ivanrichards

Hello and welcome back to the blog after a break of several weeks. This time we are looking at a classic hard rock guitar amplifier from the USA, manufactured back in the early 1990′s: the VHT Pittbull amp. This example has obviously had a hard working life, and our repairs actually started with the cabinet, which was beginning to fall apart. The stated fault from our customer was there was a pop, then nil output. No fuse had actually blown, and all the valves tested OK, so the problem was in the circuitry itself.

All of the circuitry in the Pittbull is mounted on two large printed circuit boards (p.c.b.’s) which run pretty much the width of the chassis. Unfortunately, they overlap and things are complicated by the board mounting arrangements, so it is a very time consuming job removing these boards for repairs, plus it’s fiddly, with the ever present danger of introducing additional faults that weren’t there in the first place. Even worse, the status LED’s that indicate which channel is active, are mounted below the boards where your fingers can’t get to them to line them up with the holes in the front control panel.

Examination of the boards didn’t reveal any obvious component failure, but a check of the DC conditions with a DVM indicated problems with the low-voltage (LV) power supply, ie the +/- 18V DC rails (regulated). Apparently the signal path includes some solid-state circuitry, hence the complete loss of signal, and there is also some switching/control circuitry which is IC based. Unfortunately all the device identifiers have been intentionally rubbed off the IC’s ! The original VHT company definitely didn’t want to make servicing their products any easier than necessary. This is a design approach that we have never ever understood or endorsed.

A silicon diode, part of the bridge rectifier for the LV supplies, had gone short-circuit, also causing the failure of filter capacitors and a regulator IC. We replaced the identified components, and also upgraded a few electrolytic capacitors while we had the board out. The LV supplies were now completely functional. As part of our routine electrical safety checkover, we noticed the mains fuseholder was wired incorrectly – strange to find such a basic error in a modern amplifier. We reversed the fuseholder connections and carried out a PAT test, and this amp now complies with Workcover NSW electrical safety standards.

Having reassembled the chassis and reloaded the valves as supplied (with a very necessary rebias of the output stage – running too hot), the amp was given a power output test. This amp is quite a monster, with 540V DC supplied to the anodes of the EL34 output valves (like some very old Marshalls). We measured 35V into an 8 ohm dummy load which equates to 150 watts ! This is pretty impressive, considering the amp claims to be 100 watts. A burn-in test revealed no further problems, and the final play test showed this amp to be a much more versatile beast than you might have assumed.

As a sweeping generalisation, we have found the “clean” channel in multi-channel switching amps to be a real disappointment, with all the emphasis placed on the high-gain performance at the expense of clean and crunch tones. In this Pittbull Classic model, the three channels each offer two modes or gain levels and all the settings are useable. The “lead” and “crunch” channels do admittedly share the same EQ, but the very useable “clean” channel, with its own dedicated EQ, more than makes up for this compromise. Rock on ! Many thanks to Joss for supplying us with the subject matter for this blog. IR.

We discussed in a much earlier blog that we provide service & repairs for the David Eden brand of bass amplification. In today’s blog we are showing the “World Tour Series” WT800 model as an example, but this blog probably applies to most of the range at this time.

We recently implemented a MOD to this amp at the request of our good customer, Mr Paul Disspain. We can’t claim any originality for this as the idea came from an example on the David Eden user forum. Initially we thought the idea was a bit overkill, but now wholeheartedly agree this is a MOD that greatly enhances long term reliability.

Commercially made amplifiers often have little design short cuts or oversights where the design team momentarily dropped the ball, and David Eden is no exception. Specifically, the low voltage power supply in this model has been done “on the cheap”. Rather than design dedicated low volts supply rails from their own transformer winding, the design simply drops the power amp volts supply rails by way of inserting series resistors, designated R62 & R64.

The end result of this technique is that the voltage dropping resistors generate a LOT of heat, even though R62 & R64 each comprise a pair of 5 watt ceramic body components. We are talking about enough heat to desolder the connections over a period of time. This heat is constant, the whole time the amp is powered on. The resultant poor connectivity gives rise to intermittent faults – not necessarily a total loss of signal, but unpredictable drops in signal level and functionality which mysteriously correct themselves.

The intention of the MOD is to get the low voltage supply dropping resistors off the printed circuit board altogether. No more heat being absorbed into the board represents a big improvement in reliability. The next obvious step is to up the power dissipation rating of the resistors. If we use aluminium clad resistors, then these by design are suitable for mounting onto a heatsink. A pair of 15 watt resistors each for R62 & R64 (ie, four in total) mounted to an aluminium heatsink, suitably mounted to the bottom section of the amp chassis, in the space between the front board and the toroidal transformer will require only short lengths of hookup wire to connect back to the board.

You can see in the photos that we have to strip out the front board completely from the chassis. Removing & replacing the printed circuit board without introducing any additional problems is a time consuming element of this work. We need access to the solder side of the board to clean up any problems, remove the old solder and possibly repair any heat damaged tracks & pads.

While we have the board out of the way, we can mount the heatsink, taking care to leave enough clearance to actually be able to reinstall the board. This task is fiddly enough as it is, without the resistors in the way. We don’t want the heatsink hard up against the transformer or the interconnecting wiring.

The final photos show the board reinstalled and the interconnections between the aluminium clad resistors and the former positions on the board for R62 & R64 completed. The final arrangement still creates some heat, but nothing at all like the original design. With one channel only driven into a 4 ohm resistive load at a time, the power amps deliver 480 watts, which translates into a nominal 400 watts per channel with both channels driven into 4 ohm loads.

The whole job takes at least 2 hours to complete. It is a worthwhile improvement in reliability for those owners of the original David Eden amplifiers who intend to keep their amps for the long term.

A Silverface Fender Champ amp, serial # A34003, date of manufacture August 29th 1972, was purchased on Ebay by Jessica, a young lady just starting out in the music industry here on the NSW Central Coast, and is the subject of this blog. This was a smart purchase on Jessica’s part, but required a major overhaul to bring the amp up to performance standard, which of course we were happy to do.

This amp was purchased off Ebay from the USA, so first and foremost we needed to replace the 110V power transformer with a 240V primary replica Champ transformer, plus install a 240V 3-core power cable and a 3-pin plug, including the installation of a dedicated chassis earth connection. The fuse also needed to be changed to a suitable value for 240V operation.

Next item on the agenda was the speaker – the original 8-inch alnico magnet speaker was doing this amp no favours at all. There are replacement/upgrade speakers for the Champ available from Jensen, Weber & WGS. We chose the WGS model, which is a significant upgrade compared to the originals. Seriously folks, WGS are making some great replacement and/or upgrade speakers at very competitive prices. We did have to enlarge the speaker mounting holes somewhat, to mount the new speaker on the existing bolts.

The Champ was now much louder and more toneful than the day it left the factory. There were just some relatively minor issues remaining to be corrected, ie noisy electronics. The pots and jacks were cleaned, as always, with Caig FaderLube and DeOxit. The power supply capcitors were actually 4 x 20uF/475V DC in a single chassis-mounting metal can. Fortunately, these capacitors are being manufactured again by CE Distribution (USA), so we were able to replace this unit, and maintain the original power supply design and appearance.

The photo on the left shows a capacitor that should be automatically removed before running these amps on the 240V mains. All that remained now to complete the overhaul, was to install spring clips to the octal valve sockets (to prevent upside-down valves from falling out), and to carry out power output and electrical safety tests. The amp delivered 4 watts into 4 ohms, at the onset of clipping.

The photo on the right shows you how we could date this amp so precisely – a Fender factory worker named J. Castillo date stamped the amp AUG 29 1972 ! regards, IR.

Welcome back to the blog, everybody ! We haven’t published a new blog for about 6 weeks, but things have been rather busy at the Richards Amplifier Company – Australia. Well, it was quite a surprise when we were recently contacted by our very good customer Victor, to say that he had acquired a Vedette EL34 amplifier, built back in the 1960′s in Sydney by ZJM Industries.

This is the first ever Vedette amplifier that we have encountered, and only the third amp by ZJM, so not many appear to have survived. For a more detailed discussion of ZJM Industries, please check our blog dated June 26th 2012, titled the Challenge guitar amplifier.

The Vedette is loosely based on the Fender “Blackface” amp designs, pre-reverb, but with EL34/6CA7 output valves and those big Aussie transformers that we noted with the Challenge amp. The sweep of the controls on the Vedette is a bit different, and the midrange is a bit bigger. The standout feature is probably the valve-driven tremolo, which gives the amp a lot of character.

While the EL34 & GZ34 valves had been replaced at a previous service, many important service issues had been completely overlooked. The photo to the right shows we replaced a couple of power supply caps that were well past their use-by date. The speaker jack & #1 input jack on the tremolo channel also required replacement. The tremolo didn’t work at all, which required a new 12AX7 plus a cathode bypass capacitor. The prescence control was no longer functional, as a result of someone’s experiment, and required rewiring.

But the most significant outstanding problem was that this amp was electrically unsafe to use ! We installed all new 240V 3-core cable & 3-pin plug, plus a fuse in series with the active (brown) lead, installed on the chassis rear panel, and also a dedicated chassis earth connection (not a shared connection). Now the amp is safe to use.

The wiring in the Vedette is pretty rough in places, and the sweep of the tremolo and eq pots is a bit frustrating for the user, with most of the action compressed into a few degrees of rotation. But, actually the amp works fine and also background noise is quite low, in spite of the random routing of wiring, which pretty much breaks all the rules for audio electronics.

Once again, we appeal to anyone out there who has some knowledge of the ZJM guitar and amp building enterprise in Sydney back in the 1960′s, to make contact and perhaps we could publish a more complete history. We will be back soon with a new blog publishing some photo’s of Victor’s new Richards Blue Mood series 12W 6V6 amp.

A new customer for us, Peter Timperley, delivered his Laney LC30 II 1×12 combo to the workshop for a general service, but with a special request. Peter wanted to restore the sweet tone that previously existed with this amp, but also wanted to know if we could install a full power/half power switch. The obvious way to implement this mod would be to switch out one pair of the quartet of EL84 power output valves, and correct the impedance mismatch that this introduces by utilising the impedance selector switch on the rear panel.

In other words, we need to increase the primary impedance that the EL84′s require from 4000 ohms a-a (anode to anode) for a quartet, to 8000 ohms a-a for a pair. We can achieve that by driving the internal 8 ohm speaker from the 4 ohm connection to the output transformer (see photo #5). That is not the full story, however, as this amp employs a cathode-bias configuration for the EL84′s, so when we switch to half-power we also need to change the bias circuitry appropriately, otherwise the EL84′s will be quite seriously under-biased, resulting in certain destruction.

There is not much depth from the rear of the cab to the rear panel of the amp chassis, so we selected a low profile DPST (double-pole, single-throw) rocker switch to mount on the rear panel. One pole is used to switch out (in this case) the outer pair of EL84′s in low-power mode, leaving the inner pair activated for both scenarios. The second pole is used to switch in an additional cathode-bias resistor (plus cathode bypass capacitor) in series with the existing p.c.b. mounted bias components. By this method, the bias voltage is maintained exactly the same, regardless of mode of operation.

Creating a 19 x 22mm hole for the rocker switch on the rear of the chassis really isn’t much fun at all, unless you happen to own a hydraulic hole punch. The cathode pins (pin 3) on the EL84 9-pin sockets were rewired, with the inner & outer pairs now having separate paths to the switch. You can see from the photos, the additional cathode-bias components are wired on a small tagstrip installed conveniently near the valve sockets. One end of these components will be wired back to the original cathode bias resistor, on the main board, and from there to ground (earth).

We added some Dymo labelling on the rear panel to finish the job. How successful was the mod ? The final power output test resulted in 32 watts/8 ohms full-power, and 17 watts/8 ohms half-power. The tonal quality in half-power mode was in no way inferior to full-power, in fact it might have been a tad sweeter ! Peter contacted us after taking the amp home to confirm he was very pleased with the result. As part of a general service, prior to implementing the mod request, we replaced corroded input & output jacks, cleaned all pots with FaderLube, replaced a dodgy Standby switch, and replaced some high voltage electrolytic capacitors with selected upgraded caps. IR.