Cold clipper question.

[Colas LeGrippa]

Hi to all !
In one of my builds (don't ask me the schem. diagram, it's not drawn yet)  the 4th half of the 2 12ax7 preamp tubes is hooked up with a 10k resistor at the cathode to gnd, which causes the corresponding anode voltage to raise drastically over  300V. In order to maintain the plate of the other half of that tube (which k is tied to gnd with a 820 R resistor )at 250V, I would have to split the B+ supply, with 2 different supply resistor values and 2 x 20uF  caps, for example.
Does it exist such a configuration
In a preamp ? I would be curious to examin the schematics


Colas

[sluckey]

I would be curious to examin the schematics

And we would be curious to examine your schematic.   

[Colas LeGrippa]

You don t need the schem. to answer my question. but you don't have to. I ll find the answer by myself.
Thanks anyway

[Dave]

You could use another dropping resistor on the power rail and a smoothing cap, or you could power it from another existing node, further down the chain. I have seen amps that powered 6 or seven triodes from one node. Sometimes you can't get away with it because of some interplay between stages, other times you can, you just have to try it and see.


Dave

[Colas LeGrippa]

Thanx buddy I was goin to give it a try.
Definitely I have to add a dropping resistor from the power rail right to the plate of the cold clipper side. More, I want to try a 33k cathode resistor  without damaging the 12ax7.


Cheers !


Colas

[PRR]

At low ("cold") current, 12AX7 can stand a lot more than 300V.

[HotBluePlates]

... a 10k resistor at the cathode to gnd, which causes the corresponding anode voltage to raise drastically over  300V. In order to maintain the plate of the other half of that tube (which k is tied to gnd with a 820 R resistor )at 250V, I would have to ...

I don't understand your question.

  - Plate A rises to >300v, and has a 10kΩ cathode resistor
  - Plate B could be at any voltage due to its plate load resistor & 820Ω cathode resistor.

     - If 820Ω is the "magic cathode resistor" then dial-in plate voltage by changing Plate B's plate load resistor.

[Colas LeGrippa]

I admit my question is a bit confusing, sorry for that.


I just wanted to get a fixed voltage on the A side of a 12a...7, let's say 250V, while changing the value of the k resistor on the B side to obtain 250V as well. This can't be done without  2 different B+ supply. So, if I change the power rail dropping resistor to yield 250V on the A side with a 820R on it's cathode, I will get to high a voltage on the B side plate with a 10 or 33k cathode resistor.
PRR tells that a 12A....7 can stand a voltage way over 300V at low current. So there should be no problem, electronically speaking. But my ears tell me that a cold clipper sounds better at lower voltages.
So, what would be a better choice between 2 different dropping resistors with their corresponding capacitors (to supply the 2 halves A and B ) or the same dropping resistor with a higher plate resistor on the cold clipper side ?


Hoping it's clearer now.
Thanks to all
Sluckey, my mood was not optimum yesterday.

[ac427v]

Sounds like an interesting amp. I could visualize the situation better with a schematic including the power supply dropping resistor string and B+ voltage.

[HotBluePlates]

... PRR tells that a 12A....7 can stand a voltage way over 300V at low current. So there should be no problem, electronically speaking. ...

Scroll to the schematic for this MC-30 and look at the 12AX7 driver tube.  There's 490v plate-to-cathode, and I've got a pair that have been running without complaint since the 1950s.  So no worries about voltage.

... This can't be done without  2 different B+ supply. ...
So, what would be a better choice between 2 different dropping resistors with their corresponding capacitors (to supply the 2 halves A and B ) or the same dropping resistor with a higher plate resistor on the cold clipper side ?

I would assume changing the plate load resistor would alter the cold clipper's sonic performance more than "250v vs 300v" so I suppose I'd recommend 2 different dropping resistors in the B+ supply, for 2 different-voltage nodes.

As long as there's a stout filter cap between the dropping resistor & plate resistor, you can raise/lower the dropping resistor as much as you like to test out different supply voltages for that stage.

[Colas LeGrippa]

Thanx HBP  for your complete and satisfying answer !


Colas