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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. Please enquire.

While on the subject of Carr amps, contact us at amptech@ihug.com.au 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.

Mobile number for urgent enquiries is 0418 862 034 (within Australia), and 61 418 862 034 (international).

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.