Archive for the ‘Amp Repair’ Category

MODs to the Fender ‘Super 60’ amplifier

May 14, 2016

Super 60 016Hello again. The Fender Super 60 was introduced in the late 1980’s as one of the dreaded ‘red knob’ series of amps, which weren’t always favourably received. This model caused some confusion for people who purchased one, as it sounds nothing like the traditional Fender amplifier models. It has a few admirers, to be sure, but the model was eventually deleted. On the plus side, this is a 60 watt 1×12 amp in a compact combo (although quite heavy as a result), with ‘clean’ and ‘overdrive’ modes plus reverb (solid-state driven, not valve/tube). The ‘clean’ tonality is good, it’s just not quite the same as a ‘blackface’ Fender.

Super 60 011On the minus side, the ‘overdrive’ mode is not that great, and the clean/overdrive tonalities and levels really don’t match up. This is due to the amount of shared circuitry, which is re-configured using opto-couplers. The bias supply voltage is not adjustable, leading to complications when installing a fresh pair of 6L6GC output valves. So, the service tech has to change resistor values as required to achieve the desired result. This is really tedious ! However, the PCB can be modified to accept a bias trimpot.

Super 60 012Which leads us to serviceability issues with these amps. As you can see from the 2nd and 3rd photos, all components (apart from transformers) are mounted to either the large main board (PCB), or the smaller front panel board. Unfortunately for the service tech, these boards are kind of ‘back-to-front’ with the solder side of the board facing upwards, and the component side facing downwards, so they cannot be inspected without stripping out the boards. The valve sockets are mounted direct to the main board.  The three preamp valves are not especially a problem, but the heat from the pair of output valves will to some extent end up being absorbed into the chassis and the main board.

Super 60 013This customer’s amp was delivered to us with low output level, a couple of badly damaged pots, and broken jacks. The input and footswitch jacks on this model are the very brittle and easy to break PCB mounting types from the 80’s and 90’s. The pots have a solid shaft with a flat section, and the original knobs naturally match this shape. The ‘clean’ mode level was considerably lower then the ‘overdrive’. We would have to completely strip out the boards anyway, so this was the perfect opportunity to implement some MODs to try and improve the ‘overdrive’ qualities plus balance up the two modes a bit better. This ‘balancing act’ doesn’t achieve absolute perfection, but is a definite improvement over the stock amp. We would suggest leaving the 3-band EQ as it is, to maintain the ‘clean’ tonality, even though the EQ is less than ideal for the ‘overdrive’ mode.

Super 60 015For those of you who have access to a schematic we modded the following components: R167 to 82K, C102/C105 to 0.01uF, R103 to 100K and R158 is the resistor to be changed for different bias levels, or else remove altogether if a trimpot is to be installed. We rebiased for 2 x 35ma Ik at B+ of 487V DC. Biasing this amp to run much hotter would be a mistake, as per the preceding paragraphs. The end result is still a compromise, but the changes will result in a fuller signal range and also more saturation available for a more modern ‘lead’ sound. Upgrading the preamp valves will improve the tone of the amp in both ‘clean’ and ‘overdrive’, and will smooth out the spiky distortion characteristic and reduce hum/noise. We have more Fender MODs to be published soon. IR.

Super 60 017p.s. the previous owner of this amp discarded the red knobs, but installed some really cheapo looking ones in their place; we ended up matching up some Fender vintage reissue black knobs, which look much better.

p.p.s. some comments on the web forums mistakenly attribute this amp to the Paul Rivera era at Fender – this is not the case, this design is dated 1988 – well after Rivera moved on to manufacture his own products. IR.

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.

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6L6GC conversion for the DeLuxe Reverb Reissue (DRRI)

April 24, 2016

DRRI Project P1020347Welcome back ! Hopefully we are now back into a routine of publishing a fresh blog every week. In this week’s blog, we discuss a major upgrade project recently completed for the (Fender) ’65 Deluxe Reverb Reissue, which we will refer to as the DRRI. The guitarist in one of Sydney’s best loved and longest surviving blues bands has been playing through a Twin Reverb for decades, but decided to downsize to a lighter amp for the smaller rooms, which makes complete sense, and invested in a new (2015) DRRI.

DRRI Project P1020345As you can see from the photos, this particular Limited Edition DRRI looks very smart indeed, with the early ’60s Brownface amps cosmetics. Also included in the Limited Edition package was a Jensen (new production) speaker. All other aspects of this amp were identical to the standard edition DRRI.

DRRI Project P1020346Unfortunately, the tonality presented by this amp was very thin and rather shrill. Overdrive characteristics when the amp was turned up (no pedals) were actually pretty awful. We attribute a lot of this to the choice of speaker. Just as critically for our customer, the lack of headroom with this amp and speaker combination defeated the whole purpose of investing in this particular amp, as it was virtually unuseable even for the smaller gigs.

DRRI Project P1020343The first step in this project was to upgrade the speaker, as is so often the case with combo amps. Choosing a speaker is not as easy as it sounds – there are a bewildering number of specialist guitar speaker options out there now. WGS offer some excellent choices at very reasonable prices, and we ended up choosing the G12C from their American Vintage range. We could just as easily have chosen the G12C/S for a smoother, softer top-end, but this model does seem to complement the DRRI voicing and adds a couple of dB in efficiency which will help with the headroom problem. Most importantly, the G12C fills out the missing lows and mids of the DRRI and behaves itself very well under overdrive conditions. The 75 watt rating of this speaker allows a healthy overload margin.

DRRI project P1020322Having restored the basic tonality of the amp with our new speaker, in consultation with the customer we devised a major upgrade project to increase the headroom available by implementing a conversion from the existing 6V6GT output valves to the larger 6L6GC output valves. To take full advantage of the larger valves we also replaced the existing transformer set.

DRRI project P1020320The available space and the need to allow clearance for the 12-inch speaker restricts our choices somewhat, but by re-orientating the output transformer and choke just slightly, we could install the power transformer, filter choke and output transformer set as per the 60’s Vibrolux/Tremolux series 6L6 amps. Such a reproduction transformer set was available at very reasonable pricing from Magnetic Components of Chicago, USA, although the cost of shipping adds quite a bit these days.

DRRI Project P1020337Such an upgrade will increase the power output of the amp to between 30 and 40 watts, as per the original amps of the abovementioned series, without a huge increase in the weight of the DRRI. It will be necessary to modify the bias circuitry slightly (one resistor) to increase the range of the bias adjustment to accomodate the 6L6 output valves. We also upgraded the main high-voltage filter capacitor to the good quality F&T brand.

DRRI project P1020321This was not an inexpensive project by any stretch of the imagination, but much, much cheaper than selling the amp on and reinvesting in a more suitable amp. We have replaced the shrill and brittle tonality, as presented by this example, with a very warm, full sound, still characteristic of the best examples of the Fender Deluxe Reverb, and increased the headroom significantly. This is now a fine performance amplifier which will serve our customer well !

DRRI Project P1020344

Correcting electrical safety issues in a Bad Cat ‘Classic Cat’

April 9, 2016

Bad Cat 0159Hello again. In this week’s blog we look at correcting some electrical safety issues in a Classic Cat 1×12 combo from Bad Cat amplifers, of the USA. This particular Bad Cat would appear to be a limited edition, maybe even a one-off custom order, and we could find no reference to it at all on their website.

Bad Cat 0163Most Bad Cat and Matchless amplifiers are derived in some way from (or inspired by) vintage 1960’s made-in-UK VOX amplifier circuits. This model is completely different, and is obviously derived from 1950’s Tweed/Fender and Valco/Supro amplifier designs. Classic Cat controls and functions are really sparse – just a pair of input jacks plus volume and tone controls. That’s it ! The design follows the classic Supro pattern of a pair of 5881 or 6L6 output valves, cathode-bias, for a nominal output of 24 watts rms at the onset of clipping.

Bad Cat 0164There is a single 12AX7 gain stage followed by interactive volume control and single (“tweed”) tone control. The effectiveness of the tone control depends on the position of the volume control and also offers no control at all over bass or lower midrange frequencies. This stage is followed by a 12AX7 valve employed as a “paraphase” (self-balancing) phase-inverter, very similar to some mid-1950’s Fender (“tweed”) designs. However, this type of phase-inverter is not known for achieving perfect balance or low distortion, and so adds to the raw vintage character of the amp’s performance.

Bad Cat 0154Our good customer Danny brought this amp into the workshop to resolve some electrical safety issues. The most obvious sign of trouble is the broken IEC 240V power inlet. Further investigation revealed a lost earth connection, plus a loose mains fuseholder which could not be tightened. This latter component was also not compliant for 240V in the first place. We replaced the IEC inlet and installed a dedicated chassis earth point. We replaced the dodgy fuseholder with a compliant component and installed a T1A fuse.

Bad Cat 0155The amp now passed the Workcover NSW electrical safety test with flying colours, meaning that we achieve an earth test of less than 2 ohms from the earth pin of the 240V plug to any exposed metal part on the amp. You can see from the next photo the IEC inlet and fuseholder wired correctly, plus the dedicated earth connection to the chassis. The final photo shows the internal layout of the chassis, and hand-wiring Bad Cat/Matchless style.

Bad Cat 0158We would like to thank Danny for his continued custom, and for supplying us with the subject matter for this blog. We offer world-class service, restoration, repairs and advice re most boutique level valve (tube) musical instrument amplification.  It HAS been a lifetime obsession.

 

Bad Cat 0156Please check out our earlier blogs re amplifiers by Matchless, DR-Z, Landry, Jackson, Morgan, Matamp and others, plus our own Richards custom design amplifiers.

Repairs to the Peavey Classic 50 in Australia

April 4, 2016

Classic 50 P1020263Welcome back to the blog, everyone. We have been servicing the Peavey Classic Series 30 watt and 50 watt combo’s since their introduction back in the 1990’s and we are in fact still the A.M.I. authorised Peavey warranty repairer (valve/tube amps only) for NSW. Since the 1990’s the 30 watt 1×12 combo has overtaken the 50 watt models in terms of popularity as they provide reasonable performance in a compact package. Reliability has been questionable at times – we have had to repair printed circuit boards (PCB’s), replace transformers, valves and 9-pin valve sockets, and so on. Bias for the EL84 output valves is non-adjustable, which means some amps run rather hot.

Classic 50 P1020262The subject of this blog is a more recent 2×12 model delivered to us by a local music shop, reported as losing output and nil reverb level. It may not be immediately obvious from the photo, but the speaker connection tagstrip, normally pop-riveted to the speaker basket, has come adrift, causing unreliable operation. This is a problem we have witnessed many times with the cheaper OEM speakers supplied by Eminence of the USA to various amp manufacturers. The only reliable solution is to carefully drill out the remaining body of the pop-rivet and refasten the tagstrip by other means.

Classic 50 P1020264In the meantime we isolated the ‘nil reverb’ problem to the reverb tank itself, which is mounted in a vinyl bag at the bottom of the cab. The only way to gain unrestricted access to both the problem speaker and the non-functional reverb tank is to remove the speaker baffle completely from the cab. This is what is known as a ‘floating baffle’, with mounting screws along the top and bottom front of the cab, which must be removed. The amp chassis has to be removed before this can happen, of course.

Classic 50 P1020267Once the baffle is removed and placed face down on the workbench, we can repair the speaker. We choose to refasten the tagstrip with nut/bolt/lockwasher rather than another pop-rivet. It takes longer but will survive the demands of rock’n’roll guitar. The other speaker on this 2×12 baffle is checked OK.

Classic 50 P1020269Before we re-install the baffle into the cab, we will repair the reverb tank. One of the transducers tests open-circuit (O/C) and is not repairable, so we will replace the unit altogether. This is not a bad thing as we will upgrade the reverb with one of the ‘MOD’ tanks, The reverb will sound much better than it did previously.

Classic 50 P1020270Reverb tanks are available in various combinations of impedances and delay times, so it is important to select the correct unit for the individual amp. In the Peavey Classic Series, the reverb tank is driven by an IC, requiring a different input impedance than for example a Fender ‘Deluxe Reverb’. The replacement tank is placed within the vinyl bag, which provides some degree of isolation from vibration, which in turn is screwed into the bottom of the cab. The baffle and amp chassis are each reinstalled into the cab, and the reverb cable reconnected within the amp.

Classic 50 P1020272We had previously tested all the valves (tubes), external to the amp, and also carried out a power output test and electrical safety test. The final play test revealed a huge improvement, and the amp was returned to the music shop. Thanks for checking out the blog and we are located here in Gosford (Wyoming) NSW for all your valve (tube) amp service needs, including custom design. IR.

the Marshall Class 5 in 2015

December 14, 2014
the C5 head after VVR mod

the C5 head after VVR mod

Hello and welcome back to the blog. Only a couple more weeks now and the year 2014 will be history. Our most popular amp mod’s during 2014, apart from the perennial Fender Blues Junior and Pro Junior mod’s, would have to be to the Marshall Class 5, aka C5. We have completed mod’s/upgrades to a number of these amps now with consistently good results for our customers.

 

the C5 head after VVR mod

the C5 head after VVR mod

Basic mod’s to the C5 start at $200 inclusive (Australia), and include installation of the VVR module and upgrade of signal path and tone control capacitors. For those who really love their C5, we suggest looking at upgrading to the Mercury Magnetics transformer and filter choke set. For a more detailed description of our C5 mod’s, see our December 2013 blog.

However, to summarise – the capacitor upgrades result in more effective EQ and a tonality more like the favourite old-school Marshalls, for example the “Plexi” series amps. The VVR module works as a power attenuator, reducing the output of the amp to quiet conversational levels. Very handy for apartment dwellers. The most suitable location for the VVR module, both electronically speaking and for user convenience, is directly behind the Class 5 logo. But, at least now we have located a source of traditional looking Marshall style control knobs with set screws, to suit the solid shaft of the VVR control. Many thanks to all our customers from 2014 !!

the Landry 100 watt amplifier in our workshop

August 3, 2014
the Landry amplifier

the Landry amplifier

Hello again. We are now offering world-class (non-warranty) service and repairs to Landry amplifiers of St. Louis, Missouri USA. We should point out that we have no official relationship with Landry amplifiers, so we cannot accept any warranty repair claims without the endorsement of Landry or their Australian sales representative. The Landry amp is a relative newcomer to the boutique guitar amp scene. We endorse the fact that these amps are totally valve (tube) circuitry (apart from solid-state rectifier), and are completely hand-wired and assembled, presumably by one person.

the Landry amplifier

the Landry amplifier

Landry are producing only one or two models, very obviously voicing their amps towards the classic British/Marshall sounds (in both channels), but with a number of enhancements and more modern high gain sounds available as well. There are two independent preamp channels, each with its own 3-band passive EQ. The ‘clean’ channel is voiced similar to a JTM45/Bassman style amp, and the ‘distortion’ channel is voiced similar to a JMP series amp, but with additional gain available via a footswitch. After the channel switching takes place the signal is routed via a valve-driven series FX Loop.

the Landry amplifier

the Landry amplifier

The master volume is of completely conventional design, following after the FX Loop, but works very effectively in this amp design. The big surprise in a Marshall inspired amp such as this, is the inclusion of a valve (tube) driven reverb of very acceptable sound quality. Certainly much better than the reverbs ever were in Marshall amps over the years ! The phase inverter, power amp and power supply stages are very similar to what you would find in a 100 watt JMP Marshall.

Landry turret board and hand wiring

Landry turret board and hand wiring

The power and audio output transformers are in fact the very excellent 100 watt JMP style models by Classic Tone/Magnetic Components of Chicago USA. The claim on the Landry website that the ‘clean’ channel delivers sparkling blackface tones is of course an exaggeration. This channel delivers the fatter JTM45/plexi/’59 Bassman type tones, and there is absolutely nothing wrong with that at all.

setting the bias on this amp is very easy

setting the bias on this amp is very easy

Our good customer, Steve Edmonds brought this 100 watt head into the workshop last week, stating that the amp had lost some of its magic. Please see our post of 25/2/2014 re the Steve Edmonds band, and new project Mescalero. Inspection of the chassis revealed no apparent problems, but testing all the valves (tubes) separately from the amp did reveal most of them were in a degraded state, suffering from microphony, low gain, etc. Apart from a Tung Sol (reissue) 12AX7 in position V1, all other valves (tubes) were Chinese 12AX7B and EL34B. Now this amp is really only a few months old, but Steve is currently the hardest working guitarist we know, generally playing four gigs per week.

Landry chassis: six preamp valves in total

Landry chassis: six preamp valves in total

As a sweeping generalisation, Chinese manufactured valves (tubes) can quite often sound very good indeed when brand new, but can also lose their quality relatively quickly. Their 12AX7’s especially seem to become not only microphonic but also rather harsh and brittle. No doubt they will improve their quality of manufacture over time, the Russians certainly have. We installed a mixture of JJ, Electro-Harmonix and Tung Sol (reissue) valves, pre-tested and graded in our workshop, making an effort to select the ideal valve for each position. For the output stage we installed a matched quartet of Tung Sol EL34B, which have been certainly the most reliable of current production EL34’s (in our experience), although they do sound different to other EL34’s – therefore the customer’s personal tastes have to be taken into account in these situations.

the Landry chassis

the Landry chassis

Rebiasing the Landry for new output valves is easy and convenient – I wish the major manufacturers would take note ! All four EL34 cathodes are connected together to ground via a 1 ohm/10 watt resistor, which in turn is connected to external DVM test points. Simple ! The amp passed its power output and burn-in testing with flying colours, and it seems that Steve is very happy with what he was hearing at last week’s rehearsal. Many thanks to Steve Edmonds for his continued support and for supplying us with the subject matter for this blog. IR.

another Laney AOR Series ‘Pro Tube Lead’ amp in our workshop

July 20, 2014
the Pro Tube lead 100 head

the Pro Tube lead 100 head

Hello and welcome to the blog. Once again we have completed major repairs to a Laney (UK) “AOR Series” amplifier – model name Pro Tube Lead, this time a 100 watt head for customer Ben Rabey. We previously published blogs on 14/5/2013 and 10/7/2012 describing repairs and MODs to Laney amps in detail, including the AOR Series 50 watt combo, and also the LC30-II combo.

 

Pro Tube Lead 100 head sitting on a Hame speaker cab

Pro Tube Lead 100 head sitting on a Hame speaker cab

The fault description from our customer was that the amp plays OK for a while, and then the output drops markedly ! Upon removing the chassis from its sleeve, the first things we noticed were that the mains fuseholder was loose and could not be tightened satisfactorily, plus the amp was seriously under-biased resulting in a much higher than normal current draw by the quartet of EL34 output valves (tubes).

 

removing and reinstalling the board is quite a task, but has to be done !

removing and reinstalling the board is quite a task, but has to be done !

We replaced the fuseholder altogether with a current production unit that complies with contemporary electrical safety standards. We also installed the correct value HT fuse – T1A. Following this, the amp passed electrical safety testing to Workcover NSW standards. This is all important stuff, and should never be overlooked. We tested the quartet of EL34 valves and all the 12AX7 preamp valves externally to the amp with no apparent problems revealed.

access to much of the wiring is under the board, but available space is very tight !

access to much of the wiring is under the board, but available space is very tight !

After re-installing the valves (tubes), we rebiased the EL34’s to a sensible current draw. The amp tested both very low and very distorted output signal into a dummy load. We observed a good drive signal to the EL34’s from the phase-inverter stage, with a signal injected at the FX Loop ‘return’ jack, so we had every reason to suspect that the output transformer had broken down internally. We quoted Ben on replacing the output transformer plus other tasks, and the project proceeded.

 

the new output transformer installed on the Laney chassis

the new output transformer installed on the Laney chassis

As we are an authorised repairer for Marshall amplification, we therefore have access to the full stock of Marshall spares for current and vintage models. It just so happens that the 100W output transformer for the JCM800 series, designated C2668 (by Dagnall Electronics), is a perfect match and perfect fit for the 100W Laney. This is hardly a surprise, as the Laney is very obviously based on the JCM800 design, but with additional gain and additional features.

repairs completed and board reinstalled

repairs completed and board reinstalled

Getting access to the wiring to carry out this task is not easy, as much of the wiring passes underneath the single large printed circuit board (PCB). Definitely not mil-spec wiring, but the JCM 800 amps are exactly the same (although their wiring is somewhat tidier). Removing the board requires removing a number of fasteners, plus also removing all the front panel controls, as all these controls are hard-wired to the board, as you would expect.

new 'touch proof' mains fuse installed

new ‘touch proof’ mains fuse installed

The replacement C2668 transformer wiring of course has a completely different colour code to the original Laney unit, which required some additional investigation to complete the installation successfully. While we had both sides of the board exposed, this was the perfect opportunity to replace the bias supply filter caps, plus some of the low-voltage supply components (as per the previous Pro Tube Lead repair job). You can see a burn mark on the component side of the board under a power resistor in the low-voltage supply, so this appears to be a common problem in these amps.

rear view Laney 100W head

rear view Laney 100W head

For those interested in technical matters, this amp was serial # 2685, and appears to have been built in 1988. After installation of the new transformer and rebias of output valves (tubes), we had a high-tension voltage supply of just under +470V, and a bias voltage of -38V. The amp delivered 43V into 16 ohms @ the onset of clipping = 115 watts. Was the blown audio output transformer a direct result of the under-biased output valves ? We will never know – sometimes these are just random events. Many thanks to Ben for his continued custom. As always with these models – once set up and running properly the amp sounded huge !   IR.

Servicing the Carr ‘Sportsman’ amp in Australia

July 5, 2014
the Sportsman logo

the Sportsman logo

We have had a relationship with Carr amplifiers of North Carolina, USA, going back several years to when the former Bondi Intermusic Australia began importing these amps. Of course Bondi is long gone, so therefore we are no longer the national warranty service agent as the brand has been taken over by another distributor. However, with the full approval & support of Carr USA, we are still providing post-warranty after-sales-service to the entire range.

the Carr Sportsman 1x12 combo

the Carr Sportsman 1×12 combo

We have previously published blogs re the Carr Artemus and other models in the range. The Sportsman model is a relative newcomer to Australia, and this is our first servicing experience with this model, for new customer Matt. The design of the Sportsman follows on from Carr’s most successful models, being somewhat like a hot-rodded “blackface” Princeton Reverb amp. That’s the best way we can describe it, although there are some differences, particularly in the phase-inverter and power amplifier circuits.

the Sportsman front panel

the Sportsman front panel

The power amplifier is designed around a pair of 6V6 output valves, employing a form of compound bias – combining both fixed bias & cathode bias, with a power output @ the onset of clipping of about 16 watts into a 16 ohm load. This arrangement seems to work well in practice, although when Fender (as a division of CBS) tried something similar in the late 1960’s the results sounded so bad they had to implement a redesign of the “Silverface” models.

the Sportsman from the rear

the Sportsman from the rear

The power supply uses solid-state rectification, as opposed to valve (tube). The “headroom” control is actually a master volume with some tone shaping, following on from the final preamp gain stage. As per the Two-Rock amp discussed in the previous blog, this control is intended to manage the gain structure of the amp, rather than introduce distortion, hence the “headroom” label.

Sportsman chassis & speaker

Sportsman chassis & speaker

The reverb on this amp is exemplary, which is a real contrast with the many current production guitar amps offering really poor reverb !! There is no tremolo on this model. The chassis is completely hand-wired. Matt reported that although the amp was still functioning, the sound of the amp had deteriorated markedly. We examined the interior of the chassis looking for any signs of component failure or drift, but chassis checked out A-OK.

Sportsman 006We carried out a full re-valve and once more the Sportsman was delivering the kind of attitude that Matt was missing. The one and only concern we might have with this amp is the minimal ventilation for the horizontally mounted 6V6 output valves, which are biased to run quite hot. We have to thank new customer Matt for providing us with the subject matter for this blog. I think the only remaining question for Australian guitar players would be – could you buy an amp with a duck on the front panel ??    IR.

Sportsman 008

Service centre for Two-Rock amplification in Australia

July 5, 2014
Two Rock 001

Two-Rock EXO15

Hello and welcome back to the blog after a long break. We are now offering world-class (non-warranty) service & repairs to the range of valve (tube) amplifiers from Two-Rock guitar amplification of California, USA. We should point out that we have no official relationship with Two-Rock Amplifiers, so we cannot accept any warranty repair claims without the endorsement of their Australian distributor.

Two-Rock EXO15

Two-Rock EXO15

We recently serviced the Two-Rock EXO15 head (serial # 62) for Sam, one of our regular customers – the amp is featured in the accompanying photos. Sam advised us that the amp ‘just stopped’, which is a fairly common amp fault description, and in most cases would suggest a valve (tube) or other component failure, or at least a blown fuse.

 

 

6V6 powered EXO15

6V6 powered EXO15

The EXO15 is a single-channel, 15 watt 6V6 powered and valve (tube) rectified class AB amp, designed in an all-metal enclosure,  with a pair of 12AX7 preamp valves (tubes), one of which functions as the phase-inverter. The preamp design is somewhat similar to the “clean” channel in a Dumble guitar amp, as per the Two-Rock heritage, and the 3-band passive EQ is also voiced along similar lines.

5AR4/GZ34 rectifier

5AR4/GZ34 rectifier

The master-volume is placed directly after the 2nd gain stage, and is therefore intended more for managing the gain structure than generating massive amounts of distortion (a design approach which we endorse). The master feeds a passive FX Loop, for time-based FX devices that can accomodate line levels. In spite of the name & description, the “contour” control is in this case a simple passive high frequency roll-off, as per the vintage VOX AC15/AC30 amps. We heartily endorse the inclusion of a GZ34/5AR4 valve (tube) rectifier in an amp of this power rating.

under the chassis view from the front

under the chassis view from the rear

Anecdotal evidence from the customer suggests this hasn’t been a particularly reliable amp. As this is our first service job on this particular amp, we are not familiar with its previous service history. Assembly is a combination of circuit boards and hand-wiring. All connections to valve sockets, jacks/pots/etc are hand-wired which of course we endorse. Components are of commercial standard but not “boutique” standard, with no apparent design problems.

under the chassis view from the front

under the chassis view from the front

We replaced the blown mains fuse, and also the output valves & rectifier valve just to be on the safe side. Our initial choice of JJ 6V6 output valves was not a success, as they are too tall for the cover to fit back on !! We ended up installing a matched pair of 6V6GT by Electro-Harmonix, which have been pretty reliable at the voltages within this amp (+440V DC in our Wyoming NSW workshop). We rebiased for a sensible current draw with the 6V6’s (this may have been the problem all along ??), and measured the power output @ the onset of clipping = 22 watts.

ventilation grille at the top of the reassembled enclosure

ventilation grille at the top of the reassembled enclosure

We were looking for any other factors which may have contributed to the unreliability factor, at Sam’s request. When we looked at the speaker output jacks (4, 8 & 16 ohms), we were really disappointed at the nondescript quality of the jacks installed, and their current condition. We installed a trio of the very fine Switchcraft jacks, which grip the speaker plug very firmly for a positive connection. H/R (high resistance) or O/C (open circuit) speaker connections can cost you not only your output valves, but potentially also your output transformer. Always use the best available jacks & plugs for such critical connections.

Thankfully, no further problems have been reported with this amp. Many thanks to Sam for his continued custom, and for supplying us with the subject for this week’s blog !   IR.

the mighty HIWATT DR103

March 30, 2014
Hiwatt DR103 serial 12465 April 1978

Hiwatt DR103 serial 12465 April 1978

Hello & welcome back to our blog. We recently carried out service & repairs to a 1978 Hiwatt [UK] DR103 amplifier which obviously has had a long & hard working career in rock & roll bands. This DR103 is the 2-input model with active mixing of the normal & brilliant channels, as distinct from the late 1960’s/early 1970’s 4-input model with passive mixing of the two channels. We suspect that both versions of this model have their fair share of admirers.

DR103 front view of chassis

DR103 front view of chassis

This amplifier was working but had issues with microphony and very scratchy pots – some pots had dead areas on their sweep, so were barely useable. The amp had been modded by persons unknown further back in its history. The high frequency response of the brilliant channel had been boosted to ear-drum shredding levels, and was to all intents & purposes unuseable. Apparently the previous owner had only ever played on the neck position humbucking pickup of his 335 style guitar, hence the need for extreme high frequency pre-emphasis.

1978 Custom Hiwatt 100

1978 Custom Hiwatt 100

In addition to this problem, the amplifier had high levels of hum & noise. We started out by giving the chassis a quick clean up and tightened up a loose transformer. We checked the fuses, and as is so often the case in older amps, they were not offering any protection at all as they were the wrong values ! We installed new fuses, carried out an electrical safety check [PAT test], then moved on with the main part of the service.

DR103 chassis rear view

DR103 chassis rear view

All pots [excepting bass & treble] were in very poor or damaged condition, so we replaced them with those excellent CTS pots. One source of unwanted noise came from the input jacks – they weren’t shorting to earth [ground] when the guitar lead jack plug was removed. We cleaned all input & output jacks, plus the 9-pin valve sockets with DeOxit, which corrected the remainder of the intermittent problems.

DR103 interior view of chassis after replacement of 5 x pots

DR103 interior view of chassis after replacement of 5 x pots

Two only of the preamp valves were tested as being faulty and/or microphonic, so we replaced those accordingly. All other valves, including the quartet of EL34’s tested OK. This amp has obviously been in regular use since manufacture in 1978, and the various electrolytic capacitors in the power supply still appeared to be serviceable. As the customer’s budget did not allow for complete replacement of electrolytics, we left them alone.

the DR103 nameplate tarnished by years of rock&roll

the DR103 nameplate tarnished by years of rock’n’roll

There was still a remaining source of hum within the amp, and this turned out to be a lack of earth [ground] reference for the 6.3V AC heater filament supply. The pair of 100 ohm resistors that provided a virtual centre-tap for the 6.3V AC looked perfect but measured O/C. We fixed this problem with the installation of a pair of 100 ohm 1 watt resistors. The final issue to be resolved was removing the MODs to the front end of the amp.

 

the mighty 1978 Hiwatt DR103

the mighty 1978 Hiwatt DR103

This DR103 already possesses a very bright voice, especially when plugged into the brilliant channel. The amp had been modded with a double layer of additional brightness, so the brilliant channel was unbearable with a Strat or a Tele. We restored the front-end circuitry to original spec, removing both MODs. How does this amp sound now ? Just fantastic, actually, although at 100 watts I would get thrown out of every gig we do these days, not to mention making my existing tinnitus problem even worse ! It’s just so British, with a warm & fat midrange, sweet & crisp top-end, and a tight but powerful bottom end. I love the balance of tone across the 6 strings of the guitar, but unfortunately for this amp to sound at its best, you have to play bloody loud.

So, we heartily recommend vintage Hiwatt amps as an investment for all you amp collectors out there – they were so well made in the first place, and are a pleasure to work on from the techo’s perspective. We are currently building several custom amps from 15 watts to 60 watts [to a customer order], that while not a carbon copy of the Hiwatt, are very strongly inspired by the Hiwatt and will deliver the classic British tonality at more sensible volumes. These amps are offered with a choice of EL34, KT66 or KT88 output valves. We also offer a single-ended KT88 model at 15 watts output.

Thanks for checking in again, and there’s plenty more amp talk to come. IR.