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Fractal Audio AMP models: Class-A 30W (Vox AC-30)

yek

Moderator
Moderator
* EDIT: Up-to-date information is available in Yek's Guide to the Fractal Audio Amplifier Models *


Class-A 30W: based on Vox AC-30

From Beatles to nowadays rock stars, everybody plays Vox amps. Brian May, The Edge, Rory Gallagher, Radiohead, R.E.M., Status Quo, Kings of Leon, Ritchie Backmore etc. And etc. And for some reason unknown to me, Vox is the tone of choice in P&W.

Vox is owned by Korg. The original Vox company (Jennings) also builds the original amps, under the name of JMI.

The Vox AC-30 flagship amplifier came to life in ’59 because the existing AC-15 simply wasn’t loud enough. The AC-30 doubled the number of power tubes.

The Edge’s famous amp is a ’64 Top Boost AC-30/6 model. A ‘60-’63 Vox AC30 occupies spot #18 on Vintage Guitar’s list of most valuable amps:

“The hallowed AC30 is a seminal tone template for the ages.”​

The AC-30 originally was a rather dark two-channel amp, with a single tone control. The “Brian May” tone. People used treble boosters to, duh, boost treble, which also added gain. The Top Boost circuit, aka “Brilliance”, was added later, adding Bass and Treble controls, creating the well-known Vox “chime”. More background information. See also the comments in the Class-A 15W write-up.

The Vox is often cited to be a Class A amp. Read the Class-A 15W write-up for an explanation of Class A and Class A/B amps.

The AC-30 runs on EL84 power tubes. Originally the AC-30 had an EF86 preamp tube in one channel and ECC83 (currently: ECC83/12AX7) in the other one. Vox dropped the EF86 because of reliability issues. There’s no EF86 in the current AC-30, although you can order it (custom). Our AC-30 model defaults to 12AX7 preamp tubes.

The AC-30 has High and Low inputs per channel. If the original amp has High and Low inputs, the model is always based on the High input. To get the equivalent of the Low input, decrease Input Trim.

Vox amps are ”no negative feedback” amps. This means that the Damping parameter in the model is set to zero. When Damping is 0, the model’s Presence becomes a Hi-Cut control.

Some of Fractal Audio’s AC-30 models are based on an AC-30 Handwired. This amp has ECC83/12AX7 preamp tubes, Normal and Top Boost channels, a Bright switch on the Normal channel, and a Hot/Cool switch on the Top Boost channel, and a Master volume control with the option to bypass it. The amp was modeled with Master Volume bypassed, IIRC. The “Hot” mode bypasses the amp’s EQ for more gain.

It seems (but I don't know for sure) that the other two models are based on a non-Top Boost AC-30.
  • Class-A 30W: Normal channel of a non-Top Boost AC-30
  • Class-A 30W BRT: Bright channel of a non-Top Boost AC-30
  • Class-A 30W TB: Top Boost channel of the AC-30 Handwired
  • Class-A 30W HOT: Hot mode of the AC-30 Handwired
The controls: Volume, Tone (model: Hi-Cut), Bass/Treble on the Top Boost channel, Bright switch on the Normal channel of a non-Top Boost AC-30.

Since the “Hot” mode of the Handwired amp bypasses the amp’s EQ, you should leave all tone controls at default in that model for authenticity.

To increase gain you can of course turn up Input Drive. What also works well with Vox amps is boosting the signal at the input stage. For example, by increasing Input Trim, or by adding a Drive such as FET Boost or SDD.

Cliff talking about the Handwired amp:

"Note that the particular AC-30 used for the model has this scratchy high end. This is reflected in the high Dyn Pres value. It's a hand-wired version and that can lead to this type of sound as the parasitics cause boosting of the very high frequencies when the amp is driven hard. The obvious solution is to turn Dyn Pres down."​

Older Top Boost AC-30s have a design flaw. When Bass control is maxed, there’s a very noticeable midrange notch. Cliff:

"I intentionally limited the midrange notch since, IMO, it's a design flaw in that tone stack. But you can recover that behavior by setting the Mid control fully CCW. Modern AC30's don't have this flaw. They use a Fender-style tone stack with a fixed mid resistor."​

A Vox combo comes with two 12” AlNiCo Blue speakers or Celestion G12M greenbacks. Blue refers to the color of the speaker’s metal. AlNiCo speakers are often used when bright tones are needed. Click here for AlNiCO stock cabs.

Several Cab Packs offer additional Vox / Class-A / AlNiCo IRs. Look them up here. “Tom’s Mix” in Cab Pack 4 is a pair of Vox IRs, liked by many.​




 
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yek

Moderator
Moderator
Praise and Worship.

It seems that in the USA a lot of guitar players play weekly for large congregations.
 

edo

Power User
ah ok, got it!
BTW, i just read for the first time about the dyn presence and its high setting yielding excessive brightness, and just noticed that the ac30tb dyn presence defaults at 1.... was it higher in previous fw's??
 

yek

Moderator
Moderator
I don't know if the quoted statement is still valid for that current model.
 

simviz

Member
And just for you @yek (now I know you are also a fan), here is a quote from an interview with Rory Gallagher in Guitar Player - March 1978, where he discusses the AC30:

"For years I used a Vox AC-30, which is the best all-around European amp I've ever come across. I still have it. The Shadows used to use them, and the Beatles used them, so you know it was the popular amp. But I found that when using the treble booster, that along with the treble boost you got a built-in gain, because the transistors were fairly primitive. If I used the normal input—which was very bassy, as opposed to the brilliant input—I could get that nice rough edge without getting into a very fuzzy sound. I used that for years.......some of those hundred-watt stacks really only give you about fifty watts, while an AC-30 is a genuine thirty watts. Instead of a wall of sound, where you lose your song because it's spread out among eight speakers, I’d rather see a small amp turned to 8 or 9 and really hopping off the chair."
I've spent a long time with the "Class A - 30W" model and the "Treb Boost" drive, trying to replicate some of that Rory magic. I still have a lot of practicing to go.............:)
 

yek

Moderator
Moderator
I've spent a long time with the "Class A - 30W" model and the "Treb Boost" drive, trying to replicate some of that Rory magic. I still have a lot of practicing to go.............:)
Instead of the Treble Boost, try the Mid Boost type. It may work better, regardless of its name.
 

javier

Inspired
I have a doubt about the AC-30 model. Model Class A 30W TB default value for low cut is at 482Hz (AX8 firmware 8.00 beta). Is that OK specifically to emulate the real thing? But the model Class A 30W low cut is at 10Hz (the standard default value for nearly every amp).
 
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