5F2-A mod to add "deep" switch

[bloom]

Hi

I would like to add a mod on the 5F2-A I am building: I want to add a "deep" or "boost" switch, whatever that is called, to the pre-amp stage. What I intend to to is put a different cap at the cathode of the 12AX7. The schematic puts a 25uF-25V with a 1.5K resistor, then to ground. Can I just have a switch that will select between the 25uF cap and another cap, say 0.68uF. Keeping the same resistor? or shall I match a resistor that would go with the new cap I use? If so, how to choose the resistor value? Thanks

[Mandopicker]

I would set it up so that the .068 is in the circuit all the time and the 25uf 25v can be switched in and out.

[JHow]

If you change the resistor, you are changing the bias voltage - I think the correct design path is to choose the resistor you like (1,500) , then pick the cap to fully, or partially, or no cap at all, to bypass the resistor for the frequencies you want. In practice, switching the caps should give you a good bit of variation in sound.

Check out this article, round about page 25 where it talks about formulae for selecting cathode cap value and half-boost freq.

http://www.freewebs.com/valvewizard1...Gain_Stage.pdf


Hmmm, looks like this was also covered earlier today....

http://music-electronics-forum.com/t4847/

[PeteG45]

This sounds exactly like what Dave Hunter does in his two stroke amp which is based closely on a 5F2A? I'm building a cross between a 5F2A and a two stroke (kind of by accident) and I have a three way switch that goes between no cap a .48uF cap and a 25uF cap and it uses a 1500 ohm resistor. I haven't finished it yet but it is straight out of Hunter's design.

[soontobe1968]

Boost switch on the 2 stroke 5f2a

http://www.tonecentral.net/gearpics/...Layout-180.JPG