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Those of you who are regular readers of our blogs will know that we have visited the subject of providing quality service to Rivera amplifiers (USA) in a couple of previous blogs. This time, we are not discussing a fault as such, but addressing a design limitation with a (relatively) simple MOD.

As we have discussed previously, the classic Rivera amp models have two discrete preamp channels, one with ‘British’ voicing, and one with ‘California’ voicing, plus master reverb, FX loop and prescence controls. Both channels have gain boosts, activated from ‘pull-on’ functions on the front panel controls. On the USA (green LED) channel this is known as ‘Ninja Boost’.

The Ninja Boost feature when activated provides a substantial gain boost. In a live performance situation, activated from the footswitch unit, this gain boost is actually unmanageable, which is the whole point of this blog. We were recently approached by a local Central Coast (NSW) rock guitarist, Mr Greg Taylor, to provide a MOD so that the level of boost can be preset to a more appropriate level from a suitably located additional control.

The gain boost in the USA channel takes place between the 2nd & 3rd gain stages, where an interstage attenuator is partially bypassed by a fixed resistor when the boost is activated. Each gain stage uses one half of a 12AX7 twin-triode valve (tube). This attenuator and associated switching circuitry is located on the main board.

Inspection of the chassis and board revealed that we could snip one end of the abovementioned fixed resistor, close to the input grid of the 12AX7 gain stage. Having done this, we could run a pair of shielded cables to a pot located in a spare position on the rear panel. The pot is wired up as a variable resistance in series with the existing fixed resistor, meaning that only 2 of the 3 lugs are connected. Thus we control the amount of boost by varying the insertion loss of the attenuator circuit. Because this is a series attenautor, as opposed to a volume control, a pot with a linear or reverse-logarithmic taper is more suitable than a logarithmic taper, giving a smoother sweep.

We use a high quality shielded cable (eg, Canare), rather than ordinary hook-up wire, so as not to introduce any additional hum & noise when the boost is activated. The shields of both cables are earthed (grounded) at one end only, in this case to the body of the pot, to avoid creating an earth-loop (ground-loop). Heatshrink is used at both ends to prevent accidental short circuits, and also to create a stable connection to the resistor that we snipped at one end – the other end of this resistor is still connected to the board. The whole job is neatly routed into the existing wiring and held in place with nylon cable ties. Although we have performed an intrusive modification, the amp has not been degraded in any way, from the perspective of reliability or additional noise.

So how did the MOD perform in the final play test ?? The amount of boost is now variable from a just perceptable boost at minimum (anti-clockwise) setting, to the original level of Ninja Boost at the maximum (fully clockwise) setting, all in a smooth, linear sweep preset from the rear panel. Thanks to Greg Taylor for his continued custom and for the many challenges he has provided us over the years ! Coming soon we have a blog about the legendary Wasp amplifiers, modding current Fender amps for blues-harp players, and mods to the Fender Blues Junior amps.

Hello everybody ! Welcome to this week’s blog. It’s been a busy October long weekend 2011, with gigs for the 5-piece band plus the duo, so this will be one of the less detailed blogs, but hopefully still of interest.

Actually, we have been repairing Orange Amplifiers for years, but not the warranty claims. Bondi Intermusic (Sydney) have sent us quite a few over the years for servicing. With the possible exception of the made-in-China Tiny Terror series, the Orange amps have been consistently well made and have presented very few servicing issues. The modern Orange amp has a particular sound, which is quite different to the original amps from the 60’s/70’s, which had a huge clean sound, and baxandall EQ, which is not everybody’s cup-of-tea. The recent amps have more conventional passive/interactive EQ and a contemporary “British” sound that is more appropriate for grungier styles ( to our ears anyway).

If you are the lucky owner of an original Orange or Matamp from the 60’s/70’s (and most definitely the current Matamps, which are an excellent boutique-grade amp from the UK), and you want your amp serviced/overhauled/restored by someone who knows what they’re doing and has a genuine passion for this kind of work, then please contact Ivan Richards Audio.

Orange amplifiers are distributed in Australia by the same people who distribute Gibson & Epiphone guitars. If you have a warranty service claim there is a procedure to be followed. First thing is to contact the dealer who sold you the product. They will book it in with the distributor. A repairer to resolve the warranty claim will be nominated by the distributor on the basis of both competency and geographical location.

The AD30TC 2×12 combo amplifier in the photos belongs to a regular customer of ours, a working muso (with a day gig), based on the Central Coast of NSW. The almost brand new amp just “stopped”, and the warranty repair was referred to us via Turramurra Music (Sydney). The reason soon became clear after removing the chassis from the cab (not a quick job, by the way) – one of the internal fuses was “blown”. The fuse protecting the 6.3V AC heater supply to the valves (tubes), to be precise. However, in this case the blown fuse was not indicative of a bigger problem, it was just one of those things that happen, usually at the most inconvenient time. We replaced the fuse and subjected the chassis to a power output test, burn-in test, electrical safety test, and final play test/functionality test. All OK !

In the final photo, it may not be very clear from our crap photography in workshop lighting conditions – but we are observing a sine wave on the oscilloscope, as part of the power output test for this amp. This is one procedure for checking for abnormal amplifier behavior under load and also accurately measuring the audio power actually delivered to a nominated load, for example – 8 ohms.

By the way, just a suggestion – if you have a bad back – don’t invest in one of these 2×12 combo’s ! They are seriously heavy. More next week – Ivan R.

This week’s blog examines a major repair completed this week to a MusicMan 130W 2×12 combo, purchased brand new from Harry Landis Music in Sydney (Australia), and still owned by this customer. These 1970’s manufactured MusicMan amps were among the best of the American amps of that time, and were certainly built to last the distance. They were built to a very high standard by people who knew what they were doing. Probably the most popular and most collectable model was the 65W 2×10 combo.

The highly reliable and great sounding MusicMan chassis was let down to some extent by pretty average speakers, especially in the bass guitar models. You will often see that the original speakers have been replaced, sometimes quite early in the working life of the amp, or in the case of the amp heads you will almost always see them hooked up with a different brand of speaker cab. Not surprisingly, the MusicMan amps strongest appeal was their great “clean” tone, reminiscent of the very best of the Fender tones. When it comes to overdriven sounds………..forget it !  You will need your pedals for that.

What style of guitarists love their MusicMans ? Most probably those playing country rock, rockabilly, old-school rock and blues and any traditional roots music styles. The 130W model is clean enough for keyboards, too. This particular example was damaged some years back at the Goulburn (NSW) Blues Festival. It was quickly patched up to get through the gig, but this amp as is would never pass an electrical safety inspection that meets Workcover NSW compliance.

You can see from the thumbnails there are quite a number of outstanding service issues to be resolved. Firstly, there was some doubt as to whether the power transformer itself was 100% functional, as it had temporarily been rewired for half-power only. The wiring and general internal appearance of the transformer looked pretty crusty. I had to suss out the colour code for the primary connections in order to make any sense out of the situation.

Having achieved that, the first step was to replace the damaged 3-core 240V flex and 3-pin plug, and to make a new chassis earth connection which would achieve compliance testing. This involved rewiring the fuseholder as well. Beware: the fuseholders in some older amps are not wired in a way that meets electrical safety standards, ie they are wired so that the outer part of the fuse remains active, even if the fuse is blown. If this is the case, the wiring should be reversed. The fuse should also be wired in series with the 240V active (brown), NOT the neutral (blue). Refer all such issues to an experienced and trusted technician !

The next step was installing a new power switch on the front panel. This is actually a 3-way SPDT toggle switch with a centre-off position, labelled HI/OFF/LO. What is actually happening is the incoming 240V AC can be switched to either one of two primary voltage taps, which gives higher or lower internal DC voltages to power the EL34 output valves (tubes). In all cases we insulate the high voltage connections with heatshrink.

The unusual design feature of these amps, apart from a completely solid-state preamp, is the fact that on the HI power setting, the EL34 output valves have +700V DC on their anodes, and +350V DC on their screen grids. You cannot install cheapo valves in this amp and expect them to last very long ! Stick with brands that have a proven track record in reliability, for example JJ are a good choice for this amp. The next step was to replace a number of electrolytic capacitors in the high voltage (HV) and low voltage (LV) power supplies, all of which were still original and starting to look very crusty indeed.

The HV supply requires a pair of 100uF/500V capacitors in a voltage-doubler configuration for the 700V supply, and a pair of 22uF/500V capacitors after the filter choke for the screen grid and phase-splitter supplies. For the bias voltage supply we needed some 100uF/100V capacitors. The HV supply caps are mounted in their own “can” on the top side of the chassis.

All the capacitors were bedded down in some silicone to keep them secure and to take the strain off the leads. For DIY people out there, always make sure that you use “neutral cure” silicone, ie non-corrosive, or you might be in for a nasty surprise in the long-term.

Equally as important as the HV supply capacitors, is the replacement of the LV supply caps, which in the case of the MusicMan amps, shares some common circuitry with the bias voltage supply for the EL34 output valves (tubes). Rock-solid biasing of the output valves will depend on the quality of the bias voltage supply.

Well, to wrap it all up, I cleaned up the input and output jacks with DeOxit, and installed a JJ 12AX7-S in the phase splitter valve position, and installed a matched and burned-in quad of JJ EL34 valves in the power amp sockets. I set the bias for the EL34’s for 25ma cathode current each at 700V HT, as per the original spec. This is quite adequate at 700V. A power output test and burn-in test is then carried out – the amp is hooked up to an 8 ohm dummy load and a signal generator connected to the input jack. Observation of the sine wave output into 8 ohms reveals a clean signal and a power amp that reaches the onset of clipping at approx 120 watts rms. The power amp drops into “class-B” as we get closer to maximum power output, as expected.

The final play test was fine – the reverb and tremolo both work OK, although the solid-state reverb is not comparable to the valve-driven reverb on a Fender Twin, of course. You may notice the customer had removed the MusicMan logo (which was a bit toy-town looking), and the top strap handle, which he didn’t trust to carry the weight. The original speakers were replaced long, long ago with a pair of Etone 12-inch speakers, manufactured in Sydney. As a proud Aussie, I would like to say that the Etones sound great, but the reality is these speakers do absolutely nothing for this amp. They sound rather flat and overly bright. In order to demonstrate this to my customer, I played a TELE through the amp as is, then hooked up my workshop speaker cab, which is loaded with a 100W 12-inch Jensen NEO, and the difference in tone, complexity, and sheer volume was quite obvious. Perhaps this will be the subject of a future amp/speaker upgrade ?? See you next week – Ivan.

Yes, we specialise in the service & repair of boutique, ie hand-wired, all-valve (tube) musical-instrument amplification, including Dr-Z from the USofA. The subject of this week’s amp repair blog is a Dr-Z MAZ38 (Senior) 2X12 reverb combo, shipped up to our Wyoming (NSW) workshop from Sydney by a regular customer.

This particular MAZ38 was blowing fuses. There are actually two fuses fitted on the rear panel, one is the 240V mains fuse, the other is a secondary fuse for the high voltage supply, and would normally be around the 0.5A (500ma) value. In this case the mains fuse was blowing, although when I examined the HT fuse I found an 8A fuse rather than a 0.5A fuse, which would result in negligable protection.

The fact that the mains fuse was blowing tends to suggest a fault in the power supply, and this proved to be correct. The fault was isolated in a series of diagnostic steps to the power transformer itself, which had developed an internal short-circuit. The quad of EL84 output valves was still serviceable, but the GZ34 rectifier valve was definitely short-circuit. Was the GZ34 the original problem, or the transformer ? We shall never know, but this was developing into an expensive repair job for the customer.

Removing the faulty transformer and installing a replacement was going to be a tricky operation, as access to where wires need to be disconnected and subsequently reconnected is limited. Most of the internal space within the chassis is taken up by a single large board holding the majority of the resistors and capacitors, with hand-wired connections from both sides of the board. The standard of construction as well as components is high, as you would expect, but access to some components for service is poor.

It turned out upon investigation, that the power transformer is rated at 230V AC. But, here in OZ our mains voltage is actually 240V nominal, but is often quite a bit higher than that, especially over in W.A. There was going to be a delay sourcing the original transformer from the factory anyway, and we wanted to move quickly on this job, so at the customer’s suggestion I selected an after-market replacement power transformer. I chose to install a Vox AC30 replacement manufactured in Chicago, Illinois USofA, by Magnetic Components Inc, and marketed under their “Classic Tone” series. I have sourced a transformer from this manufacturer previously with success, so I had no hesitation to use them again.

I originally read about Magnetic Components in TQR, by the way. Check out the transformer photos ! This monster is spec’d as per the original Vox AC30’s, but most importantly it is rated for 240V AC, by means of connecting the two 120V primary windings in series. In spite of the imposing bulk of this transfmr, I was able to locate it on the existing MAZ38 chassis, with a couple of additional mounting holes, and nylon grommets to feed the wires safely through the access holes. After making all the necessary connections, which included adding a pair of 100 ohm resistors to provide a centre-tapped ground connection for the 6.3V AC heater supply, and also installing a new JJ GZ34 rectifier valve (tube), we powered up the amp without any further dramas.

The amp chassis was subjected to the usual range of quality-control testing: power output test into a resistive load, burn-in test, electrical safety test (to Workcover NSW standard), and final play test into the amp’s own speakers. I was pleased to find that the new power transformer remained relatively cool during all this, one obvious advantage of bulky 50’s/60’s spec’d units with a lot of iron !

Many thanks to Shane for his continued custom and for providing us with this week’s amp repair blog. Please visit us again soon for more amp and pedal blogs, we try to create a fresh blog every week, time permitting.

Regards, Ivan.

A pair of Fender “Champs” arrived at the workshop this week for servicing. Actually, one of them is a Silverface Bronco amp, which to all intents and purposes is a Fender Vibro-Champ. The other is a current manufacture (ie, China) Champion 600 amp. Both amps have very similar electronic design excepting that the Vibro-Champ has valve (tube) driven Tremolo, and a valve (tube) rectifier, whereas the Champion 600 has solid-state rectification, printed-circuit board construction and a preset tone-stack so there is only a volume control presented on the front panel. The Bronco has a much larger cabinet, which helps enormously to develop a bigger sound.

The Bronco needed a fairly comprehensive service. This not only included replacement of all valves (tubes), but also damaged pots (potentiometers), ageing electrolytic capacitors, and a 9-pin valve socket which was no longer providing reliable connection. Our usual electrical safety standards inspection and testing revealed that the 240V 3-pin plug was due for replacement, too.

The Chamion 600 was powering up but nil output, ie the lights are on but nobody’s home, as they say. The cathode-bias resistor for the 6V6 valve (tube) in this amp is a little under-rated for the job, especially if the 6V6 suffers a catastrophic failure, so I replaced it with a ceramic-body 7 watt resistor intended for the most demanding environments. I use these resistors where appropriate in my own Richards custom amps. The 2 x boards checked out OK, as did the jacks, interconnects, and so on, but the 2 x valves (tubes) were cheapo examples of Chinese origin, and the volume control was very scratchy indeed, so these items were replaced.

Both these amps have small, very cheapo speakers, so naturally the amps sound much bigger and better through a decent quality external speaker. However, many people love how compact and lightweight these amps are, and of course 1000’s of recordings over the decades have been done with the Champ amp. So in the final shoot-out, how did these amps compare ? The basic tonality is very similar. They common circuit design is quite low gain, so you have to turn up the volume control most of the way to get much of a vibe happening. The Bronco/Vibro-Champ has a bigger sound with greater prescence and throw, and the advantage of 2-band EQ, but most of all it has a great valve-generated tremolo which is superior to any stompbox tremolo that I have heard so far. The tremolo is of the direct-modulation configuration, which varies the bias and gain of the 2nd stage of the preamp 12AX7 valve (tube). For tremolo lovers, it doesn’t get much better than this.

For all owners of small valve (tube) amps such as those described above, I offer world-class professional amp repairs from my workshop located in Wyoming (Central Coast) NSW. We have couriers delivering and picking up to this site every working day, so please consider the level of service and value add that we can supply. That’s all for today’s BLOG, please check in every week or so for more news and info. regards – Ivan.