ivanrichards

the Pro Tube lead 100 head

Hello and welcome to the blog. Once again we have completed major repairs to a Laney (UK) “AOR Series” amplifier – model name Pro Tube Lead, this time a 100 watt head for customer Ben Rabey. We previously published blogs on 14/5/2013 and 10/7/2012 describing repairs and MODs to Laney amps in detail, including the AOR Series 50 watt combo, and also the LC30-II combo.

Pro Tube Lead 100 head sitting on a Hame speaker cab

The fault description from our customer was that the amp plays OK for a while, and then the output drops markedly ! Upon removing the chassis from its sleeve, the first things we noticed were that the mains fuseholder was loose and could not be tightened satisfactorily, plus the amp was seriously under-biased resulting in a much higher than normal current draw by the quartet of EL34 output valves (tubes).

removing and reinstalling the board is quite a task, but has to be done !

We replaced the fuseholder altogether with a current production unit that complies with contemporary electrical safety standards. We also installed the correct value HT fuse – T1A. Following this, the amp passed electrical safety testing to Workcover NSW standards. This is all important stuff, and should never be overlooked. We tested the quartet of EL34 valves and all the 12AX7 preamp valves externally to the amp with no apparent problems revealed.

access to much of the wiring is under the board, but available space is very tight !

After re-installing the valves (tubes), we rebiased the EL34’s to a sensible current draw. The amp tested both very low and very distorted output signal into a dummy load. We observed a good drive signal to the EL34’s from the phase-inverter stage, with a signal injected at the FX Loop ‘return’ jack, so we had every reason to suspect that the output transformer had broken down internally. We quoted Ben on replacing the output transformer plus other tasks, and the project proceeded.

the new output transformer installed on the Laney chassis

As we are an authorised repairer for Marshall amplification, we therefore have access to the full stock of Marshall spares for current and vintage models. It just so happens that the 100W output transformer for the JCM800 series, designated C2668 (by Dagnall Electronics), is a perfect match and perfect fit for the 100W Laney. This is hardly a surprise, as the Laney is very obviously based on the JCM800 design, but with additional gain and additional features.

repairs completed and board reinstalled

Getting access to the wiring to carry out this task is not easy, as much of the wiring passes underneath the single large printed circuit board (PCB). Definitely not mil-spec wiring, but the JCM 800 amps are exactly the same (although their wiring is somewhat tidier). Removing the board requires removing a number of fasteners, plus also removing all the front panel controls, as all these controls are hard-wired to the board, as you would expect.

new ‘touch proof’ mains fuse installed

The replacement C2668 transformer wiring of course has a completely different colour code to the original Laney unit, which required some additional investigation to complete the installation successfully. While we had both sides of the board exposed, this was the perfect opportunity to replace the bias supply filter caps, plus some of the low-voltage supply components (as per the previous Pro Tube Lead repair job). You can see a burn mark on the component side of the board under a power resistor in the low-voltage supply, so this appears to be a common problem in these amps.

rear view Laney 100W head

For those interested in technical matters, this amp was serial # 2685, and appears to have been built in 1988. After installation of the new transformer and rebias of output valves (tubes), we had a high-tension voltage supply of just under +470V, and a bias voltage of -38V. The amp delivered 43V into 16 ohms @ the onset of clipping = 115 watts. Was the blown audio output transformer a direct result of the under-biased output valves ? We will never know – sometimes these are just random events. Many thanks to Ben for his continued custom. As always with these models – once set up and running properly the amp sounded huge !   IR.

the Sportsman logo

We have had a relationship with Carr amplifiers of North Carolina, USA, going back several years to when the former Bondi Intermusic Australia began importing these amps. Of course Bondi is long gone, so therefore we are no longer the national warranty service agent as the brand has been taken over by another distributor. However, with the full approval & support of Carr USA, we are still providing post-warranty after-sales-service to the entire range.

the Carr Sportsman 1×12 combo

We have previously published blogs re the Carr Artemus and other models in the range. The Sportsman model is a relative newcomer to Australia, and this is our first servicing experience with this model, for new customer Matt. The design of the Sportsman follows on from Carr’s most successful models, being somewhat like a hot-rodded “blackface” Princeton Reverb amp. That’s the best way we can describe it, although there are some differences, particularly in the phase-inverter and power amplifier circuits.

the Sportsman front panel

The power amplifier is designed around a pair of 6V6 output valves, employing a form of compound bias – combining both fixed bias & cathode bias, with a power output @ the onset of clipping of about 16 watts into a 16 ohm load. This arrangement seems to work well in practice, although when Fender (as a division of CBS) tried something similar in the late 1960’s the results sounded so bad they had to implement a redesign of the “Silverface” models.

the Sportsman from the rear

The power supply uses solid-state rectification, as opposed to valve (tube). The “headroom” control is actually a master volume with some tone shaping, following on from the final preamp gain stage. As per the Two-Rock amp discussed in the previous blog, this control is intended to manage the gain structure of the amp, rather than introduce distortion, hence the “headroom” label.

Sportsman chassis & speaker

The reverb on this amp is exemplary, which is a real contrast with the many current production guitar amps offering really poor reverb !! There is no tremolo on this model. The chassis is completely hand-wired. Matt reported that although the amp was still functioning, the sound of the amp had deteriorated markedly. We examined the interior of the chassis looking for any signs of component failure or drift, but chassis checked out A-OK.

We carried out a full re-valve and once more the Sportsman was delivering the kind of attitude that Matt was missing. The one and only concern we might have with this amp is the minimal ventilation for the horizontally mounted 6V6 output valves, which are biased to run quite hot. We have to thank new customer Matt for providing us with the subject matter for this blog. I think the only remaining question for Australian guitar players would be – could you buy an amp with a duck on the front panel ??    IR.

Two-Rock EXO15

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.

Two-Rock EXO15

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.

6V6 powered EXO15

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.

5AR4/GZ34 rectifier

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.

under the chassis view from the rear

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.

under the chassis view from the front

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.

ventilation grille at the top of the reassembled enclosure

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.