A well kept secret: the Yamaha G100 212 II

April 2, 2017

Yamaha G100 212 II

Yes folks, one of the best kept secrets in the wide world of guitar amps is the late-70’s to early-80’s Yamaha G100 model (and similar) amps. They appeared in three series of models – series I, II & III – our pick would be series II which are very robust and obviously built to last the distance. Yamaha’s cosmetic approach is very conservative, but also quite distinctive.


check out the chassis – Yamaha G100 II

Yamaha obviously marketed these amps to compete primarily with the Roland JC120 series amps, and also the Fender ‘Twin Reverb‘. You can pick the Fender ‘Twin’ influence in the G100’s tonality, but in our opinion the G100 has a somewhat warmer tone and very powerful and flexible EQ. There are two channels, as you might expect in such an amp, each with volume treble/middle/bass controls and some ‘pull-on’ options. Common to both channels is (spring) reverb and a 3-knob parametric EQ.

Yamaha’s power amp module can be removed for repairs

This is a channel-switching amp. Channel A is warm and clean. Channel B has additional gain on board for some overdrive. Like any solid-state amp from this era, clean sounds are the main strength of the Yamaha. However, you could employ this amp effectively for most playing styles, excepting hard-rock, grunge and metal, etc. The power amp is modular and plugs into the power supply board. The module is fastened to the chassis via the large heatsink and is easily removed for repairs. The power amp design is quite an old-school 1970’s design, ie quite LO-FI, with a single-ended power supply which necessitates the use of a large output coupling capacitor to the speakers to block the DC voltage present at output (ie an electrolytic capacitor). Perhaps these are contributing factors to the Yamaha’s warmer sound ??

the G100 chassis is very serviceable

This example G100 was recently acquired by our regular customer, Ian Astill. We inspected the chassis and boards, as well as testing for electrical safety. The amp came up trumps apart from some very scratchy pots. Getting access to the pots for cleaning is not especially difficult. We removed the board behind the front panel and gave the offending pots a blast with FaderLube. These pots are the commonly used PCB mounting split-shaft/splined-shaft which facilitate a push-on style of control knob. Unfortunately, the shafts are often damaged and bent. Even worse sometimes the previous repairer has glued on some knobs, making removal without destroying the pot quite a challenge ! The bent shafts can be carefully straightened so there is no excuse for this approach.

parametric EQ on a guitar amp

Some say the original Yamaha speakers play an important part in this combo’s tonal response. If you are looking at acquiring one of these amps, this is probably something you should check out first. The parametric EQ is not easy to use – it takes time and effort and some experimentation to get a result. But the controls can be set for 12 o’clock and instead rely on the very functional channel (passive) EQ instead.


A well known user of these amps back in the day was American jazz guitarist Mike Stern. Americans Pat Metheny and Robben Ford are also said to have used these amps. I can recall seeing some Aussies using these amps back in the day including Rick Springfield. There is a rumour that these amps were designed by Paul Rivera, but the power amp design looks exactly like a standard Yamaha design, so who knows ?? The standard of assembly is very high, it’s little wonder these amps have survived so well, requiring very little in the way of service and repairs – mainly just damaged pots. IR.

MUSIC MAN 210-RD-100 guitar combo amp

March 28, 2017


Welcome back to the blog. We continue to carry out service & repairs/restorations to the still popular original production amps from the Music Man company, built in the USA during the late 1970’s – early 1980’s era. This example is the 210-RD-100 model 100 watt 2 x 10 combo from the early 80’s period, owned since new by Peter Tos, and gigged heavily back in the day in Sydney’s nightclub scene.

front panel after new standby switch installed

The amp was delivered to the workshop with a broken standby switch, which is a 3-position toggle switch which selects either off (standby), high power or low power functions. Therefore, the switch needs to be a single-pole/double-throw (SPDT) with centre-off position, capable of switching high voltages. The standby switch achieves this functionality by routing higher or lower AC voltages to the voltage-doubler rectifier/power supply.

rear view new standby switch installed

In the high position, the power supply delivers approx 725V DC HT (no signal) to the anodes of the pair of 6L6GC output valves (tubes) !! No wonder this amp configuration easily achieves 100 watts audio output from a single pair of output valves. The screen grids are powered from half the HT voltage (otherwise they would certainly self-destruct). The valves are run very close to pure Class-B or in other words they are biased for very low current draw (no signal).

rebuilding the voltage doubler power supply

Having dealt with the primary problem, we knew from previous experiences we would need to check the condition of the high voltage electrolytic capacitors in the voltage doubler power supply. As expected, they had both ruptured and were no longer capable of doing their job. We replaced them with a pair of those very fine F&T 50+50uF/500V capacitors, bedded down in silcone for stability. The original capacitors have axial leads, so we have to hand wire the F&T vertical mounting units to the existing eyelet board.

another view of the voltage doubler eyelet board

The 500V F&T units in series give us a nice safety margin of 1000V DC maximum rating compared to approx 725V DC (as measured) high volts supply. We didn’t get a good earth test result, so we replaced the 240V 3-pin plug. The amps pots & jacks were cleaned with FaderLube & DeOxit, respectively. The Music Man amps were known primarily for their clean (“enhanced” Fender-style ??) tones, and this model is no exception. The overdrive channel produces what could be best described as 80’s distortion tones, which is nothing to get too excited about. The channel switching arrangements work very well. The original footswitch unit still works fine !! The 3-band EQ plus bright deep switches are shared by both channels.

the only valves in this amp – a pair of 6L6GC output valves

You may have noticed there are only two valves (tubes) in this amp design – the pair of 6L6GC output valves ! There are no pre-amp valves – the gain & signal shaping functions are performed by IC/op-amp’s. Even the phase splitting function to provide a pair of drive signals to the output valves, is performed by IC’s. How can an IC provide the level of drive signal to a 6L6GC valve you might wonder ?? Well, it’s achieved by configuring the 6L6’s in grounded grid mode of operation, whereby the drive signal is applied to the cathodes of the 6L6’s, utilising a pair of small NPN power transistors.

the well designed board still all original, as are the pots & jacks

The control grids still have a bias voltage applied, but they are grounded as far as signal is concerned. This arrangement works quite well, although you miss the warmth & colour of a valve phase-splitter stage. The only drawback that we have seen with this configuration in a guitar amp is that in the event of a catastrophic failure of an output valve, the drive transistors will surely be destroyed. The earlier 1970’s Music Man amps utilised a 12AX7 valve for the driver/phase splitter stages. Those amps are naturally considered more desirable, but lack the channel switching functionality. Thanks – IR.

the ruptured pair of high voltage filter capacitors

(the original footswitch still works fine !)

Restoration of an original Peavey 5150 EVH model amplifier

December 17, 2016
the 5150 head after the big clean up

the 5150 head after the big clean up

We recently received at the workshop one of the original Peavey 5150 dual channel high gain amps, which were the result of Mr Eddie Van Halen partnering with Peavey amps to produce his own signature model. In more recent times, Eddie has chosen to collaborate with Fender to launch his very own brand: EVH. This particular example of the 5150 reached us in very poor condition, with virtually no level from the rhythm channel and very low level from the lead channel. No serious service work had been carried out on the amp for years.

a total of five 12AX7 preamp valves (tubes)

a total of five 12AX7 preamp valves (tubes)

After a big clean up we removed all valves for testing and assessment. Most of the pots (potentiometers) were extremely scratchy and cutting in & out at certain points on their sweep, ie not at all useable, so we gave them a thorough cleaning with FaderLube. Only two of the five pre-amp valves (tubes) tested satisfactory, the others testing low gain and very microphonic. In the 5150 design, there is so much gain in the circuitry, far more gain than most styles of guitar playing would ever require, that wideband noise & microphony become major issues. Therefore, we have to carefully select the 12AX7 valves to be installed, and perhaps also select specific valves for specific positions in the signal path, ie V1 to V5 inclusive. You will notice in the photograph that we added rubber rings to V1 to provide additional dampening. The pre-amp valves are accessed by removing a cover on the rear panel.

octal sockets 'pop rivetted' to chassis

octal sockets ‘pop riveted’ to chassis

Only two of the quartet of Sovtek 5881 output valves tested satisfactory, but we were able to match up a quartet of 5881’s using a pair ex-stock. The wafer base Sovtek 5881 valves work quite well in amps like the 5150, as the tone of the amp is really being produced in the high gain pre-amp. Carrying out a quick power output test with our matched quartet of 5881’s installed, we observed a very asymmetrical waveform on the oscilloscope, indicating that possibly one 5881 was not contributing to the output. The next logical step would be to remove the output valve board from the chassis to examine it for damaged PCB tracks, octal sockets or associated components. We discovered that the board is fastened in place by ‘pop riveting’ the octal sockets to the chassis. HUH ?? It’s bad enough that the output valves are mounted to a PCB, but the only way to remove the PCB is to drill out the ‘pop rivets’ with the obvious danger of small bits of metal going everywhere under the chassis. This really is bad design practice (not Eddie’s fault though) !

the underside of the output valves PCB

the underside of the output valves PCB

Fortunately, in this case we were able to prove that the screen grid resistor for one single 5881 had gone O/C (open circuit) and it just so happened to be on that area of the board which was easily accessible from the ‘copper’ side of the PCB. In other words, we could replace this failed component without removing the PCB. You have to get lucky once in a while ! We chose to install a 100 ohm 7 watt ceramic body resistor designed for harsh conditions, as per the photo.

creating an adjustable bias supply

creating an adjustable bias supply

OK – so far we have cleaned the cab & chassis, inspected inside the chassis, cleaned the scratchy pots, replaced valves as required, corrected a fault on the output valve PCB & replaced a damaged 240V 3-pin plug. The remaining tasks are rebias the output valves, final power output test & electrical safety test. With this quartet of 5881’s, the amp is way overbiased with each 5881 only drawing about 12ma. Unfortunately the bias voltage is fixed at the factory at an arbitrary level which makes little sense. HUH ?? The only plausible explanation I can come up with for this decision is that Peavey marketing wanted you to buy their branded sets of output valves for these amps.

the 5150 power supply

the 5150 power supply

Consulting the 5150 schematic revealed that we could replace the final (fixed) 15K resistor in the bias supply (R68) with a 25K trimpot. R68, for whatever reason was covered in the hot glue applied at the factory, so we had to clean up that area of the main board, to enable us to snip off the resistor leads. Then we could connect leads from the trimpot to the final bias filter capacitor, before bedding down the trimpot in a blob of silicone. We set the trimpot to about 14K, ie slightly less than R68 measured, which was a good starting point for rebiasing the amp to a reasonable current draw. Power output was measured at over 100 watts into 16 ohms @ the onset of clipping.

5150 loaded with 5881 valves (tubes)

5150 loaded with 5881 valves (tubes)

We last discussed the 5150 series of amps in our blog do we cater for the metal head ? published Sept 2011. At that time the 5150 II was the model generally given the thumbs up by our metal addicted customers. Our favourite MOD for these models is the installation of a 10 Henry filter choke in the +500V DC power supply, manufactured by Mercury Magnetics, which cleans up the DC power very effectively. We no longer carry out warranty work on Peavey products, but we will continue to provide world-class service & repairs to your favourite old Peavey EVH amps and the Classic 20/30/50 series of amps. Thanks for checking out our blog. IR.

Service centre for “65 Amps” in Australia

December 11, 2016

65-amps-p1020689Hello 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 65 Amps guitar amplification of California, USA. We should point out that we have no official relationship with 65 Amps, so we cannot accept any warranty repair claims without their endorsement.

Lil' Elvis chassis - note the high-quality transformers by Mercury Magnetics

Lil’ Elvis chassis – note the high-quality transformers by Mercury Magnetics

Our customer, singer/songwriter Shane Nicholson delivered this “65 Amp” to our workshop for a general service & revalve. This model is called the Lil’ Elvis  and is a 1×12 combo of around 12 to 15 watts output, loaded with the very fine Celestion G12H30 70th Anniversary speaker. The valves (tubes) employed in this design are as follows: 3 x 12AX7 for the preamp, tremolo & PI stages, 2 x EL84 output valves (cathode-bias) for the power amp, plus a 6CA4 rectifier. The latter choice indicates there will be considerable sag/compression (ie similar in effect to the famous Marshall 18 watter).

chassis layout and wiring - not as obsessively neat as you might expect but very serviceable indeed

chassis layout and wiring: not as obsessively neat as you might expect but very serviceable indeed and superior to any PCB based amp in that regard

The circuit design of the Lil’ Elvis appears to incorporate design elements inspired by several vintage American amplifier designs, including the Fender Princeton and the simpler Ampeg models, featuring a single passive tone control. The 2nd 12AX7 stage is unusual – a bit like adding in a cathode-coupled phase-inverter into the signal path, prior to the actual split-load phase-inverter. This stage no doubt contributes to the unique tonal qualities of the Lil’ Elvis. The valve-generated tremolo is very effective. A passive FX Loop (ie a series loop) is incorporated into the design in such a way that it does not degrade the signal path with additional circuitry. However, if your FX device requires to be driven by a low-impedance source, then this loop will not be suitable.

65-amps-p1020685The “65 Amps” Bump circuit gives a significant boost, primarily to the mids and lows. You would probably set this one way or the other prior to playing or recording a song, rather than footswitching, due to the size of the boost and the fact that there is a very obvious “pop”, as the switching appears to be changing DC conditions for one of the gain stages, as well as increasing the gain. The cosmetic appeal of the amp is very distinctive. We are glad to see the cabinet is of sufficient proportions to give a reasonable low frequency response, ie this is not a ‘boxey’ sounding amp like some of the competition.

65-amps-p1020687We gave the amp chassis a good appraisal, with power output test, electrical safety test etc. A full revalve was called for but there were no other issues, all good !  65 Amps are evidently rather precious about which brands of valve (tube) go into each position, but we tried the Tung Sol (current production) versions in each case (except the 6CA4) and it sounded very good with that choice, although that brand might not be the best choice in every amp that is out there – we have to keep an open mind.

NTE2973 mosfet device

NTE2973 mosfet device

Finally, the “master” control, in spite of the hype on the website, is actually a power amp attenuator which works by reducing (attenuating) the high voltage supplied to the preamp and output valves (excepting of course the tremolo oscillator). The design is similar to that used in a number of amps, including our own 10 watt EL34se single-ended model, employing the NTE2973 mosfet device. It works very effectively, but as we have said before, it does inevitably change tone and dynamics at the lower settings. It is still a good feature to have with lower powered amps, which will often be used in home studios, etc.

Thank you again, Shane, for your continued custom. We look forward to providing world-class service and repairs to more 65 Amps models in the future. IR.

Upgrades to the (made in China) AC50 and AC100

June 19, 2016

VOX China P1020388Hello and welcome back to the blog. This week we discuss an upgrade project for Sydney indie band guitarist, Cameron Birt. Cameron and the boys have built their guitar sounds around the Chinese-built VOX AC50 and AC100 amp heads, but they are well aware that these amps could be significantly improved from stock with some important component upgrades.


VOX China P1020384

We put together an upgrade project scope and quotation for the band, which would deliver maximum results for the $$$$ spent. Our recommendations were accepted and orders placed to source the agreed upgrade components.



VOX China P1020385

Our agreed upgrades were as follows:  output valves – current production Mullard EL34, a pair for the 50W and a quartet for the 100W, suitably matched and the amp rebiased accordingly. Preamp valves – current production Mullard 12AX7 (‘large plate’) for the V4/PI stage positions, and Mullard 12AX7/CV4004 (‘short plate’) for positions V1 to V3 inclusive, in both amps. At the time of this project, Evatco (Queensland) had a good stock of these valves. Transformers – replacement of output transformers and filter chokes in both amps. Power transformers left in situ.


VOX China P1020386While we install and recommend Mercury Magnetics transformers for their lush tone, for this project we chose to use transformers and chokes by Magnetic Components of Chicago, USA, marketed as ‘Classic Tone’. This choice achieved a cost-effective solution. The chosen transformers provided the closest physical dimensions as well as the required electrical specifications.

VOX China P1020387For the 50W head, we installed the Classic Tone part # 40-18025, and for the 100W head the part # 40-18026. The latter is the same transformer used in the Landry made in USA boutique grade heads, previously discussed in this blogsite. For both heads we installed the part # 40-18058 filter choke, which required the drilling of new mounting holes.

We avoided the complication of replacing capacitors in the signal path for this project – upgrading components on double sided boards is time consuming and not much fun at all. Apart from that, there is insufficient space allowed for our favourite caps. Magnetic Components provide detailed dimensions, etc, on their website which is of great assistance with these projects. Never underestimate the impact that a good filter choke can have on an amp’s performance. We have installed the Marshall style 3H (3 Henry) choke by either Mercury Magnetics or Classic Tone in many many projects now, always with positive results (yes, even the dreaded Marshall JTM600/JCM600 series amps).

We should point out that the recent production (China) VOX AC50 and AC100 have absolutely nothing in common with the famous amps of the 1960’s that the British Invasion groups conquered the world with. The current amps are definitely ‘modern’ designs with multiple gain stages and channel switching, more akin to a Marshall amp than a VOX. The end result on these two amps was a big “thumbs up” from Cameron and the boys with the VOX amps sounding great and exceeding expectations. We offer world-class service/MODs/upgrades on all VOX amps in 2016 and beyond ! Thanks for checking in to the blog – see you next week. IR.

Service centre for the Little Big Muff

June 13, 2016

Little Big Muff P1020477Hello again. This blog refers of course to the original Little Big Muff pedal by Electro-Harmonix from the 1970s, and not the more recent version which is basically a Big Muff in a smaller enclosure. This pedal is much loved by its owners, including our customer Adam, for its rarity and its outrageous sound. The Little Big Muff circuit comprises four NPN transistor cascaded gain stages, with each stage filtering and shaping the guitar signal before sending it to the next stage. The “Tone” switch allows two only extremes of either treble or bass tonality. The circuit component values are very similar to the well known Big Muff pedal.

Little Big Muff P1020471Adam shipped his LBM down to us from up in Lismore, Northern NSW. The ongoing problem with his pedal has been massive amounts of hiss – the hiss is of a “whooshing” nature, and renders the pedal unplayable. This pedal has been the subject of attempted repairs at least twice previously. Our immediate impression from trying out the pedal was that there was some instability in the circuitry in addition to the excessive levels of white noise. Please note – there is always going to be some white noise with the LBM as the design is very similar to a vintage Big Muff with the “Fuzz” control turned fully clockwise.

Little Big Muff P1020470First things first !! There were several electro-mechanical issues with this pedal to be corrected before we concentrated on the circuitry/PCB. The “Tone” switch, bypass footswitch, and input jack were all going intermittent, very much confusing the issue. The input jack was also wired incorrectly (!). We replaced all three items plus the DC IN jack (3.5mm min. jack socket) and the battery clip. All were at the end of their useful life.

Little Big Muff P1020472The pedal was now working as intended, but still with mega amounts of white noise. Turning our attention to the PCB (printed circuit board), we found that the 1st gain stage had been modded. We restored this stage to original spec, which included replacing the BC239 NPN transistor. The BC239 hasn’t been manufactured for donkey’s years, so may be difficult to source, but we have kept a private stock for these repairs. If you substitute a higher-gain device for the BC239, then the pedal probably won’t sound the same.

Little Big Muff P1020473Still noisy ! We ended up replacing all four NPN devices with BC239’s. A number of signal path capacitors had been previously replaced and we weren’t happy with what we saw, so we replaced them again with known types plus a few carbon composition resistors that we suspect had gone noisy and drifted off value.

Little Big Muff P1020476Now the PCB needed a tidy up and repair. We can’t help but notice that some PCB repairs to vintage pedals these days are not working out very well due to the application of too much heat and definitely too much solder. Surely everyone these days is using a temperature controlled soldering tool ?? The tracks and pads are being damaged by too much heat, or else taking too long on each solder joint. Too much solder might create the impression of a long lasting solder joint but in fact is  creating leakage paths across tracks or even the occasional short-circuit. It’s important to clean up after a job like this, in other words all the solder flux residue on the PCB and excess solder. This particular PCB had so much flux residue we couldn’t clean up completely – much of it was probably from original manufacture, but various attempts at repair through the decades have added to the mess.

Little Big Muff P1020481So – in the end, how did it sound ?? Pretty awesome, no wonder these pedals are so collectable. We did a direct A/B comparison with a Big Muff clone, with the “Fuzz” (aka “Sustain”) control set to max clockwise, as per the description above and the level of hiss was very similar, as was the performance of the pedal – taking into account the different “Tone” settings. Our favourite was the very fat bass setting of the “Tone” switch. We hope that Adam is enjoying his Little Big Muff, the pedal is now as quiet as we could achieve. See you next week for the next blog. IR.

MODs to the Fender ‘Super Twin’ amplifier

May 22, 2016

Super Twin P1020234Howdy there folks – welcome back to the world of valve (tube) amps. This week we look at servicing and especially modding a (relatively) rare Fender amp introduced in the mid-1970’s: the 180 watt (!) Super Twin 2×12 combo. The marketing people at Fender must have decided that higher power amplifiers were the way of the future as several models were redesigned for an increased power output during this time, including the “Twin Reverb” and the “Bassman”.  The Super Twin employs a sextet of 6L6GC output valves, with a HT voltage of over 500V DC, in ultra-linear configuration, to achieve the rated 180 watts output. This amp is not recommended for anyone with a bad back – it is seriously heavy !

Super Twin P1020236The design of the Super Twin features an elaborate EQ system that includes the conventional Fender 3-band passive EQ located between two 12AX7 preamp stages as per the Twin Reverb, plus a 5-band graphic EQ and a presence control, both active, driven by a 12AU7 valve. This is not a parametric EQ as is sometimes suggested on user forums.  The EQ is also footswitchable. The power amplifier is driven by a conventional long-tailed pair 12AX7 phase-inverter stage, followed by a 12AT7 cathode-follower stage to provide a low impedance drive to the sextet of 6L6GC output valves. There is also a distortion control which uses both halves of a 12AX7 valve, which is so awful that nobody ever uses it.

Super Twin P1020232Unfortunately, this amp is just too loud to be of any practical use for most guitarists. It would make a great bass amp if it was a bit more powerful. The amp stays clean until you reach the rated power and the overall tonality is rather glassy. BUT, we have a solution to offer which halves the output power and warms up the tone somewhat. Our standard MOD for these amps is to rewire 4 out of the 6 output valve sockets to reconfigure 2 pairs of 6L6GC to triode mode of operation. The remaining pair stay wired as tetrodes. At 90 watts this amp will still be very loud, but useable with a warmer, more musical sound.

We have implemented this MOD recently for Heath Crawley of Aussie indie bands Children Collide and Vanuatu Scalps, and for Nash Chambers, well known as a music producer and artist manager. In both cases there were numerous other issues to be addressed, as might be expected in a 40 year old guitar amp. General servicing includes replacing the high voltage electrolytic filter capacitors, replacing noisy and microphonic preamp valves, cleaning of pots, jacks and valve sockets, electrical safety compliance, and noisy or leaking capacitors in the signal path, including the graphic EQ which employs inductors and capacitors in five series resonant circuits.

It has been suggested on some user forums that the Super Twin was designed by Paul Rivera, but this is definitely not the case. Thanks for checking in to the blog, and we offer world-class service, repairs, restorations and upgrades to most Fender musical instrument amplifiers. I.R.

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.

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.