Tango and Monolith Magnetics just to mention those I have actually used.
MM will make any custom transformer you might want.
Pieter, why "should" that be the case ? By design, you can get rid of the DC imbalance, well, maybe 0,5mA is left, is this what you aim at ?
hi Peter,
I know (both about huge imbalance as well as my bias unit
, I am curious about what you define as "DC imbalance".The smaller the gap, the better the transformer.
best,
Guido
It isn't unrealistic. They provide a measurement. Same Tango and other Jap stuff. They provide measurements of frequency response not simply claims....
I found the data sheet:
http://www.monolithmagnetics.com/sites/default/files/datasheets/Interstage-transformers/datasheet%20IT-03%2025%20interstage%20transformer%20prelim%20rev%2001.pdf
The core is grain-oriented silicon steel, not the best for low core distortion. Primary's inductance is 75 H, sufficient for -3 dB points at 3 Hz and 7 Hz with source impedance of 1.5 and 3.3 K, respectively - but with one caveat: small signal. I mean really small, something like 50 mV.
Core saturation voltages at 20 and 15 Hz are meaningless: for low distortion, transformer must operate far away from saturation. To show maximal signal voltages, honest manufacturers specify impedance, frequency range and signal level in dBm. With meaningless core saturation voltages, there is absolutely no clue as to what is the maximum signal level at specified low frequency points. An unsophisticated customer may come to believe that transformer is capable of handling 140 Vrms at 20 Hz, which is definitely not the case.
Leakage inductance of 0.20 mH in a gapped 75 H transformer is bogus. Most likely it is 20 mH at best, with all the interleaving that one can do. Measuring leakage inductance is not straightforward. Without knowing how they came up with the number, it is difficult to judge the validity of this claim.
So why frequency response extends to 260 kHz? The likely answer is, because of capacitive coupling between highly interleaved primary and secondary. Unloaded secondary in the test favors capacitive coupling. If this is 1:1 transformer, the more meaningful test would be one with 1.5 K source and 1.5 K load. I bet under such test it would be far, far away from 260 kHz.
As other offerings from modern manufacturers, this is low impedance transformer that will match tubes with Rp of 1 - 1.5 K. This transformer cannot be evaluated and compared to other transformers because the manufacturer obfuscates instead of providing industry standard characteristics.
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It depends on the real quality. Even Sowter transformers that use best grade M6 EI laminations have low distortion, at least an order of magnitude lower than the tubes at all levels below saturation.
With larger signal frequency response improves!
Applied voltage is independent of impedance it only depends on turns, core cross-section, frequency, form factor and induction (AC and DC). So core saturation is useful for those who know what to do with it....
If one wants the dBm level can use on-line calculators.
The leakage inductance might be a print error. Who knows? What I know is that the frequency response is correct for the other model I have. So I have no reason to doubt it and really don't care about the actual leakage inductance if I have the FR.
The test is done with 100K load and grounded secondary. 1.5K load doesn't make any sense because it will never be the actual load. That is not the application it has been designed for. Total nonsense.
Again with this impedance nonsense! There is no such thing. I can use it with a 5K plate resistance 10Y without a problem. High and low frequency roll-off will start earlier but still more than enough. High frequency behaviour for sure quite better than most OT's.
I think you are not in the position to evaluate that transformer because you don't know what you are talking about.
Now can I see any proof of the perfomance of this wordeful vintage transformer of yours? Does it just barely make 20 KHz in its typical working conditions and has all sorts of other limitations??? Or is this just your claims vs a datasheet with measurements from a reputable manufacturer? I see this is becoming quite a sport on these shores....
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http://www.junkbox.com/electronics/utc_transformer_catalog_1963.pdf
p.42: Hipermalloy transformers with nickel iron core
HA-107
Application: PP plates to PP grids, split primary and secondary.
Primary impedance: 30,000 Ohms plate to plate.
Secondary impedance: 80,000 Ohms plate to plate, 1:1.6 turns ratio overall.
1 dB from: 30-20,000.
Max level dBm: +28
Max level mW: 600.
Unbalanced DC in primary: 0.25 mA.
Chart on p. 43 shows -0.5 dB at 20 Hz and -1 dB at 20 kHz. Please note that this is at the rated primary voltage of 134 Vrms. Also, this transformer has Faraday shield between primary and secondary, which eliminates capacitive coupling.
You will much oblige me if you deduct (using online dBm calculators, as you suggest) and post the Monolith transformer's dBm at the specified frequency range.
BTW, UTC was the industry leader in specialty transformers in the US from 1932 to 1966. Not garage operation of two enthusiasts with dubious engineering credentials.
p.42: Hipermalloy transformers with nickel iron core
HA-107
Application: PP plates to PP grids, split primary and secondary.
Primary impedance: 30,000 Ohms plate to plate.
Secondary impedance: 80,000 Ohms plate to plate, 1:1.6 turns ratio overall.
1 dB from: 30-20,000.
Max level dBm: +28
Max level mW: 600.
Unbalanced DC in primary: 0.25 mA.
Chart on p. 43 shows -0.5 dB at 20 Hz and -1 dB at 20 kHz. Please note that this is at the rated primary voltage of 134 Vrms. Also, this transformer has Faraday shield between primary and secondary, which eliminates capacitive coupling.
You will much oblige me if you deduct (using online dBm calculators, as you suggest) and post the Monolith transformer's dBm at the specified frequency range.
BTW, UTC was the industry leader in specialty transformers in the US from 1932 to 1966. Not garage operation of two enthusiasts with dubious engineering credentials.
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You are comparing apples with oranges. The Monolith in your link is for SE operation only. It can work with DC current up to 25 mA. The PP version IT-02 has even better frequency response and 250H inductance!
As I suspected it really is -1 dB at 20KHz. That's really modest for PP-to-PP transformer by today's standards no matter what UTC was 50 years ago and there is nothing strange about it.
140Vrms @20Hz means 23dBm into 100K or 28 dBm into 30K to compare with the UTC transformer above (keeping in mind that is with 25 mA DC bias vs no DC bias!). When manufacturers specify impedance for IT transformers they usually mean source impedance that will give best compromise with smoth response. If no impedance is specified it is better because it usually means broader application range. If source impedance and turns ratio are specified secondary impedance is redudant...
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What rated transformer impedance means
Transformer impedance is the load that transformer with properly terminated secondary presents to the signal source, while complying with specified frequency range and signal level. For example, 30K:80K transformer rated 30-20,000 Hz and 28 dBm will present a load of 30K to output tubes when secondary is terminated with 80K resistor, without degradation of stated frequency response or signal amplitude capability.
30K primary does NOT mean that 30K is optimal source impedance. From the point of transformer distortion, the lower source impedance, the better. 30K is a good load for a PP pair of tubes with Rp of 10K or less. A 10K triode will see a load of 40K, which is good value for low distortion triode operation.
Of course, transformer will work with higher source impedance, but this will increase transformer distortion, shrink the bandwidth, and lower maximum signal level. To what extent - who knows.
This is why knowledge about the whole set of three transformer ratings, including impedance, frequency response, and maximal signal level is necessary for matching the transformer to a particular circuit. Drop one out of the three, and you are in the dark.
Transformer impedance is the load that transformer with properly terminated secondary presents to the signal source, while complying with specified frequency range and signal level. For example, 30K:80K transformer rated 30-20,000 Hz and 28 dBm will present a load of 30K to output tubes when secondary is terminated with 80K resistor, without degradation of stated frequency response or signal amplitude capability.
30K primary does NOT mean that 30K is optimal source impedance. From the point of transformer distortion, the lower source impedance, the better. 30K is a good load for a PP pair of tubes with Rp of 10K or less. A 10K triode will see a load of 40K, which is good value for low distortion triode operation.
Of course, transformer will work with higher source impedance, but this will increase transformer distortion, shrink the bandwidth, and lower maximum signal level. To what extent - who knows.
This is why knowledge about the whole set of three transformer ratings, including impedance, frequency response, and maximal signal level is necessary for matching the transformer to a particular circuit. Drop one out of the three, and you are in the dark.
Some people fall easy prey to all kinds of advertising hype. Looking at the Monolith web page, one one can see typical examples of unsubstantiated sales pitches:
..".for anyone seeking the highest degree of sonic purity"; "design philosophy by using mathematical modelling but it also uses even more refined winding schemes"; "special winding/insulation techniques (Varilay) which result in absolute sonic purity and unseen specifications"; "These components represent the pinnacle in transformer design", "You will NOT find this quality level anywhere else"...
This is all blabber without substance. They want their customers to believe that somehow they found ways to make a transformer free of well-known design compromises. Miraculous winding techniques. Cutting edge materials. Give me a break.
..".for anyone seeking the highest degree of sonic purity"; "design philosophy by using mathematical modelling but it also uses even more refined winding schemes"; "special winding/insulation techniques (Varilay) which result in absolute sonic purity and unseen specifications"; "These components represent the pinnacle in transformer design", "You will NOT find this quality level anywhere else"...
This is all blabber without substance. They want their customers to believe that somehow they found ways to make a transformer free of well-known design compromises. Miraculous winding techniques. Cutting edge materials. Give me a break.
I know that and it is both old fashioned and limiting!
If the primary impedance is 30K plate-to-plate how can it see 40K???
A 10K triode is already high impedance for IT operation....
Inductance, P/S ratio, primary voltage at lowest frequency and frequency response for a given source (or leakage L + shunt C if no FR and source impedance) are more general and useful paramenters.
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