i was disappointed with TMB tone stack (slope resistor + vox/fender/marshall classic network)
then i've tried james/ passive baxandall and had some fun;
then a friend introduced me into the voight TS for a more musical approach;
after all, i've realized that the james/bax TS had its flaws:
too lossy... either before or after gain stages there was unpleasant side-effects... muddy bass, distorted treble;
SO, I CAME UP WITH AN IDEA (but i ain't sure if someone else already did it in the past, much likely so...)
tube amp-wise, either with or without master volume, the 'gain/input volume/drive/volume' potentioneter is usually placed right after the first triode, between v1a and v1b;
so, if we wire the input gain potentiometer with the treble potentiometer in the fashion of 50's style / gibson les paul wiring, we would end up with an effective treble bleed insurance for low volume settings + early treble filtering, if wired exactly like a 1 pickup volume/tone 50's wiring les paul guitar
the benefits of this approach would be more clarity/less treble loss on lowest gain settings, plus less high frequencies distortion on the later stages (highs tend to sound better on clean amps, leaving the mids to be distorted/overdriven)
then, right after this volume + treble a la 50's wiring style, we just add a bass cut potentiometer, with signal coming to the middle lug, shorted with any outer lug 1 or 3, and a capacitor, lets say, 2.2nf with one leg on lug 3 and another on lug 1
this early bass cut could improve clarity, save energy (all bass frequencies demand alot of amplification power) so the little bass left to be amplified could be kept as clean, so it would sound like a clean fender without mud or farting
so in resume it would end up with clean highs and lows... overdriven mids, less components, less loss, yet simplier to build
it seems to be too good to believe, so why this design isnt seen as much in amps and tube pedals?
i hope this can be of help
best wishes
//LoKoMaN
then i've tried james/ passive baxandall and had some fun;
then a friend introduced me into the voight TS for a more musical approach;
after all, i've realized that the james/bax TS had its flaws:
too lossy... either before or after gain stages there was unpleasant side-effects... muddy bass, distorted treble;
SO, I CAME UP WITH AN IDEA (but i ain't sure if someone else already did it in the past, much likely so...)
tube amp-wise, either with or without master volume, the 'gain/input volume/drive/volume' potentioneter is usually placed right after the first triode, between v1a and v1b;
so, if we wire the input gain potentiometer with the treble potentiometer in the fashion of 50's style / gibson les paul wiring, we would end up with an effective treble bleed insurance for low volume settings + early treble filtering, if wired exactly like a 1 pickup volume/tone 50's wiring les paul guitar
the benefits of this approach would be more clarity/less treble loss on lowest gain settings, plus less high frequencies distortion on the later stages (highs tend to sound better on clean amps, leaving the mids to be distorted/overdriven)
then, right after this volume + treble a la 50's wiring style, we just add a bass cut potentiometer, with signal coming to the middle lug, shorted with any outer lug 1 or 3, and a capacitor, lets say, 2.2nf with one leg on lug 3 and another on lug 1
this early bass cut could improve clarity, save energy (all bass frequencies demand alot of amplification power) so the little bass left to be amplified could be kept as clean, so it would sound like a clean fender without mud or farting
so in resume it would end up with clean highs and lows... overdriven mids, less components, less loss, yet simplier to build
it seems to be too good to believe, so why this design isnt seen as much in amps and tube pedals?
i hope this can be of help
best wishes
//LoKoMaN
Attachments
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Low-loss tone controls were used in some bargain amps (because they had very little gain to spare), do a search on cheap, low-end guitar amps and you should find some examples.
cool, this seem to be a good idea, i'll peek into it to see if i catch any extra insight!
in my case, i like to remove as much components as i can, such as attenuators, cathode bypass caps, also lowering plate resistors to 68k and rC to about 820 ohms... most of this mods greatly improve the dynamics/feel as well as the tone quality, but some of the gain goes away...
I'm with you on the 68k plate resistors.
In Merlin Blencowe's guitar preamp design book, he talks about the change in distortion spectrum when you change the plate resistor. We're talking about what the triode does before you reach outright clipping - just the subtle nonlinearity that creates the "clean" sound.
In a nutshell, large-value plate resistors produce very clean sound, and as the resistor value is lowered, you get more distortion.
One example: when the Fender Excelsior came on the market around 2011 or 2012, the preamp was found to produce extremely clean (almost solid-state sterile-clean) sound. The triode gain stages had 220k plate resistors, if I recall correctly.
The last time I used a 12AX7 in a (friend's) build, I suggested we try 68k plate resistors, and both of us preferred the slightly more "tubey" sound they produced.
-Gnobuddy
lol m8, i never thought that by lowering the RP there would be more distortion... i've seen somewhere that lower rp leans toward less gain and more headroom
but considering what you've said, i think you're right, because a lower rp will pump less signal per momentum from the tube, leaving room and time for the signal to collide within itself in the valve'a guts, creating exquisite distortion... i've noticed exactly what you described: less sterile clean signal, thickened by some sort of effect that only a tube could introduce
very intresting... before your quote i thought the thickness in the signal could be a product of bigger, cleaner headroom, but it makes alot of sense, since clean tones are described as 'crystal clear' - somewhat thin, cold, sterile: very different from a low RP
but considering what you've said, i think you're right, because a lower rp will pump less signal per momentum from the tube, leaving room and time for the signal to collide within itself in the valve'a guts, creating exquisite distortion... i've noticed exactly what you described: less sterile clean signal, thickened by some sort of effect that only a tube could introduce
very intresting... before your quote i thought the thickness in the signal could be a product of bigger, cleaner headroom, but it makes alot of sense, since clean tones are described as 'crystal clear' - somewhat thin, cold, sterile: very different from a low RP
It's true, though. For the same input signal, there will be more harmonic distortion in the output, when a lower Rp is used. Even though the gain is reduced!
The attached images show what is happening. I've plotted three different loadlines on the same 12AX7 curves, corresponding to 100k, 200k, and 50k plate resistors. On each line, I've plotted a dot corresponding to the bias point, and two adjacent dots corresponding to +1V and -1V voltage swings at the input.
Because the triode curves are not equally spaced - they are further apart as you go up the graph to higher plate currents - the distance - these three dots are not equally spaced. The +1V dot is a little further away from the central dot, than the -1V dot is. This slight asymmetry causes the lovely "tubey" clean tone that guitarists with good ears want so much.
Given the unequal spacing of the three dots on each loadline, there is a simple mathematical formula that I used to estimate the amount of 2nd harmonic distortion.
As you can see, different load lines (values of Rp) tilt the load line one way or the other, and this changes the amount of asymmetry, and therefore, the amount of (slight) distortion in the clean tone. The lower Rp is, the more distortion there is!
The second graph shows why the valve Hi-Fi crowd like to use constant current sources for plate loads, rather than just a resistor. A constant current source is like an infinitely large resistor as far as AC signals are concerned - and as you can see, it lowers distortion so much, that I couldn't even calculate it from the graph.
What is the opposite of a constant current source? A constant voltage source, which has zero resistance to AC. And a cascode arrangement of two triodes comes very close. If we want maximum "tubey" cleans from a single triode stage, we should be wiring it as a cascode! The last graph shows what we can expect if we use a half-12AX7 as the lower device in a cascode.
-Gnobuddy
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Have you actually measured the distortion on a HP 334 or equivalent? I have a 12ax7 breadboard, i can do this if needed. I would keep the gain the same though.
If you can do the measurements, by all means do! I'd love to see your results.
Small signal gain is going to vary with Rp, so I don't see how we can keep it constant.
For this graphical analysis, I kept Vgk the same at (-1.5V) for all three loads. B+ is also the same at 320 V. Finally, the input voltage was also kept the same in all three cases (2 Vpp in every case.) So this a pretty fair apples-to-apples comparison.
I have also done a similar graphical analysis keeping the same operating point, but different Rp values, i.e., keeping Vgk, Vp, Ip, and Vin the same.
To keep Vp and Ip the same when Rp varies, B+ has to be allowed to vary as needed. (All three load lines intersect at the operating point, but have different slopes, so they intersect the current and voltage axes at different points, so B+ will be different in each case.)
This second graphical analysis also leads to the same conclusion: there is more distortion for the same input signal and same operating point when Rp is smaller, as long as the device is kept out of clipping.
-Gnobuddy
Keep the Output Level the same.
Gain can always be trimmed elsewhere. Output level is precious.
I can see a case for keeping the output level the same (we might adjust our guitar or stomp-boxes to produce the same speaker volume at the same knob settings on the amp.)
I can also see a case for keeping the input level the same (plugging the same guitar or guitar + pedalboard into the amp without touching any volume or gain knobs.)
I think it would be interesting to see measurements for both these cases.
-Gnobuddy
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