Archive for the ‘Amp Repair’ Category

Service Centre for Jackson Ampworks in Australia

February 22, 2014
Jackson Ampworks 'Britain'

Jackson Ampworks ‘Britain’

Hello, welcome back to the blog. We are now offering world-class (non-warranty) service & repairs to the range of valve (tube) amplifiers from Jackson Ampworks of Keller, Texas USA. We should point out that we have no official relationship with Jackson Ampworks, so we cannot accept any warranty repair claims without the endorsement of their Australian distributor.

Jackson Ampworks 'Britain' amp head

Jackson Ampworks ‘Britain’ amp head

We recently serviced the Jackson ‘Britain’ amp for one of our customers, who is a guitar tutor and professional player – the amp is featured in the accompanying photos. The Jackson amps are built in Texas, but are British-voiced. The concept of the Jackson ‘Britain’ is broadly similar to our own Richards ‘Expressionist’ amplifier models – there are two discrete preamp channels, voiced as per vintage VOX amps.

Jackson 'Britain' rear panel removed

Jackson ‘Britain’ rear panel removed

CH-1 is based on the EF86 pentode preamp valve (tube), and CH-2 employs the more familiar 12AX7/ECC83 valves and is effectively the top-boost channel. The EF86 is famously rather prone to microphony in guitar amps, but sounds absolutely wonderful in the right situation. The ‘Britain’ power amp section is switchable between a pair of cathode-bias EL84 valves for low power, and a pair of fixed-bias EL34 valves for full power.

Jackson 'Britain' chassis

Jackson ‘Britain’ chassis

The Jackson amp is very compact – about half the size of most comparable amps, ie amps in the 30 to 50 watt range. This amp depends absolutely on forced-air cooling for its survival, so if the fan ever stops working the amp needs to be serviced a.s.a.p. Some of the amp functions normally carried out by passive components appear to be implemented with the use of solid-state devices, which are mounted to vertical heatsinks, also in the forced-air cooling path.

'Britain' chassis: front view

‘Britain’ chassis: front view

Fortunately, for this service job there was nothing more serious than some noisy valves, which we replaced accordingly with a full check & test. The only problem we encountered was that the light aluminium chassis was quite resonant, at a frequency that was very close to the resonant ring from certain brands of EL34 power output valve that we tried in the amp. This was so noticeable that we substituted several brands before settling on the least resonant combination.

Jackson 'Britain' underneath the chassis

Jackson ‘Britain’ underneath the chassis

Finally, we were happy with the end result and our customer was pleased to get his amp back, working correctly again with unwanted noise at a minimum. You can see from the photo to the right that the amp is indeed hand-wired and the chassis layout is very tight indeed. Access to some components is limited or not possible at all without stripping out the electronics from the chassis.

Jackson 'Britain' chassis

Jackson ‘Britain’ chassis

 

Hardware components are by Carling, Switchcraft, Alpha and others – the standard of electronic components is also good. So there we have it – our very first experience of servicing the Jackson Ampworks product ! These amps look and sound great, but are relatively new to the Aussie music industry so we cannot pass any comments with ref to long term reliability issues. If you are in New South Wales and own a Jackson amp that needs servicing, please contact us @ ivan@ivanrichards.com

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another Morgan amp in our workshop

January 27, 2014
Morgan RCA35

Morgan RCA35

Welcome to our first blog for the year 2014 ! Another Morgan amp came to our Wyoming workshop for repairs a while back – this time the RCA35 model. We previously discussed repairs to the Morgan SW50 back in January 2013. The RCA35 is voiced quite differently to the SW50, and is intended to be a ‘clean headroom’ amp, according to the Morgan website. A pair of cathode-biased 5881 output valves delivers 30 watts output @ the onset of clipping.

Morgan RCA35

Morgan RCA35

Our good customer, Josh, had purchased this amp about 6 months previously and advised us he was getting some very strange sounds, almost like an octave-fuzz pedal. At our first attempt to diagnose this, the RCA35 behaved perfectly so we were none the wiser. This situation happens sometimes ! After Josh got the amp back home again, of course it played up immediately, so he recorded the sounds on his ‘phone & sent them to us, and indeed the amp sounded like there was an octave fuzz pedal happening.

Morgan RCA35

Morgan RCA35

Back in the workshop, I strongly suspected that this degree of amp instability must surely be down to a faulty electrolytic capacitor (or capacitors) in the amp’s high voltage power supply. Josh was preparing to head off for a short tour in central western New South Wales with a band, so we couldn’t take the risk of leaving an intermittent fault in the amp. Therefore we took the decision to replace all four HV (high voltage) power supply caps, plus the cathode bypass cap for the output valves, just to be sure.

Morgan RCA35

Morgan RCA35

You can see from the photos that the original capacitors were of standard consumer electronics quality, and of recent production. There were no tell tale signs of leakage or faulty manufacture. We installed a set of those very fine F&T capacitors, bedded down in some silicone (neutral cure) for stability. Sprague ‘Atom’ capacitors would also be an ideal choice although their larger physical size sometimes limits their application in service work. The list of capacitors reads as follows: 2 x 30uF/500V, 2 x 22uF/500V plus 1 x 100uF/100V.

Morgan RCA35

Morgan RCA35

We also tightened a loose fuseholder and changed the fuse size from 2 amp to 1 amp as in Australia our mains power is obviously 240V (sometimes higher), and labelled the rear panel accordingly. The manufacturer and their local distributor really should have taken care of that particular issue. With the high voltage supply reading +430V DC, we measured 15.6V into 8 ohms = 30 watts @ the onset of clipping. We were pleased to hear that Josh got through his tour OK and there were no further problems with this amp !

Morgan RCA35 original HV capacitors

Morgan RCA35 original HV capacitors

Morgan RCA35 upgrade caps installed

Morgan RCA35 upgrade caps installed

Margan RCA35 after completion of repairs

Morgan RCA35 after completion of repairs

MODs to the HIWATT 20H amplifier

December 10, 2013
HIWATT 20H

HIWATT 20H

Recently we implemented some home-grown MODs to a HIWATT 20H amplifier for Rick Altavilla, now on guitar for home-grown Central Coast band Sparrows – fronted by the one & only Scotte Woods (formerly of One Dollar Short). Rick purchased this amp new, fully expecting that it would sound like a scaled-down version of the great original HIWATT amps. Unfortunately it sounds & performs nothing like an original HIWATT. This is not a cheap amp either – it’s actually a rather expensive one, even allowing for the fact that it is hand-wired from the UK. To say that the guys from Sparrows were disappointed with this amp would be an understatement.

HIWATT 20H

HIWATT 20H

The 20H has some design problems in common with the Marshall C5 discussed in our previous blog. Much of the available gain is thrown away due to some bizarre design choices and the 3-band passive EQ just doesn’t work at the desired guitar frequency ranges. Indeed, the 20H sounds really flat.

Basically, you could consider this amp to be a kind of modern AC15, with a couple of sequential gain stages, a buffer stage driving the EQ, a pre-PI stage master-volume, and a zero negative feedback power amp employing a pair of cathode-bias EL84/6BQ5 output valves. Such an amp design potentially should sound mighty fine.

HIWATT 20H

HIWATT 20H

The power amp & PI (phase-inverter) stage were OK – no apparent problems there, so we concentrated our attention on redesigning the preamp, within the constraints of the existing electronics & layout, to achieve a useable amp. We reconfigured the gain stages to get some drive & balls from the amp, & revoiced the EQ as per the C5 to achieve a useable & effective sweep from all controls. The 20H now sounded like a baby “British” amp from the 60s golden-era, which is not a bad result at all !

Thanks Rick & Sparrows. IR.

major overhaul 40 y.o. Orange “Graphic” 100W head

June 10, 2013
Orange Graphic 100

Orange Graphic 100

Welcome back to the blog ! This week we look at a major overhaul to a battle-damaged classic from around 1971 – the Orange ORS100 “pics-only” head. Our very good customer David Challinor, guitar & vocals from the band Sounds Like Sunset, had recently acquired this vintage masterpiece, but in its existing condition the amp was quite unstable and of dubious electrical safety.

Orange Graphic 100

Orange Graphic 100

Dave contributed the Vadis/Galaxie amp that we blogged back in January 2012, which has been one of our most widely read blogs (for Australian readers). We have previously discussed the servicing of Orange amps back in April 2012, October 2011, & May 2011, but those amps were products of the contemporary Orange company. This is our first ever blog of an original Orange from 40 years ago.

Orange Graphic 100 internal prior to overhaul

Orange Graphic 100 internal prior to overhaul

Any guitar amp this old will have numerous issues to be resolved, including electrical safety, some components (eg electrolytic capacitors) will be well past their use-by-date, and also there will have been repairs carried out on-the-run which may well have to be corrected to produce a stable amp which performs as originally intended.

This amp is designed to run its quad of EL34 power output valves at a very high 525V DC, similar to some very old Marshalls from that era. This places additional stress on modern production EL34’s, as well as being a potential source of problems for printed circuit board designs.

Orange Graphic 100 internal prior to overhaul

Orange Graphic 100 internal prior to overhaul

Sure enough, there had been a melt-down in this amp, probably many years ago, which resulted in permanent damage to the p.c.b., with some tracks & pads having lifted, required the circuit to be completed with wire links. This had been repaired reasonably well previously, and we carried out some additional repair work in this area. You can see from the photo immediately above, the amp had been modded with the addition of zener diodes to lower the EL34 screen grid voltage. Quite amazingly given the high DC volts, there was no bias adjustment available on this amp.

Orange Graphic 100 rear view of chassis prior to overhaul

Orange Graphic 100 rear view of chassis prior to overhaul

Our agreed strategy was to restore the amp where possible to the original design, and update selected components, plus create an adjustable bias supply for the EL34’s. In Dave’s words: if you are able to drag the amp from being a potentially lethal museum piece & back to its former glory, then please proceed ! The trickiest bit was removing the p.c.b. from its position without introducing any more problems. These amps were really well made, built like a proverbial British tank, but employed single core wire, which as it ages becomes rather brittle and prone to breakage.

Orange Graphic 100 chassis front view prior to overhaul

Orange Graphic 100 chassis front view prior to overhaul

We cleaned up the copper side of the board and replaced all 9 x electrolytic capacitors, together with some 2 watt carbon film resistors. We installed a bias trimpot in place of a fixed resistor, which gives  a very broad range of adjustment, permitting the installation of various EL34 alternatives, eg E34L or KT77. We bedded down the high voltage capacitors in a blob of silicone for stability. We replaced the 4 x worn out pots and added an earth wire to the rear of the pots to improve shielding.

Orange Graphic 100 checking electrical safety issues

Orange Graphic 100 checking electrical safety issues

External to the board, the pair of vertical mounting 100uF/500V main power supply caps were replaced. All jacks & 9-pin valve sockets were cleaned with DeOxit. V1 & V2 were replaced with JJ 12AX7-s valves, which would be a reliable choice given the rather high cathode voltage in the V2 phase-splitter stage. The EL34’s tested OK and were retained. We found that the earth connection to the terminal block in the photo to the left had a stripped thread, which meant that the earth connection could not be tightened ! We replaced the terminal block and added a multi-strand earth wire connection direct to the chassis.

 

Orange Graphic 100 HV capacitors replaced & board repaired

Orange Graphic 100 HV capacitors replaced & board repaired

One unusual feature of the ORS100 design, especially compared to the published schematics of vintage Orange amps available on the internet, is that the huge filter choke actually carries the entire HV current draw of the amp, not just the screen grids & preamp valves. This probably has a positive impact on the amp’s performance when driven into clipping. However, the down side is that the voltage on the EL34 screen grids is too high. As a general rule of thumb, the voltage on the screen grids should be lower than that on the anodes, so we increased the value of the 4 x screen grid resistors, and installed those W22 series enamel body resistors which are intended for the harshest conditions.

Orange Graphic 100 new capacitors plus bias trimpot installed

Orange Graphic 100 new capacitors plus bias trimpot installed

The customary earth shield between the input jacks & the output transformer connections was missing, presumably lost at an earlier unknown repair job. Given the proximity of the input jacks to high voltage (HV) wiring, this was a contributing factor to the amps instability issues. Luckily we had a suitable shield in stock which we had manufactured for our own Richards amps, and you can see it has been installed in the photo to the left. After hours of work we were finally in a position to carry out a PAT test and a power output test. The result was 30V/8 ohms = 112 watts @ onset of clipping.

Orange Graphic 100 repairs completed

Orange Graphic 100 repairs completed

What did the ORS100 sound like ? This amp is designed to be played loud ! We gave it a blast in the workshop, and the more we turned it up, the better it sounded. The treble, bass & presence controls are very effective, but we really preferred the 6-position FAC switch on maximum anti-clockwise for a maximum full-bodied tone. There may not be too many venues left where you can actually use this amp to its full capability.

This overhaul exceeded the original budget by quite a margin but Dave seemed to be very excited by his new amp acquisition. Here are Dave’s initial reactions to the amp as conveyed by text message:

HI Ivan  – the Orange was wonderful, thanks ! Enormous sound. I love it !!

Once again the Orange was phenomenal at rehearsals. It’s my new favourite amp. I think it’s because it handles pedals so well too. I really, really love this amp & as always your work is top notch.

Thanks Dave, we love that kind of talk. IR.

MODS to the Marshall JTM600/JCM600 series amps

May 19, 2013
JCM600 range from Marshall website

JCM600 range from Marshall website

We are now implementing mods & upgrades to a broad selection of guitar & bass amplification, primarily valve (tube) but occasionally solid-state as well. The most common examples are the Fender Pro-Junior & Blues Junior models, however we have been selectively modding Marshall amps since the 1980’s. In this blog we have a quick look at the Marshall JTM600 & JCM600 series amps – it would appear that the differences between the two are limited to cosmetics & choice of speakers, the electronics remains the same for both series.

JCM600 combo

JCM600 combo

These amps remain among the more obscure of Marshall’s amp offerings of the last 20 years or so, very few guitarists would have played through one of them, let alone owned one. We do have a few Central Coast (NSW) customers that quite enjoy these amps, including “Gazebo”, the man who inspired this blog by commissioning us to mod his JTM60 amp head (we modded the “clean” channel only).

JCM601 model

JCM601 model

These amps don’t have a great reputation for reliability. We have replaced several transformers over the years, for example. The first batch of JTM600’s were blowing the main high voltage filter capacitors during the warranty period, although this problem has obviously been resolved. They are not particularly easy to work on – getting the circuit boards out and re-installing them is quite time consuming. Operating the EL34 power output valves (tubes) in the horizontal plane rather than the more conventional vertical plane is just asking for trouble. You have to choose your EL34’s carefully – with some brands there is the potential for an internal short-circuit to occur, if the heater filament sags for example. Please note, this is not normally a problem with the 12AX7 preamp valves. Regardless of your choice of valves, most of the heat from the power output valves goes straight up into the amp.

the JTM30 combo

the JTM30 combo

Over the years, we have tried to come up with ways to “bullet proof” these amps. Not all EL34’s like high voltages on the screen grids, for example. Installing a small DC powered cooling fan may assist the issue of heat build up within the chassis. The JTM30 model has presented less problems, but has one stupid design mistake – a non adjustable bias supply voltage, which complicates replacing the 5881 output valves unnecessarily.

Stock, the 60 watt amps sound a bit thin and a bit sterile. The “lead” channel develops a type of distortion that sounds like it came from a stompbox, rather than from a valve amp. The main focus of our mods is to convert the “clean” channel to a circuit sounding as close as possible to a Marshall model 1987, ie a blues/crunchy tone with more fat bottom end and crisp high end. This is achieved by substituting selected components (resistors/capacitors) with more traditional “plexi” circuit values. This is what we achieved for “Gazebo” just this week. Once the job was completed, the “clean” channel was suddenly very useable for many styles.

With the “lead” channel you could easily go overboard, spending hours rebuilding the whole circuit. The simplest approach would be to remove the clipping diodes for a more natural valve (tube) medium distorted tone, although having done this it may be necessary to make some changes to balance the circuit for the now much higher signal levels.

Modding the clean channel as described, plus limited mods to the lead channel is a cost effective way to significantly improve the performance of these amps. We need to allow a minimum of 2 hours labour for this work, plus materials. We would have to quote re specific improvements to the power amp, eg adding a cooling fan, etc. Regards – I.R.

Repairs to the Laney AOR Series ‘Pro Tube Lead’

May 14, 2013
Laney AOR50 1x12 combo

Laney AOR50 1×12 combo

This Laney 50 watt 1X12 combo, designated the Pro Tube Lead model, was part of Laney’s AOR series valve amplifiers, manufactured in the 1980’s. Basically, these amps are Laney’s answer to the Marshall JMP & JCM800 series master-volume amps, but with some additional features including ‘pull-boost’ on some controls plus a footswitchable gain boost. The combo model also incorporates reverb (solid-state driven). There is more gain available than in a standard JMP or JCM800.

Laney AOR50 rear view

Laney AOR50 rear view

You can see the speaker in this combo is in its own sealed cab, which gives the amp a more solid ‘bottom end’ than a conventional open back, but which also makes the amp more directional. These amps are very loud !

 

problems on the board are causing our hum

problems on the board are causing our hum

Chris Jones delivered this amp to us from the ‘Blue Mountains’ (a beautiful location west of Sydney, Australia) – the problem was that a huge hum/buzz was ever present in the output. Some previous repair work had been done by a music shop with no improvement, and Chris, possessing some electrical know-how, had also attempted to isolate the problem. The scope of this repair job also included a valve replacement/upgrade, and also replacing some ageing high-voltage electrolytic capacitors. There was also an electrical safety issue, ref a loose fuseholder on the rear panel, which we replaced with a more robust type. The photographs were taken by Chris, by the way.

low voltage power supply faults on this board

low voltage power supply faults on this board

Upgrading the valves (tubes) and selected components in the high voltage circuitry did nothing to cure the hum problem. We next examined the low voltage power supplies. The +/- 15V DC rails that power the IC’s ref the reverb & FX Loop circuitry were unbalanced, so clearly could be a source of hum entering the signal path. We rebuilt the +/- 15V rails by replacing the zener diodes and filter caps.

There is yet another (single-ended) low voltage DC supply which powers the switching circuitry. This is another possible source of hum entering the signal path, and in fact turned out to be the main culprit. We noticed that the DC rail powering the switching was reading much higher than the schematics indicated. We also noticed differences between component values as listed on the schematic and what was actually on the board ! We rebuilt this low voltage supply strictly as per the schematic, and the voltage readings were now correct, plus there was an absence of hum ! We were quite concerned that the switching IC’s might have been damaged by the over-voltage, but the amp’s switching functionality checked out fine.

These repairs to the Laney AOR50 were happily completed successfully and on budget, allowing for an estimated 3 hours labour, plus parts as required. The wiring and assembly in the Laney is much rougher and untidier than the equivalent JCM800. To get to the copper side of the p.c.b., all the front panel controls have to be disassembled in order to flip the board over, but it has to be done for a neat job that will hold up over time. For collectors of 70’s & 80’s Brit valve guitar amps, this model is worthy of your consideration. Here are a couple of comments received from Chris after he took possession of the repaired amp……………..

I just tried her out, it’s the best it’s ever sounded, I would have liked you to try it out wound right up with the boost pulled, it’s unbelievably loud.

Hi Ivan, well I’ve put a few hours on it today, ran perfect, no hint of any problems. I can’t believe how good it sounds, I can see why they (Laney AOR) are very popular nowadays.

service & mods to the Peavey ‘Valve King’ 100 amplifier

May 9, 2013
Peavey Valve King 100

Peavey Valve King 100

Hello. This week we received at the workshop one of the more recent Peavey made-in-China valve amp heads for repair. One of the ‘Valve King’ models, this is the VK100. Stated fault was – the amp just stopped. We have been repairing Peavey amps since the 1980’s, by the way, including warranty work, but we have noticed a steady decline in the serviceability of their amps and will only accept non-warranty repairs these days.

Valve King 100 head

Valve King 100 head

The amp indeed would not power up, and examination of the mains fuse, actually the only external fuse, revealed that it had not blown, so the next step was remove the chassis from its sleeve. The fuse for the HT (high voltage) supply, F201, is located on the main p.c.b. Obviously one or more of the 6L6GC power output valves had gone S/C (short-circuit), causing the HT fuse to blow.

Valve King 100 head

Valve King 100 head

Unfortunately, F201 is a miniature glass fuse directly soldered into the p.c.b. (printed circuit board), so easy replacement is out of the question – the board has to be stripped out, ie removing all control knobs, nuts, screws, etc. Is this for real ?? As this fuse is likely to blow several times during the life of this amp, the first MOD we will implement is to wire the fuse to an external fuseholder on the chassis rear panel, easily accessible to all. The HT fuse is T1.6A. You can see this MOD completed in the photos, plus the Dymo labelling.

Valve King circuit boards

Valve King circuit boards

One of Peavey’s favourite design tricks is to wire the valve (tube) heater filaments in series rather than parallel, even including the output valves. There can be various combinations of series heater arrangements in the one amp. Without a detailed schematic diagram on hand, this can be very confusing, especially in amps with a higher number of valves, eg the 6505 combo. Basically, if you remove any valve from its socket, the other valves (or possibly just some of them) will not power up. For each Peavey model, we build up a file of such idiosyncrasies so that we don’t get caught out twice.

HT fuse MOD has been implemented

HT fuse MOD has been implemented

So, we have reached the point when we intend to install a matched quad of JJ 6L6GC output valves. These valves should sound very good in this style of amp. The next logical step after installing any output valves is to rebias the amp. However, the bias supply in this amp is not adjustable, nor is there any provision to monitor the current draw of the 6L6’s. HUH ?? The idea that anyone would design a 100 watt valve amp with a non-adjustable bias supply in this day & age is beyond comprehension, but that is the situation with the VK100.

miniature pcb fuses & bias resistor MOD

miniature pcb fuses & bias resistor MOD

The amp initially appeared to be slightly under-biased, so we replaced R211 with a different value which resulted in a correctly biased quad of JJ’s. Next time this amp comes in, we will install a 25-turn trimpot in place of R205, which will solve the problem once and for all. We also installed 470K resistors across high voltage supply capacitors C202 & C204, as these caps were holding their charge (nearly 500V DC) long after the amp was switched off, complicating the service job. This has no effect at all from the customer’s point of view, but makes life easier for the next service tech.

VK100 rear panel

VK100 rear panel

The remaining tasks are an electrical safety test (PAT test), power output test and burn-in test, with a final play test to complete the job. The amp delivered 30V into an 8 ohm resistive load, which translates to 112 watts. When we first attempted a power output test, we couldn’t get a good clean sine wave output, even from injecting our test signal at the FX Loop ‘return’ jack. This was most perplexing, until we discovered the mysterious Texture pot on the rear panel, near the speaker jacks.

the mysterious 'Texture' control

the mysterious ‘Texture’ control

The Texture control claims to sweep the power amp from Class-A (c.c.w.) to Class-A/B (c.w.) mode of operation. This is complete nonsense ! What it actually does is convert the amp from normal push-pull (Class A/B) operation to a kind of quasi single-ended mode by shunting away some of the signal from one of the two outputs of the phase-splitter stage to ground. This only roughly approximates the behaviour of a single ended amp and naturally the resulting waveform is somewhat distorted, which is what caused our initial confusion. The only way to carry out a meaningful sine wave test is to set the Texture control to fully clockwise (c.w.). Well, how does the amp sound with a decent set of valves ? Not too bad actually – the ‘clean’ channel sounds very Fendery, with a slightly different EQ sweep, and the ‘lead gain’ channel actually starts off with considerable distortion levels, so any kind of ‘crunch’ tones are just about impossible, but will probably please the younger guys, and the reverb is also pretty good for a short tank driven by IC’s rather than valves. We also provide world-class service to the Peavey EVH amps and the well known Classic 30/Classic 50 models. Thanks for checking in to the blog – I.R.

Upgrading your amp’s power transformer to 240V AC

March 27, 2013
Mercury Magnetics power transformer (example)

Mercury Magnetics power transformer (example)

During the times that the Aussie $$$ has achieved parity with the American $$$, or close to it, there has been a surge in Aussie musicians purchasing amplifiers & other electronic devices direct from the USA, either from Ebay auction or sometimes direct from dealers. No doubt, even taking shipping costs into account, this has saved those musicians a considerable amount of money.

MM transformers (example)

MM transformers (example)

However, there is a potential trap for the unwary when sourcing mains-powered devices from another country. Many of these devices will be wired for 120V AC mains operation, but of course here in Australia our power at the wall socket is in fact 240V AC (sometimes higher). Many unwary purchasers have simply plugged their latest acquisition into the 240V and have at the very least blown the fuse, but in some cases causing considerable damage to the device in question.

MM power transformer, laydown style (example)

MM power transformer, laydown style (example)

One possible solution is to source a stepdown transformer from local suppliers, ie this is a transformer within a suitable metal casing that converts the voltage from 240V to 120V AC nominal. There would would normally be a protection fuse and an American style power socket for your appliance (there may be stepdown transformers out there for other voltages, for example Japanese 100V AC).

Vibrochamp 240V transformer installed

Vibrochamp 240V transformer installed

Anyone who goes down this path should definitely have their rig checked out for electrical safety, ie is the amplifier earthed to the 240V 3-pin plug ?? The reason we have brought this to your attention, is that we have uncovered a few examples of the earth not being extended through to the appliance (ie, the amp or other device). These discoveries were made during routine servicing of the amps in question, and came as quite a shock (no pun intended) to the owner.

Vibrolux power transformer installed

Vibrolux power transformer installed

The most obvious disadvantage of this approach is the bulk & weight of the stepdown transformer, especially when powering a 100 watt valve (tube) amplifier. Plus, it’s an additional item that has to be carried around to gigs & rehearsals. Nevertheless, we have a number of customers who continue to use stepdown transformers, especially owners of Mesa amps, due to the inflated cost of Mesa 240V transformers in this country.

PAT testing Pro junior

PAT testing Pro junior

Ultimately, the logical & professional solution is to upgrade the factory installed power transformer to a 240V model of the same spec and mounting arrangements. Note that there are some amps that have internal transformer connections for different voltages as standard, so rewiring is relatively straightforward. We can carry out an appraisal of your amp for you, to quote on the appropriate solution.

Magnetic Components AC30 power transformer

Magnetic Components AC30 power transformer

We have successfully upgraded transformers to models with global power wiring as supplied by Mercury Magnetics, Heyboer, Magnetic Components, Mojo, Hammond and so on. In the case of a toroidal transformer, we would normally consider a suitable replacement from Harbuch of Sydney, depending on the time frame.

Note:-  for those of you out there that may have a Carr amp purchased direct from the USA and therefore operating on 120V AC via a stepdown transformer, we can offer a permanent solution by installing a 240V power transformer supplied by Carr amps via their Australian distributor, Bondi Intermusic. Please enquire.

While on the subject of Carr amps, contact us info@ivanrichards.com for world-class service and support, including full backup as required from Carr amps, USA. Our workshop is located in Wyoming (Gosford), New South Wales, just north of Sydney. Alternatively, you can make arrangements via Bondi Intermusic of Oxford Street, Bondi Junction (Sydney). Mobile number for urgent enquiries is 0418 862 034 (within Australia), and 61 418 862 034 (international).

MODS & upgrades to the PRO Junior amp

March 3, 2013
Fender Pro Junior amplifier

Fender Pro Junior amplifier

One of the services we happily provide to our customers is the implementation of widely publicised MODS to the Pro Junior (and also Blues Junior) amps by FMIC. Indeed, modding current production guitar amps has turned into something of a growth industry in recent years (what does that say about the quality of commercially manufactured amps these days ??). We published a blog re MODS & Upgrades to the BJR back on April 25, 2012. All previous blogs, going back to #1, are archived & accessible on this site. Having completed quite a number of such MODS, we are in a unique position to advise customers as to the most effective strategy for their particular requirements.

speaker upgraded but chassis still original

speaker upgraded but chassis still original

The Pro Junior (& BJR) amps are relatively inexpensive to begin with, so spending a few hundred $$$ bringing them up to performance standard is easy enough to justify. The most significant upgrade to either amp would be replacement of the stock speaker, a 10-inch driver in the case of the Pro Junior, and a 12-inch driver in the case of the BJR. The most obvious choice would be one of the Weber models, WGS are also a strong contender – 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 stylist, we also recommend one of the Jensen NEO models, for their balanced frequency response and high power handling/late break-up.

Weber upgrade speaker installed

Weber upgrade speaker installed

The example amp in the photos suffered from a constant hum, regardless of control settings , etc. The volume & tone pots were already scratchy, and the output valves were running hot enough to discolour the printed circuit board (p.c.b.). After some consultation the customer requested that we upgrade the power supply filter capacitors, implement an adjustable fixed-bias supply with a 50K 25-turn trimpot mounted to the p.c.b., install a Mercury 3H (Henry) choke in the high voltage supply, and replace the original output transformer with a Mercury upgrade unit specifically designed for these amps.

MM output transformer & choke compared to the stock transformers

MM output transformer & choke compared to the stock transformers

You can see in the photo to the right, the MM output transformer is significantly larger then the stock unit. We had to cut some tracks on the p.c.b. to wire the choke in series with the EL84 screen grid supply, and also to replace a fixed resistor in the bias supply with our Bourns 25-turn pot, so this would automatically void the manufacturer’s warranty where applicable. We installed four of those very fine F&T axial 500V DC electrolytic capacitors, and bedded them down on the p.c.b. in a blob of silicone for stability. We have effectively doubled the capacitance of the main reservoir capacitor, but maintained the same amount of capacitance for the preamp.

MM choke & output transformer now installed

MM choke & output transformer now installed

While we had the amp pulled apart we took the opportunity to replace the scratchy pots and tidy up the lead dress of the interconnects, all general housekeeping type stuff that makes a difference to the final result. Once the amp is reassembled we carry out an electrical safety check to Workcover NSW standards. In other words, we measure the earth resistance from the 3-pin plug to any exposed metal part of the amp.

upgrade capacitors installed

upgrade capacitors installed

We reassembled the amp and adjusted the bias volts supply for a sensible level of EL84 output valves current draw with no signal. In this example, we check the current draw by measuring the voltage drop across the primary of the output transformer. Please do not try this at home !! Always remember there are potentially lethal voltages present within a valve guitar amplifier. The power output test achieved approx 16 watts into 8 ohms, @ the onset of clipping. The previously high level of background hum & noise was now greatly reduced to an acceptable standard.

Bourns 50K 25-turn bias trimpot installed

Bourns 50K 25-turn bias trimpot installed

In the photo to the left, you can see where the bias trimpot has been installed, underneath the parallel-connected pair of high-voltage filter capacitors. We have taken the trouble to dress the various leads to avoid any unintentional feedback paths within the circuit. Our customer, Peter G., reported back to us that he was very please with the results. In particular, the performance of the amp was much improved when driven to “crunch” levels, as distinct from the purely “clean” performance. We attribute this to the greatly improved quality of filtering of the high voltage supply to the EL84 screen grids. This supply is now filtered by a “choke” as well as the existing resistor/capacitor arrangements. When the power amp is driven into clipping, power supply hash from an under filtered supply can enter the signal chain via the screen grids. The preamp circuit also benefits from this upgrade.

safety check with PAT (portable appliance tester)

safety check with PAT (portable appliance tester)

The starting price for the basic MODs to the Pro Junior & BJR is around $180.00 AUD parts & labour. Obviously, upgrading hardware items such as transformers and/or speakers adds expense to the project. The list of customers of note who have had their amps modded includes James Black of the SBS TV RocKwiz Orkestra, James is one of Australia’s most in-demand musician/producer/musical-director professionals. IR.

A tale of two RIVERA amps

February 2, 2013
Rivera Jake Studio 1x12 combo

Rivera Jake Studio 1×12 combo

Hello again. In this week’s blog we revisit the amps of Paul Rivera. We previously published blogs re Rivera amps on Dec 16, 2011 (more about Rivera amps: the Ninja Boost MOD), Nov 4, 2011 (the Rivera Bonehead amp), and Jun 19, 2011 (do we cater for the jazz guitarist ?). There are now 65 blogs published, so there’s something there for all amp & pedal enthusiasts. The two amps we are discussing are the Rivera Jake Studio Combo, which we assume is named after a prominent American session player, and the Rivera Thirty Twelve, which is also a 1×12 combo.

Rivera Jake Studio 002Our mission statement with the Jake was to replace the broken impedance selector switch, and install our now almost famous Ninja Boost MOD. You can see that the impedance selector is a rotary 3-position switch situated on the rear panel. Replacing the switch was pretty straightforward, but unless you want to MOD the rear panel, you have to source the exact switch from Rivera, via one of their distributors.

Jake Studio Combo Ninja MOD

Jake Studio Combo Ninja MOD

You can also see from the photos that there isn’t a whole lot of spare room on the rear panel of the Jake model, so we had to install the variable boost pot (potentiometer) in the remaining space betweeen the FX Loop and the end of the chassis. We are getting more proficient at implementing this MOD now, having now installed a few, in various models from the range. We always use highest quality shielded cable, in this case bedded down in a blob of silicone, to keep microphonics to an absolute minimum.

Ninja Boost MOD

Ninja Boost MOD

Apart from a different location, this MOD was implemented in exactly the same way as in the other Rivera amps we have done, with the same very smoothly controllable boost function as the end result. The strap handle was dangerously worn so we automatically replaced that. The exposed metal strip could easily slice someone’s hand open.

Rivera Jake Studio 005To finish off the job, the pots needed cleaning with Faderlube and the input jacks (x2) were R/S, cutting in & out pretty badly. The best option was to replace them altogether. Unfortunately, the jacks are a printed circuit board (p.c.b.) mounting type, specific to Rivera amps. The jacks are earthed to the chassis as well, ie non insulated.

the slightly road worn Rivera Jake Studio Combo

the slightly road worn Rivera Jake Studio Combo

We wanted to install the traditional heavy duty Switchcraft open construction jacks, as per vintage Fender amps, so we dispensed with the input jack p.c.b. altogether and hand-wired the new jacks. Perfect ! Both these combo’s feature the standard Rivera features of a USA-voiced preamp, a British-voiced preamp, a pull-boost on each preamp, an FX Loop and a reverb tank. It seems that there are relatively small differences in the circuit that differentiate the various models.

the Rivera 30/12

the Rivera 30/12

Our second example is the Rivera Thirty Twelve combo, and in this case a much more complex set of problems to resolve. The stated fault from our customer was the amp had gone low level, generating miscellaneous noises, plus the reverb & volume pots had suddenly gone very noisy ! Initially quite a bewildering set of symptoms. When we powered the amp up, we found nil output from either preamp, but some very low level output via the FX Return jack.

Rivera 30/12 chassis but not the correct power transformer

Rivera 30/12 chassis but not the correct power transformer

The 12AX7 preamp valves tested substandard and would have to be replaced, but this was not the main problem. The first thing we noticed after removing the chassis from the cab was the non-original power transformer ! It was huge, and presumably an Aussie-made unit that was installed by persons unknown after the original factory unit went faulty. There was evidence of considerable damage to the power supply end of the main p.c.b., with lifted pads & tracks, so there had been some sort of meltdown that went along with (or maybe caused) the transformer failure.

Rivera 30/12 main board

Rivera 30/12 main board

Some p.c.b. repairs had been attempted, but the results were not what you could describe as professional. We see some extremely inept repair attempts and this one is not the worst, but is well up there. The replacement power transformer actually looks more suited to a 100 watt amp. Remember, this is a 30 watt EL34 power amp and we are expecting to see a high voltage supply in the range of 380V to 400V DC maximum. This particular transformer was delivering over 500V DC, and yet no attempt had been made to upgrade the capacitor voltage ratings in the power supply circuitry, or configure pairs of capacitors in series to achieve the necessary voltage rating, plus the bias supply needed to be rejigged to give the wider range of bias adjustment required to compensate for the higher operating voltages. In other words, the amp was permanently under biased, resulting in very hot running output valves.

Rivera 30/12 low voltage power supply

Rivera 30/12 low voltage power supply

The EL34 output valves were at the end of the road, as you would expect, so we supplied a new matched pair, as well as some selected 12AX7 preamp valves. We set about rebuilding the high voltage supply and the bias supply, within the constraints of a permanently damaged board. Installing series pairs of 350V DC caps is the simplest way of achieving a higher voltage rating. None of this exlains the complete lack of output from the preamps. The low voltage supply was also damaged – the regulated +18V supply rail was down to only a few volts. Replacing the +18V regulator IC did not make a huge improvement, so the next step was rebuilding the regulated and unregulated low voltage supplies.

rear view of reassembled amp

rear view of reassembled amp

In case you’re wondering the significance of the low voltage supplies to a Rivera amp, the 12AX7 preamp valve heater filaments are powered in a relatively complex arrangement of series connections to the low volts supplies, as opposed to the more conventional parallel arrangement from a 6.3V AC supply. Each 12AX7 should read approx 12V DC across pins 4 & 5. Therefore losing the +18V DC rail was the reason for nil output from the preamps, ie several 12AX7’s were not functional under these conditions.

the Rivera 30/12 repairs completed !

the Rivera 30/12 repairs completed !

Well, at the conclusion of several hours of work, we were able to bias the amp for a reasonable operating point @ 515V DC anode supply. Initially, on sine wave test into a dummy load, the output waveform was unstable, but we installed a 100pF/1kV cap across the anodes of the phase-inverter stage and that settled things down. The amp now delivers 20V into 8 ohms = 50 watts. Colin will have to take care not to blow the stock speaker which is only rated @ 35 watts. All functionality on this amp is now restored, we just hope the board holds up in the longer term for the customer.