Frank Zappa’s Guitar Amplifiers- Want to take a peek inside?

Frank Zappa was many things to many people: an outspoken critic, a cunning songwriter, a skilled performer, a voice for free speech in music, a family member. Our focus today is on Frank Zappa, the guitarist and sculptor of some amazing guitar tones across his storied career.

Image courtesy of Zappa Family Trust

Frank knew a lot about guitar tone, he had a sharp focus on how to achieve the sound he was seeking.  From his days with Captain Beefheart and the Mothers of Invention through decades of albums and touring, his guitar tones were always bespoke and articulate. His effects and amplifier system grew and changed over the years to accommodate his broad musical landscape and thirst for the perfect sound. His application of studio effects was also ever-changing, his “Ma Bell” effects rack was refrigerator-sized and loaded with tons of studio-grade gear that he would use to create his unique sound palette.

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Frank Zappa’s Vox and Marshall amplifiers

He used a lot of gear, but one thing stayed true from the mid-1970s onward: Frank loved his Vox Super Beatle system tied-in with two Marshall half-stacks. One head was a Marshall Major, the other was a 100 watt JMP.  Frank used the Vox Super Beatle in his earlier years, the Marshalls began their duty beginning in the early 1970s. You can see this rig in action in the video below, shot at The Palladium in New York in 1977.

We had to make certain that this rig was operating properly for an upcoming recording session, so we removed this venerable artifact from the exhibit and rolled it into the workshop for a good look and road test. The goal of the recording session was to collect impulse responses through Frank Zappa’s rig, giving his son, Dweezil, the opportunity to recreate Frank’s tone live on stage. This Vox and Marshall rig was a huge part of his sound and Dweezil wanted to bring it to the world.

Frank Zappa’s rig in our workshop

We determined that the whole rig was in very good condition considering its age and touring history. A few of the JBL K120 12” alnico drivers were blown, so we replaced them with other K120s from our Zappa collection, tightened up some screws, and the cabinets were good to go. The heads required some special focus, but we safely fired them up and looked forward to hearing the outcome.

Aaaaand the outcome was freaking awesome!

Dweezil stepped up and took each of the amp rigs for a spin, the wall of sound they created sent an off-the-charts energy through our live room. Each amp sounded totally different but equally awesome, Frank’s guitar sound must’ve been epic in a live scenario. Check out the video clip below, you can get a good idea of how it sounded!

The recording session went off without a hitch, it was a top-shelf studio geekfest. Fractal Audio Systems was in the house to help dial-in the tones and collect the impulse responses. We ended up using Freddie Mercury’s Neumann U67 tube mic through a Helios mic pre/eq from the early Rollings Stones Mobile Studio. The session went so well that Dweezil was able to use the impulse responses to bring Frank’s “Hot Rats” album to his live audiences, complete with a tone befitting Frank himself.

We invited amp-guy Bryan Parnell of Retro Sound Works to take a deep dive inside Frank’s guitar system and give us a technical analysis. He took a good look at the gear and submitted this report.

Zappa’s unique live tone consisted of a combination of speakers, amplifiers, unconventional settings, and ultimately skill. Taking a somewhat deeper technical look into his amplification, we can see a few things that might have contributed to this.


For starters, we cannot help but gravitate towards the Marshall Major. This amp, with the 200 Watt Lead “Canada” circuit, has a test date of December 9th, 1972. Interestingly enough, the amp has newer Marshall 100 watt JMP spec transformers installed with new mounts, while retaining its massive GE 6550A output tubes, just as later US-spec 100 Watt JMP’s would have had. This may have been due to failure of the notoriously fragile original transformers, or in an effort to better match the sound output of his 1971 100 Watt JMP. The preamp was fitted with Ruby 12AX7A’s, known for low-microphonics. Marked settings can be seen on the faceplate in red marker, showing some unconventional choices such as the Bass control being set to 10, along with conservative volume settings of 3 and 4 for each respective control.


The large original power supply choke was retained, as was the original impedance selector switch. Some of the filter capacitors have been changed. All tube sockets are US-made ceramic units, possibly to prevent arcing. One externally visible mod is what appears to be a pre-phase-inverter “master gain”, functioning as a somewhat crude master-volume, allowing for strictly preamp distortion, without driving the phase inverter. This results in a sharper breakup, with less warmth, and less even-order harmonics, unlike the sound of cranked up power tubes. If you have played a mid-70’s master-volume equipped Fender, you are familiar with pre-phase-inverter master-volume circuits.


Accompanying the Major, we find an early 1971 Marshall JMP MK2 100W Lead amp. This amp is mostly original, with the exception of Czech (JJ or Tesla produced) Ruby EL34 power tubes, Ruby 12AX7A, and a “Master Gain” installed in 1978, as noted on the service tag on top of the amp. The amp appears to retain its original filter capacitors, and also shows two sets of settings marked on the faceplate. One of these shows once again the Bass control set to 10, and a high setting of 8 on the Volume I control.


Both of these heads were run through a variety of cabinets, including two pre-1969 Marshall 1960 cabinets front-clip-loaded with high-efficiency alnico-magnet JBL K120 drivers. Not to mention the cabinets were recovered with iconic red carpet, with the grills replaced with expanded mesh, likely to protect the speakers, as the cabinets were never cased. The JBL K120 was one of the loudest musical instrument speakers available in its day, and had extended frequency response. They also were easily capable of handling the hundreds of watts a cranked JMP can put out.


Finally, we encounter the oddball in the room, a solid-state Vox Super Reverb Beatle amplifier, with its accompanying 4×12. This amplifier shows no signs of modification, besides a quarter-inch input jack located on the right side of the control panel, with no indicated function. The amplifier has relatively conventional control settings, and features various affects, blendable reverb which is channel assignable, and a frequency generator for tuning use. 

Its accompanying cabinet features four more JBL J120’s, in two ported 2×12 sections. Each section is relatively large for a 2×12, and the rectangular port is formed by where a tweeter would have once sat. The rear of the cabinets are lined with 1 inch dense insulation. I can only imagine this cabinet has a deep percussive tone, with plenty of bass and treble to spare. It has its original trolly cart, and is also covered in red carpet.

You can find more intel on Bryan Parnell at