T&T x Vox AC15
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 straight forward normal channel which uses one half of an 12AX7 & the classic ‘top boost’ channel which has an additional 12AX7 amplifier & cathode follower circuit driving a Treble and Bass EQ circuit.
The power section consists of an LTP phase inverter & 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.
We replaced the power filter capacitors with high/audio grade Nichicon electrolytics & in the signal path we’ve used Panasonic film & Silver mica caps.
Pre-amp cathode bypass capacitors have also been upgraded to our current favourite Nichicon muse bi-polar caps.
Critical resistors were changed to Vishay 1 watt metal film & leads dressed and routed to minimise background noise & interference. The reverb circuit has also been rebuilt to be a bit smoother and less ‘in-your-face’ than the original.
One major feature we felt is missing from the AC15, is being able to easily jumper the ‘Normal’ & the ‘Top Boost’ channels together, so we’ve added channel linking via a push-pull volume control on the ‘Normal’ channel *.
Normal volume pot up -> channels linked
We also increased the gain from the V1 stage, giving the first stage of the preamp more ‘omph’ & a more open & dynamic sound. This also means the two channels are also now more balanced in output level when the channel jumper is engaged.
We took the Lyle Cadwell’s (aka Psionic) mods as a reference & changed the power supply filtering to be more like the original 60’s version, using a ‘branch’ topology rather than the usual series. Whilst this reduces the filtering to the V1 preamp valve, it raises the plate voltage slightly and gives a more ‘lively’ & dynamic feel.
Revoice
We’ve added switchable voicing by pulling up the ‘Top Boost’ Treble control.
Vintage tone stack & channel linking engaged.
Down is the stock/modern AC15 tone stack which has tighter bass with greater mid-bump.
Vox AC15 - Modern tone stack
When pulled up, the tone stack switches to the original/Vintage EQ circuit, which give more travel over the treble control and a bigger mid-scoop.
Vox AC15 - Vintage tone stack
Harsh trebles have been smoothed out by replacing the stock ceramic bright caps with Silver Micas, keeping the classic Vox chime, but without the ice pick highs.
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.
The amp was otherwise in pretty good condition, we gave it a good clean, repaired any scuffs & re-dyed the tolex.
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.
Updated Normal channel preamp to produce more gain and dynamics.
Switchable Vintage & Modern tonestack via push-pull ‘Top-Boost’ volume control.
Normal and Top-Boost channel jumping via push-pull ‘Normal’ channel volume control*.
Lead dressing and re-routing to reduce background noise and hum.
Tolex repair and re-dye.
Custom made gold metal vents to replace stock plastic/black.
We have another AC15 in stock, ready to have the T&T treatment, so let us know if it’s something you’re intrested in, or if you have an AC15 that you’d like us to upgrade.
* Footswitchable control in development. Once ready, we’ll happily retro-fit this for no extra cost.
