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Build blog: T&T x Vox AC15
Stock
A vintage & a modern classic, the Vox AC15 (and it’s larger pal the AC30) is the ‘other’ sound of the 60’s. Whilst there have been changes in the design from the original 60’s JMI AC15 to this modern version, the core sound is still the same.
This 2011 version has two channels, a fairly low key normal channel & the classic ‘top boost’ channel, a LTP phase inverter, and a power section of two cathode biased EL84’s, producing 15 watts through a Celestion Greenback speaker.
Rebuild
The quality of these amps is very good straight out of the box, however there’s enough scope to carry out a pretty comprehensive overhaul of this amp. All the stock capacitors (apart from some 10pf's) were replaced with high/audio grade Nichicon electrolytics, Panasonic film and TDKs ceramics. Critical resistors have been upgraded to Vishay 1 watt metal film, & leads dressed and routed to minimise background noise & interference.
We took Lyle Cadwell’s (aka Psionic) mods as our reference & changed the power supply filtering to be more like the original 60’s version, whilst this reduces the filtering to V1 in the preamp, it raises the plate voltage slightly and gives a more ‘lively’ & dynamic feel. We also increased the gain from the V1 stage, giving the first stage of the preamp more ‘omph’ & a more open & dynamic sound. The two channels are also now more balanced in output.
Revoice
The harsh treble has been smoothed out by replacing the stock ceramic bright caps with Silver Micas & the amps voicing is switchable between modern & vintage EQ. By pulling up the Treble control the circuit switches to match to the original/Vintage circuit, by allowing less mids to bypass the volume control & adding and tightening the bottom end in the tone stack.
Vox AC15 - Modern tone stack
Vox AC15 - Vintage tone stack
We’ve also added the ability to jumper that two channels together. By pulling up the Normal Channels volume control, the input signal will go through both signal paths and can be mixed through the individual channels volumes. This feature is great if you like to use your guitars volume control to go from sparkly cleans to raucous overdrive.
Refurbish
Vox amps look pretty cool already so we didn’t go for a full refurb, however we weren’t impressed with the rather cheap looking plastic vents so we blinged it up with some custom made gold mesh ones. Otherwise as the amp was in pretty good condition, we gave it a good clean, repaired any scuffs to the tolex, and added shielding to the back of the cabinet.
Summary
Upgraded electrolytic filter capacitors to Nichicon audio grade.
Upgraded signal path components to Silver mica & Panasonic film capacitors.
Current handling resistors changed to 1W Vishay metal film resistors.
Power filtering changed to Vintage spec branch/parallel topology for better playing dynamics.
Switchable Vintage & Modern tonestack via push-pull ‘Top-Boost’ volume control.
Switchable Normal and Top-Boost channel jumping via push-pull ‘Normal’ channel volume control*.
Lead dressing and re-routing to reduce background noise and hum.
Updated Normal channel preamp to produce more gain and dynamics.
Custom made gold metal vents to replace stock plastic/black.
* Footswitchable control in development. Once ready, we’ll happily retro-fit this for no extra cost.