Adding a cold cathode stage to this preamp

[Jon434]

I'm looking to experiment with this preamp, just to learn and gain some experience in what works, what doesn't.. what I prefer etc.

The two main things I'm looking at is having switchable cathode resistors or caps. I made a schematic to show what the preamp I'm working with. It's my backup amp, that I'm using to learn and experiment with.

If anyone can help, the main question I have is.. would switching the cathode resistors have a big impact on tone? The characteristics of the gain? And secondly.. would a cold cathode stage of 10K (Marshall) or 39K (Soldano/Dual Rec) work with this preamp, or would I have to adjust other values to compensate? I would be looking to put this switch on the third stage.

I'm basically looking to reduce the overall gain of the amp to a more classic overdrive setting, but have some switchable mini switches on the front panel, like "voicing" switches, that could enable the higher 90's metal gain if required.

I'm learning as I go here, but any tips or feedback would be appreciated.

[Enzo]

Unless it is a common usage in Oz, "cold cathode" is a type of tube. Such tubes do not have heaters.

More:
Cold cathode - Wikipedia, the free encyclopedia

I am going to assume you mean a cathode with a larger than average cathode resistance. One thing to remember, it is all about the circuit, not the parts. A 10k cathode resistor all by itself is not the whole story. it always goes into the context of the entire stage.

[Malcolm Irving]

That type of stage is sometimes called a 'cold clipper'. It's a common cathode stage with a large cathode bias resistor. I think 39K is about as large as it gets. (No doubt folk will correct me!) This biases the stage very cold, so that the quiescent operating point is much closer than usual to cut-off. When the stage is overdriven, a lot of even harmonics will be generated.
Your third stage is decoupled from the previous and later stages by decoupling capacitors, so I think you can experiment with the cold bias there without any worry. All the stages interact towards producing the overall sound, so it could be worth trying it out by 'tack' soldering in a few alternative values to see how you like the sound.

[Jon434]

Yeah thanks for that, cold clipper is what I meant.

I guess the easiest thing would be the try a few values and see what I like.

Yeah I need to learn more about these individual things fit into the context of the big picture and how they all fit together.

[Chuck H]

You may need to adjust the voltage divider after that stage to compensate for different gain levels with different cathode resistors. Not hard to do but it would require an extra switch pole for each switchable selection.

[Enzo]

Remove the resistor, and clip a pot in its place wired as a variable resistor, then you can vary the resistance to your heart's content and when you decide on a setting you like, then unclip it and measure the resistance. The install a resistor close to that value. WHile the amp is running, such a pot will sound scratchy when you turn it, due to the small DC voltage, but since it won't be permanent, all we care about is the tone.

[Jon434]

Very good ideas, makes a lot of sense, thanks.

In terms of the voltage divider, would it be best to take notes and measure the voltages on the grid going into the forth stage (if I made these changes on the third) before changing anything, and then comparing it after the changes, and use that as a sort of baseline to start bringing it to stock value and calculate the resistor change required? Or just going by ear? What would be the hints that these needs changing? Oscillations/squeeling or just a poor quality to the gain, too fuzzy like a fuzz pedal etc? If I can find the sweet spots for both, having a duel pole switch changing both at the same time seems like a nice idea.

Am I right in saying that the coupling and cathode bypass caps are more about altering EQ characteristics and especially bass while at the same time blocking the DC out? While the grid, cathode and plate resistors alter the bias of the tube and the gain characteristics more? I'd like to have switchable plate resistors but the high DC voltage isn't ideal.

So I guess the grid and cathode is a good place to start making some changes and see what I like. Starting off with a pot would save a lot of time. Guess I could do the same with the voltage divider if needed, but yeah. Trying to plan it out ahead of time, so that I know what I'm doing once it comes time to do it.

[defaced]

Tune the amp by ear. You can use the basic principals about what a component does, and how adjusting it up and down effects the sound as a guide, but a cascaded gain stage amp is a complicated beast. You'll learn more in a shorter time frame if you just try stuff and take notes. Given the third stage configuration right now, I expect significantly increasing the cathode resistor will make the distortion sound more "open", for lack of a better term. More like a Recto, less like an Engl.

If an amp is stable and you like the way it sounds, then you're using the right values. I accidentally botched a rotary switch mod I did on a SLO preamp and ended up with two 10M resistors in the signal path. Not at all what I intended, but the amp had a really cool lo-fi clean sound with those values. They stayed because it sounded cool and gave me something no other amp had.

This is a stage by stage break down of the OD channel on the SLO. You might get a good feel for how this type of amp works by giving those pages a read.
Soldano Super Lead Overdrive - First Stage

[Chuck H]

To add to what defaced said... I might go ahead and take AC measurements at the V4 grid as you propose so that you can calculate the voltage divider values, BUT, the final tuning will need to be done by ear. A wave clipped on one side only (cold clipping), and is sort of wonky in it's audible "effect" WRT how it will affect the stage as compared to symmetrical wave form. So the final adjustments should be "what sounds right".

Or... This is where Enzo's suggestion makes for a brilliant short cut. It would allow you to use a pot as a variable load for the voltage divider. You can just twiddle the pot until each tested cathode resistor test "sounds right" and then lift the grounded leg of the pot and measure from there to the CT. That would be your load resistor value for the divider circuit that corresponds to the cathode resistor choice in the circuit.

The gain of that stage with a center biased cathode resistor using your current divider circuit will be about 18ish. Changing to something like a 39k cathode resistor will reduce the gain to maybe 3 or 4! So you'll need some range for divider compensation. Your current divider values are 470k/470k. That 470k series resistor might not allow for much range adjustment without getting into multi mega ohm load selections when high cathode resistors are used on V3. Maybe change the series resistor to 150k and use a 1M pot wired as a variable resistor for the adjustable load.

JM2C from a low tech designers perspective

[Jon434]

Cheers guys, that's very helpful.