Single tone knob question

[imaradiostar]

You're so right! I played an old gibsons and looked at some schematics and simulated it the way it's hooked up in the Dr Z amps- so you're right, it's not the same critter. I apologize to anyone who read my bad info and to Dr. Z!

I need to give credit where credit is due!

Wired in like the gibson circuit gives the high pass effect but no low pass. Connecting the control Dr. Z style gives treble rolloff at the extreme.

jamie

[dehughes]

So, I tried the Gibson/Matchless wiring for my tone knob, and it sounded...well...like a really good midrange pot should. If I needed a new midrange pot, I'd wire it up this way, but being as I want a tone pot I'm looking elsewhere.

I then wired it up like the Carmen Ghia, and liked that much better. A wide sweep of tonal options...

Next up I might try the Moonlight tone pot wiring, but I'm inclined to give this Carman Ghia wiring a thorough shot first, as it seems okay.

In case you're wondering, the amp I'm working on started off as a Brown Deluxe-ish project and now is setup as following:

- Parallel channels, one 6SN7 (triodes in parallel, single volume knob only), the other a 12AX7 triode (volume and tone), summed into the other 12AX7 triode a la the 6G3 schematic

- Long tailed PI with switchable feedback loop

- Cut control

- Cathode biased 6V6 pair

- 5AR4 rectified

The problem I'm having is that the two channels are effecting each other in that whatever I do to the 12AX7 side changes the tonality of the 6SN7 side. This is what I was hoping for to a certain degree, but it's far more complicated than I expected due to the two channels' interactions. I thought they'd be more separate and only would be blended when the volumes were up, but I'm finding that the 12AX7 side's tone pot wiring and setting changes the vibe and tone of the 6SN7 side, even when the 12AX7 side's volume is turned completely off. Thus I'm always having to find the happy compromise when I change one thing on either channel...which makes finding the "right" thing much harder...

[dehughes]

Yeah, that's what I prefer too- just the mid control from the 30rvt. The plot I posted was that control with a 500k pot instead of the 250k in the original schematic. You can easily scale it to make it work a little better with higher plate loads like a pentode.

I think it works well with a phase inverter and power section that's breaking up just a little bit. The cut control can serve to take the edge off of things- really a great combo for natural sounding low gain amp distortion tones. Not so much gain that it doesn't work with a few pedals out front...but enough to make it obvious that it's not 100% clean and add some interesting harmonic content.

jamie

One question for you: What would a 1M pot do to the perceived usefulness of this Carmen Ghia tone pot? I left my 1M in there as that was what the 6G3 tone pot used...and it seems to really kick up the bass and thin out the treble on the opposite extremes...so I'm wondering what difference a 500k or 250k would bring about.

[imaradiostar]

It seems to shift the whole control up or down slightly- looking at the simulation plot I don't know that the difference would even be audible between 500k and 1M at most positions. I think the 500k makes the knob a little more responsive and shifts the response up about a quarter octave at a given position. 250k does the same but a little more extreme, with a slight loss across the range. I'd say try tacking large value resistors across the outer lugs of the pot and see if it makes a positive difference. I don't know that I'd bother swapping the pot if it wasn't easy to get to and I had another one just laying around.

I'm confused- are the channels summed using 220k resistors like the 6g3? I would have thought that would have removed a lot of the interactivity of the controls. Are they summed before the second ax7 triode or after?

jamie

[dehughes]

Okay. Thanks. I swapped in a 500k and it didn't make that much of a difference.

I do like this Carmen Ghia setup, but it is a bit tooooo scooped and bass heavy, especially with the Brown Deluxe values all around it.

I'll give the Moonlight tone setup a try next...then possibly one of the Big Muff variants.

The bummer with this amp is that I can't seem to get the inherent tonality straight. In changing the 12AX7 side I seem to be changing the 6SN7 as well. I have the two channels summed via two 220k resistors before the other 12AX7 triode (a la 6G3), which solved my problem I was having before when I had both channels summed at the PI input (was getting a closed down tone on the 12AX7 due to running a 500pf cap across the summing resistor for the 6SN7 channel when summed at the PI). Running both 6SN7 triodes in parallel and summing them into the other 12AX7 triode solved that issue, but I still seem to be getting harshness from the 12AX7 side....much like what one would expect if they bypassed a TMB tone stack (r.e., high, harsh, gainy midrange). Granted, cranking the 12AX7 side made the amp sound killer (mini-Marshall), but I figured I could tinker a bit and get the 12AX7 side more versatile for non-cranked tones as well.

Oh well, the life of an incessant tinkerer....

[tubeswell]

Hi de hughes

Remember that the answer is not all in the tone stack. Picking up on your remark about the scoopiness being too scooped, if you want to lift some bass out of the whole circuit use a smaller cathode bypass cap and/or coupling cap at v1. For example if you have 25uF ck in there now try 2.2uF, this has a subtle effect of taking out boominess throughout the whole amp, including how the bass responds in the tone control(s)

[dehughes]

OKAY, now we're getting close...

I wired up the "Moonlight" tone stack and it seems to sit nicely as a "mellower" Brown Deluxe tone stack.....that is, it takes the edge off of the midrange harshness found in the typical 6G3 tone wiring.

However, I think I'd like a bit more "air" on the top. How would I go about opening up the top end a bit on the Moonlight tone setup? Bright cap bypassing a resistor somewhere?

[tubeswell]

OKAY, now we're getting close...

I wired up the "Moonlight" tone stack and it seems to sit nicely as a "mellower" Brown Deluxe tone stack.....that is, it takes the edge off of the midrange harshness found in the typical 6G3 tone wiring.

However, I think I'd like a bit more "air" on the top. How would I go about opening up the top end a bit on the Moonlight tone setup? Bright cap bypassing a resistor somewhere?

PM simcha (delft) - its her tonestack, she'll be able to tell you if anyone can

[dehughes]

Hi de hughes

Remember that the answer is not all in the tone stack. Picking up on your remark about the scoopiness being too scooped, if you want to lift some bass out of the whole circuit use a smaller cathode bypass cap and/or coupling cap at v1. For example if you have 25uF ck in there now try 2.2uF, this has a subtle effect of taking out boominess throughout the whole amp, including how the bass responds in the tone control(s)

You know, I believe the whole amp is balanced save for the tone setup on the 12AX7 side. I played it today in practice and the 6SN7 side felt and sounded GREAT. The 12AX7 side just needs a bit more air on the top end of the tone knob range and it's good.

The reason I think the 12AX7 side is in balance save for the tone pot sweep is that with each iteration of tone pot wiring I've found the 12AX7 side to do it's thing to whatever degree the tone wiring would allow. That is, it sounds and feels like a Brown Deluxe that's being put through various filters (which in reality, is exactly what's going on). I'm finding that certain tone setups are better than others, AND that the underlying tonality seems to be good....just that certain setups bring out this tonality in more versatile, pleasing ways than others.

So, I'm confident in the voltages, caps, and resistors around the stage(s) themselves, but I'm not yet settled on a tone pot wiring that brings the most out of the circuit. Who knows, maybe I'll come full circle back to the Brown Deluxe wiring......(I hope not...)

[kiwi]

OKAY, now we're getting close...

I wired up the "Moonlight" tone stack and it seems to sit nicely as a "mellower" Brown Deluxe tone stack.....that is, it takes the edge off of the midrange harshness found in the typical 6G3 tone wiring.

However, I think I'd like a bit more "air" on the top. How would I go about opening up the top end a bit on the Moonlight tone setup? Bright cap bypassing a resistor somewhere?

Hi Dehughes, and Tubeswell. Somewhere, I have alternative values for the Moonlight stack which give a different voicing. I'll see if I can find them and post back here. Won't be till next week because I have a bent bouzouki and a neck reset on a Guild to finish before Tuesday. Also have a couple of other 1-knob EQ's which I worked out for other amps. I'll see what I can dig up. Short answer is if you want a bit more air in a simple amp with Moonlight stack, try giving it a bit of high frequency boost around one of the tube gain stages. One subtle way of giving the feel of subtle HF boost is to lower bass and mids evenly by a few dB - for example by putting a resistor of a few hundred ohms or maybe 1k in series with the typical 25uF cathode bypass on first stage, and then put trial value film caps across this resistor Try .01uF, 0.1uF, 1uF and see which sounds closest to what you want, then juggle resistor and cap values to fine tune. You do lose a bit of low and mids gain this way, but some amps have a little to spare. Cheers, Simcha.

[dehughes]

Hi Dehughes, and Tubeswell. Somewhere, I have alternative values for the Moonlight stack which give a different voicing. I'll see if I can find them and post back here. Won't be till next week because I have a bent bouzouki and a neck reset on a Guild to finish before Tuesday. Also have a couple of other 1-knob EQ's which I worked out for other amps. I'll see what I can dig up. Short answer is if you want a bit more air in a simple amp with Moonlight stack, try giving it a bit of high frequency boost around one of the tube gain stages. One subtle way of giving the feel of subtle HF boost is to lower bass and mids evenly by a few dB - for example by putting a resistor of a few hundred ohms or maybe 1k in series with the typical 25uF cathode bypass on first stage, and then put trial value film caps across this resistor Try .01uF, 0.1uF, 1uF and see which sounds closest to what you want, then juggle resistor and cap values to fine tune. You do lose a bit of low and mids gain this way, but some amps have a little to spare. Cheers, Simcha.

Hey there! Thanks for the reply.....I'm looking forward to what you post back...

As for putting caps across the cathode of a gain stage...that's not a bad idea, but I do like the gain in the 6G3 circuit, just not the tone control of a 6G3 circuit. Hence my looking into alternate designs.

Any particular recommendations for your tonestack in a 6G3 setup? There sure are a lot of lows and low mids to spare there....but that's part of the fun once it gets cranked. I'm hoping to keep the general, gainy, raucous vibe of the 6G3 there, albeit with the ability to clear it up a bit with a twist of the tone knob.

[HBamps]

I have tried the Gibson GA30 tone knob and I feel it's too scooped, even with substitutions. Look at the Matchless Chieften mid control. It's very similar to the GA30 one but wired up a little different. The big difference is it doesn't sccop the tone nearly as much. use different resistors and/or cap to adjust the scoop depth and position.

Thanks to imaradiostar for all the help with Spice.

[dehughes]

I have tried the Gibson GA30 tone knob and I feel it's too scooped, even with substitutions. Look at the Matchless Chieften mid control. It's very similar to the GA30 one but wired up a little different. The big difference is it doesn't sccop the tone nearly as much. use different resistors and/or cap to adjust the scoop depth and position.

Thanks to imaradiostar for all the help with Spice.

Perhaps I'm missing something, but to my eyes the GA30 and Chieftan midrange controls are wired up the same...

[imaradiostar]

Perhaps I'm missing something, but to my eyes the GA30 and Chieftan midrange controls are wired up the same...

agreed. I'm probably missing something again so feel free to point it out.

Have you tried a different 12ax7?

I forget- using a bright cap across the volume?

Maybe add a conjunctive filter like the bright chan of a marshall plexi or the reverb blend circuit from a blackface fender?

jamie

[dehughes]

agreed. I'm probably missing something again so feel free to point it out.

Have you tried a different 12ax7?

I forget- using a bright cap across the volume?

Maybe add a conjunctive filter like the bright chan of a marshall plexi or the reverb blend circuit from a blackface fender?

jamie

Different 12AX7s...yup, no real change.

Bright cap across the volume...good idea!

Conjunctive filter...please explain...or post examples, as I'm still not getting it when I'm looking at schematics. I see bright caps on top of mixing resistors....but wouldn't that bleed highs from the other channel?