Hello and welcome back to the blog after a long break. We are now offering world-class (non-warranty) service & repairs to the range of valve (tube) amplifiers from Two-Rock guitar amplification of California, USA. We should point out that we have no official relationship with Two-Rock Amplifiers, so we cannot accept any warranty repair claims without the endorsement of their Australian distributor.
We recently serviced the Two-Rock EXO15 head (serial # 62) for Sam, one of our regular customers – the amp is featured in the accompanying photos. Sam advised us that the amp ‘just stopped’, which is a fairly common amp fault description, and in most cases would suggest a valve (tube) or other component failure, or at least a blown fuse.
The EXO15 is a single-channel, 15 watt 6V6 powered and valve (tube) rectified class AB amp, designed in an all-metal enclosure, with a pair of 12AX7 preamp valves (tubes), one of which functions as the phase-inverter. The preamp design is somewhat similar to the “clean” channel in a Dumble guitar amp, as per the Two-Rock heritage, and the 3-band passive EQ is also voiced along similar lines.
The master-volume is placed directly after the 2nd gain stage, and is therefore intended more for managing the gain structure than generating massive amounts of distortion (a design approach which we endorse). The master feeds a passive FX Loop, for time-based FX devices that can accomodate line levels. In spite of the name & description, the “contour” control is in this case a simple passive high frequency roll-off, as per the vintage VOX AC15/AC30 amps. We heartily endorse the inclusion of a GZ34/5AR4 valve (tube) rectifier in an amp of this power rating.
Anecdotal evidence from the customer suggests this hasn’t been a particularly reliable amp. As this is our first service job on this particular amp, we are not familiar with its previous service history. Assembly is a combination of circuit boards and hand-wiring. All connections to valve sockets, jacks/pots/etc are hand-wired which of course we endorse. Components are of commercial standard but not “boutique” standard, with no apparent design problems.
We replaced the blown mains fuse, and also the output valves & rectifier valve just to be on the safe side. Our initial choice of JJ 6V6 output valves was not a success, as they are too tall for the cover to fit back on !! We ended up installing a matched pair of 6V6GT by Electro-Harmonix, which have been pretty reliable at the voltages within this amp (+440V DC in our Wyoming NSW workshop). We rebiased for a sensible current draw with the 6V6’s (this may have been the problem all along ??), and measured the power output @ the onset of clipping = 22 watts.
We were looking for any other factors which may have contributed to the unreliability factor, at Sam’s request. When we looked at the speaker output jacks (4, 8 & 16 ohms), we were really disappointed at the nondescript quality of the jacks installed, and their current condition. We installed a trio of the very fine Switchcraft jacks, which grip the speaker plug very firmly for a positive connection. H/R (high resistance) or O/C (open circuit) speaker connections can cost you not only your output valves, but potentially also your output transformer. Always use the best available jacks & plugs for such critical connections.
Thankfully, no further problems have been reported with this amp. Many thanks to Sam for his continued custom, and for supplying us with the subject for this week’s blog ! IR.