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There is no “magic” involving an external power supply, so there are a lot of options. If you want something nice, but affordable, do have a look on e-bay. Do a search for “adjustable power supply”, and you will see a lot of nice units with meters etc. built in. Myself, I built a power supply using a transformer from my ‘spare parts supply’, and a small DC buck step down board, again from e-bay. Remove the on-board adjusting pot, and replace with a 25-turn precision pot for fine control and add one of the V/A meter units. It is cheap, and works great! Lots of options depending on how much money, or how much work you are willing to put into it.
Cheers, Bill
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With all due respect, I do appreciate what you are saying. But….
This all assumes that the tube specifications are absolutes, which they are not by far. Manufacturing tolerances vary widely, and is recognized in the data sheets right up front. Where the filament specifications are given, you will often see a minimum and a maximum or a tolerance. For a typical 6.3V filament it is usually something like +- 0.6V. That means the manufacturer says the tube will perform as stated with a filament voltage of 5.7 to 6.9 Volts. Yes, emissions, lifespan, and performance are all affected by the filament voltage, and in part this tolerance is why some tubes fail before others, and possibly why some audiophiles roll tubes until they find something that sounds just right to them. Tubes are far from an exact science, so while testing I agree it is important to get it “right”, but don’t get too hung up on the micro details. It’ll drive you nuts!
Bill
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For the record, I do not disagree with you, I’m just trying to play devil’s advocate to highlight the tolerances in some of the specifications. On that note….
Why focus on the filament voltage? The argument can be made that the filament current is as, if not more important. In many applications (most notably TV sets) it is indeed the current that is controlled. If the heat produced in the filament is proportional to the electron emissions, and the resistance of the filament is known, than the current would be king. But I suppose that is the clinch, it is the filament resistance that is the variable. My power supply has a Amp meter as well, and for reasons you eloquently described earlier (measure filament Voltage at the socket, not at the regulator) the argument can be made to go with the current since that will be the same throughout the circuit (Kirchoff’s current law). No worries about losses in the wiring.
As I said, I am just throwing out food for thought, it is all too easy to get hung up on one issue and lose perspective of the larger picture. There is no clear right or wrong, do what works for you within the tolerances provided and you will be close enough. Calculating tolerances properly is a whole discipline in itself, and that is a subject for another time. Interesting discussion though, I hope it is making some readers think about this.
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I’m not saying that, considering modern technology, we should not do the best we can in regulating heater voltage, (or current, or power). However the valve/tube testers of yesteryear, including the various AVO models, just set a nominal ac voltage for a typical current. So the unloaded transformer voltage was a little on the high side, to compensate for wiring resistance, switch resistance and transformer winding losses. Notice the rather wide tolerance limits too.
From the AVO IV service manual.
Craig
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Craig,
The information concerning the Avo tester is not surprising or unusual. The reality of "tube testers", no matter how complex or sophisticated, is that they are really only "good" at doing one thing. And that is; determining whether or not a tube under test is suitable for service. To the contrary, regardless of the number of knobs, meters and dials, they are unsuitable for revealing the actual electrical conduction characteristics of vacuum tubes. This is why modern, highly accurate methods of accessing tube characteristics, including "curves" have been developed, often with the final objective of "tube matching". Examples are, of course, the "utracer" and Steve Lafferty's "VTA". The accuracy of these machines is directly dependent upon the techniques used in building them. If an accuracy of 10% or 20% is good enough, then certain shortcuts may be used. If higher accuracy is desired, then more complex (and possibly more expensive) techniques or circuits will be needed.
---ART---
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