Power triode bias by tube rectifier

MrCurwen · 15th September 2010, 06:55 PM

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?

SY · 15th September 2010, 08:12 PM

High dynamic impedance, that's the major problem.

MrCurwen · 15th September 2010, 08:18 PM

Please explain.

SY · 15th September 2010, 08:37 PM

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.

Lingwendil · 15th September 2010, 08:42 PM

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...

MrCurwen · 15th September 2010, 08:52 PM

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?

SY · 15th September 2010, 08:55 PM

Well, think about it this way: why do you bypass your cathode resistors?

MrCurwen · 15th September 2010, 09:18 PM

You mean that a vacuum diode wouldn't prevent regenerative feedback? Ah. Feeling a bit slow right now.

SY · 15th September 2010, 09:37 PM

Bingo. At AC, it will act like a large (250-300R for this example) resistor.

MrCurwen · 15th September 2010, 10:20 PM

So I guess I'm stuck with solid state diodes or larger than life film caps.

Well, thanks for clearing this up.

15th September 2010, 08:12 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	High dynamic impedance, that's the major problem. __________________ If there's a sucker born every minute, where do the rest of them come from?

15th September 2010, 08:37 PM	#4
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?

15th September 2010, 08:55 PM	#7
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Well, think about it this way: why do you bypass your cathode resistors? __________________ If there's a sucker born every minute, where do the rest of them come from?

15th September 2010, 09:37 PM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Bingo. At AC, it will act like a large (250-300R for this example) resistor. __________________ If there's a sucker born every minute, where do the rest of them come from?

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15th September 2010, 08:18 PM	#3
MrCurwen diyAudio Member Join Date: Feb 2010 Location: Helsinki	Please explain.

15th September 2010, 08:42 PM	#5
Lingwendil diyAudio Member Join Date: Apr 2008	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...

15th September 2010, 08:52 PM	#6
MrCurwen diyAudio Member Join Date: Feb 2010 Location: Helsinki	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?

15th September 2010, 09:18 PM	#8
MrCurwen diyAudio Member Join Date: Feb 2010 Location: Helsinki	You mean that a vacuum diode wouldn't prevent regenerative feedback? Ah. Feeling a bit slow right now.

15th September 2010, 10:20 PM	#10
MrCurwen diyAudio Member Join Date: Feb 2010 Location: Helsinki	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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