| Motivation I like small things, and I suppose this extends to guitar amplifiers too. For one reason or another, I've just never been drawn to the idea of a huge-ass Marshall Megastack pumping away behind me, causing me irreparable hearing damage, and generally just flooding a room or a stage with way too much volume. I'm also a firm believer in making a soundguy earn his keep: get a good sound at a modest volume, and let the PA do the heavy lifting. I'm also a bit of a wuss, and I don't have roadies to help me carry said megastack. I'm also a fan of Vox amps. There's just something about the pompous look of them and their gritty, never-quite-completely-clean shimmery sounds. Some of my favourite guitar sounds, such as the Edge's earlier work, rely heavily on a Vox AC30 being driven just ever so slightly into overdrive; the sweet spot of the amp. One day, I'll probably break down and get myself a proper AC30 or an AC15, but until then, I'm content to play with some of the smaller/cheaper members of the amp family. For a couple of years now, I've been playing with a Pathfinder ministack (2x10) and an "AC1"; a battery-operated 1 watt replica of an AC30. The Pathfinder is quite an excellent solid state amp. One could describe its sound best as an "inspired approximation" of the ACXX sound, though there are obviously limits to how well a solid state amplifier relying on opamps and diodes for its overdrive characteristics can emulate an all-valve Class A design. The "sparkliness" and some of the grit in the overdrive is there, but the amp's transitions from clean to crunch are sometimes a bit rough. The 15 watts of the Pathfinder into the 2x10 Celestion-loaded cab was barely enough to keep up in a full band rehearsal setting, however. At one point, I experimented with swapping the power amplifier IC of the Pathfinder head to a TDA2050 from a TDA2030, effectively boosting the wattage from 15 watts into about 30 watts. And then I had an idea: the Pathfinder 15 and the discontinued Cambridge 15 amps (hybrid valve/solid state design) shared the same cabinet and chassis size. Theoretically, I could just swap out the Pathfinder chassis and pop in the Cambridge one and be off to the races, so to speak. The Cambridge 15 is
outwardly physically identical to the combo version of the Pathfinder
15. It's a 1x8" amplifier with a small Celestion "Bulldog" speaker, but
the Cambridge uses a 12AX7 valve/vaccuum tube in conjunction with the
usual opamps and diodes in its preamp stage. Theoretically, at least,
valves impart more musical overdrive and distortion characteristics
than transistors do, and they also add a bit of harmonic colouration to
overall sound. In practice, the difference between the Pathfinder and
the Cambridge are not night
and day: they are quite similar. If anything, the Cambridge has a bit
less fizziness in its overdrive, and is a bit more lively sounding, but
the Pathfinder has a superior clean sound.
ModificationsIn any case, I acquired a used Cambridge 15, and immediately set out to plan swapping amp electronics, but then realised how big the Cambridge's cab was, compared to its tiny little speaker. This got me thinking. What if I kept the amp intact, but performed the power amp upgrade, and swapped the speaker for a good-sized 10"? Crazy-talk, really, but screw sanity. The first thing I did
was to pick what might be the most powerful 10" guitar speaker known to
man. Eminence makes a line called "Redcoat," meant to emulate the
"British" sound, and the Ramrod is a 10-inch speaker in the Redcoat
line, rated for 75W, with a sensitivity of 100 dB (the 8" Celestion in
the Cambridge is probably 92-93, and the 10s in the Pathfinder bottom
are likely 94-96).
To make the speaker fit, I had to create 4-5 mm more depth in the cabinet, and shape one of the reinforcing pieces of wood a bit. The dremel and my leftover mahogany veneer from the September 2004 guitar project came in quite handy :) The power chip upgrade was a straightforward job, as the TDA2030 and TDA2050 chips are virtually identical. I ought to read through both spec sheets more carefully to see if I need to change the resistance and capacitors in the power amp circuit to better bias the mess, but at the moment, I'm feeling lazy, and I don't really relish the idea of tracing a PCB circuit without a schematic. I also clamped on an extra piece of heatsink metal, just for good measure. Finally, I swapped the stock Sovtek 12AX7-WA valve out for a JJ ECC83S. A touch warmer, but again, not a night-and-day difference. (update: I now use a JJ ECC803S). Some notes:
The Body of Evidence Sound sample (overdrive on the
first bit, and the delay on the second is supplied by my Zoom 606)
 Notice the size of the Ramrod compared to the 8" Bulldog. The Ramrod is a beast.  Ready for reassembly  Close-up of the mahogany shims I made by gluing ten layers of veneer together. I made eight of these shims in total!  In action
Results The amp is now a
completely different beast, and in a band rehearsal situation, it's had
no problems keeping up, with plenty of headroom. Most importantly, it
sounds pretty damn sweet. Good things do come in small packages.
DisclaimerThis is not a detailed "how-to" guide on modifying a Cambridge amp. No guarantees, etc, are made for anyone who wants to try this for himself! |
