• We would like to remind our members that this is a privately owned, run and supported forum. You are here at the invitation and discretion of the owners. As such, rules and standards of conduct will be applied that help keep this forum functioning as the owners desire. These include, but are not limited to, removing content and even access to the forum.

    Please give yourself a refresher on the forum rules you agreed to follow when you signed up.

Fractal Audio AMP models: Class-A 15W TB (Vox AC-15)

yek

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


Class-A 15W TB: based on Vox AC-15

From Beatles to nowadays rock stars, everybody plays Vox amps. Brian May, The Edge, Rory Gallagher, Radiohead, Kings of Leon, Ritchie Blackmore etc. And etc. And etc. Did you know that the famous James Bond theme was recorded on an AC-15? And for some reason unknown to me, Vox is the tone of choice in P&W.

Vox is owned by Korg. While the original Vox company (Jennings) now builds the original amps again, under the name of JMI.

The AC-15 existed before the AC-30. The AC-15 was not loud enough, so Vox made a louder version: AC-30. The AC-15 is still being made by Korg, eh sorry, Vox. A ‘ 60-’65 Vox AC15 Twin occupies spot #21 on Vintage Guitar’s list of most valuable amps:

“The Vox AC30 garnered more attention for years, but in this age of reduced stage volumes and project-studio recording practices, the smaller AC15 has become even more desirable in some players’ estimation. Dick Denney’s creation benefited from several design elements that were specifically intended to flatter the voice of the electric guitar, rather than merely amplify it, and these babies achieve that in spades. Plug into the stout EF86 pentode preamp channel for creamy, milkshake-thick tones or the ECC83 channel for more jangle and sparkle, and you quickly hear what all the fuss is about.”​

The AC-15 and AC-30 originally were rather dark amps, with a single tone control. The “Brian May” tone. The Top Boost circuit was added later, adding gain and Bass and Treble controls, and the well-known Vox “chime”.

The Vox is often cited to be a Class A amp. I’m not an engineer so I’ll just quote Korg/Vox about the difference between Class A and Class A/B amps:

“In order to understand the difference between these two types of amplifier designs (there are others) you must first understand a little about tubes.

The most basic tube used as an amplifier consists of three elements: Cathode, Plate and Grid. The Cathode is heated (by the heater, another element in all tubes except in very old designs where the cathode is the heater) and forms a cloud of negatively charged electrons. The Plate has a positive charge that attracts the electrons. The Grid is the audio input to the tube and usually controls the flow of electrons.

Amplification happens when a signal is applied to the grid that allows for and controls how much current flows through to the plate. Because the signal voltage is relatively low and the plate voltage is relatively high (as supplied by the power supply), the small changes produced by the audio signal at the Grid appear much larger at the Plate, hence amplification.​

In a Class A circuit, a positive voltage is applied to the Grid, which controls the flow of electrons. In this circuit design current is flowing at all times through the tube.

In a Class “AB” design a negative “bias” voltage is applied to the grid, which will cause the tube to “shut off” when the audio waveform is below a certain point. Meanwhile there is another tube and associated circuit that is turning on before the first one turns off and is reproducing the rest of the waveform. In short these two tubes share the job of reproducing the full audio waveform.​

Each type of design has its advantages and disadvantages.”​

Class “A” advantages:
– The tube is ready to amplify the signal at all times.
– The signal is instantaneously amplified because the tube does not have to “wake up: from a less than full operational state.
– A 30 watt Class “A” amp will sound louder than a 30 watt Class “AB” amp.
– Because current is maximum at all times, the amp will have a smooth compression.
– There is not a lot of headroom because of the lower plate voltages used in Class “A” amps.
– Instantaneous amplification and smooth compression make for an amp that is responsive to the touch: the amp feels good and playing it is a satisfying experience.
– Combined with EL84’s in push-pull operation, the amp will emphasize high order harmonics and the amp will “sing”.​

Class A disadvantages:
– Maximum current at all times means that the tubes are being strained even without playing.
– Shorter tube life.
– Lower power rating than a Class “AB” amp with the same tube configuration.
– Power transformer needs to be upgraded in order to handle the high current demands.​

Class “AB” advantages:
– Longer tube life because the tubes are “idling” with lower Plate Current.
– Higher power ratings with the same tube configuration.
– More headroom.
– Tighter bass response.
– Less continuous demand on the power transformer.​

Class “A/B” disadvantages:
– Not as “responsive” as a Class “A” amp.​

Let’s carry on with Wikipedia:

“The Vox isn’t really a Class amp. It’s a Class A push-pull amp with cathode bias. The high bias condition is believed by some to be the source of the amplifier's famous immediate response and "jangly" high-end, though the lack of negative feedback, minimal preamp circuit, simple low loss tone stack, and the use of cathode biasing on the output stage play at least as large a role, if not larger. It's the Celestion "Blue" speakers that are integral to the AC30, and also contribute much to the sound of the unit.”​

The AC-15 runs on EL84 power tubes. Originally the AC-15 had an EF86 preamp tube in one channel and ECC83 (currently: ECC83/12AX7) in the other one. There’s no EF86 in the current AC-15 and AC-30 models, you have to resort to boutique alternatives for that (Matchless, Morgan). Our AC-15 virtual model defaults to 12AX7 preamp tubes.

It 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.

So what does “negative feedback” mean? Negative feedback sends a bit of the signal coming out of the amplifier back to the input of the power amp. This cleans up tube distortion, but also causes loss of harmonics.​

The original AC-15 had two channels. The first channel provided the "Vibravox" effect, a combination of tremolo and vibrato, plus Volume and Brilliance. Channel 2 had a Volume and a Cut control (represented by the Hi-Cut parameter in the model). Later models with Top Boost added Bass and Treble controls. More recent models offer Normal and Top Boost channels with additional options (Bright, Master Volume etc.).

I do not know on which particular AC-15 our model is based, other than that it has the Top Boost circuit.

Personally I prefer the AC-15 model just a tiny bit over the AC-30 Top Boost model, although I can’t really explain why.​

A Vox typically feeds AlNiCo Blue speakers or 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. I like #011: Buddy's 1x12 Bludo Mix.

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.​





 
Last edited:

6stringscott

Inspired
This whole series has been great Yek, but I learned more than normal from this one. Thanks!

Now I want to play with negative feedback to better hear and compare with how I think it will sound after reading about it.

And I want to try an AC15 with EF86 tube to compare creamy sounds with a Morgan.
 

brianv4

Power User
the ac-15 with #F083 cab has been my vox tone since I first got a Fractal. very versatile amp and the tone sits well
 

Smittefar

Fractal Fanatic
Must give this one another spin. I always thought, I was a Vox guy, but initially I did not bond with any of the Top Boosts in the Axe
 

thunder100

Inspired
Hi Yek

Reviews alone are a masterpiece of its own.

THANKS

and let one explore again long lost sound ground

Roland
 

simviz

Member
Great article and a nice layman's version of Class A and Negative Feedback.

You have to mention Hank Marvin from the UK group 'The Shadows' when talking about Vox amps. I believe he was the one that asked Vox to make a 'louder' AC15 in the late 50's so he could hear himself above the screaming fans.

Isn't @StratnVox the Shadows expert around here?
 

warlockII

Experienced
For some reason, the big brother to them, the AC50 never seems to get the love like it's little brothers. The AC50 came about as the screaming girls were overpowering the AC30's the Beetles were using...crazy thinking that an AC30 could ever be not loud enough.
 

Smittefar

Fractal Fanatic
Ended up with a great sounding patch, where I finally found those beautiful Vox cleans - I must admit that I am not so much a fan of Vox drive, but coupled with the Treble Booster and/or the Zen Drive, it shines :)
My favourite stock IR was 154 — 2x12 ACROX MIX (TAF)
 

StratnVox

Inspired
Great article and a nice layman's version of Class A and Negative Feedback.

You have to mention Hank Marvin from the UK group 'The Shadows' when talking about Vox amps. I believe he was the one that asked Vox to make a 'louder' AC15 in the late 50's so he could hear himself above the screaming fans.

Isn't @StratnVox the Shadows expert around here?
Yes indeed! The Shadows were using Vox AMPS before the AC15. A friend of mine who is a collector has a prototype of the TV faced AC15 with a 15" Goodman speaker. The early AC15 were all EF86 tube for the Normal Channel. When Hank Marvin wanted more power for their larger audiences in 1961, Dick Denney produced the AC30/4 using the EF86 tubes again but with the Celestial Blues. This amp proved troublesome because the EF86 tubes became microphonic when higher volumes where required so Dick reverted to the 12AX7s. They produced several notable hits using the EF86 AC30/4 - a great sound.

The Shadows were the UK Pioneers from 1960 to 1963, not only as a backing group for Cliff Richard but also making guitar instrumentals a genre. Many of those people mentioned in the OP revered Hank Marvin and regarded his contribution as being the foundation to their careers.

The TOP boost was requested by Hank Marvin to overcome the loss of "bite" after the 12AX7 become the preamp tube. All recorded in Dick Deeny's book - The Vox Story with an introduction by Brian May. 1993 Library of Congress Catalog No 93-070580

Reading Chapter 2 - 4 reveals the progress from 1958 prototype to the AC30/6 - the top boost model that most people know today.

Great work Yek on this series. Really helps!
 
Last edited:

philipacamaniac

Fractal Fanatic
I wonder if Cliff and Co. would be willing to give some insight into the specific AC-15 modeled (vintage? Handwired reissue? etc).
 

6stringscott

Inspired
Negative feedback is now one of the first amp parameters I tweak when I've been setting up a patch. I like to jack that way up, which makes an edge-of-breakup signal more clean and edgy/cut-throughy without harsh overtones. Then I can squeeze in a little more input gain but still maintain the sound definition.

I'm no pro, so that's my layman's effort to describe it. Thanks again Yek.
 
Top Bottom