ivanrichards

Back in the 50’s & 60’s, one of the largest musical instrument retailers in Sydney, Australia, was Nicholsons. I vaguely remember their premises in a prime location on George Street, in the CBD. They indulged in some importing & distribution of instruments, but we don’t know the full scope of this activity, and they also had some guitars & amplifiers manufactured here in Sydney.

Nicholson’s house brand was in fact Challenge, and they marketed various guitars & amps with that brand name. The guitars were possibly cheap knock-offs of Strat style guitars, and were likely sourced from Japan, as well as locally. The Challenge amplifiers were much more interesting and were seemingly destined to survive longer than the guitars.

The subject of this week’s blog is only the 2nd Challenge amp we have ever seen, since taking up amp servicing on a full-time basis, but obviously they are still out there in unknown numbers. The Challenge amps were designed & built for Nicholsons by ZJM Industries, located in Waverley, in Sydney’s eastern suburbs. The primary consideration for Nicholson’s would have been to produce the house brand products at the lowest possible cost.

The ZJM story is indeed a very colourful one. ZJM stands for Zoltan Julian Mielnik, and he was a Polish immigrant to Australia in the aftermath of WW2. He was rumoured  to have been in the Polish Army, possibly in their Intelligence Service, possibly was even a prisoner of the Russians and sent to Siberia. Some even suspected him of being a spy ! How does someone from such a background end up building valve guitar amps, and respraying guitar bodies, winding pickups, etc etc etc, in Sydney suburbia ?? We don’t have any of the answers to this mystery, but we would like to hear from anyone who does.

The amp that inspired this blog, was received from customer Greg as a very unknown quantity, requiring a full appraisal to decide whether a restoration job was justified. The most important thing, initially, was to prove that the two huge transformers were functional, which they were, so then we could arrive at an agreed $$$ figure to complete the job to get this amp useable and meeting electrical safety standards. We still had no idea what this amp would ultimately sound like, so it was a bit of a gamble for Greg, but one that paid off in the end.

As you can see from the photos, the amp is wired up in a seemingly fairly random manner. Low budget amps from this late 50’s/early 60’s period show scant regard for contemporary electrical safety standards – quite often there isn’t even a fuse. This amp required complete rewiring of the 240V side of things, with a new 3-core mains lead, 3-pin plug, the installation of a grommet and cable clamp, and the installation of a fuseholder & fuse. The mains earth connection requires its own dedicated nuts/bolt/solder tag – not a shared connection.

Every electrolytic capacitor looked original and needed to be replaced, which we did. Interestingly, all the signal capacitors are the now famous “mustard caps”, renowned for their great guitar amp tone. Many Aussie manufactured domestic electronics employed these capacitors in the 60’s. The larger caps are bedded down in silicone (neutral cure). A general clean up of jacks, valve sockets & pots was required, with some new pots installed. New valves installed were a 12AX7 in V1, a matched pair of EL34 output valves, plus a 5U4 rectifier valve.

The use of off-the-shelf transformers (rather than OEM units) in this amp is a dead give away that only small quantities of this model were ever going to be built. The transformers lack any kind of identification, but they look exactly like the multi-purpose units sold in the day through electronics wholesalers, and probably manufactured by Ferguson, A&R, or similar companies. The transformers are physically quite large, and were comfortably over-engineered for the job – no wonder they have survived.

This amp lacks any kind of a model name, model number or serial number, so we have no idea where it sits in the ZJM heirarchy of amp designs, or even approximately when it was built. We just know that it sounds great. Neil Young would love this amp. The front end is very similar to the famous tweed Deluxe amp as used by Neil, with a very simple circuit and just two volume controls and a single tone control, in this design common to both channels. The two volume controls interact with each other as per the tweed amps. This amp also has a very sweet sounding tremolo on board, which modulates the bias for the output valves.

The big difference between this amp & the tweed amps, apart from the big Aussie transformers, is the power amp stage, employing a pair of EL34/6CA7 output valves in a Class-A/cathode-bias configuration. These valves are drawing a lot of current, before you’ve played a single note, yet the transformers remain relatively cool. The power amp develops 20 watts/Class-A into 8 ohms, at the onset of clipping, and sounds huge. Greg assures me that when he got to turn the amp up flat-chat, it sounded even better.

You may have noticed only two of the original control knobs have survived – the others are generic. The Challenge sticker has also been removed at some point. The only way we knew for sure that this actually was a Challenge amp was by direct A/B comparisons with another Challenge guitar amp ( a 15 watt 1X12 combo). The grille material looks the same as that used on the Moody amps, also of Sydney (Lidcombe), which initially caused some confusion.

Due possibly to the complete lack of high frequency pre-emphasis, and the huge midrange response, this amp sounds particularly fine with Telecaster style guitars, and other brighter instruments. You know – we Aussies were making very unique & great sounding amps as far back as the late 1950’s (and even pedal steel guitars as far back as the late 1940’s), and yet hardly anybody remembers or knows anything about any of this stuff. So much of it seems to have disappeared off the face of the planet (apparently). It goes without saying, we would love to hear from anybody who has any more information about the Challenge amplifiers and also their builder, ZJM Industries. This is all part of our musical heritage, so let’s try to keep it alive !   Best regards, IR.

A customer brought in a recent acquisition – a late 1970’s Silverface Fender Princeton Reverb 1×10 combo amplifier, serial # A816986, for a complete overhaul & safety check. This amp was obviously in pristine condition, probably stored somewhere nice & dry in recent years without much use, and has rarely, if ever, been serviced.

This could be a blessing in disguise, as the amp has remained all original with no dodgy repairs or modifications to deal with. Ben advised us that the amp was farty and all the pots were very scratchy and cutting in & out. Before we even got to an appraisal of the chassis and electronics, we discussed with Ben the options for a suitable upgrade speaker unit, as the stock speaker in these amps is very ordinary, especially after 30+ years.

As you can see from the photo on the left, Ben accepted our recommendation to supply and install the Celestion G10/Vintage in this amplifier. Regular readers of this blog will know that we enthusiastically recommend certain speaker units by Weber, WGS and in certain applications Jensen Neo 10 & 12-inch speakers. We will also be trying out some Scumback speaker units in the coming months. However, in this amplifier upgrade the G10 is a very cost effective choice, which fills in the previously missing details in the lows and low mids, without sacrificing the top end.

The first thing we noticed about the chassis, apart from the fact that everything including the valves (tubes) appeared to be original, was that the voltage selector was incorrectly set to 220V instead of 240V. These voltage selector switches have a pretty rubbery feel to them, and it’s quite easy to knock them off the required setting. It does mean that this amp was being operated with higher internal voltages than normal. We replaced the intermittent power on/off switch with a Carlingswitch, and rewired the 240V primary so that the fuse is in series with the active (ie, brown) wire. We installed the correct value fuse for 240V mains, and replaced the 240V 3-pin plug. The amp passes the Workcover NSW electrical safety test without further problems.

We cleaned all the pots with FaderLube, and the input jacks with DeOxit. This cured the issues with scratchiness. The main power supply capacitor was well overdue for replacement. This particular can-mounting capacitor is actually four capacitors in one single metal can, which is of course earthed (grounded) to the chassis. Fortunately these capacitors are being manufactured again by a USA based amp parts distributor, and we replaced the 20+20+20+20F/475V can.

Before we could install a fresh matched pair of 6V6 output valves, a small upgrade was required. These amps were never originally built with screen grid resistors, which is NOT a great idea with modern valves (tubes).  We installed a pair of 470 ohm 5 watt wire-wound resistors directly on to the octal sockets, and rearranged the wiring accordingly. This will help the 6V6’s to survive by limiting screen grid dissipation when the amp is driven hard, and also will contribute to amplifier stability.

We set up the amp with a new matched pair of 6V6 output valves, and found that they were under biased, ie the quiescent no-signal current drawn was too high, exceeding the ratings of a 6V6 before actually playing a note. The bias supply in this amp is non-adjustable, so required paralleling of a pair of resistors in the circuit to set the desired bias voltage. An acceptable no signal current draw with the voltage conditions present in this amp is approx 22 to 24 ma per 6V6, which equates to around 9W anode dissipation per valve. We also installed a pair of spring clips to firmly hold the valves in their sockets.

The final power output test and burn-in test revealed a power output into an 8 ohm resistive load, at the onset of clipping, of 11.75V/8 ohms = 17 watts (rms). The clipping of the output signal is quite assymetrical, which is a characteristic of this particular amp design, or more specifically the type of phase-splitter employed. Don’t worry about the technical details – it sounds good, that’s all that really matters.

So, how does it sound ? Well, the amp sounds very good at conversation levels, which is what I think the customer was anticipating, and it packs a punch as well, thanks to that speaker upgrade. The sound is very balanced now across the guitar spectrum, as compared to before. The reverb and tremolo are fine, with somewhat less depth than some of the bigger Fenders. The negative comments that apply to some of the Silverface Fenders, in particular from the late 1970’s, don’t seem to apply to the Princeton.

The schematic for the Princeton doesn’t seem to change very much at all from the Blackface to the Silverface models, so one possible theory is that the Princeton was considered so low down in the Fender amplifier heirarchy, that the engineering staff just left it as it was. Whatever the reason, I am quite impressed with the end result of this service and upgrade, and had a bit of a blast through it with a couple of my Telecasters. Many thanks to Ben for his custom and for supplying us with the subject matter for this week’s blog – IR.