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Question on 'source impedance'.

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Hi, when the spec sheet says source imp. value of, say 5k., this is refering to the plate resistance of a suitable tube with current and voltage values that it operates as 5k? Thanks! Tony

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Re: Question on 'source impedance'. - Jim Doyle 22:33:15 10/23/02 (1)
It's very confusing. My interpretation of the specs is that the
value == Zin Primary when the secondary windings are left unloaded.
As an example, the LL1660/10mA in Alt S (1:1.125 SE-SE) has a
Zin of 14k. This would compel me to use a tube that has an Rp
less than this value. I'd use a tube with Rp = 1/2Zin or 1/3Zin.
In other words, something like a 8532W with Rp=4400 ohms or paralleled 6N1P sections.
If you use the LL1660/10mA in other modes - i.e. 1:2.225. Zin=3.5K.
In this case, I'd be tempted to use a 5842 tube with Rp=1100 or
a 6C45 with an Rp of 900 ohms.
If you load the primaries (i.e. class A2), you need to account
for the load impedance reflected back into the primary.
I'm waiting to pickup a real oscilloscope (not my sound card scope)
and a function generator - so I can actually see what I am doing.
- Re: Question on 'source impedance'. - KevinC 04:45:04 10/24/02 (0)
  < value == Zin Primary when the secondary windings are left unloaded.> >
  This is true.
  < Zin of 14k. This would compel me to use a tube that has an Rp
  less than this value. I'd use a tube with Rp = 1/2Zin or 1/3Zin.> >
  Why use a tube with Rp = 1/2Zin? You can use tubes with a wide variety of Rp values, so long as they are about equal to or less than Zin. My experience has taught me that there will be sonically acceptable results that way and that the "sound" of the individual tube type will contribute most strongly to the sonic impression that results. It does help sonically to move from the LL1660 to the LL1671 when using really low Rp triodes like the 2A3 or 6BX7. The better match obviates some of those out-of-band resonances.
  Be careful about getting too hung-up on the oscilloscope trace for the deciding vote in your quest for good sound. It's wise to pay attention to that if you are looking to get a good review in Stereophile. Look at the October issue with the review of the VAC preamp, the line stage of which was designed by me and uses Lundahl transformers. That's where good test results count! Then recognize that the reviewer wasn't disappointed with what he heard, because he doesn't have a 'scope, just ears. In fact, he was enthralled. I shouldn't be so difficult on the subject of testing; I have a bench full of 'scopes and distortion analyzers, but I just don't let them determine what I listen through.
  Kevin
Re: Question on 'source impedance'. - KevinC 18:19:56 10/23/02 (0)
You are correct. A source impedance equal to or less than the specified value will prevent in-band high frequency roll-off. However, if the plate impedance of the tube is markedly lower than the specified value, you can expect to see some transformer resonances in the region above 20kHz. These can be reduced to a substantial degree by loading the transformer secondary with resistors, but too much loading can "deaden" the sound, so approach loading as a listen-and-change activity.
Kevin

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