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

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

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

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.

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

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

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

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.

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

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

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

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

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

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

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 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.

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)

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)

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

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

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

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

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).

A degree of common sense will keep your amp in top form

and the following tips should help you and your amp

run smoothly and safely for years  

1) Use your Standby switch

Put simply, the Standby feature found on most valve amps lets the heaters in the valves warm up, whilst not actually putting signal to them. Turning your amp on and off puts strain on the valves, so using the Standby where possible will prolong valve life.

2) Don’t let your amp get damp

For both safety and reliability, keep your amp dry. Storing the amp in a garage or playing outdoors can affect components and cause noisy operation. Never put drinks on an amp or play in the rain – there are dangerously high voltages present within valve amplifiers, and the resulting currents can cause terrible injury, and even death.

3) Check your plugs

The wires in any plug can work loose, with disastrous results. Again, because of the currents involved, shorting wires can lead to shortened lives ! Beware !

4) Use a residual current device (RCD)

An RCD will trip the power should problems occur, or if it recognises faulty wiring in a venue, for example. Every plug board used should ideally have its own RCD. If in doubt, don’t play !

5) Change your power output valves regularly

Valve amps have two sets of valves for pre-amplifier stages and power amplifier stages (some amps boast valve rectifiers as well). Change your power output valves annually if you are constantly using your amp, but never singly – one incompatible valve will bring the others down very quickly.

6) Make sure your speakers are connected

Before powering up your amp, be absolutely sure that a working speaker cable (never a screened guitar cable) is fitted between the amp and speaker cabinet. Speaker emulators and attenuators such as those made by THD, Dr-Z, Marshall, Palmer and others are fine, as they safely simulate the speaker load presented to the amplifier.

7) Don’t touch hot valves

Avoid the pain that a severe burn can cause – and the inconvenience of blistered fingertips.

8) Impedances must match

Your amp’s performance and valve life will be compromised if the impedance is not set to match that of the speakers in the cabinet. Adding extension cabinets will affect the overall impedance. For instance, adding an extra cabinet of the same impedance as the existing one will halve the overall impedance – two 16 ohm cabinets require the amp to be set to 8 ohms. Two 8 ohm cabinets require the amp to be set to 4 ohms.

9) Volume down before powering up

Big valve amps can give out damaging sound pressure levels (SPL’s). Sudden loud noises can cause permanent hearing impairment, so always have the Master Volume down before switching ON.

10) Care when lifting

Due to their hefty construction and big transformers, valve amps are usually heavier than their solid-state equivalents. Never lift an amp or cab with a bent back – keep it straight and bend from the knees. If it’s too heavy, get help.

Warning !

Valves [vacuum tubes], transformers, capacitors, amplification circuits found within a guitar amplifier operate at high voltages that can cause permanent injury, disability or death. Valves [vacuum tubes] operate at high temperatures that can cause severe burns. Never attempt to repair, modify, test, work on or touch electronic equipment unless you are trained or otherwise qualified to do so. Likewise, never remove a protective cover from electronic equipment unless you are trained or otherwise qualified to do so.

Warning !

Do not remove the amplifier rear panel, the amplifier chassis, the amplifier fuses, the valves [vacuum tubes], or any other part of the amplifier with the 240V AC mains supply connected.

]]> http://ivanrichards.com.au/2013/03/08/10-ways-to-protect-you-and-your-amp/feed/ 0 ivanrichards http://ivanrichards.com.au/2013/03/03/mods-upgrades-to-the-pro-junior-amp/ http://ivanrichards.com.au/2013/03/03/mods-upgrades-to-the-pro-junior-amp/#comments Sun, 03 Mar 2013 09:36:00 +0000 ivanrichards http://ivanrichards.com.au/?p=2413 ]]>

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

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

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

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

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

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

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)

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.

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.

Our 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

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

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.

To 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

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

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

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

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

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

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 !

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.

the VHT Special 6

Welcome back to the blog ! Back on December 7th 2012, we published a blog describing repairs to a VHT Pittbull Classic valve (tube) guitar amplifier. The Pittbull was a 3-channel, 100 watt monster built about 20 years by the original VHT amplifier company in the USofA. Since then, the original owner sold on the rights to the VHT name, and the modern VHT amp is made in China, and is generally a much lower powered amp.

the VHT Special 6

We recently repaired a VHT Special 6 ULTRA for Steve, a regular customer. This amp is loosely based on the well known Fender “Champ”, but features an additional 12AX7 gain stage, with the inclusion of  separate low-gain & high-gain input jacks, so the player can bypass the high gain stage, and also an output attenuator and additional tone shaping switches. The power amp is the traditional single-ended 6V6 valve output stage, employing cathode-bias, ie the amp is self-biasing. Power output is 5 watts @ the onset of clipping.

the VHT Special 6

The speaker is unfortunately only a cheapo Chinese unit, but the cabinet is much more generously proportioned than many other similar amps, so we would assume that upgrading to a quality replacement speaker would result in much improved performance, particularly ref the lows & low mids. Another obvious upgrade would be the valves themselves, which is where this blog is heading.

rear view

The stated fault on this amp was “it just stopped”. The external & internal fuses were not blown, and the DC voltage conditions appeared normal, but the output stage was non functional. We soon realised why. The Chinese 6V6 had gone S/C (short-circuit) internally, destroying the 470 ohm cathode-bias resistor in the process, which acted like a fuse. The chassis in this model is folded in a U-shape so getting to the parts of the circuit you need to get to with a soldering iron is time consuming, adding to the cost of repairs. However, by dropping out the octal socket and twisting its position we could remove the blown resistor and install a replacement W22 series 7 watt ceramic body 470 ohm, our preferred type for demanding conditions.

the chassis is removed & on the workbench

While we had the chassis out we took the time to correct a few “dry” solder joints, which would have caused intermittent problems in the long-term. We can confirm that these amps are actually “hand-wired” in China, and the quality of assembly is no worse than what we are seeing from the major guitar amp manufacturers. Naturally, the larger hardware items such as transformers, speaker, etc, are cheaper units, but this is only what you would expect from a budget amp.

we have replaced the cathode bias resistor

To complete the repair, we installed a new (re-issue) 6V6GT output valve by Tung Sol, one of the best of the current production 6V6′s, plus at Steve’s request we replaced V1 with a 5751 preamp valve. This reduces the preamp gain by about half, allowing a much wider range of clean to crunch tones to be easily obtained. Steve was very happy with the end result. So there we have an example of the contemporary VHT guitar amp. Many thanks to Steve Molenaar for his continued custom & loyalty. IR.

the Morgan SW50 head

Many of you will already know that for many years now we have specialised in the repair, service & restoration of hand-wired valve (tube) musical instrument amplification, including many of the “boutique” grade amps that have appeared on the scene since the 1990′s. This is our first customer repair job for a relative newcomer to the “boutique” scene, the Morgan amp from California.

rear view – Morgan SW50

The Morgan amps have enjoyed an increased profile in recent times, and have appeared in numerous YouTube demonstrations. This particular model is known as the SW50, and is a stripped-down, single channel dual 6L6 powered amp delivering approx 45 watts @ the onset of clipping. The power supply is quite conventional, employing silicon diodes for rectification. The transformer & filter choke set is by Mercury Magnetics.

rear panel removed

The Morgan website implies that the design of this model is loosely based on the clean channel of a customer’s early 1970′s Dumble amp. Clearly, the design was revoiced & refined quite significantly to become the SW50, which is primarily a clean, percussive amp with very late break-up. The 2-band EQ is more interactive than a “Blackface” Fender, for example. The bright switch is very effective at low settings of the volume control, the bass switch is obviously Dumble inspired but unfortunately primarily boosts very low frequencies and is best left off.

top/rear view of SW50 chassis

The SW50 is actually quite dark running with the volume control set high and the level (ie, master volume) set low. Conversely, the SW50 is quite bright with the volume control set low, and the level set high – this is when the bright switch is most effective. There is more than enough bottom-end with the bass switch off. The preamp employs a single JJ 12AX7 valve (tube), plus a 2nd JJ 12AX7 for the phase-inverter stage.

chassis interior view from front

This particular Morgan was creating some weird resonances on selected notes being played. We discovered that V1 was microphonic, and the pair of SED 6L6GC output valves (tubes) was in very poor condition, with a much weakened internal structure vibrating at selected frequencies. The amp was under-biased, meaning that the 6L6′s were running far too hot for long term reliability.

chassis interior view from rear

We installed selected replacement valves & rebiased for a more reasonable current draw. On power output test there was no crossover distortion evident and we measured 19V/8 ohms which is 45 watts, @ the onset of clipping. Mains voltage here in Wyoming NSW is approx 245V AC. I should point out that at this point in time we have no official relationship with Morgan amps, we are not an authorised warranty agent. However, we are now offering world-class non-warranty service/repairs to all Morgan amps for Australian customers. Regards – Ivan Richards.

The photos show Dean’s new Richards amp sitting on top of Jonny’s favourite well played in Marshall 4×12 speaker cab, but we demo’d the amp through a variety of differently voiced speakers. Some amps only sound at their best when connected to specific models of speaker, but this amp sounded great connected to any speakers we had access to. We originally discussed the assembly and design concepts of this amplifier in some detail in our earlier blog dated July 17th, 2012. There are two important details that differentiate this example of the Blue Mood series amps from other custom orders, apart from the choice of 6V6 output valves.

Firstly, this amp includes the optional extra of a valve-driven FX Loop, please see the original blog. Secondly, Dean chose the Mercury Magnetics reproduction of the original Fender “Brownface” DeLuxe 6V6 audio output transformer. This transformer has only a single 8 ohm secondary winding, so there is no impedance selector on the rear panel as per most models. However, this limitation is more than compensated for by the trademark Mercury sound that is smooth, exceptionally musical, warm and balanced. Sweet, rich, detailed and seductive !

So………………what is the intention of the Richards Blue Mood series amps ? To equal or surpass the tones of the great late 50′s tweed and the early 60′s blonde and brownface guitar and bass instrument amplifiers. The Blue Mood gives you finer control over your tones and less power supply noise than the originals. The amps are designed around 6V6 or 6L6 power amp stages, although we do also build a 30 watt EL34 model with a more ‘British’ voicing, as a tribute to the great TW style amps. In the latter case, the control panel and chassis layout remains unchanged. The EL34 model will be the subject of a forthcoming blog. Regards – Ivan R.