VOX -- Valvetronix

Change the 12AX7 power tube to EL84.� Drop the stereo features.
Develop more of each amp model around the Valvetronix' power-tube dist instead of around its preamp dist.� Incorporate the Power Scaling approach and power tube biasing of London Power/Kevin O' Connor.
Eliminate the existing Master Volume control.� Move the Power Selector to the top, and label it Master Volume.� Make the Power Selector have more settings, including 0.1 watt.
Enable driving an external guitar speaker without using the built-in speakers.

Improve the Distortion Voicing

Provide separate tone controls: Pre-Dist Bass, Mid, Treb; Post-Dist Bass, Mid, Treb, Presence.
Enable separately selecting the amp model and cab model.
Make the amp computer-programmable.� Display the channel name.
In the GUI, support 9-band Pickup EQ, Preamp EQ, and Spk EQ.

Improve the Effects

Move Phaser before preamp distortion.� Add wet/dry blend control, or add dry signal to reduce intensity.
For Echo and Reverb, provide a Tone control; or reduce the treble of the Digital echo, and increase the treble of the Tape echo.
For Wah, make the unused knob control the intensity or Q width.

-- Michael Hoffman, BSEE, Amptone.com

Whether 12AX7 as Power Tube Produces Good Distortion

Designers so far tell me that a 12AX7 makes a bad-sounding power tube; I plan to produce mp3 proof of how the distortion of the VOX 12AX7 circuit sounds (despite nearly everyone else's glowing reviews and marketing hype).�

I'll use the Reamp technique to create an A/B mp3 clip of true power tube distortion response vs. preamp-tube-as-power-tube distortion response.� Just because a circuit has a power tube doesn't mean that it produces good-sounding, musically useful power tube distortion.� It's good the industry moved from solid-state to preamp tubes (initially failing to distinguish between preamp vs. power tubes) and then finally has come to appreciate the central need for power tube distortion, but it's time now to make the distinction between bad power tube distortion and good power tube distortion, including circuits that control how a given power tube is voiced.�

VOX's marketing touches on power-tube voicing somewhat, but does not touch on the issue that loudly screams out, "Doesn't a 12AX7 used as a power tube sound bad?� Isn't there a distinction between a good- vs. bad-sounding power tube distortion?"� And neither have the reviewers showed much critical testing of the result: they haven't isolated the 12AX7 power-tube distortion circuit and evaluated whether it sounds as it should, in isolation.� Despite VOX reconfiguring the 12AX7, we need mp3 demo's of how that "power-tube" distortion sounds, in itself, separate from preamp distortion voicing tricks, as you switch between amp models to reconfigure the 12AX7 "power tube".

Redundant Volume Controls

The idea of "keeping the Master Volume dimed" on this amp would mean keeping Volume and MV dimed.� They are additive.� The Volume is programmable (storable).� The Master Volume isn't.� The Volume and Master Volume might be on different sides of the fx loop (don't recall for certain, offhand).� The Volume and Master Volume are both between all preamp processing and the tube power amp.� They are in series.� To get maximum power tube distortion, you need to max the Vol and MV both; Gain also adds to the level, as does Compression with high level.� An EQ as booster in FX loop also adds to the power tube distortion level.

I'm not sure (don't recall offhand) where Reverb stands in relation to these two volume controls.

Master Volume Control Affects Power-Tube Distortion, Yet Isn't Programmable

All volume controls in the amp that affect the distortion level must be programmable.

About the non-programmable Master Volume: given that guitar amp tones involve some selected amount of power-tube distortion, you certainly ought to be able to store *all* the volume settings (that is, all of the volume settings which affect the amount of power-tube distortion), and then have the final listening volume be set on-demand aside from the programmed settings.� You'd naturally assume that this amp can do this -- but it can't; that's part of the set of mis-designs in this amp.� The MV isn't stored.�� The back-panel 4-position wattage switch is rightly non-programmable.� There's little justification for having both a Volume and MV control, in addition to the Wattage control; Volume and MV should be combined into one programmable control (unless they are needed for adjusting pre-/post-fx loop levels), and the Wattage control should be made continuously variable and more accessible.

Design Not There Yet

These amps will be surpassed by similar designs that make more sense.� Because they'll be overshadowed by amps that have these design changes, this first generation of Valvetronix amps is unlikely to become classics and retain value.� A version two per my design changes could become classics.� A real power tube, combined with programmable volume levels (affecting distortion), and better listening-level control (wattage), would make this amp what it ought to be.

The Back-Panel Jacks Fail, Going Silent

It's possible the latest amps have higher quality jacks.� One could contact Vox to inquire � has anyone opened up the latest units and the older units to compare the jacks?

The notorious problem with these amps: the jacks on the back panel fail to close.� The poor-quality FX jacks and Headphone jack are in series with the signal path.� If these don't close, the speaker goes silent.� You have to make these close, or have to make them complete the signal path.� There should be a popular mod to replace these with mil-spec jacks, maybe Switchcraft -- in truth, Vox needs to do a factory recall for this severe, universal problem.� These switching jacks all fail, especially after they are used (which opens their contacts).� This is a grave parts-choice quality error.

The temporary fix is to rapidly jerk a 1/4" plug in and out of the back-panel jacks, or jumper the FX loop jacks for a reliable, switch-always-open signal path.

Official Description

official product page - "VOX Valvetronix digital modeling amplifier -- The best of all worlds".

Digital technology plus a valve power amp.

A Collaboration between VOX and Korg.

Hybrid digital modeling amplifier.

The preamp sections of the Valvetronix combos rely on Korg's proprietary REMS(tm) technology. REMS (Resonant structure and Electronic circuit Modeling System)

Valve Reactor Technology -- The power amp section, as well as the relationship and interaction between the output valves, output transformer and speakers, are a critical part of a valve amp.

The power amp can change itself.

This is a modeling, valve, power amplifier. The Valve Reactor power amp is a valve power amp with an output transformer electronically coupled to a solid-state power circuit.

The output power can be configured. When you select an amp model, the output switches automatically between Class "A" and Class "AB", and affects whether there is a negative feedback loop at all, and if so, how much and what kind of feedback.

http://www.voxamps.co.uk/products/amps/valvetronix/valvereactor.htm -- "In the Valve Reactor power amplifier, a 12AX7 dual-triode valve (12AX7s are typically used as preamp valves) is connected directly to an output transformer. Our amp designers had to develop a way to raise the output level to one which could actually drive speakers to stage volume levels while maintaining the integrity of the valve-amp sound and feel. We accomplished this by creating a circuit of proprietary design dubbed the VOX VariAmp Power Circuit. The output transformer is connected to this new VariAmp Power Circuit which uses Constant Current design and Reactive Feedback technology.

"The VariAmp Power Circuit cannot be overdriven, is totally transparent and can be configured to be 1, 15, 30 or 60 Watts. The VariAmp Power Circuit does not color or change the signal in any way and the resulting output tone is pure. It also ensures that the output signal maintains the high dynamic range associated with traditional valve amps- something most solid-state amplifiers simply cannot deliver, and one of the primary reasons why valve guitar amps normally sound and feel so much louder and more alive than solid-state amps boasting the same output power rating.

"The Reactive Feedback technology used in the VariAmp Circuit "reads" the impedance curve of the speaker and then reports this reactive information back to the secondary side of the output transformer. This information is fed back to the primary side of the output transformer and therefore changes the loading on the valves, another important part of the vital role an all-valve power amp plays in the creation of traditional valve tone."

http://www.voxamps.co.uk/products/amps/valvetronix/valvetronix.pdf -- brochure

Effects:

Compressor

Acoustic Simulation

VOX Wah

Auto Wah

U-Vibe

Octave Divider

Treble Boost

valve Overdrive

Fat Overdrive

Fuzz

These appear before the preamp models.

The Modulation, Delay and Reverb effects sections can be used simultaneously. Like in a pro guitar amp/rack, these effects sit between the preamp and power amp sections. The Modulation Section includes models of popular Chorus, Flanger, Phaser, Tremolo and Rotary effects. The Delay Section delivers Delay, Tape Echo and Multi-Head delay. A Tap button is provided for setting the delay time. The Reverb Section has three types, Spring, Plate and Room.

Each section can be controlled independently from the top panel, or, in the case of Modulation, from the optional VC-4 Foot Controller.

"Signal path is a critical factor in the design of a modeling amp and another reason why the new AD60VT and AD120VT are capable of accurately modeling the sound, tone and feel of so many desirable amplifiers and effects."

Guitar Player magazine, Oct 2001, page 17, 4 page ad.

This is a 3-stage amp rig with EQ coupling between the speaker and tube power amp.

VOX Valve Reactor architecture:

1. Preamp

2. 12AX7-based tube power amp

3. Solid-state final amp

Feedback (from final-amp's output to the tube power amp's load and to the tube power amp's input) to affect tube power-amp response; designer can affect EQ of power-tube saturation here)

Guitar speakers

New material added December 2002

Is there a URL listing information about each amp model in the Valvetronix, such as which specific amp is targeted, and whether the Tone controls are pre or post the "Gain" distortion? Yes - this page, now.

How about the 12AX7 distortion -- has anyone o-scoped it out to confirm whether and when the 12AX7 tube (per channel) distorts, and the character of that power-tube saturation? I'm confident that it happens when Gain, Volume, and Master are past 2:00 -- the added distortion is in fact the 12AX7 power tubes.

Are there just 3 configurations of the 12AX7 power amp, as detected by injecting a high signal into the FX Return jack while turning the Amp Type selector and listening to the resulting EQ spectrum? I have concluded that there are 3 distinct settings here, that may interact with other circuits uniquely.

Does the headphone jack have flat response, or is there a speaker emulation filter curve to make it sound like the built-in speakers? This question arises by wondering about using the Headphone jack as a Speaker jack, to disconnect the internal speakers and send to a remote cab only, as an alternative to building tab-to-1/4" interconnects. I have concluded that the headphone jack is halfway between a raw "power-tube preamp" sound and a speaker-simulated sound; it seems to have a bit of speaker simulation but not much.

What is the character of the Celestion speakers provided with this amp -- is it more of a full-range speaker, or more of a classic guitar speaker? I have concluded that these are standard guitar speakers, not near-fullrange, because I can swap heads and cabs treating this like any other guitar amp and cab, with decent sound resulting.

Processing Chain as Described by VOX

Processing Chain: [official chain as described by VOX -- but practically, as far as distortion voicing, there's more action in the preamp model module than the "power amp model" module]

Guitar

One of 10 Stomp boxes

One of 16 Preamp models [practically, this is the modelled amp's preamp distortion and power-tube saturation]

One of five Modulation effects [practically, this is like placing chorus/phaser/flanger *after* conventional power-tube saturation, not after conventional preamp distortion]

One of three Delay effects

One of three Reverb effects

Virtual Effects Loop

Analog Master Volume

Power amp model [presumably you can get 12AX7 power tube distortion, class A or A/B, here, model dependent in some respects]

Celestion guitar speaker(s)

3 Power Amp Models Detected

Using direct-inject of white noise into the fx loop return, I seem to hear just 3 distinct power-amp models here as I turn the amp model selector.

Power-Tube Distortion Placement in the Chain

You can make the power tube (one per side) distort by turning the "Gain", "Volume", tone, and "Master" controls very high, and then I venture that the distortion is not quite as smooth and gradual as an actual power tube such as EL84. This is post-delay distortion, so heavy echo with heavy 12AX7 power tube distortion causes dissonant beats as you bend a note. Why didn't VOX put delay after the power tube and dummy load, as is standard for a dummy-load rig?

In practice, if you avoid that 12AX7 power tube clipping, you can think of the "power-amp model" (it sounds like there are just 3 distinct ones) as a mere final eq. Reread the marketing lit in that light: "The VariAmp Power Circuit [solid-state amplification after the 12AX7 power tube saturation] cannot be overdriven, is totally transparent ... The VariAmp Power Circuit does not color or change the signal in any way and the resulting output tone is pure." Echo is before power tube saturation, but that's a universal problem solved only by adding echo at the mixer or after the dummy load in a 3-stage amp rig like Van Halen.

Direct-inject of a loud signal into the fx loop return (between echo and 12AX7 power tube), then cranking the "Master" knob, produces 12AX7 power-tube distortion. I need a signal generator and scope, to see at which stage distortion is being introduced. I trust that when you crank the "Gain", "Volume", and "Master", and tone stack, the final clipping is actually the intended 12AX7 power-tube saturation, and not some solid-state buffer.

This amp has two distortion stages:

o The "Gain" distortion, which I think of as preamp distortion and power-tube saturation rolled into one as in the POD or SansAmp

o The 12AX7 power-tube saturation that, with dummy load, is immediately before the final solid-state power amp.

Cranking the Volume and Master controls does cause 12AX7 power-tube distortion but that I suspect, as I've always held, that EL84s would probably sound better. So to speak, swap the 12AX7s with EL84s instead.

The Headphone jack enables you to hear the 12AX7 distortion, indicating that the headphone jack is not positioned like in a POD, but later, after power tube and dummy load.

An interesting exercise is to turn up the reverb, then crank Volume and Master to produce that dubious-quality 12AX7 power-tube distortion. This produces strong distortion after strong reverb, which causes reverb ducking; when you hit the strings, no reverb is audible at all, then as the notes die, the reverb swells up.

Also, strong echo followed by strong 12AX7 power-tube distortion causes the expected beats as you bend a note. This may be one reason some people like real room reverb better than built-in reverb that's positioned before power-tube distortion: reverb followed by distortion sounds unnatural, largely because of this ducking effect. Real room reverb is always after the power-tube saturation and speaker distortion, so there is no reverb ducking; there's always more headroom for real room reverb to respond to the amp's peaks.

Odd Placement of FX Loop

The Valvetronix is just a POD followed by a 12AX7-based tube power amp, dummy load, solid-state amp, and guitar speaker -- but with more use of pre-dist tone-stack placement. Why didn't VOX put the time fx between the dummy load and solid-state amp, and put the fx loop there? The POD's cab-sim filter would be mostly put after the dummy load, in this equivalent configuration.

The Mysterious Built-in Power Attenuator

Somewhere near the final power amp in there is an unfathomable quasi power-attenuator that only has 4 steps: 1, 15, 30, and 60 watts. For fine-tuning the volume to the speaker, and for private cranking, you can follow that "power attenuator" with another, per side, or insert a dummy plug into the Left FX Return jack to not use the left output side. I'm sure the architecture is not power tube> built-in power attenuator. I think it's:

power tube

dummy load

solid-state power amp, with gain controlled by 4-step "power attenuator"

Probably the best chain to represent this, to avoid the impression that this is a soak, is:

power tube

dummy load

4-position attenuation switch

solid-state power amp

Placement of Tone Controls and Outboard EQ Pedals

In the official chain description in the marketing material, they never mention the tonefully crucial placement of the tone controls -- turns out, they are placed before the "gain" distortion, for the non-Metal amp models, but after the "gain", for the Metal amp models. I did find one, just one clue, a passing mention, in the manual that indicates or hints by implication, that the tone controls are before the "Gain" distortion in some amp models, but are after the "Gain" distortion in other amp models.

If you have two eq pedals, hook up this chain:

eq pedal 1

amp's tone controls (for amp model that has tone controls before "Gain" distortion)

amp's "Gain" distortion

amp's tone controls (for amp model that has tone controls after "Gain" distortion)

eq pedal 2 (in amp's fx loop)

12AX7 power-tube saturation

solid-state power amp

When the tone controls are before "Gain" distortion, eq pedal 1 is technically redundant with the tone controls.

When the tone controls are after "Gain" distortion, eq pedal 2 is technically redundant with the tone controls.

For any given amp model, one of the EQ pedals is redundant, and one is needed. In a Metal-type amp model, the tone controls are present after the "Gain" distortion, so the only way to control the "Gain" distortion voicing is by the eq pedal 1. In a Blues-type amp model, the tone controls are present before the "Gain" distortion, enabling you to control the "Gain" distortion voicing, but the only way to control the equalization after the "Gain" distortion voicing is by the eq pedal 2.

For maximizing the Tone of this amp, use at least one EQ pedal with this amp -- for Metal-type models, place the EQ in position 1 (amp's Input jack); for Blues, position 2 (fx loop). To most fully control the Tone, use two EQ pedals, one before the guitar amp and one in the guitar amp's fx loop, and realize that the amp's tone controls are sometimes at the same "redundant" placement as one of the EQ pedals, depending on whether you're using the built-in distortion pedals and depending on whether the tone controls are "pre" or "post" (controlled by which Amp Type is selected). Even if an amp's tone stack *can* be used to dial in the exact desired sound, an EQ remains valuable for precise and visualizable control over the curve -- important for learning and experimenting.

Processing Chain in a Valvetronix Amp, Expressed in Standard Format

Optional additional EQ pedal placement is indicated.

[guitar's tone control - eq1]

[eq pedal -- eq1]

amp's Input jack

1 of: comp, od, dist, fuzz, phaser ("U-Vibe")

tone stack (bass/mid/treb controls are here for some Amp Type settings) -- eq2]

preamp distortion (per model) - "Gain" control

tone stack (bass/mid/treb controls are here for some Amp Type settings) -- eq3]

chorus, delay, reverb

"Volume" control

"Presence" control [eq3]

fx Send jack

[eq pedal -- eq3]

fx Receive jack

"Master" level ctrl

power-tube saturation (12AX7) (class A or A/B per model; eq3 here is affected by Amp Type selected)

solid-state power amp

4-position "power attenuator" of sorts

cable with female tabs

guitar speaker w/ male tabs (hard-wired to head, using tab connectors)

Line Out jacks, or mics on speaker

[mixer, eq 4]

Here is a simplified chain showing alternating EQ and distortion stages. First, list the distortion stages:

amp's built-in distortion pedals

amp's Gain distortion

amp's 12AX7 power-tube saturation

Then, insert EQ stages between each distortion stage, and identify how/when you have control of them:

eq1 (controlled by guitar's Tone control or outboard EQ pedal)

amp's built-in distortion pedals

eq2 (controlled by amp's tone controls if selected Amp Type has "pre-Gain" tone placement)

amp's Gain distortion

eq3 (controlled by amp's tone controls if selected Amp Type has "post-Gain" tone placement. And, controlled by outboard EQ pedal in FX loop, and by Presence control, and by 1 of 3 EQ curves depending on which Amp Type is selected)

amp's 12AX7 power-tube saturation

eq4 (controlled by outboard mixing board)

To enable driving a Hot Plate power attenuator and an isolated guitar speaker, with mics, mixer, and headphones in the control room, you can build two sets of connector cables as follows:

Male tabs on one end, inline 1/4" mono jack on other end.

Female tabs on one end, inline 1/4" mono plug on other end.

It might be possible to use headphone jack to disconnect int spk and send to remote spk only - is that flat response, or is there a speaker emulation filter curve? Also, what is the character of the Celestion speakers provided with this amp -- is it more of a full-range speaker, or more of a classic guitar speaker?

In my last testing, with an eq before the preamp and another in the fx loop (between the model "Gain" distortion stage and 12AX7 power-tube saturation), I liked the sound of headphones in the headphone jack, better than the sound coming from the amp's guitar speakers; I got the impression that the amp was voiced using the headphone output rather than the provided speakers.

With headphones, you can hear 12AX7 power-tube saturation when you crank "Gain", "Volume", tone, and "Master".

Amps Modelled, and Tone Stack Placement for each Amp Type

Amp Type	Actual amp modelled	Tone stack loc.	Power amp model
AC15	VOX AC15	pre	B
AC15TB	VOX AC15TB	pre	B
AC30	VOX AC30	pre	B
AC30TB	VOX AC30TB Top Boost	pre	B
UK BLUES	Marshall JTM-45	pre	B
UK '70S	Marshall Plexi	pre	V
UK '80S	Marshall JCM-800	post	V
UK '90S	Marshall JCM-900	post	V
UK MODERN	Marshall DSL-2000	post	V
RECTO	Mesa Boogie Dual Rectifier	post	V
US HIGAIN	Soldano	post	V
BOUTIQUE OD	Dumble Overdrive Special	pre	B
BOUTIQUE CL	Dumble other model	pre	B
BLACK 2x12	Fender Twin Reverb	pre	T
TWEED 1x12	Fender Princeton	pre	B
TWEED 4x10	Fender Bassman	pre	T

Key:

Tone stack loc. -- "pre" means the tone controls (Bass, Mid, and Treble) are before the "Gain" distortion, which is actually a pretty fully modelled amp distortion just like in a Line 6 POD or Tech 21 SansAmp. "post" means the tone controls are after the "Gain" distortion. Practically, a nickname for the "pre" configuration is "Blues", and for "post" is "Metal".

Power amp model -- when inject high white noise into FX Return, turn down Gain/Vol/Treb/Mid/Bass/Presence, turn Master to Max, and rotate Amp Type knob while listening to headphones, there are only 3 distinct EQ curves audible. B means bass is prominent, T means treble is prominent, and V means both bass and treble are prominently present.

To determine whether the Tone controls are before or after the "Gain" distortion: turn the Gain control to about 75%. Turn Midrange to 50%. Crank Bass and cut Treble, then crank Treble and Cut bass -- if the distortion becomes crusty and then liquidy, without the heard treble or bass changing in volume, the tone controls are before "Gain" distortion. Also try turning down all 3 tone controls -- if the volume linearly decreases accordingly, the tone controls are after "Gain" distortion.

In other modelling amps, such as the Line6 Flextone, are the Tone controls ever placed before the amp distortion? Yes - the treble control is pre-dist on some Line6 models, giving you a 1-band EQ pre-dist and a 3-band EQ post-dist. The more extensive "pre" placement in the Valvetronix is part of what is good or at least realistic sounding in the Valvetronix design. A classic Boogie amp has more like a 4-band pre-dist EQ, and 5-band post-dist EQ.

Some Great Basic Amp Tone Settings

Using two 7-band EQ pedals, just like with the actual amps modelled. This is how to do custom mods of the provided models, just as the stock amps modelled sound better when modded or run with a couple EQ's plus the mixer's EQ or a custom cab.

Issue: which pickup to use. Generally, for Heavy Rock, I use the bridge pickup double-coil. For Blues Rock (SRV, Hendrix, later Floyd guitarist (Waters?)), single-coil neck. Things seem to group neatly:

Left-hand of Amp Type dial

Single-coil neck pickup

Tone stack is pre-distortion

Right-hand of Amp Type dial

Double-coil bridge pickup

Tone stack is post-distortion

With a pickup with too much upper treble, a long coiled guitar cord could help compensate -- here we see that the greats *did* use a virtual EQ pedal.

Basic Setup for Pro Studio Setup in Home Studio

Gear Chain:

Guitar: a single-single-double pickup guitar. neck pickup is bright, excessive upper-treble

BOSS 7-band EQ pedal [eq1]

VOX Valvetronix amp - Guitar In jack

amp's tone stack and Gain distortion

FX Send Right jack

BOSS 7-band EQ pedal [eq2]

FX Return Right jack

FX Return Left has dummy patch cord inserted to turn off left speaker

amp's tube power amp, dummy load, and solid-state power amp

Right speaker output wires are disconnected from the right speaker tabs and are connected to a tabs-to-phone-jack patch cord

Remote multi-miked speaker (Celestion Blue) in the other room in an isolation enclosure

Amp output switched to 1W per side (can turn up to 15, 30, or 60 watts if conditions and speaker wattage permit, for more speaker distortion)

Mixer: 6-channel mini Behringer

Mics: SM57, Condenser

Headphones: AKG closed-back cheapo full-size

The mics are 1" in front of the grille, 50% from the center to the edge, pointed at 50% from the center to the edge.

Great Settings Using 2 EQ Pedals and Miked Speaker

Classic Electric Blues Tone

Guitar: neck singlecoil pickup

EQ1: boost lo-bass, cut mid-bass, boost mid-mid and upper-mid, cut upper-treble

Amp Type: AC30TB

Treble 6 (out of 10)

Mid 10

Bass 4

Gain 8

Volume 10

Presence 7

EQ2: boost lowest bass 1 notch, highest treble 1 notch

Master 7

Mixer: mics mixed to center

12 KHz boosted

2.5 KHz boosted

80 Hz boosted

I'm listing Gain after the tone stack here because that's where it is with this amp type.

Turn Volume and Master up so that some sparkle/fizz is heard, but not so much it's ratty and torn sounding. Here's where I wonder if an EL84 would sound better, with slower onset of distortion.

No "Pedal" selected, though moderate compression sounds great (may need boost of treble in tone stack).

UK 80s - Singlecoilneck (aggressive quasi-clean stack)

Guitar: neck singlecoil pickup

EQ1: 0db, -10, -9, -4, 1, 5, 1, 0db level

Amp Type: AC30TB

Gain 1:30

Volume 3:30

Treble 2:00

Mid 11:30

Bass 10:30

Presence 1:30

EQ2: 2.5db, 2, -1, 0, 2, 4, 4, 0db level

Master 2:00

Mixer: mics mixed to center

12 KHz boosted

2.5 KHz boosted

80 Hz boosted

Using a Power Attenuator with the Valvetronix

This worked well on my second try a year later: hooking up a Hot Plate power attenuator to a VOX Valvetronix amp (with an EQ pedal before the amp and another in the FX loop) even though the last stage is a solid-state power amp. Unlike how I would design it, the Valvetronix' volume affects its amount of 12AX7 power-tube distortion. It effectively has a power attenuator built-in (each output channel puts out 1, 15, 30, or 60 watts), but like most built-ins, it has far too few steps, and the lowest (1 solid-state watt) is still a hundred times too loud for private playing levels, when getting power-tube saturation.

Compared to when I first got the amp (I got the first unit that was received from VOX by the major guitar store):

o I now have better connectors (speaker-tabs to phone jack convertor)

o A better understanding of the EQ aspects of the Hot Plate

o A meta-attenuator (a wall plate speaker attenuator chained after the Hot Plate) to use Hot Plate -16dB fine-tune controller for its inadvertant side-effect of acting like a treble control as well as the intended volume functionality.

o A better understanding that for some Amp Type settings (modelled amps), the Tone Stack is before the "Gain" distortion on the Valvetronix. (I also confirmed that at least some of the tone controls in Line6 products are before, not after, the "gain" distortion.)

o A clear idea that the tone explorer will *always* want to put an EQ in an amp's FX loop together with an EQ before the amp. This idea of putting an eq pedal in the amp's fx loop is sort of an old idea for me, but somehow I've now fully moved to that paradigm -- the idea comes first but the paradigm shift can occur later, where you really "get it". However, at the same time, it's important to remember that this is exactly equivalent to eq>dist>eq pedals skipping the amp's preamp and injecting directly at the amp's FX Return jack. When you bracket the amp's preamp distortion with two EQ pedals, you're just doing the secret classic pedal chain of eq>dist>eq but using the amp's distortion instead of a dist pedal -- and for amps without an effects loop, but with a tone stack, my equivalent chain is eq pedal > amp's preamp dist > amp's tone stack as relatively feeble "2 1/2-band EQ".

o A clear idea that the tone explorer will *always* want to try a power attenuator after a guitar amp's power amp -- even with a solid-state amp.

I really like this 2-EQ + Hot Plate approach -- instant virtual amp mods. I'm trying to get the power attenuator makers to offer more passive eq controls than the Hot Plate's 2-position treble and 2-position bass switch (and its inadvertant treble control that's a side-effect of the -16dB fine-tune volume control).

The remote-miked isolated guitar speaker: I *finally* have it set up *well*, after years -- with decent mics and mixer and a dedicated great guitar speaker. And -- this is a somewhat new development -- I drive it always with a power attenuator. I can also drive the guitar speaker in the control room.

I can push the speaker in the iso-cab hard and mic it, while simultaneously pushing the guitar speaker in the control room just a little, by using chained power attenuators (a wall-plate attenuator near 8 ohms drawing from one of the Hot Plate's two Speaker jacks). The idea here is that ideally I'd dial-in a sound that sounds great in the control-room's guitar speaker (typically in the combo amp I'm using) and also sounds great in the line-level signal at the board, from the multi-miked loud guitar speaker in the other room.

I also always use this with EQ bracketing preamp distortion, whether using a distortion pedal, an amp's preamp distortion, a POD, the Valvetronix' "Gain" distortion (amp modelling).

For example, yesterday I did the chain:

EQ pedal

Valvetronix' Gain distortion (Amp Type: AC30TB or 80s Marshall)

Valvetronix' FX Send jack

EQ pedal 2

Blues Jr.'s guitar Input jack (no fx loop in this amp, unless you count the reverb return plug)

Blues Jr preamp: high but tried to keep clean

Blues Jr tone stack cranked and master volume cranked, to hit power tubes without introducing Blue Jr's preamp dist.

Hot Plate power attenuator w/ treble and bass boost

Remote iso-cab with Celestion Blue speaker

Dynamic and condenser mic

Mixer w/ tone controls

Headphones

This approach is solidly based on a pro studio setup. This setup uses:

An EL84 tube power amp driving a power attenuator

rather than:

The Valvetronix' 12AX7 tubes as power tubes driving the Valvetronix' internal dummy load then the Valvetronix' solid-state power amp

EL84 tubes are bound to make a better-sounding power amp than 12AX7 tubes. Another disadvantage of using the Valvetronix's power amp system is that it's a 3-stage amp, involving a dummy load, which blocks the signal and then the final solid-state amp has to hurry to put the snap and presence back in again. A power attenuator sends some of the original signal straight through to the guitar speaker, so it's bound to have more dynamic presence than a 3-stage amp with full dummy load and final amp.

It's ironic that VOX used a 3-stage amp architecture, suffering the liability (some loss of dynamic presence) without taking advantage of the strengths (volume-independent power-tube distortion, as in the Guytron amp). Similarly, unlike my recommended approach, they put time fx before the power-tube distortion, instead of between the power-tube distortion and the final solid-state amp.

VOX has only half answered my plea for integrating a power tube and dummy load into the middle of modelling amp circuitry, with controllable EQ on either side.

Wall-Plate Stereo Power Attenuator

One wall attenuator I tried measured as 5 ohms -- too low to work well with the Valvetronix. I tried this with the Vox Valvetronix and it only worked up to a point, then the amp made farty noises when turned up past a point (using the amp's 1W setting). However, I got good results when using the 8 ohm Hot Plate and optionally putting this attenuator after the Hot Plate, except for when you turn the Hot Plate to Bypass -- then the farty noise happens, probably because this unit is only 5 ohms and the amp expects 8 ohms or perhaps higher. Each channel's speaker in the Valvetronix is 8 ohms. You should be able to wire a load resistor in series with a (mono or stereo) wall attenuator to give the right adjustment -- try the resistor at either end to check for EQ impact. - mh

To make the Valvetronix even quieter that 1W, a stereo wall attenuator (8 ohms or higher, not 5 ohms) would be more convenient and affordable than two guitar amp power attenuators.

Power Attenuators and EQ Pedals

The VOX Valvetronix experiment worked great. I also confirmed that approach with using one EQ before the amp, two in the stereo fx loop of the amp, and two power attenuators -- one per channel. I read the Harmony Central reviews about this amp -- surprisingly favorable. So I think many people will be glad to find they can bring out even more Tone, at truly private levels at home, using EQ pedals and power attenuators. This amp works on the same principle as the Guytron -- preamp, tube power amp into dummy load, then final linear amp. The Valvetronix was a nice way to confirm the great strengths of the simple trick of starting with a standard tube amp, putting an eq pedal in front of the amp, putting an eq pedal in the amp's fx loop, and putting a power attenuator between the head and cab. The Valvetronix truly does act like a literal tube amp this way.

>I'll be interested in hearing if you wind up concluding that a hot plate on a solid state final stage is worth the trouble or not.

It is, in this case, but this case is different than pushing a solid-state power amp into distortion into a power attenuator. The attenuator is used slightly differently -- the Valvetronix' final amp isn't pushing into the dummy load so hard that that amp stage distorts; the power-tube distortion is generated earlier. It's like running a tube amp into a soak without driving the tube amp's power stage hard.

Imagine running a 10-watt solid-state amp into a soak -- this implies running the amp's solid-state power amp hard, so it distorts. That's not what's happening with the Valvetronix. Even if you turn up all knobs in the amp and insert a cranked EQ pedal into the fx loop, so that the 12AX7 power tubes output a square wave, the final solid-state amp still doesn't distort. The power attenuator is only used to cut the voltage to the speaker. Remember the amp effectively has a power attenuator too -- a switch between 1, 15, 30, and 60 watts per side. That too isn't a normal power attenuator loading down a distorting power amp; the distorting power amp comes earlier and drives its own load. My architecture chains of the Valvetronix itself should indicate a kind of power attenuator:

12AX7 tube power amp

dummy load

power attenuator sort of -- 1, 15, 30, 60 watts

I have no idea why the Master controls the amount of power-tube distortion and there's a 4-position stepped "power attenuator" control on the back. Why not eliminate the stepped control, and put the Master control (continuously variable) *after* the dummy load? That's how I would have designed the amp.

>Is part of the idea of having an EQ bracketing the pre intended to allow you to boost signal going in to pre or power tubes?

Overall boost if needed, as well as frequency-selective boost and cut.

>I'm with you 100% that having an EQ pretty much everywhere you can put one is key to getting exact sound, but I have been wondering if my need isn't more in having a way to boost signal going in to the big tubes.

If the amp has an FX loop, you could confirm that using an EQ pedal. An EQ pedal with a Level control is much stronger than the MXR EQ pedal design. Level matching is always a concern -- you don't want the EQ pedal itself to distort. In my experience so far, an EQ pedal in an amp's fx loop enables pushing the power amp much harder, and is also frequency-selective.

Why don't people know about EQ pedals? One reason is that they picture EQ controls without a Level control, but an EQ is really like a literally clean Clean Boost pedal that's frequency-selective.

>My Marshall JCM 900 has never roared the way I envisioned it would, and I've lately been looking at just what the deal is? Are the pickups in my strat not giving it enough to work with? Why does a supposedly ferocious amp sound so tame?

Use EQs as a research tool. If you want to hear what a stronger pickup sounds like, use an EQ as a clean boost before the amp. If you want to hear what pushing the power tubes harder sounds like, put an EQ in the amp's FX loop (if no loop, you have to crank the tone stack and do other tricks to restrict preamp dist while pushing power tubes harder).

Amp tone is all about alternating EQ and distortion stages, with levels controlled at each stage.

http://www.amptone.com/truesecretofamptone.htm

>Yesterday I took a tape of "Snakebite" by Alice Cooper and "Make It Real" by the Scorpions to the nearest Guitar Center and had a couple of different sales guys give a listen. I said "How can I make that sound?" Some of them had interesting things to say, but I think that overall they are not really getting very good sounds themselves and don't want to admit it. Your site, with its emphasis on power tube d. over preamp tube d. with time domain effects after all d. has certainly got me interested in that.

I no longer elevate power tube dist over preamp dist -- that was reactionary and compensative to the "preamp tube = tube = power tube distortion" fallacy. Instead, I emphasize the right voicing throughout, which mostly depends on levels and EQ curves in each gain stage. Tube types affect spongeyness -- rate of bending into clipping.

The Problem with Modelling Literal Amps

The modelling amp concept will partly give way to a new goal of producing good sounds, rather than producing accurate renditions of quirky detailed behavior of amps. I don't want ten classic amps; rather, I want an amp that can produce the best sounds of those amps but with more control and range. The "modelling a literal guitar amp" concept is not the final ideal.

We need to model ideal sounds, not literal amps. To do that, we need programmable pre- and post-distortion EQ, and need to more like model modded and reengineered improved versions of old amps, and stop trying to literally mimic the flaws and quirks of actual amps. For example, people got a better sound than an AC30 by using a treble booster -- so what we should model is the sound of an AC30 used with a treble booster.

Most old amps needed band-aids to bend them to produce their best sound. A better modelling amp would incorporate these band-aids into the controls and basic voicings. This means a step back to the mid-90s approach to presets, which targeted album sounds rather than literal amps.

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