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In Reply to: RE: Operating SE trans just at limit current... posted by SomeJoe on June 06, 2007 at 09:44:42
Hi Somejoe:
If available you really need the advice and knowledge of the builder of your iron. Here is why;
there is a lot of confusing ways to state dc current values and limits on a SE output transformer.
Some companies print a large number and call it--- Maximum DC current or such. But this number uses ALL of the available flux density of the transformer--- leaving no magnetic headroom for the AC signal. It is or can be essentially a measure of how much DC current by itself the trans can absorb before reaching it's saturation point.
You'll note that some companies publish a BIG dc current number--- but do all of their own testing at a much smaller DC current level. this is in practice the real dc current level that the trans will comfortably support if you value fidelity.
Not picking on anyone or any company--- but as an example--- look at some Tango specs for SE outputs. They will print a larg MAX value of DC current--- but when you look at where they test the unit it is most times at about half of this value.
Any transformer only has a certain amount (call it X) of flux density available. If you consume all of this flux with the dc current then you will have little or no capacity for the transformer to handle the AC (your music).
Another good clue to use--- is see if the manufacturer shows under what conditions they get their advertised L figure. Use that as a moreso reliable guide as opposed to just using the max number that may be printed.
MSL
Follow Ups:
Hello Mike and thanks for the info - it is very much appreciated.
Here is the inductance vs current graph for the James trans. What do you think of it? I am affraid that the inducatance might fall dramatically with current over 100mA.
Aside from that, should i decide to build a push-pull, is it possible to design a 10K PP tranny with good performance?
Thanks.
Joris
Hi Joris:
:::Here is the inductance vs current graph for the James trans. What do you think of it? I am affraid that the inducatance might fall dramatically with current over 100mA.:::
Sticking with just practical considerations I would share your concern---
It's tempting to go into a more detailed analysis of the chart and what it's ramifications are or are not. But, rather than me Monday morning quarterbacking someone else's work--- my sense is that their own data points toward 100 madc as not being ideal or well suited for this device.
And keep in mind that the above chart was generated with a half of volt rms (which (if I did my math right) would be 5/100ths of one milliwatt power level assuming a 5K primary impedance) at 60 hertz--- none-the-less this is a good test to document the small signal response of the transformer--- but this graph does not show you the large signal response of the trans. If you have both the small signal (for demonstrating incrmental inductance)and the large signal analysis together--- that's even better :=)
Sticking to the wholly practical level--- my advice would be to use it somewhere in the range that they show--- from about 4 mils up to about
68 mils--- and notice that this trans may give it's best performance at about 40 mils (which is firmly situated on the L plateau of this device)--- which is about what a 45 tube wants to see. Other advantage is that the 45 tube is not going to tax this trans so much from an AC flux density point of view--- overall, the lower the total flux(ac and dc combined) the less core losses you will have and generally, keeping the total flux low, will also lead to reduced levels of core generated harmonic distortion.
::::Aside from that, should i decide to build a push-pull, is it possible to design a 10K PP tranny with good performance?::::
much depends on how you are going to define "good performance". within my
purview of what constitutes a "good" transformer--- then, yes, it is entirely possible to build a good quality 10K pp trans. In some ways it will far outshine any conventional single ended airgapped transformer--- for instance, in terms of providing a stable non-reactive impedance for the tube to work into--- also note that parafeed SE outputs generally show marked improvements in this area.
If you evaluate performance say using Peerless' exciting current test\analysis then again most PP and parafeed SE outputs are going to look good compared to most conventional SE airgapped output tranneys.
From a small signal freq response analysis (the most commonly used barometer of quality--- though I don't agree with it being the singular or even most important parameter) I've seen smaller power level PP output tranneys with 12K pp primary impedances that had good FR from sub 20 hertz to greater than 100KHZ at plus or minus 1 db.
As George Harrison said--- "It's all up to what you value"...
hope I've been some help--- bottom line (in just my opinion of course) I'd use your trans with something like the 45 tube--- if my tube were operating at 100 mils I'd look for another tranney or find a different set of operating points for the tube which you have in mind.
Remember that--- even if the tube's plate curves demonstrates an operating point at 100 mils that looks sweet and of low distortion--- the graphical analysis is assuming you do in fact have the resistance load specified across all frequencies and that it is a pure resistance. Once you use a transformer as your load impedance--- those ideals are just simply that--- wholly academic ideals--- real world transformers are reactive devices (even the best of them) and real world transformers have core generated harmonic distortions to add into the analysis--- you want to find a place on the tube's curves where your real world transformer will exhibit it's best behaviour. That might not be the same sport or set of operating points as if this load impedandce were truly purely resistive.
again--- I hope I've been helpful.
MSL
Thank you Mike for the detailled reply. Although i did like the sound of my former 2A3 SET amp (built with the James tranny), the cost of SE grows exponentially with power. A 10K - 15K PP trans would give me nice choices of op points, while providing lots of power. Plus the hum from the DHT would cancel to some extent. I guess I'm heading toward a push-pull project...
Thanks again.
Joris
Joris
From the graph you showed I would think they
recommend only 68mA maximum current with a signal
(see the arrow on the graph at 68mA).
120mA is the saturation current with no signal.
Inductance wouldn't be the limiting factor.
Core saturation would be the limiting factor.
I would think about parafeed in this instance.
DanL
Hello Dan
I tend to agree with you. So back to plan B - parafeed. The parafeed cap is a concern, but i had good results in the past with a motor run bypassed with a teflon.
Thanks for your input.
Joris
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