V76 Sowter choke question

[tomas.borgstrom]

I just finished a unit with two V72 (left) and one V76 (right). They sound great and are very quiet. I've compared them to original units and I think the sound quality is just as good. I actually like my V76 more since I've removed the filters and therefore get a more open sound. Before I get to the question I would like to brag about the complex switch I built for the V72s. It allows me to switch between the circuits of V77, V72, V72S, V72S EMI and V78 (I know they have different transformers etc). Besides the gain flexibility the sound changes when I switch. I like them all. The clearest difference is V77. Without the positive feedback the sound is a lot cleaner and the top sounds more open.

Now to my question. The voltages of the V76 match the schematics very close except for the plate where it's about 40-50V higher. The only conclusion I can make is that the Sowter chokes are not perfect clones of the original (or the values on the schematics are wrong). Would you leave it as it is or add a resistor in series before the choke? In other words. Whats the most important? Correct plate voltage and bias or avoid the increased distorsion and altered load of the tube with the added resistor?

 

[Dwarf Mc'Dougal]

Hi Tomas,
Sounds like the DCR of the Sowters isn't the same as originals. Not a big deal as long as inductance is high enough.
I think the V76 original chokes have DCR of about 18K and 7-8K respectively but it's been a while since I measured them.

Anyway, the bias of pentodes is set by the screen grid voltage, so if the screens are about where they should be then the bias will be fine.
Which choke loaded plate is higher, tube no. 2 or tube no. 4?
The E83F (no.4) has a max plate voltage of some 210V so if it's higher than just over 200-210V then I'd be inclined to add a resistor before the choke to bring it down.
If the E83F screen grid is at about 140V then the plate current will be about 10 - 11mA with the cathode resistor as shown, so just use ohm's law for your resistor value. Screen will be about 1/5th the current of the plate fwiw

Tube no. 2's anode, even if high, should still be within the limit of the tube and it would be your call on whether to fudge it down or not.
A higher plate V will increase the tube's gain, but since both stages (1+2, and 3+4) have loop feedback this translates into more feedback voltage being applied when the loop is closed. Stability may be an issue here, but maybe not. You'd want to check this to be sure.
Hope that helps.
Nice job