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ro7939
05-15-2010, 06:10 PM
How much, if at all, does the output transformer of a tube amp contribute to the ideal, classic electric guitar distortion/overdrive/sustain effect?

I'm considering building a completely custom combo amp with custom vertically offset bipolar speaker. The unlikely goal is something below about 35 lbs providing both a dead-quiet pure/clean tone (sounding like the best, most delicate, cost no object acoustic nylon and steel string guitars only louder) and similarly superb distortion/overdrive effects for electric guitar. Though no amp has achieved this lofty goal I'm quite the dreamer and have achieved some milestones in the past.

Starting with the preamp: Do we all agree that a properly designed tube preamp is ideal, and the best SS is a far distant second, for the above-described electric guitar effect? Please correct if necessary. I'm consulting with the best tube designer/engineer I’ve met to see if one tube preamp could fill both the above-described acoustic and electric requirements. 2nd choice is a 2-ch preamp: Ch1 for clean/acoustic (tube or SS), Ch2 for electric guitar distortion/overdrive/sustain (tube only).

Power amp architecture:

The power amp architecture can only be selected after confirming to what extent the output transformer of a tube amp contributes (or does not contribute) to the ideal electric guitar distortion/overdrive/sustain effect.

Recently a reliable industry source described a special Mesa Boogie tube power amp with dual output transformers (or possibly one transformer with dual primary and/or secondary taps). The industry source said the MB user could switch between either one or two output transformers (or taps) depending on the desired distortion effect.

If the tube preamp can supply all the requisite distortion effects, I lean toward an OTL (output transformer less) tube power amp. This is because, compared to SS (and contrary to street logic), OTL can provide a lower bass cutoff when driving a high impedance speaker load such as in my case (custom vertically offset bipolar system, 2x 8 Ohm wired in series for 16 Ohm). Also, when matched to the right load, OTL is in a singular class of one when amplifying acoustic guitar, blissful magic otherwise unknown and unknowable.

teleharmonium
05-15-2010, 08:15 PM
IMO the output transformer contributes significantly to distortion and compression. I can't quantify how much, but there's a big difference between amps with small OTs and amps with "big iron"/"overbuilt" OTs and otherwise similar circuits.

As to your preamp question, IMO it makes surprisingly little difference to a traditional non master volume amp design, whether the preamp is tube or solid state. The tube preamps in those types of amps are clean sounding. If low noise is a major consideration, I'd actually suggest giving serious consideration to solid state. Everything I like about amp distortion comes from PI and power amp distortion, as was made clear to me by the Vox 730.

Hearing it, with the built in germanium fuzz off of course, I don't think anybody would peg it as a solid state preamp. If the power amp is tube, the amp sounds like a tube amp. If the power amp is solid state, the amp sounds solid state. I have never encountered an exception to those two statements. A good tube pre into a good solid state power amp can sound better than a SS pre into the same power amp, but the overall effect is more like SS than tube.

Tube amps with no output transformers tend to require large numbers of tubes. All of the ones I have seen have been like that. It doesn't sound very practical to me.

Lower bass cutoff would not be a desirable feature to me in a guitar amp. I know it sounds good on paper, but I always want less bass in my guitar amps, not more. (If I played totally solo, it would be a different story.)

What is the motivation for doing these two different tasks in the same amp ? If it's a matter of convenience and portability, I suggest that the best amp for the acoustic might be solid state, and might not add much to the weight; whereas an OTL dual purpose amp may well be heavier than two dedicated amps.

Speakers also tend to be optimized for one job or the other. I don't think there is such a thing as a speaker cab equally suited to both apps and able to provide top notch performance in both. Traditional distorted tones, at least from certain eras, require speaker distortion; but you don't want that for the acoustic sound, and the acoustic amplification will require more clean power and thus more power handling.

Sorry to be a pessimist but I think the goal is impossible. I don't discourage experimentation as a rule, but a lot of development has gone into guitar equipment in the past and there are reasons why professionals follow certain conventions. I don't think you could achieve either of those tones under 35 pounds, much less both. Certainly not a tube amp with enough power for clean acoustic sounds, with two output transformers; that's going to exceed the weight limit right there before you get to speakers.

ro7939
05-15-2010, 09:01 PM
I am as happy as I am delighted to have read such an unexpected, in-depth reply. It did not take you long to awake me from my dream. Thank you!

Just to clarify, the alleged lower bass cutoff of OTL was in reference to high-end audio, where high-20s is normally preferred vs. the mid-30s. I get your point though. How highly do you rate performance below 200 Hz (my engineer/friend mentioned a particular Hammond output transformer starts to high-pass filter around 200 Hz).

So, moving forward...I'm cured of the all in one dream. But I still desire a custom combo amp because of the unique performance of the aforementioned vertical offset bipolar array (something I think you would dearly appreciate; the advantage is readily apparent).

For electric guitar: What particular transformer coupled tube amp circuit might you recommend I build, preferably from a kit with all upgraded parts or from scratch? Distortion/overdrive is the only desired effect. I estimate 15W into 16 Ohms will suffice. What might you recommend for 8" speakers (two will be used in bipolar array)? I could buy a used Fender Blues Jr, rebuild the amp, toss the enclosure and 12", but I estimate starting from scratch makes far more sense.

I also want a vertical offset bipolar speaker system for the acoustic amp. A well known professional (not paid to endorse) praised the just released 70W Vox AGA70 Acoustic Amp. I could mount the AGA70 atop a second (custom) speaker box with the second rear-firing speaker (must make sure the power amp will properly drive two speakers).

Your wise counsel is highly appreciated.

regards,
Jimbo

Hwoltage
05-15-2010, 09:03 PM
The reason it isn't possible to create a convincing 'valve' type or softer clipping (as opposed to hard clip like a square wave fuzz) with transistors has to do with differences between the grid of the tube and the base of the transistor which both serve pretty much the same purpose.

In a tube the anode is charged to a high positive voltage, the cathode is referenced close to ground and the grid is referenced much more negative than the cathode. The cathode if covered with a material that emits electrons when heated to a high temperature by the heaters. These electrons are negative, the anode is positive but since the grid is even more negative than the cathode and the electrons it is emitting they go screaming from the cathode through the grid (which repels them then since it is even more negative) towards the anode. When you input a sine wave at the grid that sine wave modulates the grid from a starting point that is set a certain voltage more negative than the cathode. In a typical 12ax7 that value is about 2 volts or so, but mind you it's -2 volts because it is measured relative to the cathode. The sine wave starts at -2 volts, on the upper (positive) swing brings the grid to, say -.5 volts (which limits the current to the anode since fewer electrons are attracted to the anode and more towards the grid. On the lower (minus voltage) swing the grid swings to -3.5 volts which increases the current to anode since even more are allowed through the grid and to the anode (this is why the AC voltage taken from the anode is inverted when compared to the input at the grid). Now, eventually as you increase the voltage at the grid it swings wider and wider until the grid hits 0 volts at which point the signal is clipped and current then slowly starts to flow through the input grid and down through the typical 1 meg resistor to ground. Depending how quickly the builder allows this current to flow the input will clip softly at first and then harder as the either the voltage is increased or the reverse grid current is allowed to build more and is then suddenly released to ground. Transistors lack this ability which is why they just clip with a hard edge. Now mind you this pertains to only on portion of the input signal, cutoff clipping is another issue.

As far transformers are concerned, when coupled with a speaker load that is reflected through the transformer to the primary and due to the impedance ratio of the two windings sets a particular load for the plates of the output tubes that is not unlike the load set for the preamp tubes by the typical 100k resistor. Coupled with either a cathode resistor or negative voltage at the grids this sets the operating point for the tube and it is from this operating point that one determines the amount of headroom before the tube clips. You can move the operating point around to get different values and different types of clipping but it's an issue separate from transformer saturation. However they do work together as transformer saturation lessens the efficiency of the transformer and changes the operating point, usually closer to the point of clipping. As far as how you go about setting up a transistor output section I have no idea but I don't imagine is unlike transistor preamps which again doesn't allow for softer clipping.

ro7939
05-15-2010, 09:26 PM
BTW, if Tele is right (and he is very convincing) then the marketing text at the OTL Rendition Audio website is um, "puffery" as my wife puts it. Per Rendition, their OTL amp can get the cleanest sound for acoustic and has several gain/level controls in the preamp stage for the best distortion/overdrive effects.

stratman_el84
05-15-2010, 11:40 PM
Tube amps with no output transformers tend to require large numbers of tubes. All of the ones I have seen have been like that. It doesn't sound very practical to meWell, that's true with most designs. One (sort of) exception is the SRPP (Series Regulated Push-Pull) amplifier. Here's an SRPP power amp using EL34s ias an audiophile/hi-fi amp.

http://www.pmillett.com/el34_active-load_%28srpp%29_amp.htm

The reason for the "sort of" above, is that it has a small output transformer but it carries no DC, only the audio. This means a much smaller & lighter OT. The amp linked above is only 8 watts, but Pete Millett was designing for super-low distortion, not great guitar sound/high output.

Might make an interesting project to come up with a variation on this Pete Millett design for a power amp section for guitar. Weber uses an SRPP *preamp* in their "Smokin' Joe II" P-P EL84 kit (which sounds darned good!), but that's the only guitar-amp-related application of the SRPP circuit that I know of.


Strat

TubeStack
05-15-2010, 11:47 PM
Here's a great related article at legendarytones.com: The "Overlooked Upgrade”: Guitar Amplifier Output Transformers (http://www.legendarytones.com/guitouttrans.html)

(The author and webmaster of that site is a TGP member, too.)

Hwoltage
05-16-2010, 12:03 AM
Yeah pretty much the situation you're dealing with is the speaker's need for current as it can't be driven by voltage. The reason transformers are used is because most cheap (as in number of parts) and consistent tube circuits are only able to output voltage. From there, the cheapest way to convert that voltage to current is through a transformer. Basically, current is what results from voltage through a resistance (or impedance in the case of a transformer). The reason you see a lot of glass in most transformerless amps is because they can produce current, it just takes a lot of them to make enough to drive a speaker, essentially.

teleharmonium
05-16-2010, 12:14 AM
I am as happy as I am delighted to have read such an unexpected, in-depth reply. It did not take you long to awake me from my dream. Thank you!

No problem. It's fun to have an unusual set of questions once in a while.


Just to clarify, the alleged lower bass cutoff of OTL was in reference to high-end audio, where high-20s is normally preferred vs. the mid-30s. I get your point though. How highly do you rate performance below 200 Hz (my engineer/friend mentioned a particular Hammond output transformer starts to high-pass filter around 200 Hz).

That's interesting, I'm surprised the rolloff gets that high, unless it's a very shallow angle at that frequency, like down 1 db at 200 and -3 db at 100.

Personally I usually play in a band or in a mix where I don't want to take up low end, clarity in the lows (for the overall music, not for me personally) is very important to me. So a rolloff below 200 is not unusual for me to dial in with the single band parametric in my TC sustain+, or with a parametric EQ in a recording mix. But if I played in a 3 piece heavy rock band, for example, I'd at least want the option of a pretty much full response down to about 100. The fundamental of the low E string is about 82 hz, and much of the output from a guitar signal is going to be at the harmonic at 2x that frequency and other harmonics above that, so you can make an educated guess from those parameters.

But I still desire a custom combo amp because of the unique performance of the aforementioned vertical offset bipolar array (something I think you would dearly appreciate; the advantage is readily apparent).

I might appreciate it, if I had any idea what that was !

For electric guitar: What particular transformer coupled tube amp circuit might you recommend I build, preferably from a kit with all upgraded parts or from scratch? Distortion/overdrive is the only desired effect. I estimate 15W into 16 Ohms will suffice. What might you recommend for 8" speakers (two will be used in bipolar array)? I could buy a used Fender Blues Jr, rebuild the amp, toss the enclosure and 12", but I estimate starting from scratch makes far more sense.


I'm not a big fan of the Blues Jr. Had one once. I like the Pro Junior better, but in that wattage range, I'd suggest taking a hard look at these: the Watkins Dominator aka 18 watt Marshall; the tweed Fender Deluxe, maybe even one of the earlier versions with the sweet sounding octal preamp tubes; and the original 2 channel Vox AC15 with an EF86 tube in the brilliant channel, with or without the vib/trem effect depending on how much trouble that effect is worth to you. Those are some of the all time classic designs, and are all known for smooth natural compression and distortion without excessive volume. I love them all equally, I'm lucky to own vintage examples of the two British amps so I'm more familiar with those but the Deluxe is great too.

What's your motivation behind the 8" speakers ? I like 8s in smaller amps, you might need to go with 4 of them if you're using low power vintage style speakers. Choices are limited in that size. 10" isn't going to make a huge weight difference and you would be OK with two of them as far as power is concerned. You'll get a bigger sound with wider range. 8" speakers are really nice for harmonica, or for midrangey, kind of boxey vintage sounds, which I do love, but you don't see them used in gigging guitar amps as a rule.

teemuk
05-16-2010, 12:14 AM
Buildding oft clipping mechanism to solid-state (power) amps is easy. It has been done for few decades already but popularity of such amps hasn't really been too great. There's likely several reasons such as

- It's SS so people always have some prejudice and can always say it doesn't sound like a real deal (unless they're put to the blind test, in which case they can't usually tell a difference even if the amp didn't have such soft clipping circuits at all)
- It will decrease the amount of output power because output power is rated by THD and softer transition to clipping means distortion will appear at lower power
- Such circuits only tend to appear in the "luxus" models that cost $$$, and pricey SS amps have never been able to sell in great volumes

Nevertheless it can be done and it has been done several times.

Hwoltage
05-16-2010, 12:58 AM
I built one of these circuits and tested it with my guitar through a 1500 watt Carvin power amp and 3 way mains. The result:

My dog threw up, all my milk soured and there was a small earthquake in southern California.
I remain convinced. :D

stratman_el84
05-16-2010, 02:02 AM
My dog threw up, all my milk soured and there was a small earthquake in southern California.

I'd pay good money for an amp like that! :dude

The closest I came to something with that kind of power was an ex-girlfriend. :worried

I'm just trying to see where the problem is. :D


Strat

Jerry Glass
05-16-2010, 05:18 PM
I'm not sure what you are talking about when you use the term "vertical offset bipolar array"; are you referring to an isobaric mounting configuration? If so, there are xMax considerations that need to be taken into account espacially when guitar speakers are used...two motors driving one low compliance cone could tear it from its suspension.

ro7939
05-16-2010, 09:07 PM
I'm not sure what you are talking about when you use the term "vertical offset bipolar array"; are you referring to an isobaric mounting configuration? If so, there are xMax considerations that need to be taken into account espacially when guitar speakers are used...two motors driving one low compliance cone could tear it from its suspension.

No, not even close. It's no surprise you are not familiar with it because the only known builder/designer is Duke LeJeune (yes, a descendant of the Marine HQ namesake) of www.AudioKinesis.com. He provided the vertical offset specs for my home audio system, which is a Trinaural System with three matched front speakers. Each speaker is a 2-piece monitor.

"Vertical offset bipolar" signifies a difference in height between the front-firing and rear-firing midbass drivers of a bipolar speaker system. The 2.1kHz LPXO mandates the front-firing midbass be higher, putting the rear-firing midbass down close to the floor. IIRC Duke said the greater the offset the better, though there are practical considerations, such as the forward midbass firing too high above the ears. In my case I mounted the midbass above the tweeter to increase the midbass vertical offset.

I've been in the high end audio business for decades and have heard just about everything worth hearing. I would never go back to any other type of system.

ro7939
05-16-2010, 09:39 PM
Personally I usually play in a band or in a mix where I don't want to take up low end, clarity in the lows (for the overall music, not for me personally) is very important to me. So a rolloff below 200 is not unusual for me to dial in with the single band parametric in my TC sustain+, or with a parametric EQ in a recording mix. But if I played in a 3 piece heavy rock band, for example, I'd at least want the option of a pretty much full response down to about 100. The fundamental of the low E string is about 82 hz, and much of the output from a guitar signal is going to be at the harmonic at 2x that frequency and other harmonics above that, so you can make an educated guess from those parameters.

Great stuff! Thanks again!



I might appreciate it, if I had any idea what that was !You can hear my vertical offset bipolar speaker in Logan Utah (75 NE of Salt Lake City).

bipolar: a speaker system with matched front/rear drivers wired in the same electrical polarity...contrasting with "dipolar" in which matched drivers are wired in opposite acoustic polarity. A "monopole" is what every musician is stuck with, meaning a speaker with drivers facing only forward (although bass below about 70 Hz radiates in an omnipolar pattern simply because the wavelengths are so long).

vertical offset bipolar: a bipolar speaker system having a deliberate difference in height between the forward-firing and rear-firing midbass. My speaker guru says the greater the offset the better, and I'm quite sure that the lower the frequency the greater the advantage from the offset. But there are obvious practical considerations, such as the front-firing midbass being too high above ear level.

The magic of the VOB is its sound "intensity", being defined as sound power per unit area. The way the sound fills the space is improved, separate and distinct from SPL. For lack of better words, it sounds warmer, more natural, more pleasing, with more body.

The front and rear firing sections of my speaker each have their own 8 Ohm XO, allowing me to quickly switch from mono to bipole radiation. My receiver's level steps are in a dB scale, for easy compensation of the bipolar's +1 dB acoustic advantage.

I'm not a big fan of the Blues Jr. Had one once. I like the Pro Junior better, but in that wattage range, I'd suggest taking a hard look at these: the Watkins Dominator aka 18 watt Marshall; the tweed Fender Deluxe, maybe even one of the earlier versions with the sweet sounding octal preamp tubes; and the original 2 channel Vox AC15 with an EF86 tube in the brilliant channel, with or without the vib/trem effect depending on how much trouble that effect is worth to you. Those are some of the all time classic designs, and are all known for smooth natural compression and distortion without excessive volume. I love them all equally, I'm lucky to own vintage examples of the two British amps so I'm more familiar with those but the Deluxe is great too.Wow! My wife Debra is one of the best chefs in Utah. You've more than earned one of her great meals if you ever visit Logan north Utah. Drink of your choice as long as I can find it here (not the selection we had in CA).

BTW, my brother in law plays harmonica and lives with us. He has no computer so I did what seemed like reasonable research (till you came along) and suggested he get the Blues Jr., which he just brought home new from Guitar Center....ooooops! Well, at least I can play it till I get something better!

What's your motivation behind the 8" speakers ? I like 8s in smaller amps, you might need to go with 4 of them if you're using low power vintage style speakers. Choices are limited in that size. 10" isn't going to make a huge weight difference and you would be OK with two of them as far as power is concerned. You'll get a bigger sound with wider range. 8" speakers are really nice for harmonica, or for midrangey, kind of boxey vintage sounds, which I do love, but you don't see them used in gigging guitar amps as a rule.Yes, weight was the main consideration. I'm pretty huge and strong but Debra will use the amp. The larger the speaker the larger/heavier the box, but your advice is keenly appreciated. I realized after reading your reply that the market is a self-fulfilling prophecy: few people look for a good 8", so speaker companies don't bother making speakers that no one is likely to look for or buy.

teleharmonium
05-20-2010, 03:18 PM
bipolar: a speaker system with matched front/rear drivers wired in the same electrical polarity...contrasting with "dipolar" in which matched drivers are wired in opposite acoustic polarity. A "monopole" is what every musician is stuck with, meaning a speaker with drivers facing only forward (although bass below about 70 Hz radiates in an omnipolar pattern simply because the wavelengths are so long).

vertical offset bipolar: a bipolar speaker system having a deliberate difference in height between the forward-firing and rear-firing midbass. My speaker guru says the greater the offset the better, and I'm quite sure that the lower the frequency the greater the advantage from the offset. But there are obvious practical considerations, such as the front-firing midbass being too high above ear level.

The magic of the VOB is its sound "intensity", being defined as sound power per unit area. The way the sound fills the space is improved, separate and distinct from SPL. For lack of better words, it sounds warmer, more natural, more pleasing, with more body.

The front and rear firing sections of my speaker each have their own 8 Ohm XO, allowing me to quickly switch from mono to bipole radiation. My receiver's level steps are in a dB scale, for easy compensation of the bipolar's +1 dB acoustic advantage.

Interesting. I'd like to hear something like that.

Wow! My wife Debra is one of the best chefs in Utah. You've more than earned one of her great meals if you ever visit Logan north Utah. Drink of your choice as long as I can find it here (not the selection we had in CA).

Sounds great ! I haven't been out west in some time, but one never knows.

BTW, my brother in law plays harmonica and lives with us. He has no computer so I did what seemed like reasonable research (till you came along) and suggested he get the Blues Jr., which he just brought home new from Guitar Center....ooooops! Well, at least I can play it till I get something better!

Oh that's OK, you could do a lot worse. It's a real tube amp with a not terribly complicated circuit. They respond to speaker upgrades, and I think Fender improved the op amp used for the reverb after I had mine years ago. I have high standards having been spoiled in the amp department for years, but if you aren't used to something fantastic (and, often, expensive), you can focus on more important things.