two-setting fixed bias switch?

[Luthierwnc]

I have been working on a Fenderish design that could use either 6L6s or 6V6s. What I'd like to do is dial-in a known set of each power tubes and have a switch that would remember the negative voltage for each with more resistance on the tail of the bias circuit for the bigger tubes. In this case, the other half of a DPDT switch would also change the impedance tap on the output transformer.

Here's what I've got so far. Any thoughts? Thanks, Skip

[tubeswell]

What about something like this?

[img:618:346]http://farm9.staticflickr.com/8443/7902 ... e432_z.jpg[/img]

[Milkmansound]

why not cathode bias?

[Luthierwnc]

Milkmansound: I need as much clean as I can squeeze out of this unit.

tubeswell: That will fine-tune one set of tubes but there's no guarantee the fixed tail resistor will suit the other set. A trimmer in parallel would work fine. The plan would be to bias each set in advance so I can pop them in and out without resetting the bias. Course, gotta do it for each new set but I'd have to do that anyway.

Also, the 6L6's are going to be coming in at around 4k primary and the 6V6's almost double that. That's why I was thinking about the impedance on the other side of the switch. I'll be using a 4k multi-tap trannie and an 8 ohm speaker so I figure 8 for the 6l6's, 4 for the 6V6's or 4 for 6L6's with an auxiliary speaker. sh

[tubeswell]

If the two tail resistors are set so that the permanent one hits the 6L6 bias voltage, and when they are paralleled it hits the 6V6 bias voltage, then it'll work. The pot is optional for fine tuning. On 2nd thoughts, a more accurate method is probably as follows:

[img:627:449]http://farm9.staticflickr.com/8172/7904 ... 8bbb_z.jpg[/img]

If the OT secondary tap selector is separate, you have more versatility.

[sliberty]

In both of tubeswell's diagrams, I would replace the "optional" resistor with a resistor and a trimmer in series. This would allow me to fine tune the additional resistance that is in circuit when the switch is open.

And ifvthatbswitch is dpdt, you should be able to also change OT taps. The jack would go to the center lug, and the two OT taps would each occupy one of the outer lugs.