I have a Bassman on a breadboard, had the tone stack between the first two 12AX7 stages, then moved it to after the 2nd stage (then vol pot) to the PI....Only using two gain stages, and no cathode follower. Had heard putting the stack just before the PI gives it a Marshall "plexi" sound. Dunno about that, the amp sounded great either way, a bit more overall gain that way tho.... Any thoughts or preferences? Built the AA864 power section, and preamp based on the AA165 (without the 3rd stage and its feedback 470K). Amp breaks up around 8/10, plenty of gain. Will be playing with the 100K slope resistor (try 56K) and a .022uF instead of the 0.1uF (per Tone Lizard) on the stack... Thanks!
The lower the slope resistor, the lower input impedance will be and there will be higher insertion loss. You might want to consider adding the cathode follower if you do change the slope resistor to 56k. It all depends on how you want it to sound. The FMV tone stack is quite lossy anyway.
Having the vol. after the first stage and the tone stack after the second will allow for the second stage to be overdriven easier because insertion loss before that stage will be less with only the vol. control before it. This is how the Plexi was arranged, although it had the cathode follower driving the stack.
In wanting to play with the overall sound, remember, you can also do it with coupling caps or cathode bypass caps.
In thinking about the volume right before the PI, you might be confusing it with the master vol models as this is where that went on Marshalls.
Also, if you want more breakup, you could increase the NFB resistor. That 820 ohm value allows for quite a bit of FB.
Having the vol. after the first stage and the tone stack after the second will allow for the second stage to be overdriven easier because insertion loss before that stage will be less with only the vol. control before it. This is how the Plexi was arranged, although it had the cathode follower driving the stack.
In wanting to play with the overall sound, remember, you can also do it with coupling caps or cathode bypass caps.
In thinking about the volume right before the PI, you might be confusing it with the master vol models as this is where that went on Marshalls.
Also, if you want more breakup, you could increase the NFB resistor. That 820 ohm value allows for quite a bit of FB.
Last edited:
Thanks BT - Am using the attached tone stack. That is a FMV?. On coupling caps, can you use the RC calc for hi cutoff? 1megR with 0.1uF gives a really low Hz....and is common in Fenders. And I'm not using NFB, didn't hear/see much difference. But have a 1megR stopper before the PI (instead of the 220K). Get more oscillation on a breadboard, hope to trim it down when it goes into a chassis. Need to play with NFB more. Am missing something..Thanks again!
Attachments
This is just my personal philosophy, one of my keys to 'voicing' an amp IMHO:
The guitar comes in, and immediately I want lots of harmonically-rich treble, and to generate my preferred blues-type distortion on that. That first treble kick might be as simple as a cap across an input resistor, cap across a grid's signal-level resistor, a smaller coupling cap, a cap across a cathode resistor, etc.
Generating the distortion on a treble-boosted signal seems to make more complex chord harrnonics or something...anyway IMHO it's wonderful.
BUT the result is irritating, and when two frequencies beat I like to emphasize the difference beats and not the sum beats. Any sharp edges on the waveform distortion sound "fizzy" and just irritating. So I want the tone controls AFTER any distortion generation, and to be capable of some pretty good treble-cut at various knees.
So...the guitar gets treble boosted and cut, and sounds OK. The distortion generated between the two tone shaping stages only goes thru the later, so it only gets its treble cut, for a nice "creamy" distortion.
Some people like it a little bit different, but if you want independent control of the "tone" of the distortion and the "tone" of the guitar, you need two tone-shaping circuits, one before the distortion generation and another after. That's why an Alembic preamp is so versatile with the two channels in series, the distortion is generated between the two tone stacks.
So, when in doubt, IMHO the later tone-shaping (after the distortion generation) is the critical one that needs to be adjustable. The guitar has tone controls, etc., to affect the early pre-emphasis if necessary. But I would want the adjustable tone stack to affect the guitar AND and preamp distortion, so put it as late as possible.
Just one opinion.
The guitar comes in, and immediately I want lots of harmonically-rich treble, and to generate my preferred blues-type distortion on that. That first treble kick might be as simple as a cap across an input resistor, cap across a grid's signal-level resistor, a smaller coupling cap, a cap across a cathode resistor, etc.
Generating the distortion on a treble-boosted signal seems to make more complex chord harrnonics or something...anyway IMHO it's wonderful.
BUT the result is irritating, and when two frequencies beat I like to emphasize the difference beats and not the sum beats. Any sharp edges on the waveform distortion sound "fizzy" and just irritating. So I want the tone controls AFTER any distortion generation, and to be capable of some pretty good treble-cut at various knees.
So...the guitar gets treble boosted and cut, and sounds OK. The distortion generated between the two tone shaping stages only goes thru the later, so it only gets its treble cut, for a nice "creamy" distortion.
Some people like it a little bit different, but if you want independent control of the "tone" of the distortion and the "tone" of the guitar, you need two tone-shaping circuits, one before the distortion generation and another after. That's why an Alembic preamp is so versatile with the two channels in series, the distortion is generated between the two tone stacks.
So, when in doubt, IMHO the later tone-shaping (after the distortion generation) is the critical one that needs to be adjustable. The guitar has tone controls, etc., to affect the early pre-emphasis if necessary. But I would want the adjustable tone stack to affect the guitar AND and preamp distortion, so put it as late as possible.
Just one opinion.
That is close to the FMV-Fender, Marshall, Vox. The 6.8k resistor that goes to ground is where the mid control would be. Its value simulates a mid-position setting of the missing mid control.
Yes, just use the RC formula for establishing your coupling cap values.
- Status
- Not open for further replies.