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Cathode follower Heater-to-cathode voltage problem

Verbstank

Member
2019-11-28 1:57 am
Hey all.


I am a beginner at all this and I have been trying to design my first preamp, have gotten a lot of great help from users on this forum so thank you! I am wanting to use a cathode follower so that I can run both tube and ss power sections as I have read that using the CF drops the output impedance enough for solid state power to be happy. However, I'm having trouble understanding what to do about the heater-to-cathode voltage.



Schematic attached. From what I've read and other people's recommendations, it seems my options are:



elevate heater voltage by connecting "fake" center tap to B+ and voltage dividing to get ~50v DC, but then this would mess with my 12ax7 on the same winding?


drop the B+ going to the cathode follower by adding another resistor/capacitor before the cathode follower plate?



nix the idea of a tube cathode follower and use a MOSFET instead? If so, do these just go directly into the same circuit or are modifications required? Never worked with anything solid-statey before.



Any other ways to make this work? I know some of this is simple but I'm just trying to wrap my head around it, you guys have been very patient so far
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rayma

Member
2011-04-29 8:37 pm
elevate heater voltage by connecting "fake" center tap to B+ and voltage dividing to get ~50v DC,
but then this would mess with my 12ax7 on the same winding?

No problem, this is standard practice. It's ok (whether positive or negative) if less than 100VDC,
so the 12AX7 has no problem with it. Elevate the filaments by a positive voltage of about half
of the 12AU7's cathode potential. This splits the difference of the follower's cathode potential
and the 12AX7's cathode potential (which is near zero).

Be sure to bypass the filament's derived center tap to ground with a capacitor when elevating it.
 
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Verbstank

Member
2019-11-28 1:57 am
No problem, this is standard practice. It's ok (whether positive or negative) if less than 100VDC,
so the 12AX7 has no problem with it. Elevate the filaments by a positive voltage of about half
of the 12AU7's cathode potential. This splits the difference of the follower's cathode potential
and the 12AX7's cathode potential (which is near zero).

Be sure to bypass the filament's derived center tap to ground with a capacitor when elevating it.


So tried to follow the Valve Wizard page on heater elevation, attached amended schematic, does that look right? Then I would adjust the 100k trim pot until heater reads a certain voltage above ground?


How do I tell what the cathode's voltage will be before elevation? I keep thinking it's going to be ~200v because of the B+ and load resistor, but I guess that is the voltage that the plate will be? I'm confused on that.
 

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rayma

Member
2011-04-29 8:37 pm
How do I tell what the cathode's voltage will be before elevation? I keep thinking it's going to be ~200v because of the B+ and load resistor, but I guess that is the voltage that the plate will be? I'm confused on that.

Forget the pot. the cathode follower's cathode will be at around +150VDC from the graph,
so use +75VDC for the elevation. That's 1/4 of the B+ of 300V, which makes the upper resistor
going to B+ from the filaments equal to 300k, and the lower resistor to ground from the filaments
equal to 100k. Use a 0.1uF 400VDC capacitor across the 100k for noise filtering.
 
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Verbstank

Member
2019-11-28 1:57 am
Forget the pot. the cathode follower's cathode will be at around +150VDC from the graph,
so use +75VDC for the elevation. That's 1/4 of the B+ of 300V, which makes the upper resistor
going to B+ from the filaments equal to 300k, and the lower resistor to ground from the filaments
equal to 100k. Use a 0.1uF 400VDC capacitor across the 100k for noise filtering.


Thank you Rayma!



But how did you find these resistor values? Previously when I computed the voltage drop resistor for initial secondary --> B+ I did 50v drop (350 to 300) / 18mA (current I was expecting to draw with 2 12ax7 and 2 12au7 stages) = ~2.7k resistor

Are you doing the same thing here? Want a 225v drop over a resistor and probably not a whole lot of current because it would take from HT? So small I value 0.75 divide into 225v for 300k?
 

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rayma

Member
2011-04-29 8:37 pm
You can use high resistor values because there is no loading on the divider.
Roughly a mA drawn from the supply is fine for this purpose, and 400k would draw less.
That can be split into 300k and 100k, common values.

Then 300V x (100k/(100k + 300k)) = 75V, which is halfway between the potentials
of the cathodes of the 12AX7 (1V) and the 12AU7 (150V), and also gives less than
100V difference for either.
 
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