Would it be a good idea to replace all of the resistors in a tube guitar amp (or pedal) with trimpots (set to the value of the resistor which they replace), switch the amp on, and start tweaking until it sounds best to you? (with proper insulation of course) Would just turning the values up and down slightly (within a 5-20%ish tolerance) cause any problems like deafening oscillation or blowing anything?
I suppose you would have to use trimpots with plastic (or metal oxide/foil if those exist) elements to keep noise low? Would it still be very noisy?
Thanks for reading 🙂
I suppose you would have to use trimpots with plastic (or metal oxide/foil if those exist) elements to keep noise low? Would it still be very noisy?
Thanks for reading 🙂
1: No
2: Probably not, but... maybe?
3: Maybe
4: Yes
😀
But seriously, there's such a thing as too many options. Do you know the Z.Vex Fuzz Factory pedal? The one that can sound absolutely freakin' amazing, but there are so many knobs you'll NEVER EVER find that one sound again?
2: Probably not, but... maybe?
3: Maybe
4: Yes
😀
But seriously, there's such a thing as too many options. Do you know the Z.Vex Fuzz Factory pedal? The one that can sound absolutely freakin' amazing, but there are so many knobs you'll NEVER EVER find that one sound again?
Its entirely doable but (perhaps) not very practical. You could replace all the preamp tube cathode resistors with trimpots. Most of these are low voltage points so no stress for operator or resistor but they will adjust the operating point of your preamp tubes. There probably will be ‘earthquake’ sounds when you adjust (DC voltage on the pots) but only when they are being set. You could also use trimpots on the attenuation networks between stages obviously the ones without gain/drive/volume pots in there already. Trimpots are cheap, choose them to go above and below the standard value - 2k trimpot replaces a 1k-1k5 cathode resistor - and easy to impliment. Go for it!
No point putting presets in place of anode resistors, they wouldn't like the high voltage anyway, but you could try adding the odd preset as part of the cathode resistor but a fixed resistor and preset should be used, the preset wired as a rheostat, EG the wiper tied to one end.
Using pots so you can balance AC going into the OP stage is a good idea and can reduce distortion, but this is the opposite of what most guitar amps are trying to do, they are usually set up to add distortion.
Valve amps work a bit differently to transistor amps and are very tolerant of out of spec resistors in most cases, your unlikely to change the sound much by using presets, that said if you study your amps schematic then add the odd preset you might improve or change things, but adding pots everywhere could just f*ck things up, though your unlikely to blow anything up, it's entirely possible; it's the risk you take if you start messing about and are not sure 100% what your doing. I'm all for experimentation and learning by doing so but think carefully before tinkering, I'd hate to see you writing in a weeks time that you've just blown your OPT on your Marshall lead amp head or burned your house down.
Andy.
Using pots so you can balance AC going into the OP stage is a good idea and can reduce distortion, but this is the opposite of what most guitar amps are trying to do, they are usually set up to add distortion.
Valve amps work a bit differently to transistor amps and are very tolerant of out of spec resistors in most cases, your unlikely to change the sound much by using presets, that said if you study your amps schematic then add the odd preset you might improve or change things, but adding pots everywhere could just f*ck things up, though your unlikely to blow anything up, it's entirely possible; it's the risk you take if you start messing about and are not sure 100% what your doing. I'm all for experimentation and learning by doing so but think carefully before tinkering, I'd hate to see you writing in a weeks time that you've just blown your OPT on your Marshall lead amp head or burned your house down.
Andy.
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Variable resistors may have weird behavior: they could have momentary open circuit when you turn the wiper. This could cause overbias or momentary surge, depending their position. Result: red plate or burnt OPT, plus burnt loudspeaker.
Here's my idea regarding this. Use small RC servos to control multiple trim pots tied into various parts of a circuit. Control the RC servos with a PWM to servo adapter board from ebay. Generate a PWM using a circuit with a single control potentiometer that's panel mounted.
As you rotate the main control, all the servo-connnected pots (trim or otherwise...) rotate accordingly. So, let's consider multiple cascaded tube gain stages, where the gain of each can be modified by;
1. Cathode resistor value.
2. Voltage dividing network from the previous stage.
With a single control, one could vary the parameters of several tubes worth of such circuitry simultaneously. (Without violating the "mechanical control" edict so many audiophiles take as gospel for a volume control) Imagine turning one panel control and having all your signal tubes grid biases go more negative - while at the same time, all the interstage attenuators reduce the signal coming from the upstream stage.
Or, you could flip the sense of one set (by simply inverting the PWM) and have the all tube biases go more negative, as the signal from the previous stage goes up. Talk about changing the amp's sound character with one control!
Of course, there's umpty ways to do this using electronic controls, but this idea keeps the "point to point wiring" and "real mechanical potentiometer volume control" crowd happy. While cheapness in the construction of "trim pots" is certainly a concern these days, I have to believe you could simply use real pots in the event no worthy trims can be found anymore. That's a real possibility, as there are solid state programmable versions available intended to eliminate the mechanical versions.
As you rotate the main control, all the servo-connnected pots (trim or otherwise...) rotate accordingly. So, let's consider multiple cascaded tube gain stages, where the gain of each can be modified by;
1. Cathode resistor value.
2. Voltage dividing network from the previous stage.
With a single control, one could vary the parameters of several tubes worth of such circuitry simultaneously. (Without violating the "mechanical control" edict so many audiophiles take as gospel for a volume control) Imagine turning one panel control and having all your signal tubes grid biases go more negative - while at the same time, all the interstage attenuators reduce the signal coming from the upstream stage.
Or, you could flip the sense of one set (by simply inverting the PWM) and have the all tube biases go more negative, as the signal from the previous stage goes up. Talk about changing the amp's sound character with one control!
Of course, there's umpty ways to do this using electronic controls, but this idea keeps the "point to point wiring" and "real mechanical potentiometer volume control" crowd happy. While cheapness in the construction of "trim pots" is certainly a concern these days, I have to believe you could simply use real pots in the event no worthy trims can be found anymore. That's a real possibility, as there are solid state programmable versions available intended to eliminate the mechanical versions.
Guitar Amp . . .
Post the complete schematic of your Guitar Amp on the Instruments & Amps section of this forum . . .
and see if anybody in Instruments and Amps can give you some answers.
Or better yet, post your schematic here . . .
And tell those of us in the Tubes / Valves what kind of sound you want from your tube Guitar Amp.
Smooth
Clipped and Ready to Break?
Etc.
Jazz?
Hard Rock?
Practice Amp?
Woodstock super power?
In other words, what kind of sound are you trying to achieve?
And that is a very subjective thing.
If you have a make and model of a Guitar amp that sounds good to you . . .
Either build a duplicate,
Or go back to the Instruments & Amps part of this forum.
Post the complete schematic of your Guitar Amp on the Instruments & Amps section of this forum . . .
and see if anybody in Instruments and Amps can give you some answers.
Or better yet, post your schematic here . . .
And tell those of us in the Tubes / Valves what kind of sound you want from your tube Guitar Amp.
Smooth
Clipped and Ready to Break?
Etc.
Jazz?
Hard Rock?
Practice Amp?
Woodstock super power?
In other words, what kind of sound are you trying to achieve?
And that is a very subjective thing.
If you have a make and model of a Guitar amp that sounds good to you . . .
Either build a duplicate,
Or go back to the Instruments & Amps part of this forum.
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"Variable resistors may have weird behavior: they could have momentary open circuit when you turn the wiper " That's why I suggested - "the preset wired as a rheostat, EG the wiper tied to one end." Or you can wire fixed resistors across the pot, something big like 1m in case of the wiper loosing contact.
Andy.
Andy.
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Diabolical Artificer, your idea might work pretty well. Aa for the sound, I don't really know yet, which is why I want to experiment until I find what I like. I suppose experiments like these are probably better to do on something low power and simple like a 5F1 or something like that.
To completely eliminate the risk of blowing something it seems like a better idea to turn off, adjust and turn on again.
Thanks for all the input.
To completely eliminate the risk of blowing something it seems like a better idea to turn off, adjust and turn on again.
Thanks for all the input.