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Vox AC30 Reissue
"JMI" Modifications

Psionic Audio specializes in modifying "reissue" Vox AC30s to the specifications of the great-sounding JMI era of the 1960s. We perform these mods on the following Vox amplifiers:

  • AC30TB/TBX Series
  • AC30 Custom Classic Series
  • AC30 Heritage Series
  • AC30 Custom Series

Note: we do not merely change parts to match the schematic of the amplifier as designed in the '60s. Rather, we have serviced and restored many original JMI era AC30s, and measured particular parts of the circuit in unusually good-sounding amplifiers. Many times the values of resistors in these nearly 50 year old amps have drifted from what they originally were. Sometimes this is bad, but other times the drifted components resulted in a particularly good sounding amplifier. This one has more harmonic response, this one has more compression, this one breaks up in a really cool way, etc.

Frankly, none of this is "magic" - if an amp sounds a certain way, the cause is measurable. And reproducible. We reproduce the characteristics of many different great sounding JMI-era AC30s in our modified reissue AC30s. Rather than chasing a schematic, we are providing you with a modern version of a truly excellent sounding amplifier from that era.

We use only the finest vintage-appropriate components in our amplifier modifications:

  • Mercury Magnetics Premium Upgrade and Replacement Transformers
  • Sozo Vintage Replacement Capacitors
  • Silver mica picofarad value caps
  • Filter capacitors (TB/TBX Series) from Nichicon
  • Electrolytic capacitors from Sprague and Nichicon
  • High Quality Jacks from Neutrik and Cliff as needed
  • 1/2W and 1W Carbon Film Resistors in the audio path
  • 1/2W and 1W Metal Film and Metal Oxide Resistors in the power circuit
  • Teflon coated shielded and unshielded silver wire as needed

Please read our article: "Mojo" Parts in Guitars, Effects, and Amplifiers Part 1: Resistors

Modification Packages

General "JMI" mod package: $350

  1. Changing the circuit to that of a '64 JMI AC30
  2. Upgrading audio resistors to 1W metal films
  3. Upgrading power resistors to 1W or higher metal films/metal oxides
  4. Upgrading signal path capacitors with Sozo and silver mica capacitors (JMI appropriate)
  5. Upgrading the screen and grid resistors on the power tubes to run modern EL84s at a slightly lower current for longer tube life and less heat (no audible change)
  6. Making the amp less bright and more pedal friendly (applies to both the Top Boost and the Normal channels)
  7. Adding a switch that changes the Top Boost channel preamp from the '65 and earlier sound to the '67 and later sound (optional, $15)
  8. Fixing the effects loop so it does not lose low end (only applies to the Custom Classic series, optional as needed [some don't need the fix], $20)
  9. Adding a high quality speaker jack to the bottom of the chassis (only applies to early Custom Classic amps where the speaker wires go into the chassis, optional, $30)

Mods I recommend but that are better done locally:

  1. Upgrading the stock tubes to JJ/Teslas (optional)
  2. If the amp has the Wharfdale speaker(s), replacing with a Weber Blue Dog and/or Silver Bell, or, for a lower cost upgrade, the Celestion G12H

Mercury Magnetics Output Transformer upgrade: $300 standalone/$250 in combination with the JMI package above.

This is a big part of the "JMI Sound" and we highly recommend it if it's at all in the budget. We get a special price on the MM transformers which we pass along to you - just the output transformer alone is $235 normally, uninstalled.

That price is for the Woden clone, which is the most commonly requested for the AC30. We can also do the Haddon or Albion for the same price, or the Radio Spares for an additional $50 (a darker, more aggressive sound - more of a Brian May kind of thing).

We can also install a MM choke for an additional $75, though it isn't needed in the AC30 in our opinion.

We can also install a MM power transformer for an additional $325, though again we don't think this is necessary - but some clients want to have the "absolute best" and we are glad to provide it. Again, the MM power transformer for the AC30 is normally $270 by itself, uninstalled.

Power Scaling: $300 standalone/$200 in combination with either package above

We install variable voltage power scaling, as invented by London Power amplifiers, which allows the output voltage of the amplifier to be varied, rather than using a traditional "master volume". This lets you have the sound of a cranked JMI AC30 at any output level you want (up to max power of 33-40W). At lower volumes/voltages, this also results in less heat and longer tube life. This is absolutely the best sounding way to vary the output volume in an AC30, as the phase inverter and output tubes compress and overdrive as on the original non-master volume amps.

All brand names, amp names, guitar names, etc, that are not Psionic Audio products are of course the rightful property etc of their respective owners. Used humbly with the realization that we live in the real world and people use these products, despite the best efforts of various legal departments.