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

yek

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

It seems that in the USA a lot of guitar players play weekly for large congregations.
 
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??
 
I don't know if the quoted statement is still valid for that current model.
 
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.............:)
 
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.
 
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).
 
Since it's been a long time and I'm under the impression that recent models tend to be less "idealized" than before, does the current version still omit the mid frequency notch from cranking the bass?
 
Praise and Worship.

It seems that in the USA a lot of guitar players play weekly for large congregations.
Same here in Canada. Christian musicians are selling out stadiums and play a lot of local venues as well as in churches all over. There are a lot of young people in seminary colleges. It's becoming popular again and in a way its never been from what I am seeing with these revivals popping up all over. The Music is good music. Inspired and powerful.
 
Same here in Canada. Christian musicians are selling out stadiums and play a lot of local venues as well as in churches all over. There are a lot of young people in seminary colleges. It's becoming popular again and in a way its never been from what I am seeing with these revivals popping up all over. The Music is good music. Inspired and powerful.

I agree. And I'm not a religious person by any stretch of the imagination, but some of the music and the guitarists are very impressive. I also like the way it's about pulling people together in an on-going way, which I think of as the whole original point of music.
 
I agree. And I'm not a religious person by any stretch of the imagination, but some of the music and the guitarists are very impressive. I also like the way it's about pulling people together in an on-going way, which I think of as the whole original point of music.
You might have known that I am a Christian by my post and enthusiasm in it about what I see in the churches I know. I am not here to preach or anything or try to convert anyone. I don't believe in that. I talk about my world from my view and people can make up their own minds just fine. Isn't it the only way we can make up our minds lol. By ourselves? Otherwise we are just believing what someone else tells us.

The music I find though to be so inspiring when we jam at these spiritual jams that I hear things I never hear anywhere else.
Cool story. We were doing our worship thing and something was in the air. I never heard the worship band so in sync and do on point. Perfect pitch and singing freestyle with vocal ranges I never imagined would come out of their mouths. Their voices changed I am telling you. Their instruments sounded better also. I was watching from the congregation and I started noticing even my own voice that was singing along became better. I am awesome guitarist and a terrible singer but all of a sudden I was singing in perfect pitch also, I also had this vibrato to my voice that I could not do again if I tried. I see the keyboard player take her hands off the keyboard and she said something inaudible. She is looking at the band on the stage all surprised like. I hear this really low c note ringing out like a bass synth sound. Almost like Gregorian style harmonics start to add to that low note. The place erupted. Everyone started to sync and sing perfectly and people were adding their own harmonies like pro singers. I never heard sounds like this. I know what I believe and take this story how you like but I talked to the band after and the leader told me she was not playing that low note. She asked if I could hear it actually. Everyone knew something special had happened and what we do believe is like that Bible tells us when we sing the song of our hearts to God he sings back over us. He was joining in on the music is our only explanation. Best jam I ever attended but there have been others too since then. Its becoming a regular thing actually.
 
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