I just picked up an old Peavey VT amp, has a 2x6L6 power section, and solid state preamp. Since it has "pre-out" and "pwr amp-in" jacks, I want to build a tube preamp, and plug it in, making it all tube....almost. The PI is solid state. My question: if the distortion that makes the "tube" sound comes from only the preamp and/or the power section(s), would my addition of an all-tube preamp be "true" to the all-tube amp claim? Changing the PI to tube would involve a ton of work... thanks!
The only reason "all tube" matters is to marketers. What matters to YOU is how it sounds. If it sounds Ok to you, what difference does it make if the PI is transistor?
Tube preamps do not sound the same a solid state preamps in my view. Not better or worse, just different. Go ahead and build a nice tube preamp, it will sound tube-y for sure. Whether or not you ALSO get tube sound from the power amp is a separate question. As a rule PV biased their amps quite cool, on purpose, and the result was a loud clean power amp that worked reliably. They counted mostly on the preamp for tone. You may get some tubiness from the power tubes, but you only get power tube distortion when you overdrive them. That means at flat out max power.
If it is desperately important to be able to say all tube, then you will have to replace that PI. Otherwise, a tube preamp and a pair of power tubes is good enough for me.
Besides, your preamp is a separate unit. it will be all tube, wherever you use it. Plug it into your Classic VT (I assume) and enjoy. if at some later time you decide to convert that PI to tube, then go ahead, but you don't need to do that just to have a tube preamp ahead of it.
Tube preamps do not sound the same a solid state preamps in my view. Not better or worse, just different. Go ahead and build a nice tube preamp, it will sound tube-y for sure. Whether or not you ALSO get tube sound from the power amp is a separate question. As a rule PV biased their amps quite cool, on purpose, and the result was a loud clean power amp that worked reliably. They counted mostly on the preamp for tone. You may get some tubiness from the power tubes, but you only get power tube distortion when you overdrive them. That means at flat out max power.
If it is desperately important to be able to say all tube, then you will have to replace that PI. Otherwise, a tube preamp and a pair of power tubes is good enough for me.
Besides, your preamp is a separate unit. it will be all tube, wherever you use it. Plug it into your Classic VT (I assume) and enjoy. if at some later time you decide to convert that PI to tube, then go ahead, but you don't need to do that just to have a tube preamp ahead of it.
On that note, tube preamps do not often sound the same as other tube preamps either, and same applies to solid-state preamps as well.
It's all about design. If the intention is to mimic a certain tube (pre) amp with solid-state technology then the designs will probably sound afwully similar. If the point is to design an amp that intentrionally sounds different than some other amp then they probably won't.
AC30 sounds totally different than a Fender Bassman, which is very dissimilar to something like Mesa Mark V or Soldano SLO100. Yet they are all "tube amps".
There are no quantifiable tube or solid-state "sounds" in practice. Circuits may have certain characteristics, but what and in what amplitude is matter of design. Each different design will sound unique from others.
Thanks Enzo and Teemuk! Your comments and suggestions help me decide what direction to go. I just so happen to be playing with a dual triode preamp on my breadboard, as I want to build a Marshall 18W clone, after having built only Fenders.... So now I have a 50W power section to see how they sound amplified! I will test the Peavey thru a dummy load with my scope to see where the amp distorts, and what it looks like. I should be able to rob B+ and heater power for my preamp from it as well.
Hybrid concertina with +mu/2 -mu/2 gains and true equal Z's.
May not have been my cleanest version of the AB2 deblocker.
Guitar perhaps doesn't matter, i can't find the other drawings.
Disconnect the GNF.
May not have been my cleanest version of the AB2 deblocker.
Guitar perhaps doesn't matter, i can't find the other drawings.
Disconnect the GNF.
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teemu, my point was maybe clumsily put, but it was that any tube preamp he builds will not sound like that PV op amp preamp. of course all tube amps do not sound alike, not all SS amps. What matters is the sound, not what it is made of.
http://peavey.com/support/technotes/hartley/Chapter_3.pdf
There are seven chapters, you should read them all.
Particularly in regard to the desired misbehavior of a
tube output transformer on the verge of saturation.
There are seven chapters, you should read them all.
Particularly in regard to the desired misbehavior of a
tube output transformer on the verge of saturation.
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I understood your point, but it kinda dodges the issue: If his tube preamp encloses each tube gain stage within negative feedback loop, enforces moderately symmetric clipping of the stages and on other aspects follows the overall circuit architecture of the PV preamp it will sound awfully similar. If he builds something totally different then it naturally will sound totally different.
It's not like solid-state or tube "as is" makes the thing sound and behave in certain ways.
You can make opamps with tube circuitry, but commonly people just employ plain single-stage common cathode gain stages sans negative feedback, which will behave totally different than "operational amps".
Same with tube power amps. Most follow the usual pattern: PI, often of "differential" type, is capacitively coupled to drive the power tubes, which are transformer coupled to load. The power tubes work as push-pull voltage amps (common cathode amps with transformer coupling load to plates). A generic solid-state amp often follows an entirely different design concept: Differential input drives a single-ended voltage amp coupled to push-pull current amp (emitter followers) directly coupled to load. All stages are typically DC coupled. Such circuit will behave very differently, especially in overdrive regions. It won't, for example, have the "crawling" bias resulting from interaction of grid clipping and capacitive coupling of driver stage to grids. One could build similar systems with tubes (e.g. usual "OTL" designs) but tubes are usually far more unideal for such circuit architectures because of their low gains and high impedances. A SE voltage amp direct coupled to push-pull cathode followers driving speakers directly would operate very similarly to generic solid-state amp similar in architecture. But a design like that is not a common option to choose whenever someone wishes to build a tube power amp. (Especially for guitar amp applications, HiFi might be another issue). In eitcher case, the different operating characteristics of such widely different circuits can still be mimicked to great extent with suitable designs. But you have to establish that as design goal.
Anyway, whatever he chooses to build it sounds like a fun project.
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Wait, why OTL? Did we not just read Hartley Peavey's Chapter 3 pages 4&5?
Transformer is essential, and one that can just barely do the job even better.
Solid state output transformers do exist, example McIntosh MC2100. Though
for guitar, maybe a bad example by way of being too good. You want some junk
transformer easily pushed to saturation. And need to disconnect any GNF that
would defeat those desirable flaws.
If you already own 6L6 and PV iron, I would never ditch that stage for sand.
There is no problem with sand for phase inverter, followers, or deblocker.
Preamp is a matter of personal preference. In post#6, I left you an unused
triode in that bottle. Suggesting it just volunteered.
Transformer is essential, and one that can just barely do the job even better.
Solid state output transformers do exist, example McIntosh MC2100. Though
for guitar, maybe a bad example by way of being too good. You want some junk
transformer easily pushed to saturation. And need to disconnect any GNF that
would defeat those desirable flaws.
If you already own 6L6 and PV iron, I would never ditch that stage for sand.
There is no problem with sand for phase inverter, followers, or deblocker.
Preamp is a matter of personal preference. In post#6, I left you an unused
triode in that bottle. Suggesting it just volunteered.
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OK guys, here's some pics to chew on.... Is first stage preamp output (of the Peavey VT), with 100/130 and 220mvP-P sine wave input to "Bright" channel, pre-gain pot at max. 100mv is just before clipping. Note gain stays the same (OPAmp at the supply rails), about 9 volts P-P, for gain of 90. I think I see an attempt (by design) to "round" the corners of the clip, but still see sharp enough corners (that produce those awful SS harmonics).
I built a simple fuzz box a while back, with opposing diodes, and you could "round" the corners with a cap to smooth out the fuzz. A simple way to make a more (dare I say tube-like) pleasing distortion.
Valve Wizard says the corners of the clip determine the harmonics (odd, even, good, bad?), and are part of the "tube sound". Comments? Thanks!
I built a simple fuzz box a while back, with opposing diodes, and you could "round" the corners with a cap to smooth out the fuzz. A simple way to make a more (dare I say tube-like) pleasing distortion.
Valve Wizard says the corners of the clip determine the harmonics (odd, even, good, bad?), and are part of the "tube sound". Comments? Thanks!
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If you are scoping the output of the Peavey op amp preamp, then nothing on the scope will look tubish. Not in the sense you mean.
I won't drag it into a tangent further, but while we CAN emulate op amps by building them with tubes, no one does that. They did decades ago, but not in guitar amps. And I have seen reports that someone used 17 op amps (or whatever the number was) to emulate a triode like from a 12AX7, and claim it sounds identical. Heck, I'll even believe them, but no one builds solid state versions of tube amp circuits using a hundred op amps to cover all the triodes. So can and does are two different things.
If I had to come up with one, I'd say the Peavey "transtube" transistor circuits probably come as close as anyone to building a "tube circuit" with silicon.
I won't drag it into a tangent further, but while we CAN emulate op amps by building them with tubes, no one does that. They did decades ago, but not in guitar amps. And I have seen reports that someone used 17 op amps (or whatever the number was) to emulate a triode like from a 12AX7, and claim it sounds identical. Heck, I'll even believe them, but no one builds solid state versions of tube amp circuits using a hundred op amps to cover all the triodes. So can and does are two different things.
If I had to come up with one, I'd say the Peavey "transtube" transistor circuits probably come as close as anyone to building a "tube circuit" with silicon.
And some more pics, off the final preamp out jack (effects loop out), 100 mvP-P input. No more gain, still around 9 Vp-p - I guess they just boost it 90x from the get-go, then massage it with active tone OpAmps, reverb and the "phaser" effect it comes with...But the tone controls have to be at "0" (first pic), or it goes distorted (2nd pic). Dropping the pre-gain to 8 brings the clean sine wave back. Interesting... Of course I plug in my guitar to listen to what I'm seeing (that's the whole point, right?)...
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Indeed.
By the way, I bumped into the following thread a moment ago, over at PV. Considering the tube versus solid state:
By the way, I bumped into the following thread a moment ago, over at PV. Considering the tube versus solid state:
Not quite sure to what you're referring to...
But anyway, a perfect sinusoidal signal is comprised of only single frequency, the fundamental harmonic. Unless equalizing controls operate at that exact frequency they won't alter magnitude of the waveform. Shape never alters.
...Unless the waveform is distorted, meaning it is comprised of more than one frequency, each in some harmonic relation to fundamental frequency. Now pass that signal through any filter that alters frequencies of interest (those which comprise the waveform) and the varying magnitude of those frequencies will alter waveform's shape.
EQ alters shape of distorted waveforms, it only alters magnitude of pure sinusoidal signals. That's why those tone controls seemingly do nothing for sinusoidal waveforms but seem to have a drastic effect on other waveform shapes, particularly those with high order harmonic content, like square, triangle and saw -type of waveforms.
Thanks Teemuk. I'm just trying to understand what I'm seeing happen. The tone circuits are "active" (versus passive tube amp designs), so gain increase is possible, right? I assumed they would increase gain above mid setting, and drop it below midpoint, but only act one those frequencies in their designed ranges (bass, mid or treble). I was using a 500Hz sine wave, but saw signal increase when bass or mid pot was raised. There must be a fair amount of overlap on each tone circuit....
In the PV Classic VT, that is not an active tone stack. Compare it to the typical Fender tone stack. This is fed by one op amp and then on to a second.
Go over to Duncanamps, and look up his Tone Stack Calculator, it is a free download. There you can play with carious tone stacks and watch their tonal effect in real time as you adjust the controls. A very cool thing.
Go over to Duncanamps, and look up his Tone Stack Calculator, it is a free download. There you can play with carious tone stacks and watch their tonal effect in real time as you adjust the controls. A very cool thing.
In#11, flat tops aren't.
Compensate your scope probe lately?
What the square wave built into the scope,
and recessed adjustment of probe are for.
Not that you wouldn't already know better,
just my pet peeve at work. I got idjits that
switch 10x probe to 1x mode, turn the comp
screw to no effect, and wonder why waves
never look like test procedure.
Compensate your scope probe lately?
What the square wave built into the scope,
and recessed adjustment of probe are for.
Not that you wouldn't already know better,
just my pet peeve at work. I got idjits that
switch 10x probe to 1x mode, turn the comp
screw to no effect, and wonder why waves
never look like test procedure.
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Understand what you say about probe compensation, but that in general applies in the Professional realm to very fast digital signals and such, while in this Guitar preamp , clipping a sinewave, at a quite low frequency (500 Hz) , scope compensation will have little effect, if any at all.
In this case I'd trust the scope to be showing the actual waveform.
To boot, these are mildly saturating Op Amps, not fast gates or other kind of Digital circuits with bandwidth extending into the MHz, plus that audio gain stage is very probably bandwidth limited to 16kHz or less.
And as Teemu says, waveform will be *strongly* shaped by tone controls plus typical tone tweaks we find in a Guitar amp, both to smooth it or to give it bite.
Or the popular Thrash Metal midrange killing scoop.
In this case I'd trust the scope to be showing the actual waveform.
To boot, these are mildly saturating Op Amps, not fast gates or other kind of Digital circuits with bandwidth extending into the MHz, plus that audio gain stage is very probably bandwidth limited to 16kHz or less.
And as Teemu says, waveform will be *strongly* shaped by tone controls plus typical tone tweaks we find in a Guitar amp, both to smooth it or to give it bite.
Or the popular Thrash Metal midrange killing scoop.
What I mean by "shape" gettingh altered is literally that. When a wave is composed out of many different frequencies then amplification or attenuation of any of those frequencies will alter waveform's shape.
e.g. "square wave response" test:
Consider low-pass filter:
The high frequency components of the square wave are attenuated due to filter. Consequently these frequency components are those responsible for "quick" rise and fall times within the square wave of certain frequency. They are also responsible for overall "sharpness" of waveform's "corners". When frequencies are attenuated (or amplified) the waveform changes shape.
The underlying idea is that all waveforms, except sinusoidal, are made out of multiple frequency components. If magnitude of these components changes it must also change the waveform's shape.
Also, even passive filter's can seemingly "amplify" by means of overshoots and resonance.
e.g. "square wave response" test:
Consider low-pass filter:
The high frequency components of the square wave are attenuated due to filter. Consequently these frequency components are those responsible for "quick" rise and fall times within the square wave of certain frequency. They are also responsible for overall "sharpness" of waveform's "corners". When frequencies are attenuated (or amplified) the waveform changes shape.
The underlying idea is that all waveforms, except sinusoidal, are made out of multiple frequency components. If magnitude of these components changes it must also change the waveform's shape.
Also, even passive filter's can seemingly "amplify" by means of overshoots and resonance.
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