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3-stage amp architecture for variable-SPL cranked-amp tone


The best approach for classic, quiet cranked-amp tone, with great control, is this chain:

The key sequence to remember is dummy load > eq > guitar speaker. This is my recommended best way for best tone at lowest level and at any variable higher level.

3-stage amp rigs: test results, conclusions, and revised opinions - follow-up to the present article

This is the 3-stage amp rig such as the EVH/Guytron rig architecture. This produces pretty much any amp sound. This provides full control of Tone and does not compromise the Tone at all. This trick has all of the key requirements. If you use a different load, such as an unmiked isolation cab or a raw speaker in blankets or an R/L dummy load, this will just mean you would have to set your post-load EQ slightly differently - so don't bother.

If there was ever a prime candidate for a trivial and awesome and cheap DIY project to save a large amount of money, it's a dummy load with line-level trim (meant to be followed by eq > ss amp > guitar speaker pushed over 1 watt). Don't put hot power resistors directly on top of your amp case, though - I marred my Marshall that way. (That was my first circuit - my huge mistake was making it merely a power attenuator rather than full dummy load with line-level trim pot. Then I should have sent that line-level signal thru eq, then used a separate power amp, and that way, I would have only had to push my speaker at 1 watt, not 50 watts.) This should clarify: my Tiny Tony 1.5 watt tube amp kit I built has a 99-cent 10-watt 10-ohm power resistor that doesn't even get warm - and the $2 pot across it puts out the same type of signal as a 50-watt tube power amp driving a $300 Hot Plate. Since there is no longer any reason to generate 50 watts, there won't be much need for a high-wattage, heavy duty, feature-packed, expensive power attenuator. The multi-level power attenuation circuitry is not needed - just the simple dummy-load part. Instead of several $300 products (one per impedance), THD should produce a single multi-impedance product that does just dummy load, not power attenuation -- meant to be followed by eq > ss amp > guitar speaker. A nice dummy load product should only cost $100. Better, though, is a power-tube saturation pedal, all in one unit: power supply, power tube, small power resistor, and output pot. Maybe an input pot.

A dummy load (with line-level trim) has advantages over a pure "power attenuator" of the type that only has a high-level Out and possibly a fixed-level Line Out. Beware of fixed-level Line Out. For "power attenuator" products, a full attenuation setting and a trim pot are mandatory features. As soon as you use full attenuation, you can insert post processing: another EQ stage that essentially EQ's the cab itself, and time fx.

For the preamp distortion, you must obtain several differently voiced circuits; each has different eq voicing between these 2 or 4 gain stages. You cannot place an EQ pedal between the preamp distortion gain stages, so you have to buy several distortion circuits if you want to do serious Tone emulation.

We should see an explosion of power-tube saturation pedals. I urge the industry to offer power-tube saturation pedals. With AX84 5-watt amp kits for just $160 (prebuilt for just $200), we see that a power-tube saturation pedal can be simply a $100 black box with minimal components, no preamp, no power switch, an input level pot, a load resistor, and an output level pot. A deluxe model would have more features including input and output level meters.

Tube "accelerator" at Licho's site - Piotr Liskiewicz. (hit Mute)). Note the load resistors and output level pot (to produce a variable line-level signal). Local mirror. These circuits might be designed by Tomasz Wˇjkowski at his Polish site. 5W all-tube homebrew amp - includes a built-in Volume Reducer circuit, Line Out jack, and trim pot, to drive an EQ - great feature. This amp can drive a speaker directly, but suddenly, I have completely switched from the hypothesis that the speaker should be driven directly by power tubes, to the conclusion that they should actually be separated, for more control of both the Tone and the SPL (sound pressure level).

Use a power-tube and load resistor in all chains including modelling amps. You must end the chain with an actual guitar speaker. The guitar speaker must be pushed to 1-2 watts, which is only as loud as loud should be, not "instant-eviction" loud. It's still pretty loud, to get speaker smoothing.

A miked guitar speaker pushed over 1 watt is essential. Dynamic rounding is not present at the speaker terminals, only in front of the pushed speaker cone. I do not recommend direct recording, because a pushed guitar speaker has more dynamic rounding.

On my "strong buy/test" list are products that contain a power tube, dummy load, and line-level adjustment pot, such as:

To use these properly, you must do: Many people make the mistake of eliminating the actual pushed guitar speaker, and then complain that the power-tube saturation product doesn't sound good.

To be included in this buy-recommendation list, a "power tube into dummy load" product must have a way to take the signal straight from the load resistor and trim pot; you must be able to totally bypass any speaker simulation filter, because you will be pushing an actual guitar speaker.

Always bracket these units between 2 equalizers and study the distortion voicing techniques at this site. You can turn any tube power amp into a fine but unnecessarily large and expensive power-tube saturation pedal by driving a tube power amp into a dummy load box with line-level trim pot (the THD Hot Plate is a good example of the latter).

I no longer recommend driving a guitar speaker directly by a saturating tube power amp - a mistaken, limiting formula that occurs in many of my previous articles. The speaker must be pushed above the milliwatts range, and there must be a saturating tube power amp, but it is *not* important what load the tube power amp sees (spk, R, R/L), and it is *not* important for the speaker to be directly driven by the tube power amp - they *can* be separated by dummy load, post-processing, and solid-state power amp with no loss of dynamic Tone. The speaker does not have to be driven at the same wattage as the tube power amp is putting out! Key concept: line level -> high-wattage speaker level -> line level -> low speaker level. That is:

3-stage amp rigs: test results, conclusions, and revised opinions - follow-up to the present article

Torres is now doing mods to insert the Tiny Tone 1.5 watt tube power amp stage with line-level option (load resistor) in the middle of conventional guitar amps. Phone for information. I recommended posting this online and in the catalog.

My Tiny Tone amp (the version with minimal preamp) is a success and I will install the Line Level option, and will post samples.

The "speaker tap - monitor amp - guitar speaker monitor" approach:

You can use a speaker tap box such as Red Box without using any built-in cab-sim filter. Then send it to any ol' solid-state amp then to the monitoring cabinet (containing guitar speakers). For example:

Dummy cab in closet, muffled: Fender 2x12 closed-back cab with WeberVST speakers, model x.
Red Box device, cab-sim filter turned off
Radio Shack/RCA mini stereo amplifier - 10 watts, solid state with lots of I/O & source switches.
Fender cab & speakers identical to dummy cab.

Because the monitor cab is a good standard cab like the dummy cab, this is an ideal, best-case, intriguing setup. The key point is that there is just one difference between the sound at the cone of the dummy cab vs. the sound at the cone of the monitor cab: the first is itself distorting, while the second is not itself distorting; the monitor g. speakers carry a copy of the dynamics not at the dummy cab's cones, but something slightly different: the dynamics at dummy cab's terminals. It's all about *dynamic* frequency response.

This is not quite the same as the DI signal from a Red Box device that is using its cab-sim filter. It is rather, essentially a Red Box out-jack tap device, into the control-room solid-state amplifier, driving a guitar speaker as the monitor speaker. This is the "speaker tap - monitor amp - guitar speaker monitor" approach. The clearest way to picture this is:

Marshall head
Marshall cab in closet, pushing into pile of blankets/clothes
Red Box
Solid-state Marshall 2x12 combo amp, run clean

This should sound awesome, capturing all the dynamics which are at the tube power amp out jack, and hearing that signal via barely-driven, good guitar speakers. The only compromise is the difference of dynamics at the distorted speaker cone vs. at the terminals of that distoring speaker.

Another approach which is in a way more elegant, is to tap just a little of the tube amp's out jack power using a higher-ohm variable resistor and drive the monitor cab without requiring a monitor amp.

You can't simply use a Power Brake or Hot Plate, because they have too low an impedance, which the amp will see. We want the amp to be unable to see the monitor tap or the quiet monitor speaker. This means a high resistance such as 1000 ohms, in parallel with the dummy cabinet which is around 8 ohms. Offhand, I can't think of an existing dummyload/pwr-atten/cab-sim product which can be used this way. [yes, see below] The Red Box puts out too little power to drive a speaker -- it outputs a high-power thru signal and a couple-milliwatt line-level signal, not a 1/4 watt power signal.

But it would be very easy to add a variable 1-watt out jack to a Red Box (or Aiken GoldBrick) spk tap product.

If you search the above page, you might be able to find a dummy load or spk tap product (not an 8 ohm dummy load product) which has a variable 1-watt output in parallel with the . We're doing the opposite of a power attenuator -- we want to attenuate only 1%, and send the soaked 1% to our monitor cab rather than to the dummy resistor.

We need a new type of power attenuator - needs to drive a separate 8 ohm load (dummy cab or dummy load) and have a pot which sends 0-1 watt to the monitor cab. Actually, Harry Kolbe The Attenuator is exactly this.

I love to break gear products into separate pieces, via so-called "effects loops". Now, Kolbe has done us a favor by breaking the "power attenuator" concept into such separate modular pieces, as Lexicon G2 has done with its "wraparound" preamp/processor which is separated into two pieces.

Imagine the Power Soak -- or the amazingly retro Altair PW-5 which I have inspected -- with its resistors in a separate case and the ability to use a dummy cabinet instead of the resistors.

The Attenuator has exactly the feature set that is needed. Amp In jack (from amp's spk out jack), Spk Out jack (for monitor cab), Load jack (for dummy cab or dummy load or power attenuator product at ~100% attenuation), and Attenuation pot for choosing how loud to make the monitor cab.

There are two great approaches for the red box/ dummy cab/ quiet monitor cab approach.

A. Use a Red Box spk tap, monitor amp, [time fx,] and monitor cab. This provides a great place to add effects, for the EVH 3-stage amp-rig/fx setup.

B. Use a loadless Kolbe: The Attenuator load-splitter box to divert just a little of the signal from the main, dummy cab, to the quiet monitor cab.

EVH-type 3-stage amp rig with all currently-manufactured, off-the-shelf products, no simulators/ R/L soaks:

Marshall 50-watt head
Kolbe The Attenuator load-splitter ($150)
Marshall 4x12 dummy cab firing into blankets
Marshall 4x12 cab for monitoring/miking

Approach A (spk terminal tap, mon amp, mon cab) is more complicated, less elegant, but offers far more control, because the Red Box output is line-level, enabling you to add EQ and time fx, hearing all the results in the monitor cab.


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