What value and where ?

[hebaton]

HI all, attached is a pic of the bias circuit ( and around ) of a JTM45 clone built from a Mojotone kit.
It came to me assembled and with EL34's, I replaced the OT with a Metro spec 8K primary from Heyboer to use KT66
BUt I cannot bias them, the pot will not allow to go lower than -69 MA @455V plate voltage. I understand I need to add a resistor, in series either side of the pot but I am not sure how to calculate the value and because this has some mods I don't know, not even sure where to install.

Pleas help

THank you !

PS. yes, I will re-install the gromet.

[martin manning]

Reduce the size of the 220k feeding the bias rectifier. Maybe tack a 470k across it and see where that gets you.

[Stevem]

What range of negative voltage do you get out to pin 5 on the tubes ?

[hebaton]

What range of negative voltage do you get out to pin 5 on the tubes ?

-35 to -42

[sluckey]

What range of negative voltage do you get out to pin 5 on the tubes ?

-35 to -42

That's fine for EL34s but you want to run Kt66s. They need more negative bias voltage so do what Martin said.

[hebaton]

Thank you very much, that works just fine !

[Stevem]

You will need to have up to maybe -52 available for the 66s to allow that pot to be close to center on its adjustment.

The thing to remember when you parallel resistors is that you will always end up with less then the smallest value resistor you are working with.

You should make use of the the on line calculator section up near the top of this technical section.