DR ROBERT V2 (VOX UL730) "BEATLES" Revolver & Sgt. Pepper

THE MOST ACCURATE REPLICA OF THE ORIGINAL UL730 PREAMP

Sometimes you can’t trust the internet or an old schematic! We’ve found major discrepancies between the published schematic and the real thing, such as component types and values, so we can assure this is the closest you can get to the rarest Vox amp! Following the preamp there’s a custom FET based circuit emulating the saturation produced by the UL730 EL84’s tube power amp section. Additionally, the circuit works with the same voltages found on the original amp thanks to a charge pump circuit which takes the 9VDC from the battery or DC plug and converts it to 18VDC.

(Although based on the UL730 preamp, the UL400 series (designed for bass) shared almost the same circuitry.)

The Vox® UL700 amp series were the secret weapon used by The Beatles to achieve the unique guitar tone on Revolver and part of Sgt. Peppers. They were also used by The Rolling Stones, Joy Division and Jimmy Page of Led Zeppelin

Introduced in 1966 these state-of-the-art guitar amps mixed a solid-state preamp with a tube power amp section.

Tubes were standard at that time but had the inconvenience of being voluminous and prone to fail, plus they needed bigger transformers due to impedance matching and power requirements. Transistors were the solution as they were way smaller, reliable and took much less energy to operate. So, in 1965, Vox engineer Dick Denney decided to develop a hybrid amp with a solid-state preamp and tube powered amp section.

Thanks to the relatively new transistor technology, lots of features could be added. Top of the line models included two channels with three band eq each, distortion, tremolo and reverb.

There were many variants with different power ratings, but the ones used by The Beatles were the UL730 (30w) and UL7120 (120w).

Unfortunately, aside from The Beatles, those amps weren't popularly used at the time and most of them ended being returned to Vox for their disposal.

Only a small number of UL730 units survived (around 20/30 units as some renown Vox connoisseurs state), attaining the title of being the most collectable Vox amps, with prohibitively collector pricing.

Now they are the missing link between tube and solidstate amps.

The Dr. Robert is based on the Normal channel of our Vox® UL730 S/N: 3042. We traced the original tag board circuits and measured each component’s value to make the closest replica of the amp’s soul and character.

One of the keys of its tone is the “Mids” control which is one of the most interesting mid-range eq ever found on an amp. By turning it counterclockwise, a great amount of mid-frequencies will be subtracted creating a scooped sound. On the other hand, when it's turned clockwise it will bring out all the mids and the signal will build up in gain. By tailoring the “Mids” knob you can control both midrange and saturation.

As the solid-state preamp of the UL730 doesn’t create much distortion by itself we’ve coupled it with a custom FET saturation engine to mimic the attributes of the tube power amp section. Not happy with that, we’ve gone even further creating the “Mach Schau!” section, which adds saturation to your main sound and includes an independent volume control to allow you to have both a rhythmic and a solo tone.

• BYPASS SWITCH: Engages the unit allowing the signal to flow through the Dr. Robert circuit. It also lights up the LED above it.
  
  
• VOL: Controls the overall volume of the Dr. Robert when you’re not using the Mach Schau! section.
  
  
• GAIN: This control is placed in the same position of the circuit as the original UL730’s volume control. Since none of the Vox® UL series had a master volume control the user had to turn it all the way up to bring the amp into saturation, and believe us, this thing is loud. To solve that we put the Vol control at the end of the FET section so now, the volume control on the original amp turns into a gain knob on the Dr. Robert. At lower settings the Gain knob will produce a clean tone, but as you approach the 12’o-clock mark the FET section will start entering the saturation region.
  
  
• MIDS: This is the most critical knob to tailor your sound. Unlike most equalizer circuits (which get placed after or before the volume control) Vox® separated the mids control from the rest of the tone stack in the UL730. In this preamp the signal gets amplified first, followed by the Mids control before the volume pot. After this, Vox® placed a special gain stage coupled with the Bass and Treble controls. Thus, you can control the midrange without interacting with the rest of the eq, as traditional tone stacks do.
  
  By turning it counterclockwise, it will subtract a great amount of mid-range, creating a really interesting scooped sound.
  
  Turning it clockwise will bring mid frequencies and the overall gain builds up hitting harder the EL84 tubes, creating saturation.
  
  
• MACH SCHAU! SWITCH: Activates the Mach Schau! Section. It also lights up the LED above it. (Note that the Mach Schau! function works only when the Dr. Robert’s bypass switch is activated. This function is not an independent overdrive but an extra tone of Dr. Robert’s main circuit)
  
  
• MS VOL: Independent volume control for the Mach Schau! section.
  
  The Mach Schau! Section was conceived during the design process of the Dr. Robert to make it more versatile in live situations. Every now and then, you need an extra push in terms of gain, to play solo parts or for songs you want people to shake their waist at. Taking The Beatles “She Said She Said” cut for example, there’s a lead part, clearly saturated and the main rhythmic arpeggio in a cleaner tone. With the Dr. Robert you could play both tones using the Mach Schau! section for the lead parts. The Mach Schau! extra saturation will always be determined by the Gain and Mids knob settings, meaning that the heavier your main setting is, the grittier this section will build up from. Paired with the extra gain circuit there’s an independent volume control labelled “MS VOL” to allow you a better control of this section. Easy and simple :)
  
  

  Pedal Secrets

 
BASS: Sets the amount of low frequency content reaching the FET section. The Treble and Bass controls work together in a tone-stacked manner, so they are a bit interactive.

TREBLE: Controls the amount of high frequency content of the signal before reaching the FET section. Setting this knob will have an effect over the overall gain.

MACH SCHAU! GAIN TRIM Aclam Guitars has included a Gain trim for the Mach Schau! section. This will allow you to lower the saturation and make it a little bit less aggressive. At high gain settings it’s less effective but a lower gain it will make the Mach Schau! sound a bit gentler. Try it and experiment!