I don't know if this is a simple question or not, but...
In a classic tube guitar amp like a Fender Deluxe Reverb or Twin Reverb, when you dial in a 'clean' sound at moderate old-style country music or jazz combo volume, about what level of THD is there in the signal from the amplifier (not including the speaker here)?
1% THD?
3% THD?
5% THD?
10% THD?
More?
I'm playing around with designing a guitar preamp with pre-gain and post-gain controls. With the pre-gain down at '1' or '2', I'm wondering what level of harmonic distortion should be in the output signal at that point. I figure at least 3%, 2nd and 3rd harmonics dominating, but I don't actually know.
Anyone?
In a classic tube guitar amp like a Fender Deluxe Reverb or Twin Reverb, when you dial in a 'clean' sound at moderate old-style country music or jazz combo volume, about what level of THD is there in the signal from the amplifier (not including the speaker here)?
1% THD?
3% THD?
5% THD?
10% THD?
More?
I'm playing around with designing a guitar preamp with pre-gain and post-gain controls. With the pre-gain down at '1' or '2', I'm wondering what level of harmonic distortion should be in the output signal at that point. I figure at least 3%, 2nd and 3rd harmonics dominating, but I don't actually know.
Anyone?
Its probably not quite that simple. Usually guitar amps have multiple stages that can each produce their own distortion. Since the individual stage transfer functions are nonlinear functions you can't just add them all up into one stage and expect an equal sonic result (since such adding equivalence is only a property of linear systems). Then there is the speaker, and its distortion (and some guitar speakers are used because of their specific distortion properties).
If you want to understand more about the distortion mechanisms in tube preamps, the Valve Wizard book on designing guitar and bass preamps gives a pretty good introduction to the subject. Output stage distortion mechanisms are different, but maybe more well documented in hi-fi design literature, or in other guitar amp design books.
Then there is also output transformer distortion. Cheap transformers can sound pretty bad, or in other designs reasonably good. It really depends on the specifics. There are some manufactures of transformers just for guitar amps that specialize in getting some classic and other great sounds.
All the above having been said, if you want a cheap yet decent and adaptable guitar amp sound, the schematic of something like a Fender Frontman 10G, exactly as Fender designed it, can lead to some decent sounds with very simple circuitry. Make take some experience to learn how to tweak its controls along with the guitar controls to get a good sound though. There is an easy method to start learning how to dial one in though, especially given its fixed gain distortion function.
If you want to understand more about the distortion mechanisms in tube preamps, the Valve Wizard book on designing guitar and bass preamps gives a pretty good introduction to the subject. Output stage distortion mechanisms are different, but maybe more well documented in hi-fi design literature, or in other guitar amp design books.
Then there is also output transformer distortion. Cheap transformers can sound pretty bad, or in other designs reasonably good. It really depends on the specifics. There are some manufactures of transformers just for guitar amps that specialize in getting some classic and other great sounds.
All the above having been said, if you want a cheap yet decent and adaptable guitar amp sound, the schematic of something like a Fender Frontman 10G, exactly as Fender designed it, can lead to some decent sounds with very simple circuitry. Make take some experience to learn how to tweak its controls along with the guitar controls to get a good sound though. There is an easy method to start learning how to dial one in though, especially given its fixed gain distortion function.
Last edited:
I have no measurements, but my vox ac-15 type amplifier, according to the LTspice simulator, has THD of 0.7% for a 1Khz signal at 1 watt. I don't know what you consider moderate volume, but 1 watt into an efficient speaker could get you there.
This amp has no global negative feedback.
This amp has no global negative feedback.
Thanks for your replies.
1 watt out into an efficient guitar speaker (ca. 96dB/1W) is probably loud enough for jazz trio gigs in smallish rooms. So that is a good starting point. But would you really notice 0.7% THD?
I do have Merlinb's book on designing guitar and bass preamps. That inspired me to try some things, like choosing pentode operating points to
get some compression of program peaks, using grid current in a cathode follower to add some flavor, and tailoring multiple stages in series to get a particular distortion profile.
Here's the circuit I'm toying with:
The above circuit, with the Treble, Bass and Mid controls set at 50%, the Pre-gain set to 10%, and the Post-gain set to 100%, yields about 430mV RMS output signal (unbalanced output), which is about 6X gain.
With those settings and 70.7mV RMS input, the output FFT looks like this (3.3% THD):
Too much THD? Or would that be a nice, rich 'clean' sound that would still allow playing full chords?
Since this is just the preamp, intended for direct recording into a USB audio interface with low impedance inputs, I want it to output no more than about 1V RMS into a 5k ohm load.
I'm not looking for a completely squeaky clean 'hi-fi' type of sound. I'm going for the somewhat compressed sound of a Fender Deluxe Reverb with its volume set to 3 or 4, driven by a guitar with humbucking pickup.
1 watt out into an efficient guitar speaker (ca. 96dB/1W) is probably loud enough for jazz trio gigs in smallish rooms. So that is a good starting point. But would you really notice 0.7% THD?
I do have Merlinb's book on designing guitar and bass preamps. That inspired me to try some things, like choosing pentode operating points to
get some compression of program peaks, using grid current in a cathode follower to add some flavor, and tailoring multiple stages in series to get a particular distortion profile.
Here's the circuit I'm toying with:
The above circuit, with the Treble, Bass and Mid controls set at 50%, the Pre-gain set to 10%, and the Post-gain set to 100%, yields about 430mV RMS output signal (unbalanced output), which is about 6X gain.
With those settings and 70.7mV RMS input, the output FFT looks like this (3.3% THD):
Too much THD? Or would that be a nice, rich 'clean' sound that would still allow playing full chords?
Since this is just the preamp, intended for direct recording into a USB audio interface with low impedance inputs, I want it to output no more than about 1V RMS into a 5k ohm load.
I'm not looking for a completely squeaky clean 'hi-fi' type of sound. I'm going for the somewhat compressed sound of a Fender Deluxe Reverb with its volume set to 3 or 4, driven by a guitar with humbucking pickup.
Last edited:
Its possible to get a beautiful jazz sound with a Marshall 18W clone (with good transformers, and a Celestion Gold 50W speaker in a properly dampened closed-back cab). Extra gain stages are not needed. Just turn down the tone control as needed to get a mellow jazz sound (and don't set the guitar controls at 10). Start with 5, then listen and adjust as needed. Gorgeous. And loud enough to play with a small jazz group. Big band might be another matter though. If going for a more modern more affected jazz guitar sound with an electric type jazz band, then maybe more power needed.
Also, more efficient speakers may be necessary when more SPL is needed but guitar SQ will suffer some.
Also, more efficient speakers may be necessary when more SPL is needed but guitar SQ will suffer some.
Last edited:
Well, I actually don't play that way, with the muffle-tone, roll-off-all-the-highs tone. I also don't play 'electric jazz' with effects, chorus, etc. Just the straight sound of a hollow-body electric guitar and whatever the amp/speaker combo gives it. No reverb, even. But I do prefer open back cabs with a single 12" or two 10", basically vintage '60s Jensen type (I have a couple of old Jensen C12N 12" speakers I like).
I usually play with the tone control on my old Gibson jazzbox almost all the way up, into a Fender Deluxe Reverb (w/ Jensen C12N) with its Bass and Treble controls set at about '1'. I'll fiddle a little bit, but not much. Also, I almost never have the guitar's Volume control maxed out. I typically have that at about 4 or 5, then up to about 7 or 8 for solos. I'll turn the guitar up to 10 if the band is really loud, like with horns, a loud drummer, etc.
This kind of sound (in my dreams, I know, but... "I Can Dream Can't I?")
The grand master of tone these days is Peter Bernstein (in my opinion, of course)...
It's not all about the gear, of course! Your sound is in your head and in your hands. But it's nice to feel comfortable with the sound of your amplifier.
I usually play with the tone control on my old Gibson jazzbox almost all the way up, into a Fender Deluxe Reverb (w/ Jensen C12N) with its Bass and Treble controls set at about '1'. I'll fiddle a little bit, but not much. Also, I almost never have the guitar's Volume control maxed out. I typically have that at about 4 or 5, then up to about 7 or 8 for solos. I'll turn the guitar up to 10 if the band is really loud, like with horns, a loud drummer, etc.
This kind of sound (in my dreams, I know, but... "I Can Dream Can't I?")
The grand master of tone these days is Peter Bernstein (in my opinion, of course)...
It's not all about the gear, of course! Your sound is in your head and in your hands. But it's nice to feel comfortable with the sound of your amplifier.
Last edited:
" Clean" tone for guitar tube power amp around 1 to 4% distortion
More realistic minimum 2% up to 8%
It is just numbers and distortion misunderstood regardless.
Since in guitar land " Clean" tone is basically anything not clipping.
And the beginning of " break up" is the beginning of clipping.
And of course depending on preamp will increase.
And whatever power levels being used.
Typical Output transformer Ratio is wound for more voltage output.
Or most power output. So distortion is higher.
Rather than ideal impedance for the tubes to produce low distortion.
Which results in typical less output voltage and lower distortion.
The main key to good distortion. Is typically good pre and post filters
And with " Tube" distortion or even transistor type. Being basic class A gain stage.
One side of the signal will squish or distort. So if you blast a single stage to full 20% to 30% distortion
That most players want. One side is clipping and it sounds and feels like a spongy, horrible splattery no sustain buzzy fart.
If you blast a single stage with to much gain.
Basically a very gated buzzy splat that a old fuzz pedal sounds like.
Basically you want enough gain to drive the bottom or top of the signal to start clipping.
Then run it through another tube stage. Since the output is usually inverted.
The next stage will now clip the other side of the signal. Now both the top and bottom
are distorted or clipping. Then you get the compressed or equal limiting of the top and bottom of the signal
for the Sustain and smooth compression. Of course for more intense distortion or more saturation
You need to use another gain stage to take that clipped signal to more saturation.
So again it will take another 2 tube stages to clip both sides of the signal. And the problem is
the voltage swing from the previous stage is way to high. And the next stage will be blasted
to that horrible splatter fart sound. So you pad that signal down to a reasonable level.
Just enough to distort the next stage. And again to clip both sides it takes 2 tube stages.
Of course as mentioned it is mainly filters that improve the sound.
So you typically have filters between each gain stage to knock down the fizzle.
Or basically the harsh high frequency artifact/ harmonics of a clipped signal.
Easier of course to do with a distortion pedal.
But for typical reference a " blues" distortion
A full gain tube screamer pedal is around 23% distortion
And the more extreme multi gain stage "metal" distortion
Will range from 27% to about 33% distortion.
Again it is not about blasting one stage to holy hell to get 33% distortion.
It is multiple stages and filters to get a pleasant intense distortion.
And same idea you get a clipped signal, filter and pad, then another gain stage
filter then pad etc etc and usually a more complicated final filter.
More realistic minimum 2% up to 8%
It is just numbers and distortion misunderstood regardless.
Since in guitar land " Clean" tone is basically anything not clipping.
And the beginning of " break up" is the beginning of clipping.
And of course depending on preamp will increase.
And whatever power levels being used.
Typical Output transformer Ratio is wound for more voltage output.
Or most power output. So distortion is higher.
Rather than ideal impedance for the tubes to produce low distortion.
Which results in typical less output voltage and lower distortion.
The main key to good distortion. Is typically good pre and post filters
And with " Tube" distortion or even transistor type. Being basic class A gain stage.
One side of the signal will squish or distort. So if you blast a single stage to full 20% to 30% distortion
That most players want. One side is clipping and it sounds and feels like a spongy, horrible splattery no sustain buzzy fart.
If you blast a single stage with to much gain.
Basically a very gated buzzy splat that a old fuzz pedal sounds like.
Basically you want enough gain to drive the bottom or top of the signal to start clipping.
Then run it through another tube stage. Since the output is usually inverted.
The next stage will now clip the other side of the signal. Now both the top and bottom
are distorted or clipping. Then you get the compressed or equal limiting of the top and bottom of the signal
for the Sustain and smooth compression. Of course for more intense distortion or more saturation
You need to use another gain stage to take that clipped signal to more saturation.
So again it will take another 2 tube stages to clip both sides of the signal. And the problem is
the voltage swing from the previous stage is way to high. And the next stage will be blasted
to that horrible splatter fart sound. So you pad that signal down to a reasonable level.
Just enough to distort the next stage. And again to clip both sides it takes 2 tube stages.
Of course as mentioned it is mainly filters that improve the sound.
So you typically have filters between each gain stage to knock down the fizzle.
Or basically the harsh high frequency artifact/ harmonics of a clipped signal.
Easier of course to do with a distortion pedal.
But for typical reference a " blues" distortion
A full gain tube screamer pedal is around 23% distortion
And the more extreme multi gain stage "metal" distortion
Will range from 27% to about 33% distortion.
Again it is not about blasting one stage to holy hell to get 33% distortion.
It is multiple stages and filters to get a pleasant intense distortion.
And same idea you get a clipped signal, filter and pad, then another gain stage
filter then pad etc etc and usually a more complicated final filter.
The so called discovery of how to get useful clipping with tube preamp was discovered
With early Bassman Circuits.
The original intent was a follower to drive the poor impedance of a basic tone stack.
Again by luck the gain was just about right, to where if you feed the input with more gain.
That is higher than a Guitar signal.
The first stage clipped enough but not to much that it sounded like fart garbage.
Again the lower half of the signal is only clipped then inverted.
Then the follower stage designed to drive the tone stack.
Also distorted, but then clipped the other side of the signal.
So you got a very horrible limiter basically. But both sides are clipping
and not saturated into fart garbage. And you got nice pleasant distortion.
So anything from a Soldano Slo to a boogie Rect to a peavey 51/50
is the adaptation of that circuit. Just uses more stages and filters.
Every design is different and you can play with filters etc etc
or how much you pad or dont pad the previous stage to continue saturation.
Doesnt matter if it is 100 volts or 9 volts. Your just clipping a low level guitar signal
to a certain clipping threshold. And it is just a Class A stage clipping at the bottom or top.
So it takes a second stage to clip both sides. The magic is getting the gains right.
Rather a tube or transistor, the Harmonics are the same, and the sustain the same.
The filters will be the same. Frequency is by designers choice.
The ironic thing, to get clipping with a tube, the gain has to be very very high.
To get slight clipping. It is now hundreds of volts to clip a itsy bitsy small guitar voltage.
So nothing magical, just higher voltage and a buttload of noise from high gains.
Which you will pad anyways to get pleasant clipping for the next stage.
So by luck filters are easy, your padding with a resistor. Whatever padding you want or resistor value.
You can also make it a basic filter, and the cutoff will be whatever cap value is needed to resistor value.
Or with many designs, the tonestack drops alot of signal, so that is your " padding" and they might add another
clipping stage after the tone control. Usually light clipping because to much bass will splatter or fart out the distortion.
Couple zillion guitar pre or pedals to get ideas from or understand how it has been done.
Easier to do with opamps of course. Because your clipping threshold will be the same old guitar pickup voltage
usually the same old .7 to 1.4 volts. Sound familiar....yep one silicon diode or 2.
And saturation will be the same old gain of 40 to 100. after that fart garbage.
So pad that clipped signal and do it again.
With early Bassman Circuits.
The original intent was a follower to drive the poor impedance of a basic tone stack.
Again by luck the gain was just about right, to where if you feed the input with more gain.
That is higher than a Guitar signal.
The first stage clipped enough but not to much that it sounded like fart garbage.
Again the lower half of the signal is only clipped then inverted.
Then the follower stage designed to drive the tone stack.
Also distorted, but then clipped the other side of the signal.
So you got a very horrible limiter basically. But both sides are clipping
and not saturated into fart garbage. And you got nice pleasant distortion.
So anything from a Soldano Slo to a boogie Rect to a peavey 51/50
is the adaptation of that circuit. Just uses more stages and filters.
Every design is different and you can play with filters etc etc
or how much you pad or dont pad the previous stage to continue saturation.
Doesnt matter if it is 100 volts or 9 volts. Your just clipping a low level guitar signal
to a certain clipping threshold. And it is just a Class A stage clipping at the bottom or top.
So it takes a second stage to clip both sides. The magic is getting the gains right.
Rather a tube or transistor, the Harmonics are the same, and the sustain the same.
The filters will be the same. Frequency is by designers choice.
The ironic thing, to get clipping with a tube, the gain has to be very very high.
To get slight clipping. It is now hundreds of volts to clip a itsy bitsy small guitar voltage.
So nothing magical, just higher voltage and a buttload of noise from high gains.
Which you will pad anyways to get pleasant clipping for the next stage.
So by luck filters are easy, your padding with a resistor. Whatever padding you want or resistor value.
You can also make it a basic filter, and the cutoff will be whatever cap value is needed to resistor value.
Or with many designs, the tonestack drops alot of signal, so that is your " padding" and they might add another
clipping stage after the tone control. Usually light clipping because to much bass will splatter or fart out the distortion.
Couple zillion guitar pre or pedals to get ideas from or understand how it has been done.
Easier to do with opamps of course. Because your clipping threshold will be the same old guitar pickup voltage
usually the same old .7 to 1.4 volts. Sound familiar....yep one silicon diode or 2.
And saturation will be the same old gain of 40 to 100. after that fart garbage.
So pad that clipped signal and do it again.
Last edited:
Which is quite similar to a Fender Deluxe Reverb (claimed 20W output).
We're probably thinking along very similar lines...
Vox AC30 and Marshall 18 watt or any amp will do " clean" tone.
More notorious actually for Distortion since the low wattage was easy to get breakup from the common booster pedals.
Also those are typical guitar speakers and open back.
So even though you can get clean tone. They were still open back and used basic underhung radio speaker design.
So you would still have a lot of speaker distortion.
Clean tone for guitar or lap steel or jazz is higher wattage than needed.
Hence the Well Known Clean Tone of Fender Amps.
Which is basically 50 to 80 watts over the years.
Beam Tetrode.
Fender Used closed back cabinets and used Overhung speakers or more common live sound speakers.
The notorious Special Design JBL speakers, With closed back even ported cabinets had less distortion.
And very clean high end compared to open back underhung speakers.
The lap steel classic, for very good bass and very clean high end the JBL D-130
Clean tone amps are the Showman. Which vary from no effects to Vibrato/ tremolo and Reverb
To be really loud with no amp distortion or speaker distortion
You have the Showman = 8 ohm output transformer or Designed for one 8 ohm cabinet.
Or the Dual Showman = 4 ohm output transformer or designed for two 8 ohm cabinets.
" One" cabinet was 2x15 JBL D-130
Lots of bass and clean bright top end
" Dual" would be 2 cabs or 4x15 total JBL speakers
Of the course the Deluxe cream of the Crop Fender was the Dual Showman Reverb.
It of course had 2 effects Vibrato and Reverb
And could Run 2 cabinets being a " Dual"
The combo version of the Dual Showman Reverb is also notorious with " Clean Tone"
known as the " Fender Twin"
Being rather notorious as a combo, Roland later did the Roland Jazz Chorus.
Much like the Fender Twin as a 2x12 combo.
It was a solid state, which added one more effect " Chorus"
Made possible by solid state Delay Chips.
Clean Tone is the Dual Showman or Roland Jazz chorus. End of story
More notorious actually for Distortion since the low wattage was easy to get breakup from the common booster pedals.
Also those are typical guitar speakers and open back.
So even though you can get clean tone. They were still open back and used basic underhung radio speaker design.
So you would still have a lot of speaker distortion.
Clean tone for guitar or lap steel or jazz is higher wattage than needed.
Hence the Well Known Clean Tone of Fender Amps.
Which is basically 50 to 80 watts over the years.
Beam Tetrode.
Fender Used closed back cabinets and used Overhung speakers or more common live sound speakers.
The notorious Special Design JBL speakers, With closed back even ported cabinets had less distortion.
And very clean high end compared to open back underhung speakers.
The lap steel classic, for very good bass and very clean high end the JBL D-130
Clean tone amps are the Showman. Which vary from no effects to Vibrato/ tremolo and Reverb
To be really loud with no amp distortion or speaker distortion
You have the Showman = 8 ohm output transformer or Designed for one 8 ohm cabinet.
Or the Dual Showman = 4 ohm output transformer or designed for two 8 ohm cabinets.
" One" cabinet was 2x15 JBL D-130
Lots of bass and clean bright top end
" Dual" would be 2 cabs or 4x15 total JBL speakers
Of the course the Deluxe cream of the Crop Fender was the Dual Showman Reverb.
It of course had 2 effects Vibrato and Reverb
And could Run 2 cabinets being a " Dual"
The combo version of the Dual Showman Reverb is also notorious with " Clean Tone"
known as the " Fender Twin"
Being rather notorious as a combo, Roland later did the Roland Jazz Chorus.
Much like the Fender Twin as a 2x12 combo.
It was a solid state, which added one more effect " Chorus"
Made possible by solid state Delay Chips.
Clean Tone is the Dual Showman or Roland Jazz chorus. End of story
Basically collectors that know the drill.
A Showman or Jazz Chorus would be clean tone.
End of story.
Other than that Dual Showman Reverb ( Fender Twin )
The Meaning of Dual = 2x Cabinets Sealed Back with JBL speakers.
Very little Amp or Speaker Distortion. Enough Power and speaker to float your eyeballs.
Including actual Bass Response.
Real Guitar Speakers, Rather notorious for Country Clean tone or Lap Steel.
JBL D-130
JBL D-140F with orange label for fender
Or the classic Full Orange Basket for Fender
Another notorious clean tone speaker for steel players
The Black Widow.
A Showman or Jazz Chorus would be clean tone.
End of story.
Other than that Dual Showman Reverb ( Fender Twin )
The Meaning of Dual = 2x Cabinets Sealed Back with JBL speakers.
Very little Amp or Speaker Distortion. Enough Power and speaker to float your eyeballs.
Including actual Bass Response.
Real Guitar Speakers, Rather notorious for Country Clean tone or Lap Steel.
JBL D-130
JBL D-140F with orange label for fender
Or the classic Full Orange Basket for Fender
Another notorious clean tone speaker for steel players
The Black Widow.
FWIW, I used to do jazz gigs with my Deluxe Reverb. I found it wasn't quite loud enough to stay subjectively clean sounding if the group was a boisterous quartet of saxophone, guitar, acoustic bass, acoustic drumset playing in a live room. I eventually swapped out the power transformer for a Thordarson with 150mA plate supply capacity, swapped out the original OPT for a Fender Bassman clone (w/ 8 ohm tap), and put 6L6s in the amp for a whopping 34W output (RMS, measured into an 8 ohm resistor). That was my trusted workhorse for about a decade, after which I stopped playing gigs and got a regular job.
I've also experimented with different tone control circuits, which do all sound somewhat different. I like the Fender 'Blackface' circuit's 'quack', but I also like the smoothness of Ampeg's version of the classic Bauxandall passive tone controls. But there is more to it than the tone controls, of course...
One thing I'm sure everybody's noticed about the Fender blackface amps with reverb is that the Vibrato channel always sounds better than the Normal channel. Nobody ever wants to use the Normal channel. Why is that? Well...
The Vibrato channel has three 12AX7 gain stages, with a voltage divider to knock down the gain before the last stage. As below:
The mid-1970s and later versions stuck a 1M ohm Master Volume control after the output (labeled 'OUT' above).
Let's see if I can throw that circuit into LTspice and see what it looks like.
As shown, with 0.7mV input, it yields 40V output at 3% THD. ((🙂eek🙂))
That'll blow up a soundcard input real good!
I've also experimented with different tone control circuits, which do all sound somewhat different. I like the Fender 'Blackface' circuit's 'quack', but I also like the smoothness of Ampeg's version of the classic Bauxandall passive tone controls. But there is more to it than the tone controls, of course...
One thing I'm sure everybody's noticed about the Fender blackface amps with reverb is that the Vibrato channel always sounds better than the Normal channel. Nobody ever wants to use the Normal channel. Why is that? Well...
The Vibrato channel has three 12AX7 gain stages, with a voltage divider to knock down the gain before the last stage. As below:
The mid-1970s and later versions stuck a 1M ohm Master Volume control after the output (labeled 'OUT' above).
Let's see if I can throw that circuit into LTspice and see what it looks like.
As shown, with 0.7mV input, it yields 40V output at 3% THD. ((🙂eek🙂))
That'll blow up a soundcard input real good!
Okay. If that's the case I will tell you a little more about my experience with speaker cabs. In general guitar speaker cabs tend to be badly underdamped. One reason is to make them light weight to move around. Why does proper damping of guitar cabs add weight? Its because the right way to dampen them isn't to fill them will batting material. Instead, the sides, front, and back have to have added bracing, preferably using a known tone wood. I use 1x2 clear maple strips on edge, which are actually less than 1x2 when finished. Screws and glue are used to brace plywood, or even to brace pinewood construction. By experimentation I found that sides may need two strips of maple which divide the side into three segments. Same for the top and bottom.
Keep tapping on the box in different areas with a finger knuckle to listen to how much it rings in that spot. Total damping is bad. Underdamping is less bad, but still not ideal. Ideal damping is something that has to be approached incrementally, like a chef adding perfectly the right amount of salt to Michelin 5-star dish. Front baffle and back panels have to be carefully damped but not overdamped. If done right, then closed back should sound better than open back. IOW, open back only really sounded better because it reduced some of the worst of cab resonances.
That's my experience, don't know if any interest in trying it. If interested in more details please PM and I will be happy to offer what assistance I can.
EDIT: What some people do if they need more SPL is to add a more efficient speaker in parallel with a good sounding speaker. The Celestion speakers with an H suffix mean "High SPL". They don't sound as good as something like a Celestion Gold, so sometimes one of each will be put in a two speaker cab so as to fill up a club with sound while still providing at least some of that good sound too.
Last edited:
Thanks for the offer. That's much appreciated.
I tend to dislike a hi-fi like, damped sound for my guitar sound. I like some fur in the notes.
However, there comes a point where there's just too much fur, and that's not good.
On the other hand, for me, if the sound gets too damped like a Polytone Mini-Brute, or bright and super-clean (to the point of harshness) like a Roland Jazz Chorus, then that's not good for me either.
Then I have to remember that Ed Bickert got his exquisite sound playing a Telecaster through a Roland Cube amp. (It really is in the mind, ears and hands of the player. But he did do the roll-off-the-treble thing. Not too much, though.)
Over the years, I tried some sealed cabs, and even a ported cab.,
I'm afraid my taste has taken me back to the more spread out sound of open back cabs, right where I started with my first Traynor Guitar Mate, 50 years ago (I wish I still had that amp!).
There are many really cheaply made open back cabs out there. They do sound weak and anemic. A good cab is going to be heavy, at least in my experience. I really do like the sound of a Fender Deluxe Reverb with a typical Jensen style 12" speaker in it. The baffle has to be large enough too. I once put a 12" speaker in a Princeton Amp. It did not sound good. Not enough baffle area, so the back wave canceled the direct output too much. It had no oomph, no mid-bass. The Deluxe Reverb cabinet is wider, so it sounds better to me.
Oh, and yes, any rational guitar amplifier can play a 'clean' tone. If that was all there was to it then we could use those super squeaky clean acoustic guitar amps and be 100% happy.
What would make me happy would be to have the 'clean' tone of my Fender Deluxe Reverb's Vibrato channel in a box, with a cathode follower or source follower output for direct connection to lower impedance loads.
I could figure out the power amp and speaker later.
I tend to dislike a hi-fi like, damped sound for my guitar sound. I like some fur in the notes.
However, there comes a point where there's just too much fur, and that's not good.
On the other hand, for me, if the sound gets too damped like a Polytone Mini-Brute, or bright and super-clean (to the point of harshness) like a Roland Jazz Chorus, then that's not good for me either.
Then I have to remember that Ed Bickert got his exquisite sound playing a Telecaster through a Roland Cube amp. (It really is in the mind, ears and hands of the player. But he did do the roll-off-the-treble thing. Not too much, though.)
Over the years, I tried some sealed cabs, and even a ported cab.,
I'm afraid my taste has taken me back to the more spread out sound of open back cabs, right where I started with my first Traynor Guitar Mate, 50 years ago (I wish I still had that amp!).
There are many really cheaply made open back cabs out there. They do sound weak and anemic. A good cab is going to be heavy, at least in my experience. I really do like the sound of a Fender Deluxe Reverb with a typical Jensen style 12" speaker in it. The baffle has to be large enough too. I once put a 12" speaker in a Princeton Amp. It did not sound good. Not enough baffle area, so the back wave canceled the direct output too much. It had no oomph, no mid-bass. The Deluxe Reverb cabinet is wider, so it sounds better to me.
Oh, and yes, any rational guitar amplifier can play a 'clean' tone. If that was all there was to it then we could use those super squeaky clean acoustic guitar amps and be 100% happy.
What would make me happy would be to have the 'clean' tone of my Fender Deluxe Reverb's Vibrato channel in a box, with a cathode follower or source follower output for direct connection to lower impedance loads.
I could figure out the power amp and speaker later.
Last edited:
Same with Bassman Heads.
The Normal Channel sounded better than the Bass Channel for Bass.
The Bass Channel sounded slightly better the normal Channel for Guitar.
Usually very slight difference in values for the Tone Stack
where The Treble or Bass Curve start.
If remember the " Bass Channel" had actually more highpass filter.
Probably to keep bass from farting out the speakers.
Turns out removing little bass for Guitar is helpful in some circumstances.
Then the frequency point of the treble control for bass, also works well for guitar.
Even though " looking" at the curves of the tone stacks they make perfect sense
for the instruments they are designed for.
Depends on the player.
If you play with the values it will just mainly change the cut of the Mid.
It varies from a 400 Hz mid cut to 800 Hz mid cut.
400 Hz makes sense for bass, but depends on speakers and players.
Most players like about 700 Hz actually.
800 Hz mid cut for guitar makes sense too.
Again many seem to like 400 Hz better.
Mainly depends on the player, the speakers or distortion or clean.
Why many solid state heads have a sweep able mid.
Unfortunately they usually sweep to far up and too far down.
400 to 800 Hz is a good mid cut for bass and guitar.
Peavey Solid state bass amps with simple 3 band usually have the mid cut up high around 700 / 800 Hz
And also many " modified" tone stacks for guitar put the mid cut down low 400 to 500 Hz
It is just a genre/ distortion thing. Sometimes basically reversing channels works out.
Or with Ampeg bass amps they have a 5 way switch for Mid
3 being useful 2 being stupid. The 3 useful ones about the same 400 to 800 Hz for cut.
Here's another question:
How much signal output voltage does a guitar preamp need to deliver?
I can see the preamp needing to fulfill two tasks:
1) Be able to drive a typical power amp to full volume, but not drive it into clipping all the time.
2) Be able to send a healthy, low-noise signal to the ADC of a line level audio interface for PC recording.
Most power amps have pretty high sensitivity these days, and are driven to full power with only 500mV to 1V RMS input signal voltage.
I figure a USB sound card can only handle up to 1V of input signal before frying.
Is there any reason to have the guitar preamp deliver more than 1V RMS of signal output?
How much signal output voltage does a guitar preamp need to deliver?
I can see the preamp needing to fulfill two tasks:
1) Be able to drive a typical power amp to full volume, but not drive it into clipping all the time.
2) Be able to send a healthy, low-noise signal to the ADC of a line level audio interface for PC recording.
Most power amps have pretty high sensitivity these days, and are driven to full power with only 500mV to 1V RMS input signal voltage.
I figure a USB sound card can only handle up to 1V of input signal before frying.
Is there any reason to have the guitar preamp deliver more than 1V RMS of signal output?
Also over time many players dont really touch the Mid knob. On a " 3 band" fender tone Control.
Mainly just adjust Bass or Treble.
So with the " 2 band" tone control on the Bassman Head.
The Mid control is a set resistor. Actually set to a good value.
Looking at HiWatt or Ampeg " 2 band" tone controls the Mid cut is very deep.
HiWatt is very narrow cut and actually useful, Ampeg Mid cut more wide.
Hi Watt did a custom preamp for the " OX" or John Entwistle of the Who.
Few Resistor changes, Basically Made the Hi Watt Curve closer to a Generic Bassman Curve.
As with many " Custom" Tone stacks for Guitar, you either get a Bass Channel or Normal Channel.
Or where the mid dip occurs.
Mainly just adjust Bass or Treble.
So with the " 2 band" tone control on the Bassman Head.
The Mid control is a set resistor. Actually set to a good value.
Looking at HiWatt or Ampeg " 2 band" tone controls the Mid cut is very deep.
HiWatt is very narrow cut and actually useful, Ampeg Mid cut more wide.
Hi Watt did a custom preamp for the " OX" or John Entwistle of the Who.
Few Resistor changes, Basically Made the Hi Watt Curve closer to a Generic Bassman Curve.
As with many " Custom" Tone stacks for Guitar, you either get a Bass Channel or Normal Channel.
Or where the mid dip occurs.
If for a solid state amp correct about 1 volt RMS would be full clean tone before it hits the voltage rails of the amp.
Then 1.2 to 1.4 would put the amp into clipping.
But you can change the Gain of the power amp to whatever you want.
Also pickups have different outputs. Cheap to normal guitars might be a little low.
Some humbuckers or Active very High.
So preamp typically have more gain than needed.
Or preferable for a manufacture selling a product.
Since you dont know how low or high the customers pickups are.
As noticed some amps volume knobs " Jump"
To much gain or poor value for master knob.
Or with 2 channel heads/ clean isnt jumpy of course.
Then distortion channel the gain is roaring anyways
and the master will be somewhat jumpy regardless.
Most amps have a limiter for solid state. Actually prevents excessive signal to the power amp.
And the pre amp gain and total voltage output usually pretty high.
It is really all over the map.
And depends if clean or dirty / pickup output.
Plenty of Class D junk amps now.
You cant clip a class D so the limiter circuits are more elaborate and designed
so the class D amp wont clip or actually keep it way below clipping threshold.
So you loose a lot of possible voltage swing of the amp.
Or the voltages involved dont make any sense compared to real amps.
Such as this is clipping, this is not. And I should know what that is because the power supply rails
are this much blah blah. Different ball game with D class. Garbage, Junk annoying
Does not belong in a MI amplifier. But plenty of engineers wasted about 20 years to figure it out.
You cant clip a class D so the limiter circuits are more elaborate and designed
so the class D amp wont clip or actually keep it way below clipping threshold.
So you loose a lot of possible voltage swing of the amp.
Or the voltages involved dont make any sense compared to real amps.
Such as this is clipping, this is not. And I should know what that is because the power supply rails
are this much blah blah. Different ball game with D class. Garbage, Junk annoying
Does not belong in a MI amplifier. But plenty of engineers wasted about 20 years to figure it out.
You mentioned you use a Fender deluxe reverb, wich has a negative feedback loop. The above quote makes me think you might like sound without negative feedback, the sound would be less damped, more direct.
3% THD seems like a lot more to me than you would normally see from a clean guitar preamp. But, maybe it sounds good? just experiment.