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.
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.
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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
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
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...
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...
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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
John
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.
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.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
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.
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..
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.
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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.
Oh, and kitchen scales are good for the low frequency measurement.
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?
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
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
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.
Varnishing transformers makes them immune to air humidity, prevents vibration of windings, but can somewhat increase stray capacitance.
Thanks, I had wondered what sort of ballpark we are playing in.
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
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.
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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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.
No, I don't expect much from those Hammond transformers. The leslie stuff though was rebranded Stancor so it might just be ok.
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) as parafeed OT for my el84 SE 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
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.
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
) as parafeed OT for my el84 SE 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
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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