40W Texas Tone

[bluescaster]

Thank you.

I notice on silver Face schematics (super Reverb for example) that they didn't use this ! So, it's works but it's bad to not have offset ?

[VacuumVoodoo]

PI cathodes are at ca 95V in Super Reverb so it is a bit out of spec but can be tolerated. Some european made at7/ECC81 (RFT) were rated at 100V Uf/k

[martin manning]

If you look at various tube data sheets you will see Vh-k max ranges from 100 to 200V, with older USA production typically being on the high end of this range, and certain new production tubes (Sovtek e.g.) on the low end. Early designs with cathode follower stages (5F6-A) ran close to 200V on the cathode, and require careful selection of the tube for that location if built per the original schematic. Elevating the heater string allows use of virtually any tube there.

[bluescaster]

Thank you very much.

[Structo]

Were those voltages with the tubes installed?

Strange not to see a bigger drop when you changed R40.

How is it now that you have modified it to Martin's idea?

[VacuumVoodoo]

Strange not to see a bigger drop when you changed R40.

Before change PI plates were drawing ca 3mA each, now closer to 2.8mA so there's less voltage drop on plate resistors which opposes reduction in voltage on C node.
To really get PI cathodes below 90V one would have to change plate and cathode resistor values as I suggested in earlier post.
It is not necessary to do, now that heaters are on DC offset. Unless there's a need to increase headroom in the PI stage.

[bluescaster]

I need to order somme parts to add DC on heater. I'm in hollydays for two weeks, so next step in 15 days.