Dual bias adjustment and effects on grid leak/PI load?

[Overtone85]

Hi

I have just finished building a Superlead clone where I have used the dual bias supply circuit explained here:

https://robrobinette.com/Generic_Tube_A ... anced_Bias

I have just learned that the value of the bias splitter resistors (the 220k in a Marshall with EL34s) affects the roll-off frequency of the PI output and also the signal level going into the power tube grids and NFB.
Too high of a value can lead to too much bass and blocking distortion in the power tubes.
Smaller values shunt the signal of the PI to... AC ground I guess? If I understand correctly this is how a kind of PPIMV works right? And you get less bass too.

I have implemented the circuit explained in the link above by copying all the values and adding a 10k before the rectifier diode to change the range. the two adjustment pots are voltage dividers instead of a variable resistor to ground in the classic Marshall bias circuit.
Does that mean that the resistive parts of the pots are adding up to the 220k resistors and therefore changing the load on the PI? (there is more resistance between the 220k and the output of the bias filter cap)
Does it also mean that, if the power tube are very mismatched, extreme adjustments of the pots will load the two halves of the push pull differently?

I don't have another Superlead to compare to and this is my second build (I dismantled my first build I did 6 years ago) so I don't really have a reference for how much bass I am supposed to get in the output section. I was wondering if I should decrease the value of the 220k in order to bring the PI load closer to original specs.

I cannot rebuild the bias circuit right now, the chassis is packed with other stuff I added to the amp. But I can parallel resistors to the 220ks to bring the value down.

Thanks
R

[Reeltarded]

Nope.. if you copied that it's fine. Using very mismatched tubes still might not work extremely well.. if they are off by a few mA it's ok. If one is 25 and the other is 40 it's still not great. If they are far apart, put the stronger tube on the non-inverted side for best setup, but still...

[romberg]

Yes. Some of the resistance in the bias supply circuit is also part of the grid leak resistance of the power tube grids. Just follow the path to ground from a power tube
grid to ground to see which resistance in the bias circuit must be traversed. Since power tubes have a maximum value
for this resistance (going above this risks having the tube run away and red plate) one needs to keep this maximum value in mind when selecting both the grid
leak resistors on the power tubes and the key resistances in the bias supply circuit. Like many things an acceptable compromise is required. Merlin covers this
a bit here:

http://www.valvewizard.co.uk/bias.html

Mike

[FourT6and2]

Also, the grid leaks form a filter with the PI couplers. So as you change the grid leak value, you need to also change the cap value if you want to keep the same cutoff frequency as the stock 220K/0.022uF combo. There's a calculator online at ampbooks.