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Old 15th September 2010, 06:55 PM   #1
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Default Power triode bias by tube rectifier

Just today I was thinking about my future amp project, a 45 direct coupled SE. Recently I found that diode biasing a triode (6C45's in a RIAA preamp) can be a nice thing. In the case of the 45, it would save a lot of money and inside space in the chassis, what with the size of large bypass film caps (I use Solen Fast).

So why not do it with a vacuum diode? A rectifier comes to mind - there are a number of relatively cheap rectifiers able to push thru the needed 40mA of current.

What do you think, is this smart? Can it be done?
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Old 15th September 2010, 08:12 PM   #2
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High dynamic impedance, that's the major problem.
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Old 15th September 2010, 08:18 PM   #3
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Please explain.
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Old 15th September 2010, 08:37 PM   #4
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Look at the V vs I curve for a 6X5, for example. The AC impedance is dV/dI, which looks like 250-300 ohms for that tube. Other high vacuum rectifiers are similar.
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Old 15th September 2010, 08:42 PM   #5
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Like SY said, impedance is a bear on most vaccuum diodes. I've experimented with it a little, somewhat promising results, as long as your diode drops the necessary voltage at the desired current. Finding a pair of diodes reasonably matched is the main issue. I wouldn't do it for SE, due to voltage drops changing during current peaks. On a PP amplifier it seems like it may be promising, although either way your basically using it like a hot resistor though. I dropped my experimenting and just started tinkering around with LED arrays instead. VR tubes however...
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Old 15th September 2010, 08:52 PM   #6
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I'm afraid I still don't understand what the problem would be? Please elaborate.

I've understood that class A circuits always draw the same current, so why would there be current peaks?
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Old 15th September 2010, 08:55 PM   #7
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Well, think about it this way: why do you bypass your cathode resistors?
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Old 15th September 2010, 09:18 PM   #8
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You mean that a vacuum diode wouldn't prevent regenerative feedback? Ah. Feeling a bit slow right now.
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Old 15th September 2010, 09:37 PM   #9
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Bingo. At AC, it will act like a large (250-300R for this example) resistor.
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Old 15th September 2010, 10:20 PM   #10
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So I guess I'm stuck with solid state diodes or larger than life film caps.

Well, thanks for clearing this up.
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