAT 1070 UC with primaries parallelled (it is meant to run in a mcintosh unity coupled
push pull amp :P) as parafeed OT for my el84 SE :P but,
i have done extensive listening tests, and it hands off beats mains toroids used as OPT,
and also dedicated el84 SE OPTs, which are a fraction of the size.

i figure, it uses only a small "area" of a very small B/H curve.

in any case, it is the best OPT i have here hence i use it :P

PLITRON - audio transformers - toroidal transformers - toroids - output transformers - current transformers - power transformers - medical isolation transformers - power toroids

i luuv it in this application; infra to ultra sound, clarity, etc.
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