Help Modding/Differentiating Grid Leak Inputs

[rp]

Anyone want to help me design different inputs for the grid-leak bias side of a 5C8?

Just to recap: 5C8 clone (posted here in the past). The original Fender had two channels V1 & V2 w/ 4 grid-leak inputs, one side Normal other Bright (100pf cap across pot). I kept V1a/b grip leak but changed V2a/b to cathode bias. Keep in mind the 6CS7 twin-triode has an internal shared cathode, and instead of typical switching Hi-Lo jacks on V2 and having an unused triode I first tried a switching jacks setup shown to me by Jana here to do Normal and Paralleled inputs. Cool enough, but a bit too much for this microphonic octal critter and that extra fat gain just seemed out of place in this already aggressive circuit. So V2 is now set up with resistors on each grid to mimic the Hi-Lo switching jacks.

On to the hard part: on V1 I still have two identical inputs w/ .1 cap and 4.7M, as on the schematic - boring and useless. I want to differentiate the inputs for two flavors. Top one (call it Bright) I wish to keep the same as it sounds great, what can I do with the other? I don’t want brighter but maybe make a Higher, Lower, or Dark set up?

Maybe it's as simple as changing the .1 cap to .01 but I’m pretty sure messing with the input cap value, or adding a stopper resistor in series, changes the tube’s bias and why I haven’t experimented. I really don't understand the Flexi THD at all. Maybe playing with some pf caps across things or to ground?

Again, remember the 6SC7 has that annoying shared cathode. I’m open to rewiring the whole thing for a 6SL7 if that offers better options. I have just one old pull though in unknown condition. The 6SC7s are troublesome but they do have a lot of balls, I kind of like them.

I’ve enc’d some Supro grid leak diagrams that seem to create different inputs, plus the modern THD amp.

Any ideas, insights, solutions?

[sluckey]

Maybe it's as simple as changing the .1 cap to .01 but I’m pretty sure messing with the input cap value, or adding a stopper resistor in series, changes the tube’s bias and why I haven’t experimented.

The cap is an important part of a grid leak bias circuit. It prevents the grid from having a low resistance dc path to ground through the input. This forces the grid to seek a return (leak) path through the very large grid resistor, thus developing a negative voltage for biasing.

The resistor must be a large value, but the cap value is not critical. You can put any kind of tone shaping, treble bleed, treble peak, etc., between the input jack and grid leak cap without affecting the bias on the tube.

You can find some more grid leak examples by looking through the low end amps, especially cheap Valcos.

[rp]

Playing with the input cap makes things pretty easy. What effect do these pf caps have?

[sluckey]

The 500pf in the Supro will shunt high freqs to ground. It should kill off some audible brightness.

The 22pf in the THD is a local NFB loop from plate to grid. It will kill high frequencies also, but may not be as apparent as the Supro circuit. It 'may' just be there to further reduce any RF interference or kill hi-freq oscillations.

[rp]

Tried .01, not much different than .1, just a bit less bass, maybe better that way, I might make .02 the Normal. I tried .01 w/ 500pf to ground, bit less bass and treble but nothing major. I guess I can start getting extreme w/ the cap values and just to see where it goes. Any idea how to create a "scooped" sound?

Can I knock down gain to mimic the 6db down as Fender does? If I add a 68K stopper and 68K load in front of the capacitor will this do it, and if they are in front of the cap will it not affect the bias?

[Malcolm Irving]

Can I knock down gain to mimic the 6db down as Fender does? If I add a 68K stopper and 68K load in front of the capacitor will this do it, and if they are in front of the cap will it not affect the bias?

That's right, it won't affect the bias. The guitar will also see a load impedance of 68k in place of 1M, which will reduce the brightness.

[rp]

That's right, it won't affect the bias. The guitar will also see a load impedance of 68k in place of 1M, which will reduce the brightness.

I'll report back later but this was fun and fruitful experimenting to do. Worked very very well at tempering the grid leak input w/o changing it's unique quality. The 68K/68K worked great too to drop volume, just like in a cathode bias set up, and yes I lost more high end than I'd like. Any trick to get it back, right at the stage, the vol. pot already has a 100pf across it?

For now looks like playing with the input cap isn't too special. Take away bass, take away treble and what have you got? Not much. So far THD Flexi looks the way to go, but not sure why he chose such a small value input cap, unless I misread 6n8F = 6.8nF = 0.0068uF = 6800pF.

[didit]

A more radical option -- sub 6SL7 in one socket, split half & half for grid leak & cathode bias. Use that second socket for 6SJ7 to gain distinctive different pentode input options.

Best .. Ian

[sluckey]

The 68K/68K worked great too to drop volume, just like in a cathode bias set up, and yes I lost more high end than I'd like. Any trick to get it back

Put a 100pF to 500pF across the 68K that's NOT connected to ground.

[rp]

Put a 100pF to 500pF across the 68K that's NOT connected to ground.

I knew that, but I still need hand holding. I should show more courage especially around milivolts I ask too as I usually hate A/Bing things but this is fun and I'm learning a lot. 250pf worked to restore treble and to match the High and Low, but I think I'll lower it to 100pf.

I tired the 22pf cap on the THD, the grid to plate. My guess this was put in to temper string fingering/sliding noise, which the grid-leak really emphasis. I had an 18pf, seems to work and without giving any sense of treble loss. The grid-leak input is pretty funny now. Anyone down the road looking inside will see either an exquisitely tweaked circuit or a desperate kluge. I'm feeling kind of proud so I'm claiming the former.

A more radical option -- sub 6SL7 in one socket, split half & half for grid leak & cathode bias. Use that second socket for 6SJ7 to gain distinctive different pentode input options.

I thought of that from the get go, and it's still a possibility - would be many cool tube sounds. But first I want to get a really good sounding 5C8, then if and when I get bored w/it start the heavy modding.