ivanrichards

Welcome to the blog ! This week we are taking a brief look at a relatively obscure amplifier here in Australia: the Evans – designed & manufactured by Evans Custom Amplifiers of the USA. This is not to be confused with a much cheaper Evans amp of Asian origin, possibly from the 1970′s.

Evans Custom Amplifiers are still being built in Burlington, North Carolina, and appear to be a small operation with an emphasis on quality products for specific applications in the guitar playing scene. We can definitely relate to that ! The Evans amp is obviously primarily designed for the pedal-steel player, and also jazz & country music players. The designs are 100% solid-state and offer a very clean sound with a lot of headroom.

This particular example is the JE 1×12 combo, presumably of 1980′s manufacture, judging by the syle of assembly. Our customer brought this amp to the workshop for a general service, the jacks & pots were cutting in & out, and so on. The amp chassis is very well laid out for service & reliability, in the 1980′s style, and different modules within the amp can be isolated from each other in the case of a major repair. Very few solid-state amps are made like this these days !

This amplifier is rated for 200 watts into 4 ohms, or 250 watts into 2 ohms. Very powerful and weighs half a ton ! With all this clean headroom and powerful EQ, an obvious contender for the pedal steel. The internal 4 ohm 12-inch speaker is responsible for most of the weight. The magnet is huge. The speaker appears to be an OEM design from Eminence, and would have to be rated for 200 watts.

We replaced all the front panel jacks & cleaned the pots with Faderlube. There is a potential electrical safety issue with older USA amps which incorporate a ground switch, or, as in this case, a 3-way on/off toggle switch which has two x on settings & a centre off position. The two ‘on’ positions switch in a capacitor from either active or neutral, to ‘ground’, ie to the chassis. This arrangement might be OK at 110V AC (?), but is extremely dodgy here in OZ, and does not comply with Workcover NSW regulations. This ‘ground’ capacitor should automatically be removed from any such amps.

Sure enough, when we initially plugged the amp into the 240V socket, our workshop RCD device tripped, shutting off the power. This indicates there was some current flowing to earth (ground), probably the ’ground’ capacitor was faulty. We ripped out the 3-way on/off switch & the ground cap, and installed an industrial quality double-pole 2-position (DPST) on/off toggle switch. Subsequently the amp was tested for electrical safety to the Workcover NSW standard successfully. The amp passed its power output test & final play test with ease. Many thanks to Andrew W. for his custom, & for supplying the subject matter for this week’s blog. IR.

Welcome back ! This is our third blog on the subject of service & repairs to Orange amplifiers (UK), here in Australia. This week we are discussing the AD200 model, a 200 watt rms amp head primarily intended for the bass guitarist, but actually this is one of those rare bass amps that sounds & records mighty fine for 6-string guitar as well.

This particular AD200 is serial # 035 and is signed by A. G. Emsley on 28/7/2000. We suspect that the initial production of this model was completely hand-wired, and that later production moved to p.c.b. technology, with a revised circuit design. This model has a two valve preamp & no negative feedback loop, the later models have a three valve preamp incorporating a negative feedback loop. The standard of wiring & assembly is exemplary, by the way.

The amp was brought in to the workshop by Dave from Sounds Like Sunset for a general service & checkover, having been dropped. There weren’t any signs of physical damage, but the amp did require a re-valve and some minor servicing. We tightened up a loose transformer and loose handle, and removed a couple of foreign objects from within the chassis. Remember, amps such as the AD200 will generally have around 700V DC high voltage supply, so attention to electrical safety issues is critical.

In any valve (tube) bass amp, microphony is a potential issue, so we test & select replacement valves appropriate to the customer’s intended application. We installed a JJ 12AX7 & 12AT7 in the preamp. This isn’t a particularly high gain design. This service job gave us the opportunity to try out the relatively new JJ 6550 power output valves, which have already received favourable reviews.

We installed a matched quad of JJ 6550 and rebiased the amp for 25 milliamps quiescent current for each 6550. This is adequate with an anode voltage of (in this case) 717V DC. A power output test delivered 29V into a 4 ohm resistive load at 400 Hz, which is approx 210 watts. We also carried out an electrical safety test to Workcover NSW requirements.

Well, to finish up – the JJ 6550′s did indeed live up to expectations and we have every intention of using them again. They appear to be very well made and sound very good, although we haven’t actually made any comparisons with n.o.s. 6550′s. We have had intermittent reliability issues with the JJ 6L6GC, and sometimes the JJ EL34, to the extent that we now have switched to other brands for those valves. However, other JJ models have been very reliable.

Many thanks again to Sounds like Sunset for their continued loyalty. If you are a proud owner of an AD200, we can offer you world-class service/repairs & advice re your amp, we are here in Wyoming (Gosford), just north of Sydney. If you are a bass player who loves warm, valvey old-school tones, then the AD200 may be worth checking out. Regards, Ivan.

One of the services we happily provide to our customers is the implementation of widely publicised MODs to the Blues Junior and also the Pro Junior amps by FMIC. Indeed, modding the Blues Junior has turned into a growth industry the last few years ! Having completed any number of such MODs, we are in a position to advise customers as to the best choices for their particular requirements. You can understand that the law of diminishing returns applies to modding amps, whereby customers can spend a lot extra for only a slight improvement in tone.

Nevertheless, both these amps are relatively inexpensive to begin with, so spending a few hundred $$$ bringing them up to performance standard is easy enough to justify. Our original approach to modding these amps, long before we had even heard of the Bill Machrone mods & website, was simply to install better valves (tubes) for a warmer tone, to sort out the instability issues (due to bad layout & lead dress), and to alter the fixed bias network to reduce the quiescent current drawn by the pair of EL84 output valves. We have never been happy with the Groove Tubes 12AX7 valves, for example, and some of these amps were biased so hot that the EL84 9-pin p.c.b. mounted sockets were desoldering themselves !

We heartily endorse many of the Bill Machrone (or BillM) MODs, in particular the power supply capacitor MOD, the signal path capacitor MODs, and especially the adjustable fixed-bias MOD. The replacement of the power on/off toggle switch with a 3-way toggle that gives off/standby/on is a great idea on any valve amp with a solid-state rectifier which is missing a separate standby switch, and is definitely recommended.

Many of the other BillM MODs could not be regarded as absolutely essential, and should be assessed individually as to whether they would enhance your own use of this amp. Some options include additional front panel controls, such as prescence and sparkle, an alternative boost function, and upgraded transformers – even conversion to 6V6 or 5881 operation. For owners of the earlier versions with input jacks that are prone to breakage, the Switchcraft metal input jack could be a worthwhile option.

The most significant upgrade to either amp would be replacement of the stock speaker, a 12-inch driver in the case of the Blues Junior, and a 10-inch driver in the case of the Pro Junior. The most obvious choice would be one of the Weber VST models – we can advise you on this at the time we implement the chosen MODs, as there is a bewildering array of models to choose from. If you are a jazz guy, we also recommend one of the Jensen NEO models, for their balanced frquency response and high power handling, late break-up.

The starting price for the basic MODs is $180.00 AUD parts & labour. We have allowed 2 hours maximum for this. For example, if you ordered every single MOD available, we would have to allow for at least 5 hours labour content. We can quote you re your chosen options, and we can order the recommended speaker option on your behalf. Customers of note who have had their amps modded include James Black of RocKwiz Orkestra fame & one of Australia’s most in demand musician/producer/musical directors of the last three decades, Illya Swec (blues guitarist based on NSW South Coast, solo career plus Robert Susz Continental Blues Party), Isaac Hayward (successful Sydney based guitarist/arranger/musical director, including The Rock Show with Jon English), and Paul Disspain (musical director of any number of NSW Central Coast music projects).

Here is another rarity, send to us for a complete overhaul by Bondi Intermusic – the Fender Blackface AA763 Vibrolux amplifier. Manufactured in 1963 ?? This model features normal & vibrato channels (it’s actually tremolo), with a valve (tube) complement of 3 x 12AX7/7025/ECC83, 1 x 12AT7/ECC81, a pair of 6L6GC output valves, plus a GZ34/5AR4 rectifier.

The single 15-inch speaker in this combo does not appear to be original, and has no apparent identifying codes/numbers, so is a bit of a mystery. The amp was in a filthy state as delivered, plus the chassis and front panel have a corrosion problem, so the amp has probably been stored somewhere damp for years on end. This amp has the potential to sound fabulous with a good speaker unit on board, and at 40 watts is ideal for today’s lounge gigs.

After a bit of a clean up, the first and most significant problems to be resolved are all about electrical safety ! This amp was brought into the land of Australia many years back, with a chassis wired for the USA mains voltage only. Somebody mounted a stepdown transformer in the bottom of the cabinet and wired up a pretty dodgy earth connection to this transformer – the whole arrangement would never pass the Workcover NSW electrical safety inspection, and quite frankly was an accident waiting to happen.

Our recommendation to the customer (Mark), via Bondi Intermusic, was that we remove both the stepdown and the original USA power transformers, and source a reproduction Vibrolux power transformer with an “export” primary, ie the primary winding has several wiring options for global power. This recommendation was accepted, and we installed a transformer made by Heyboer (USA), supplied by Mojo Musical Supply Co. A new 240V 3-core flex & 3-pin plug was also installed and the mains power wiring within the amp chassis rewired to an acceptable standard.

Once all the above was completed and tested, we could get on with tonal improvements. We replaced the five cathode bypass capacitors with Sprague 25uf/25V units, as per the photo. This will help bring the 12AX7 gain stages back to life. Next, we replaced the five ageing power supply capacitors with Sprague 16uf/475V units, as per the original spec, the first pair are paralleled giving 32uf for the main reservoir capacitor, which is well within spec for a GZ34 rectifier. These five caps are bedded down in some silicone (neutral cure), so the axial leads will not be stressed as the amp is bumped around, or loaded in & out of motor vehicles, etc (see photos).

All the amp’s pots (potentiometers) were cleaned/lubricated with Caig Faderlube, and all the jacks cleaned with DeOxit. It’s amazing that in an amp that’s nearly 50 years old – after this basic level of servicing, all the original pots & jacks work just fine ! If only they made stuff this well nowadays.

Someone not known to us has previously replaced all the 100K ohms anode resistors – no doubt they had become noisy. Most importantly, the bias voltage supply was repaired, with a new capacitor and the original resistor values restored. Now, we can install some new valves, as appropriate, and have a listen to this amp.

A couple of the valves tested fine, so we ended up installing a new 12AX7, a 12AT7, a JJ GZ34/5AR4 rectifier, and for the first time we installed a matched pair of the Tung Sol reissue 6L6GC output valves, which have received favourable reviews in the guitar media. We rebiased the 6L6′s and carried out a power output test. The end result was perfect – 40 watts into 8 ohms at the onset of clipping.

Well, there we have it ! This wasn’t an inexpensive repair by any definition. Over 4 hours labour plus high material costs, but it will be worth it once Mark is cashed up enough to load a good quality 15-inch guitar speaker into this amp - he will be able to enjoy the full tonality of this fine early 60′s amp.  Hope you enjoyed the blog, please check in with us again.

ps> we connected the amp up to our current workshop reference speaker – the Warehouse (WGS) ET-65, ie their clone of the famous 65 watt Celestion from 30 years ago. This speaker is like a high powered “Greenback”, with more bottom end (for want of a better description). Well, the Vibrolux really came alive with this speaker – a LOT more volume, not to mention more bottom end, more tone, more everything. Recommended !

Hello readers, this week we thought we would introduce an amp to you that is pretty rare in these parts – the Matamp C7 head. Actually, any Matamps are rare in these parts. This amp is one of a pair of C7′s belonging to James, a regular NSW Central Coast customer of ours.

We have serviced both C7′s in the past (the other head is covered with black tolex), this C7 had stopped working altogether on this visit. It turned out that the single EL34 output valve (tube) had shorted out internally, blowing the HT fuse. There was no other damage at all, so replacing the EL34 and the fuse restored the amp to its previous good working order.

The C7 is a 7 watt amp based on a EL34/6CA7 power output valve, in single-ended configuration, operating in true Class-A mode. There is an all-valve preamp, consisting of three 12AX7 dual-triode preamp valves, although the reverb drive circuit employs a TL071 IC. The reverb recovery circuit uses one of the 12AX7′s. The inclusion of reverb in this class of amp is unusual, but unfortunately the quality of the reverb effect is not sensational.

What is sensational though, is the high standard of construction and assembly compared to just about any other similar style of amp on the market. The original Matamp company in the north of England goes back to at least the 1960′s, and the C7 is built to the same bullet-proof standards as the original 120 watt heads, which were held in high esteem at the time.

The use of an EL34 output valve, as compared to the more usual 6V6 or EL84 valves, plus beautifully over-engineered power & audio output transformers, gives this amp a much bigger sound than comparable single-ended amps. The original Matamps employed Baxandall EQ (2-band), and a 6-way rotary tone switch for fine control over the bottom-end. Today’s Matamp is voiced quite differently, actually much closer to the Hiwatt/Sound City/Roost amps tonality, with 3-band interactive EQ.

Other design features unique to this amp include a (passive) FX Loop, an active Attack control, which gradually bypasses the negative feedback loop for a bigger, looser tonality, or sweep the opposite direction for tighter & cleaner – this control more than makes up for the lack of a master volume. The Record Out jack on the rear panel automatically switches in a 15 ohm/50 watt resistor across the output for recording from the amp direct, without the need to be plugged into a speaker. The LED power on indicator is actually powered from the EL34′s cathode bias supply, so gives you a visual indication that the EL34 has warmed up and is operational.

Well, there we have it. A distinctively British-voiced single-ended valve amp that possibly won’t appeal to everyone, but we would say that any of the recent Matamp models are worthy of your consideration. Although most likely intended for the home-recording guitarist, this amp is robust enough for live gigging. We definitely congratulate Matamp for not selling out and for continuing to manufacture in the UK to their traditional high standards.

Hello, welcome to the blog. In the past few months we have completed major overhauls to a pair of 1970′s Musicman combo’s. You might remember we published a blog last year regarding a major overhaul to a Musicman amp, which still belonged to its original owner. That particular blog has received hundreds of hits and continues to be read on a regular basis, so we are now attracting Musicman repair & restoration work from around the place, which is very gratifying. We have always had a high regard for the quality of construction, and the serviceability of these amps. Nearly 40 years after they were released on to the market, they are still capable of giving reliable performance, they just need some TLC from time to time, like any valve (tube) amp.

Our first example is the 2×10 65 watt model, possibly from around 1976. This was sent up from Sydney by Bondi Intermusic with the stated fault:  lights up – no power. You can see in the photo that the model number badge is attached in the wrong place on the front grille. We discovered that this was done to hide a hole in the grille material.

It didn’t take too long to realise why there was nil output from this amp, even though the amp was powering up, and the valves (tubes) were clearly operational.  The low voltage supply had failed due to a short-circuit component, which means the preamp was no longer functional.

The completely solid-state preamp relies on + & – 16V DC rails to operate. These DC voltages are smoothed & regulated by a simple circuit which includes filter capacitors and zener diodes acting as shunt regulators. Examination of the various circuit boards suggested that all the power supply filter capacitors (ie, electrolytic), for both high voltage & low voltage supplies, plus the pair of zener diodes should be replaced.

As the photos indicate, the various electrolytic capacitors have now been replaced using the very fine F&T brand caps from Germany, bedded down in some silicone (neutral-cure) for best stability. There are actually three boards within the chassis requiring this work, plus the main high voltage caps mounted within a “can” on the outside of the chassis.

There were numerous other issues to be resolved, for example the amp failed the earth test, but the replacement of the 240V 3-pin plug fixed this. All jacks were cleaned with DeOxit and all pots were cleaned with FaderLube. The “deep” switch was cutting in & out, luckily we had a black plastic rocker switch available that was an exact fit. Nil reverb effect required removal & repair of the reverb springline.

You would have noticed from the first few photos that we have replaced one of the 10-inch speakers  with a new (reissue) Jensen P10Q, 40 watt alnico magnet unit. The original speaker had an open circuit voice coil (ie, it was “blown”), but the customer didn’t seem to be aware of this. We quoted on a couple of options, but the Jensen was selected as the most cost-effective option. We often find the original Musicman speakers have not lasted as well as the electronics.

Right at the end of all this work, one of the apparently OK GT EL34′s failed, so we ended up installing a new matched pair of EL34 output valves & rebiasing the amp. These amps run in class-B mode with 700V DC supplied to the EL34 anodes, so we bias the amp to run a bit cooler than with most amps. Don’t get too hung up about whether this amp or that amp is in class-A, class-AB, or class-B or whatever, these amps sound great and there is no obvious crossover distortion which would add an unpleasant, non-musical edge to the tone.

In the final photo for this amp you see the (original) main high voltage power supply capacitors in typical condition, well beyond their expected working life. We replace these with a pair of 500V DC can mounting style capacitors, rather than axial, because of their higher working voltage (2 x 500V caps in series = 1000V) and also their higher ripple current rating, which is a significant factor in a voltage-doubler power supply as per the Musicman amps. The caps are bedded down in some silicone so they are held firmly in place.

The 2nd of our two Musicmans is like the bigger brother of the previous amp, delivering 130 watts into 4 x 10-inch speakers. The chassis & control panel layouts are exactly the same, just a much taller cabinet, and we would think the date of manufacture is very similar to the 65 watt combo. This example came from Paul, a regular central-coast customer of ours, not only for an electronic overhaul but also to have the cab stripped back & recovered with an appropriate vintage-style black tolex.

As you can see, someone has recovered this amp (badly) with a ghastly 1970′s vinyl lounge/furniture upholstery, so the amp looks a bit of a joke. The original grille has also been replaced at some unknown point in time. The upholstery has been applied with contact cement – this stuff is very difficult to remove completely. Underneath the upholstery we found there was an earlier attempt at a “natural” finish cab, ie the cab has been stained and a top coat of semi-gloss applied.

What a nightmare ! So – every item of hardware, every little cosmetic item had to be removed, and the vinyl stripped off. The cab needed to be filled & sanded before applying the black tolex, and re-installing all the bits & pieces. The baflle was removed, the speakers and the reverb tank put to one side, so the baffle could be sanded a bit, repainted flat black, and a new grille cloth installed. The whole cab was re-assembled and you can see the end result which is very pleasing. The two rear panels were long gone, so Paul helped with some plywood offcuts and his trusty router.

This combo still has the original four alnico magnet speakers, which are in good working order. The chassis required all the usual service work: tighten loose transformer, replace 240V 3-pin plug, clean jacks & pots, replaced nine electrolytic capacitors in the low voltage & high voltage power supplies, replaced damaged master volume pot & knob, replaced quad of EL34 output valves & rebias amp.

The entire overhaul took well over 10 hours labour to complete, over a period of time, in amongst the many other amp orders etc going on. Take a look at the last two photos, the end result looks terrific ! We had to use Fender style vintage repro tolex & grille cloth, as the original Musicman stuff is long gone, but it looks quite convincing and such an improvement on the way the amp looked before. Paul had some of the original and rusted metalwork refurbished & re-plated which made a big difference cosmetically.

This amp develops its best sound at quite high volume, so is probably not appropriate for that restaurant gig ! It also makes a good bass amp, as long as you plug it into an external bass cab. Many thanks to Paul D. for his continued custom.

Just when we thought it was safe to close off the final paragraph, a third Musicman combo arrived at the workshop for repairs. Only recently acquired by a local musician, this model is a different beast from the previous two amps, introduced a few years further down the track. This model is a single channel design and includes reverb, two FX loops, and most unusually, a built in phaser (spelt phasor !?!?).

Well, the phaser is a rarely used effect nowadays, but still it works quite well. The two FX Loops cater to two different signal levels, but are almost never used by the owners of these amps. The most significant difference to the earlier models lies in the fact that both the preamp stages and the phase-inverter/driver stages in the later amps are completely solid-state, implemented using both IC op-amps & discrete TO220 package power transistors. The power amp employs 6L6GC output valves, as opposed to the earlier EL34/6CA7 valves, operating in pure class-B mode, delivering about 75 watts.

The way the valves are driven is also quite different, using small power transistors to drive the cathodes of the 6L6′s. The control grids are connected to the bias supply only. You will notice from the photo here that the speakers have changed from alnico to ceramic magnet. So it’s a slightly different sounding amp to the earlier version. This amp once agin needed a general service, replacement of a damaged pot, replacement of the output valves, and replacement of the pair of high voltage capacitors, which you will see in the next two photos are in a very bad way, and well past their use-by date.

Many thanks to the three musicians, plus Bondi Intermusic, who contributed their amps to this blog, by way of submitting them for service to our  specialist valve (tube) amp workshop, here in Gosford (Wyoming) NSW. Bye for now – IR.

The subject of this week’s blog is the Marshall JMP era model 1930 10 watt tremolo combo, designated the 1930 ‘Popular’ in Marshall histories and catalogues, but designated as the model 1930 ‘Lead’ on the amp itself. This amp is extremely rare in the land of OZ (this is the only example we have ever seen), and in fact we reckon it would be a rarity just about anywhere.

Basically, the 1930 is a 10 watt combo amplifier with a single 12 inch Celestion speaker. Two channels, each with two inputs and separate tone and volume controls for each channel. A valve (tube) generated tremolo operates on all four inputs with intensity and speed controls. A tremolo on/off footswitch completes the package. The valve (tube) complement is a pair of 6GW8/ECL86 triode/pentode valves for the push-pull ouput stage and phase-inverter stage,  and a single 12AX7/ECC83 for the preamp. One half of the 12AX7 is used as an oscillator for the tremolo, so that leaves only a single gain stage in the preamp.

For those of you who are unfamiliar with the 6GW8 valve (tube), a.k.a. the ECL86 in the UK and Europe, this valve combines a pentode power valve, with 9 watts anode/plate dissipation rating, so think of it as similar to the well known 6BQ5/EL84 but with lower power ratings, plus a triode similar to one half of a 12AX7/ECC83, both in the same glass envelope, with the standard miniature 9-pin base. A pair of the these valves can theoretically achieve an audio output of up to 14 watts, in class-AB/cathode-bias operating conditions.

The 6GW8 was manufactured in Australia by AWV (Amalgamated Wireless Valve), and I’m sure in several other countries. Manufacture ceased a long, long time ago, so we have been relying on new-old-stock for years. The 6GW8 was widely used in Australia in domestic electronics as well as guitar amps under 20 watts. Typically, amps that developed about 14 watts rms, would have been sold as 20 watts (ie, “music power” rating). Aussie guitar amps using the 6GW8 would have included Vadis, Moody, Goldentone, Challenge, Diason, Kee, no doubt the list goes on and on. The pentode section was also used as a reverb driver, especially in Goldentone amps, with the triode section used for gain recovery after the reverb springline.

This amplifier came to us from Bondi Intermusic, barely producing any musical sounds at all and in need of a major overhaul/refurbishment. Fortunately, both transformers were OK and the apparently original Celestion ‘Greenback’ speaker was working fine too, although it had been reconed in Sydney at some unknown point in its history. The existing valves were definitely past their use-by date and the 9-pin valve sockets themselves were only providing intermittent connectivity, even after a blast with DeOxit.

Here are some “before” and “after” photos of the amp repairs. The first three shots show the board and hand-wiring. All three 9-pin valve sockets would need to be replaced.

Even after replacing the 9-pin valve socket, the tremolo only worked intermittently. This necessitated replacing the board-mounted trimpot, and the footswitch (which is hard-wired into the circuit).

The impementation of the tremolo effect is very similar to that in the famous Marshall 18 watt combo, but this amp design has much less preamp gain.

Because the volume and tone controls plus channel mixing components are placed before the single 12AX7 gain stage, this stage is running wide open without any post-gain volume control to moderate noise levels. Therefore, we found it necessary to install a selected low-noise “high-grade” 7025 valve in this socket.

Because we are relying on a diminishing supply of new-old-stock valves, it is next to impossible to source the 6GW8′s in matched pairs (ie, the pentode sections). You will be lucky just to get a pair of the same brand. We were able to source a pair of made-in-UK but labelled Sylvania, in their original packaging. This amp, like most similar amp designs, relies on a single cathode bias resistor, common to both (pentode) valves, to achieve the correct bias level. Due to the fact that the 6GW8 parameters might be all over the place, it would be prudent to bypass this arrangement altogether, and install two separate cathode resistors (of double the original value each), bypassed by separate capacitors. With this approach, each 6GW8 will self bias to approximately the same level, and this is exactly the solution we implemented. The original components are still on the board, but somehow we doubt that anyone will start manufacturing the 6GW8 again !

You can see from the next two photos that we have now replaced the power supply filter capacitors, firstly a can-mounting 32+32uF unit, and secondly a board mounted axial lead 30uF/500V. A couple of badly worn pots needed replacing. The new cathode bias components installed were a pair of 270R W22 series ceramic body plus a pair of Sprague Atom 50uF/50V axial lead bypass caps.

While setting up the new cathode bias arrangements we discovered an internal earth loop within the amp which was creating an unnecessarily high level of hum ! Rerouting the earth return path from the 6GW8 cathodes directly to the earth connection of the first filter cap corrected this problem, by removing this heavier current from flowing via the preamp circuitry. This amp would have been noisy right from the factory.

All amplifiers need to be tested for electrical safety compliance, especially an amp that is about 40 years old ! All that was needed in this case was a new 240V 3-pin plug. Thanks to Mark W., for sending us this unique example of vintage Marshall/JMP production. We also have a 1963 Vibrolux from Mark for a major overhaul, and attention to electrical safety issues, which will be the subject of a future blog.

I have to admit, when first confronted with this JMP, fully expecting problems sourcing quality 6GW8 replacements, I tried to persuade Mark to allow us to convert the amp to EL84 output valves. However, Mark wanted to keep the amp as original as possible, and he was proved correct in this choice, as the overhaul was completed successfully and only slightly over budget. See you next time !

Happy New Year and welcome to 2012 !!  Recently, we carried out a major repair of a Galaxie 60 watt head for David Challinor of the Sydney-based indie band, Sounds Like Sunset. The Galaxie amp is actually a rebadged Vadis amplifier, obviously rebranded for one of the major music stores, possibly Palings, possibly Nicholsons, J Stanley Johnston or Harry Landis, all situated in the Sydney CBD back in the day, but we don’t actually know the details. This was fairly common practice in the 1960′s manufacturing scene, and the Vadis amp may well have been rebadged with other brand names as well. If anyone reading this blog can shed some more light on the subject, please contact us via email, it will be most appreciated.

Vadis amplifiers were manufactured in a small factory unit in Brookvale, a northern beaches suburb of Sydney, Australia. The business name of the manufacturer was Soundcraft Industries, Pty Ltd. The name of the proprietor was Jerry Dewey, although I can’t be sure of the correct spelling. Production started in the early 1960′s and continued through to the early 1970′s, but those dates are just approximate. Vadis amps were distributed exclusively through Dynamic Music, who were located in the same general area of Sydney. No doubt this arrangement would have limited the profitability of Soundcraft Industries, but it also gave them a broad penetration of the musical instrument marketplace, as it existed at that time.

Believe it or not, we actually have a family connection to the Vadis amp. My father, Jack Richards, a guitarist and arranger on the Sydney pro music scene back in the day, lived in the North Narrabeen/Elanora area of Sydney, as did Jerry Dewey. Knowing that Jerry was in the electronics industry, my father suggested to him repeatedly that it would be a shrewd move to start manufacturing guitar amplifiers, as at that point in time there were only two serious competitors, Moody in Lidcombe (Sydney), and Goldentone in Melbourne. By the time Jerry actually started production, however, there were several new competitors starting up in Sydney, Melbourne and Brisbane. Nevertheless, the enterprise was successful.

My father also claims to have suggested the name Vadis, from the name of a Sydney nightclub, Quo Vadis (also the title of a movie). We have no idea what became of Jerry Dewey after Soundcraft Industries closed its doors.

The classic Vadis 40 watt and 60 watt heads and combos typically employed a pair of 6CA7/EL34 output valves, originally with a 5AR4/GZ34 rectifier, but later replaced with solid-state rectification. A resistor (eg 1000 ohms, wire-wound) was normally used rather than a filter choke. These models had two preamp channels, one brighter than the other, and very effective all-valve reverb and tremolo. The reverb design is very similar to that of the 60′s Ampeg amps, but using a 12AU7/ECC82 driver valve. The tremolo employs direct modulation of the 2nd 12AX7/ECC83 gain stage cathode bias, in the 2nd (reverb) channel, resulting in that classic valve amp tremolo, unobtainable any other way. The 2-band passive EQ in each channel, was sometimes as per the Vox “Top Boost” channel, but driven from the anode of the preceding stage, rather than by a cathode-follower buffer stage, sometimes a very simple “tweed amp” style, but most often used a modified Baxandall EQ, optimised for guitar, and obviously a design unique to the Vadis amp. The more expensive amps used Ferguson transformers, manufactured in Chatswood (Sydney), with cheaper alternatives used in other amps.

There were numerous other lower-powered models employing different output valves, and were generally single channel. Most of the lower powered amps employed the 6GW8/ECL86 dual function triode – pentode valves, which could deliver up to approx 12 watts output in push-pull. These valves were very common in Australian-made guitar amps and record players, etc, in the 1960′s, but are long out of production.

The speakers used in the Vadis amp were primarily MSP (Manufacturers Special Products), a division of AWA in Sydney, or Rola (later Plessey Rola), also manufactured in Sydney. The Rola 12PEG (15 watts) and 12UEG (20 watts) alnico magnet guitar speakers sounded very fine indeed and are quite collectable today. Later, Plessey Rola introduced the updated ceramic magnet versions, the 12U50 and 12P30. The 12UX50 twin-cone model was widely used in PA systems at the time (late 60′s through early 70′s).

Well, by the late 60′s live performance of rock’n'roll music was changing in a big way, with ever more powerful amps appearing on the scene, and we can imagine that Soundcraft Industries, with their mid-60′s style amps, would have been under a lot of pressure from their distributors, and from retail, to reduce costs to stay competitive. By the beginning of the 70′s the Vadis amp was still a valve (tube) power amp, but with a solid-state preamp. Tone and performance suffered compared to the older models, but the writing was really on the wall when the entire range went completely solid-state. Thus, the Vadis story came to an end.

Getting back to Dave’s amp – both the transformers in this amp had previously been replaced by the writer, installing custom wound replacements from Special Transformers, of Sydney (no longer in business). This amp has had a hard working life and has been used in live performance until quite recently. The power transformer was replaced a 2nd time by a tech not known to us, this was possibly a Mojo unit, but was unfortunately underpowered for this application and developed a short circuit, blowing the fuse of course, and leading Dave to bring the amp down to our workshop. We selected a Mojo Tweed Bassman replacement unit, as having closest to the desired current & voltage ratings, ie able to support EL34 output valves plus a GZ34 rectifier, and importantly it would fit in the available space with some additional metalwork to enlarge the rectangular chassis opening.

Apart from replacing the transformer, there were numerous other issues to sort out, as you would expect in a 45 year old amp. I’m sure the designer would be amazed that these amps are still in use, and even more amazed at the alternative styles of music being performed !! In this amp the GZ34 and its octal socket required replacement, and the 240V wiring tidied up with emphasis on a good earth connection, as per compliance with Workcover NSW standards. All these amps will require replacement of 40+ year old electrolytic capacitors, we recommend a good quality cap such as Sprague Atom (USA) or F&T (W. Germany). Some caps will need to be bedded down in silicone to keep them secure. The bias supply voltage is adjusted by means of paralleling up resistors as required, so is not at all convenient. Jacks & pots will need cleaning or replacing as required. One problem will be the condition of the 9-pin preamp valve sockets. In many cases they will be so damaged and/or corroded that replacement is the only option, which is labour intensive as the Vadis amp is wired true point-to-point, with all components mounted directly between tagstrips and valve socket pins.

The tremolo function in Dave’s amp hadn’t been working for some time, but replacement of the 9-pin socket and a couple of resistors & capacitors restored it to full working order. It sounds fabulous !! In spite of the seemingly random earthing employed in the chassis wiring, the amp is amazingly quiet, and sounds pretty damn sweet. The reverb is a bit surf’s up for our tastes, but is definitely useable – just turned down a tad. We think that Mr Jerry Dewey probably knew a thing or two about electronic design, 60′s style.

Thanks again to Dave for his continued custom, and for supplying us with such rare & unusual items to discuss here in the blog.

http://soundslikesunset.com.au

Once again – if anybody can shed any light on any of the missing pieces of the Vadis jigsaw puzzle, please make contact via email and we will publish an update as appropriate.

Regards, Ivan.

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.