Power Tube Distortion
Question:
Well, this may sound like a totally stupid question…but not knowing very much about the way tubes behave…would it be possible to make a tube distortion pedal with a power tube in it, like an el84, and starve it on lower voltage? Mark
Response:
>Well, this may sound like a totally stupid question…but not knowing very >much about the way tubes behave…would it be possible to make a tube >distortion pedal with a power tube in it, like an el84, and starve it on >lower voltage? >Mark
Sure, that’s essentially what’s done in many of the tube distortion pedals on the market. It can be done with most any amplifier tube, although 12AX’s are usually used. Mark Of COURSE it’s my opinion. Who’s else would it be?
Response:
>I’ll add my two cents. Power tube distortion can be effectively duplicated >using op amps and diodes, However, I have never heard it done >satisfactory. So, I had to come up with a circuit myself. Most of them >fall off the boat when they try to duplicate how a power tube distorts. >Can’t be done with 12ax7’s. Maybe I’ll post a little circuit for this if >there is any interest in it. > lxh2
Hi lxh2, You have an opamp/diode circuit that sounds like power tubes? OK, I’m interested. Could you post the schematic? MG
Response:
: writes:
: > : >Well, this may sound like a totally stupid question…but not knowing : very : >much about the way tubes behave…would it be possible to make a tube : >distortion pedal with a power tube in it, like an el84, and starve it : on : >lower voltage? : > : >Mark : : Yeah, you could…but they don’t sound that good when they’re starved. : Those Tube Driver things use low plate voltage, I think, but with a : 12AX7. I’ve never heard one I thought sounded good. : : Lord Valve : (Fat Willie) I’ll add my two cents. Power tube distortion can be effectively duplicated using op amps and diodes, However, I have never heard it done satisfactory. So, I had to come up with a circuit myself. Most of them fall off the boat when they try to duplicate how a power tube distorts. Can’t be done with 12ax7’s. Maybe I’ll post a little circuit for this if there is any interest in it. lxh2
Response:
> I’ll add my two cents. Power tube distortion can be effectively duplicated > using op amps and diodes, However, I have never heard it done > satisfactory. So, I had to come up with a circuit myself. Most of them > fall off the boat when they try to duplicate how a power tube distorts. > Can’t be done with 12ax7’s. Maybe I’ll post a little circuit for this if > there is any interest in it.
Hi. Is your circuit just a diode shaping circuit? Please post it. I’d like to play with it. Regards Scott
Response:
: >Well, this may sound like a totally stupid question…but not knowing very : >much about the way tubes behave…would it be possible to make a tube : >distortion pedal with a power tube in it, like an el84, and starve it on : >lower voltage? : >Mark : Sure, that’s essentially what’s done in many of the tube distortion : pedals on the market. It can be done with most any amplifier tube, : although 12AX’s are usually used. Huh? 12AX’s are pre-amp tubes, not power tubes. Nick
Response:
>>I’ll add my two cents. Power tube distortion can be effectively duplicated >using op amps and diodes, However, I have never heard it done >satisfactory. So, I had to come up with a circuit myself. Most of them >fall off the boat when they try to duplicate how a power tube distorts. >Can’t be done with 12ax7’s. Maybe I’ll post a little circuit for this if >there is any interest in it. lxh2 >Hi lxh2, >You have an opamp/diode circuit that sounds like power tubes? OK, I’m >interested. Could you post the schematic? MG
Count me in too. I want to see how you balance the harmonic generation versus input amplitude, gain some even order harmonic suppression, and in general make it sound good. Mark Amundson,
Response:
: : Ok This my power tube simulation circuit. Put it together and tell me : what you think. If you have any questions email me. The * = ground The : 100k’s are potentiometers dual ganged. : : input : .033_3.3k ___ : | | ___pf___ | | ___uf_k_ | | _| _| | _| |______ : 1M __| / | | | __| / | | | _| / | | _| / | | | : | | |/ | |_| | |/ | 100k|_|| |/ | | | |/ | | 100 | : | | | | 33K | 5.6 | | : 2.7k * 150 |_| | k | | : | | * 10k | * | : * * | * | : | __ _470__ : | | _| | | _| | | | _|33k |250k | __ to next : |___| / | 100k| __| / | d1 d2 __| / | |_.022_| ___| / | circuit : | |/ | | | |/ | | | | |/ | | uf | | |/ | : | |_.022_| : 1k uf | : | 50k : * | : * : I forgot one thing. The first two potentiometers are dual ganged …. the third one is the presence control….works just like the one on an old marshall. lxh2
Response:
Ok This my power tube simulation circuit. Put it together and tell me what you think. If you have any questions email me. The * = ground The 100k’s are potentiometers dual ganged. input .033_3.3k ___ | | ___pf___ | | ___uf_k_ | | _| _| | _| |______ 1M __| / | | | __| / | | | _| / | | _| / | | | | | |/ | |_| | |/ | 100k|_|| |/ | | | |/ | | 100 | | | | | 33K | 5.6 | | 2.7k * 150 |_| | k | | | | * 10k | * | * * | * | | __ _470__ | | _| | | _| | | | _|33k |250k | __ to next |___| / | 100k| __| / | d1 d2 __| / | |_.022_| ___| / | circuit | |/ | | | |/ | | | | |/ | | uf | | |/ | | |_.022_| 1k uf | | 50k * | *
Response:
: : : : Ok This my power tube simulation circuit. Put it together and tell me : : what you think. If you have any questions email me. The * = ground The : : 100k’s are potentiometers dual ganged. : : : : input : : .033_3.3k ___ : : | | ___pf___ | | ___uf_k_ | | _| _| | _| |______ : : 1M __| / | | | __| / | | | _| / | | _| / | | | : : | | |/ | |_| | |/ | 100k|_|| |/ | | | |/ | | 100 | : : | | | | 33K | 5.6 | | : : 2.7k * 150 |_| | k | | : : | | * 10k | * | : : * * | * | : : | __ _470__ : : | | _| | | _| | | | _|33k |250k | __ to next : : |___| / | 100k| __| / | d1 d2 __| / | |_.022_| ___| / | circuit : : | |/ | | | |/ | | | | |/ | | uf | | |/ | : : | |_.022_| : : 1k uf | : : | 50k : : * | : : * : : : I forgot one thing. The first two potentiometers are dual ganged …. the : third one is the presence control….works just like the one on an old : marshall. : lxh2 : I guess I forgot two things, the diodes are 1N914 opposite polarity paralell. lxh2
Response:
It might work, but I know for a fact that 12ax7’s do (Not power tube though)
Response:
– Hide quoted text — Show quoted text ->: : input >: : .033_3.3k ___ >: : | | ___pf___ | | ___uf_k_ | | _| _| | _| |______ >: : 1M __| / | | | __| / | | | _| / | | _| / | | | >: : | | |/ | |_| | |/ | 100k|_|| |/ | | | |/ | | 100 | >: : | | | | 33K | 5.6 | | >: : 2.7k * 150 |_| | k | | >: : | | * 10k | * | >: : * * | * | >: : | __ _470__ >: : | | _| | | _| | | | _|33k |250k | __ to next >: : |___| / | 100k| __| / | d1 d2 __| / | |_.022_| ___| / | circuit >: : | |/ | | | |/ | | | | |/ | | uf | | |/ | >: : | |_.022_| >: : 1k uf | >: : | 50k >: : * | >: : * >: ….works just like the one on an old marshall. lxh2
Mind if I give it some constructive criticism? Here’s the breakdown: 1st Stage: Gain of 2.74, decoupled high pass at 530Hz 6dB/oct 2nd Stage: Gain up to 32.2, decoupled high pass at 8.2Hz 6dB/oct 3rd Stage: Up to unity gain, decoupled high pass at 541Hz 6dB/oct for Presence Stage. Presence Stage: Up to unity gain with control. 4th Stage: Gains of 2 for 3rd and Presence with decoupled high pass at 0.32Hz 6dB/oct. 5th Stage: Gain of 48, decoupled high pass at 1.5kHz 6th Stage: Back to Back diode clipper to ground with unity gain buffer. 7th Stage: Fender/Marshall type tone stack with unity gain buffer. Comments: 1. Lack of low pass network to keep the EMI birdies away. 2. The first stage decoupling erases the low frequency response. Reminiscent of my Taiwanese designed "tortured tube/SS guitar amplifier". 3. Presence control is not presence but a treble boost circuit. 4. Stage 5 further erases the low and mid frequency response. Remember, the high open "E" string is about 328Hz. 5. Diode clippers produce even and odd harmonics symetrically but do not replicate power tube harmonic ratios at various input drive levels. No capablity for positive grid foldover "sound". 6. A farely mellow tone stack network. This is what saves your butt from all the gritty earlier stages. Also probably resuscitates a little Marshall-like sound. Questions: 1. What op amps do you use? 2. Did you A/B this against a "Marshall Stack" like I plan to do? Mark Amundson, P.S. I fear Ibanez, DOD, and MXR have nothin’ to fear.
Response:
>:(circuit) >: ….works just like the one on an old marshall. lxh2 >Mind if I give it some constructive criticism? Here’s the breakdown: > 1st Stage: Gain of 2.74, decoupled high pass at 530Hz 6dB/oct > 2nd Stage: Gain up to 32.2, decoupled high pass at 8.2Hz 6dB/oct > 3rd Stage: Up to unity gain, decoupled high pass at 541Hz 6dB/oct for > Presence Stage. > Presence Stage: Up to unity gain with control. > 4th Stage: Gains of 2 for 3rd and Presence with decoupled high pass at > 0.32Hz 6dB/oct. > 5th Stage: Gain of 48, decoupled high pass at 1.5kHz
Capacitor value of .01 uF is in error. Proper value in the lxh2 circuit is .1 uF. High pass filter effect is at 150-160 Hz, and only occurs while the diodes are conducting (during distortion, causing a distortion- dependent high-pass filter). > 6th Stage: Back to Back diode clipper to ground with unity gain buffer. > 7th Stage: Fender/Marshall type tone stack with unity gain buffer.
Circuit analysis results from Mark Amundson generally correct except for one dependent on an error in the posting of the lxh2 circuit portion posted so far. > Comments: > 1. Lack of low pass network to keep the EMI birdies away.
Circuit layout and shielding did the job. > 2. The first stage decoupling erases the low frequency response.
Most amps have some high pass filtering in the earlier stages, rolling off at least 6 dB per octave below 160 to 350 Hz. Without this, you get a more grungy sound. The 530 Hz rolloff in the lxh2 circuit is indeed more extreme than most amps have, but was done to lxh2’s taste. For a less severe rolloff like that of actual amps, increase the 3,000 PF cap to the .0047-.01 uF range. > Reminiscent of my Taiwanese designed "tortured tube/SS guitar amplifier". > 3. Presence control is not presence but a treble boost circuit.
The effect of the presence circuit in the Marshall amp being simulated is, to most ears including ours, mainly a pre-distortion treble boost. > 4. Stage 5 further erases the low and mid frequency response. Remember, > the high open "E" string is about 328Hz.
The capacitor value here is in error; it should be .1 uF instead of .01 uF. This rolloff of 160 Hz is dependent on the diodes conducting, which simulates the Marshall’s negative feedback circuit failing to correct the output transformer’s bass rolloff during distortion. For a milder version of this, increase the cap to .22 uF. This does not sound quite right without a capacitor that adds some distortion-dependent bass rolloff. > 5. Diode clippers produce even and odd harmonics symetrically but do not > replicate power tube harmonic ratios at various input drive levels. No > capablity for positive grid foldover "sound".
Actually, a symmetric clipper like the one used here produces only odd harmonics from a sine wave. We have found that the supposed desirability of even harmonic distortion is exaggerated. Some even harmonics do result from distorting an asymmetric waveform already containing some even harmonics (like most waveforms from guitar strings). We have found the best distortion to be ordinary symmetric soft clipping, and then hard clipping at higher signal levels. The hard clipping is accomplished by the previous op-amp stage clipping. To get a smaller transition range between soft clipping and hard clipping, use more than one diode pair in series to increase the threshold of soft clipping. This may be disirable since the lxh2 circuit as posted has a large range between the thresholds of soft clipping and hard clipping. We have found that other distortion effects such as positive-grid effects, while making the sound slightly more realistic, do not make the sound more pleasant. > 6. A farely mellow tone stack network. This is what saves your butt from > all the gritty earlier stages. Also probably resuscitates a little > Marshall-like sound.
We have worked long and hard to make the distortion from the op-amp stages reasonably right. Marshall amps do indeed have hard clipping. If any op-amp stages before the one driving the diode pair clips, this does not make much difference since the one driving the diode pair is already clipping. > Questions: > 1. What op amps do you use?
TLO84. BTW, these do strange distortions if the input voltage goes outside the common mode range. For example, bringing the noninverting input within about 1.5 volts or so of negative supply makes the output strangely go positive, causing an unpleasant "reversal" in the clipped output waveform. This is avoided by the voltage dividers between stages. We consider the advantages of the TLO84 (availability, price, ability to drive various loads, clipping characteristics while the inputs are in the common mode range, etc.) to outweigh the disadvantages. Please consider that the clipping threshold of one op-amp in a TLO84 package is modulated by other stages in the same package driving heavy loads. If any stage in a TLO84 is distorting, other stages in the package must drive only high impedance loads. It seems like a resistor of about 300 ohms is is inserted in series with one of the power supply pins inside a TLO84 package (or a 150 ohm resistor added to both). > 2. Did you A/B this against a "Marshall Stack" like I plan to do?
Yes. The most audible differences are: 1. The lxh2 circuit is smoother than the real amp. 2. The circuit fails to duplicate a mild distortion of the loudspeakers, probably flux-modulation distortion. 3. A 4-12 stack on a linear power amp has a much less bassy sound than it does on a distorting Marshall. We believe the actual Marshall’s output impedance rises during distortion, from the negative feedback circuit not working. Adding a resistor in series with the cabinet restores the boomier sound. Or, a band-boost circuit at the cabinet’s resonant frequency can fix this. The optimum value of a resistor to do this seems to be near or slightly above the rated impedance of the speaker cabinet. The lxh2 circuit includes a resonant bass-boosting circuit (not yet posted). The circuit we came up with theoretically does not perfectly correctly does this, but was done to lxh2’s taste. To do this correction with a parametric EQ, boost 105 Hz by maybe about 6-8 dB with a Q around 2. Adjust for best or most desirable sound. A generally only slightly audible inaccuracy is the bass boost from this not being dependent on distortion, but also occurs when the amp is running clean. 4. A complex filter circuit (not yet posted) to simulate the loudspeakers does so much better than commercially available circuits to do this (for use with flat-responding speakers), but does not do so perfectly. > Mark Amundson, > P.S. I fear Ibanez, DOD, and MXR have nothin’ to fear.
Question not asked: Supply voltage? A split supply of +/- 9 volts was used. Higher supply increases the threshold of hard clipping, a lower supply reduces the threshold of hard clipping. Threshold of soft clipping does not vary with supply voltage. You can change the transition range between soft and hard clipping by changing the number of diode pairs in the diode pair bank that does the soft clipping. More diode pairs means a higher threshold for soft clipping, as well as a higher output level during distortion.
Response:
I’m afraid you can’t built a power tube distortion pedal. You need a fair amount of voltage before the tubes will operate and to make them distored you need a lot of voltage with a lot of input gain. This is why tube distrotion pedals you a pre-amp tube. You could just slave a little amp in to another amp. (using a power attinuator or someting along those lines.) Keith Richards from the Stones just plugs an old Fender Champ amp in to a big amp and crank the little Champ to get his distortion. Your amp Friend, Andrew Okincha
Response:
>I ran lxh2’s suggested power tube simulation circuit via SPICE analysis >tools for transient and frequency responses. My conclusions so far show >that the marshall-like tone stack provides an overshoot condition that >"browns down" the previous stage’s clipper circuit (similar to Pro-Co’s >"Rat" fuzz box).
Hi Mark, Could you explain what you mean by ‘overshoot condition?’ >There is no harmonic control here resembles anything like >class A, class AB, symetrical, non-symetrical clipping power amps. All >considered, there is nothing here, circuit-wise, that distinguishes this >circuit from any fuzz box distortion generator. If you want this sound, >which is not bad by the way, take your rat or tube screamer stomp box and >add the marshall tone stack controls (bass, mid, treble).
The circuit looked pretty straightforward to me, too. But I thought that the followup comments about the diodes’ interaction with the previous cap were interesting. Personally, I’d usually put a disable switch on a tone stack like that one (or more effective mid control), cause plugging into a guitar amp would go thru two sets of tone controls, and really scoop the mids all the time. I am also a bit skeptical about the current ‘power tube distortion’ fad. I once talked with an amp designer (I think it was Fischer) who said: "I built a tube preamp with an accurate solid state power amp…it sucked. Then I built an accurate solid-state preamp with a nice tube output section… It sucked, too!" <g> Well, I found it amusing anyway. MGarvin
Response:
> I am also a bit skeptical about the current ‘power tube distortion’ > fad. I once talked with an amp designer (I think it was Fischer) > who said: "I built a tube preamp with an accurate solid state power > amp…it sucked. Then I built an accurate solid-state preamp with > a nice tube output section… It sucked, too!" <g> > MGarvin
OK, Mark…you’re gonna get me started pontificating again (and we all know how painful that can be…) with all this heretical talk about power tube distortion not mattering… 
As I mentioned to you a week or two ago, I designed what I thought was a rather decent, channel-switching amp with one clean channel loosely based on a non-master Marshall/Bassman circuit, and a high-gain channel (with no diodes!) that had lots of asymmetrical biasing and cathode followers and stuff for even harmonic enhancement, etc. Well, to make a long story even longer, I _still_ wasn’t satisfied with my stage tone, so for a sanity check I pulled out my ‘72 50W non-master Marshall head, my ‘77 and ‘78 master volume Marshalls, and an assortment of attenuators (Power Soak, Altair, Power Brake) and cranked the things up. I found that the best distortion came from the _non-master_ head. It actually had a better tone, and a smoother distortion than the master heads at any setting on their preamps. I jumpered the two channels of the non-master head and used the two volumes as "tone" controls for bass and treble balance. The real clincher came when I hooked up my old MXR Dyna-Comp in front of it for a bit of a boost. Instant Gary Moore/Michael Schenker tone. This is the rig I used to run 10-15 years ago before I decided to "improve" my tone with all this new preamp/effects stuff… The non-master head has Tesla EL34’s, and the master heads have very old Groove Tubes H-4 re-labeled U.S.A 6550’s (which are about to be replaced with EL34’s, judging from the tone comparison). Various types of 12AX7’s in all three amps. The non-master amp has the cap across the 820 ohm resistor in the preamp for more gain… I (like everyone else who’ll admit to it) went through my "rack" period and tried ADA MP-1 (a complete piece of crap), Marshall 9000 series (tube and solid-state, not too bad), Digitech (noisy garbage), Boogie Tri-Axis (buzzy and thin), and Marshall JMP-1 (the best sounding, but still too thin-toned). The best tone with these came when using a Marshall tube power amp, but it was still lacking. The funny thing about all these preamps is that they seem to sound great in the store, or at low volumes in the living room, but when you get them up on stage and mic them up they sound thin and buzzy through the PA (and through the 4×12 cab, for that matter). The best tone I could get actually came from going direct through an H&K red box. (A side note: it helps to drill a hole in the side of the red box and add a ground lift switch…I’ve gotten some pretty severe hum with a tube power amp). The thing most of these preamps have in common (besides no power stage and output transformer!) is that they try to get all the distortion from a small number of stages, or worse, from the dreaded diode clipping circuits. My experience with designing channel switching amps has been that the tone just doesn’t compare to a cranked up amp. There is _something_ that is going on in the interaction of the power amp/transformers/etc. that makes the difference between thin tone and good tone. Even the clean tone is better with a tube power stage. The preamp is important, as is the gain settings of each stage; you don’t want any one stage creating all the distortion. The end result of all this? I’m going back to a straight amp turned up. My next design will be a non-master Marshall type circuit with all-tube reverb and effects mix circuits. Barring this, I’ll just put reverb on at the board and use the Marshalls. How to get clean and dirty sounds? Two amps, one attenuator (or lower-powered amp, or both) and a speaker-level switch box, or two miked 4×12’s. And if you still aren’t convinced, get the first Van Halen album and the last one and listen to the difference between a cranked non-master Marshall and a high-gain preamp Peavey. Well, maybe that’s not a fair comparison… 
Randall Aiken
Response:
I ran lxh2’s suggested power tube simulation circuit via SPICE analysis tools for transient and frequency responses. My conclusions so far show that the marshall-like tone stack provides an overshoot condition that "browns down" the previous stage’s clipper circuit (similar to Pro-Co’s "Rat" fuzz box). There is no harmonic control here resembles anything like class A, class AB, symetrical, non-symetrical clipping power amps. All considered, there is nothing here, circuit-wise, that distinguishes this circuit from any fuzz box distortion generator. If you want this sound, which is not bad by the way, take your rat or tube screamer stomp box and add the marshall tone stack controls (bass, mid, treble). Mark Amundson,
Response:
RE:> Power Amp Distortion I read these posts with interest, not because I am an amp designer..but because I JUST found the tone I have been searching for for 12 years. I am not big fan of ultra-processed guitar sound which at times sounds more like a synthesizer than a guitar {Think Steve Vai’s sound.} It is cool at first, but seems "gimmicky" after a long period of listening. NOTE: I think Steve Vai’s a great player so don’t take this comparison the wrong way…. I’m only talking TONES here. Sometimes when this genre of players have all their compressors, harmonizers, multi-tap digital delays, 31 band graphic EQ, etc…. going , it is hard to tell it is a guitar being played. The tone I’ve always liked is that thick sustaining TONE like Yngwie Malmsteen, Hendrix on Voodoo Child, Van Halen up to the "UnChained" days. Of course of these three players, Van Halen is still prone to use a few more effects and a higher output guitar pickup… but a basic TUBE tone is common to them all. I have tried various Rackmount Pre-amp and Power Amp combinations (at least 4 different preamps and 3 different power amps in all the combinations and permutations). Sometimes the tone we pretty good, but there was NO HEADROOM. meaning there were no dynamics..even with the compessors off,these units have a leveling effect {ie.. you turn back on the volume knob or hit the strings hard and it makes no difference} This may be due to the common problem of rackmount preamps running at 50 volts or less. (Recently addressed by some manufacturers.. however I tried a Rocktron Pirannha which advertises 250 volts…YUK, it only took me 20 minutes of trying it to pass it up}. SO WHAT DID I FINALLY FIND WHICH HAS STOPPED MY ONGOING SEARCH FOR TONE.? You will probably laugh because it is so simple but…. Guitar >>>>> Ibanez Tube Screamer >>>> Noise Suppresion Pedal >>>> Marshall Model 1897X 50-watt Plexi Reissue w/EL-34 Tubes{always running full out} >>>> PowerBrake (for those times I need to keep the volume lowered) >>>> Speaker Cabinet. That is it !!! Any reverb or delay is added at mixing console. Now I can focus on writing songs and rehearsing and no more long hours spent searching for a tone I can live with. One less worry in life. I guess this is one "non-technical" guy’s way of saying…. "There really is some validity to the POWER AMP -vs- Pre Amp Only debate"
Response:
>>I ran lxh2’s suggested power tube simulation circuit… > My conclusions so far show >that the marshall-like tone stack provides an overshoot condition that >"browns down" the previous stage’s clipper circuit (similar to Pro-Co’s >"Rat" fuzz box). >Hi Mark, Could you explain what you mean by ‘overshoot condition?’
The "overshoot condition" is refered to when you compare the pre and post tone stack simulated scope traces. If you expected the tone network to act like fixed passive divider, the input and output waveforms would be identical except amplitude. But the amplitude extends beyond the expected clipping point and rounds off with asymetric rise and fall curves. I have gif of the plot if you wish. >Personally, I’d usually put a disable switch on a tone stack like >that one (or more effective mid control), cause plugging into a >guitar amp would go thru two sets of tone controls, and really >scoop the mids all the time.
A point well taken. Maybe the tone network should just have the bass and treble controls and the fixed midrange resistor. Then the circuit could be followed an active midrange boost/cut stage for all those Tom Scholtz wannabes. Mark Amundson,
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writes: > I am also a bit skeptical about the current ‘power tube distortion’ > fad. I once talked with an amp designer (I think it was Fischer) > who said: "I built a tube preamp with an accurate solid state power > amp…it sucked. Then I built an accurate solid-state preamp with > a nice tube output section… It sucked, too!" <g> > MGarvin
I am also a bit skeptical about the current ‘power tube distortion’ fad. I once talked with an amp designer (I think it was Fischer) who said: "I built a tube preamp with an accurate solid state power amp…it sucked. Then I built an accurate solid-state preamp with a nice tube output section… It sucked, too!" <g> Well, I found it amusing anyway. MGarvin O.K. try this, run a boss distortion pedal such as the ds-2, or whatever, into your tube amp with the gain turned down enough to pass a clean signal. Then cramk the amp up a bit with the master and see what happens. To my ears anyway, it still sounds pretty good. Now listen to the preamp out into your headphones or monitor. Sounds terrible, doesn’t it. Of course there are other factors to consider, such as speaker load and so forth, but this should give you some idea. Steven Keys
Response:
>Randall— how exactly do you jumper the 2 channels. >I have a ‘72 50W Marshall. (also a 1988 Dyna-Comp). I CAN’T WAIT
the easy way to do this is to put a short cord from the secondary jack of channel one into the primary jack of channel two. assuming it has 4 inputs, 2 for each channel.
Response:
writes: – Hide quoted text — Show quoted text ->I agree, but in some ways Fender may have had the answer, just buy a >different sized amp >for each size of gig – you know, a champ for practice, princeton at church, >deluxe in small >hall and so on – that way you’re always running the amp flat out. >seriously, the sound >I love the best right now is an old Silvertone (Danelectro built) 1484 35 >watt head into my >Marshall cab. Maybe pushed a little with a straight volume boost (35 dB >onboard preamp). It >compresses nicely, but still gives you that variation in sound from the tone >pot, picking >technique etc. No channel switching, but that’s what God made overdrivers >for, right?
Marshall also had a similar lineup: the cute little Mercury for practice, the 20w head for lightweight drummers, the 50w head for regular rooms, the 100w head for large rooms and the 200w Major for the masochists. Tim A great amp can make a lousy guitar sound great. A lousy amp will make a great guitar sound lousy.
Response:
>> I am also a bit skeptical about the current ‘power tube distortion’ > fad. I once talked with an amp designer (I think it was Fischer) > who said: "I built a tube preamp with an accurate solid state power > amp…it sucked. Then I built an accurate solid-state preamp with > a nice tube output section… It sucked, too!" <g> >OK, Mark…you’re gonna get me started pontificating again (and we all >know how painful that can be…)
Yeah, I know! (<g> Just kidding…Great to see you back, Randy!) >with all this heretical talk about >power tube distortion not mattering… >I found that the best distortion came from the _non-master_ head. It >actually had a better tone, and a smoother distortion than the master >… >The preamp is important, >as is the gain settings of each stage; you don’t want any one stage >creating all the distortion.
That’s kinda what I was getting at. Too much juice on the front end can create a crunchy but diode-like distortion. Too much on the power tubes can choke them off (put them into class B or worse), as you know. I don’t think that the real magic comes from any single thing. >And if you still aren’t convinced, get the first Van Halen album and the >last one and listen to the difference between a cranked non-master >Marshall and a high-gain preamp Peavey. Well, maybe that’s not a fair >comparison… >Randall Aiken
I thought you said you liked Peaveys!? Yeah, I’m pretty sure that’s exactly what you said! Goin’ for that Peavey tone! <g> (Ulp!…waiting for Randy’s retribution) MGarvin
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>>>I ran lxh2’s suggested power tube simulation circuit… >> My conclusions so far show >>that the marshall-like tone stack provides an overshoot condition that >>"browns down" the previous stage’s clipper circuit (similar to Pro-Co’s >>"Rat" fuzz box). >Hi Mark, Could you explain what you mean by ‘overshoot condition?’ >The "overshoot condition" is refered to when you compare the pre and post >tone stack simulated scope traces. If you expected the tone network to act >like fixed passive divider, the input and output waveforms would be >identical except amplitude. But the amplitude extends beyond the expected >clipping point and rounds off with asymetric rise and fall curves. I have >gif of the plot if you wish.
Hi again, Mark. That’s the plot with the blue and black traces, right? Nice artwork, by the way. Not sure about the peaking, but it’s possible to get higher peaks when filtering a square wave. That’s cause the square is all odd harmonics *in phase*, so the rising edges add, but the center (90-degrees and 270-degrees) is alternately adding and subtracting with each successive harmonic. You probably knew that, so maybe you aren’t referring to a filtering effect. Anyway, considering the above, a filter would round the edges, but possibly remove some of the subtractive components at the peaks of the waveform. That circuit really has a lot of inline silicon. It should be possible to avoid a couple of the opamp stages. There were some interesting comments on the TL084 in the designer’s followup post. I forget whether it was referring to loading or to common-mode, but I wasn’t aware of problems with the TL084. I do stay clear of 324’s and similar ’single-supply’ opamps though. Horrible artifacts under certain conditions. MGarvin
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>O.K. try this, run a boss distortion pedal such as the ds-2, or >whatever, into your tube amp with the gain turned down enough to >pass a clean signal. Then cramk the amp up a bit with the master >and see what happens. To my ears anyway, it still sounds pretty good. >Now listen to the preamp out into your headphones or monitor. Sounds >terrible, doesn’t it. Of course there are other factors to consider, >such as speaker load and so forth, but this should give you some idea.
Hi Steven, You can get a horrible sound going direct out of the external speaker jack, too (Post power section). Guitar speakers make great low-pass filters. They filter out a lot of the raspy high end. MGarvin
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