Grid leak bias switch to cathode bias

[phsyconoodler]

I built a 5B3 deluxe quite a while ago and just pulled it out of storage and popped it into a cabinet so I could play it. It's sounds wonderful clean and naturally overdriven with single coils but kinda nasty with an overdrive pedal.
  I want to change it to cathode bias but I can't find any layouts that use cathode bias with a 6SL7. I know I wouldn't need the input caps to the grid but in the grid leak version the grid one is grounded. This dual triode is not in my area of expertise so I'm not clear on how to go about the change to cathode bias.
   I see the way the phase inverter is cathode biased but why aren't both grids used on the grid leak biased version?
Any insight would be appreciated.
 

[phsyconoodler]

Duh! I'm using 6sc7s not 6sL7s.

[PRR]

> 5B3 deluxe ... ... cathode bias with a 6SL7.

?? Plan for Fender 5B3  http://www.blueguitar.org/new/schem/fender/5b3_sch.jpg

Ass-uming you wired for 6SC7s, the 1st socket should wind up looking more like the 2nd socket. Change the cathode link, pin 6, from a short to a couple-K resistor with several-dozen uFd across it. Tack a 1Meg across each 5Meg grid resistor (makes 833K, fine). Leave the 0.05u grid caps, they do no harm. You can go-back to old-skool funk by untacking the new 1Megs and tacking a strap across the cathode RC.

[phsyconoodler]

Meh.  Tried it. It does indeed take pedals well now but I miss the fat funky tone it had before.. 
  Maybe I'll drop the cathode resistor and kick it up a notch. Definitely not enough gain without pedals now

[PRR]

Did you put the large cap across the cathode resistor?

With the cap, gain should not be a lot different. Without the cap, gain is about half, which is pretty obvious.

[phsyconoodler]

I ended up using a 1k cathode resistor and a 100uf bypass cap. The gain is back and it sounds killer overdriven now.
   Still has about the same clean headroom and retained most of the funky groove of the grid leak sound..
Overall a keeper of an smp!
Thanks for the help PRR. Once I got my head away from 6sL7 I could see it. Funny how something sticks in your head. Lol
   Except for a bit of microphonics it's nicer overall than a 5E3 IMHO.

[PRR]

6SC7 is a lot like a 6SL7, or a 12AT7 or a low-gain 12AX7.....

EXCEPT: both cathodes are connected *together*! (There's actually only one cathode, two grids two plates.)

This will not work for some circuits. Can for others.

They used a phase splitter which works with a shared cathode, OK.

The stock input had both input cathodes grounded, so OK that they are really one cathode.

Here you wanted a conventional cathode-bias input (grid-leak is marginal for 1950s guitar and may flip-out with modern axes or pedals). Sharing a cathode network over two cathodes invites several kinds of trouble. In sequential stages it is unstable. With parallel (two inputs) stages it invites crosstalk (but both go to the same speaker so OK). A big cathode cap helps a lot. If one side of the tube dies, the other side will mis-bias with the half-value cathode resistor (but triodes are pretty tolerant).

[DummyLoad]

glad you got it sorted. Professor Rock-n-Roll strikes again! looks like the 5B3 might make the project list... i wonder if i could make the 6MF8 triodes work as a paraphase... hmmm. 

--pete