ivanrichards

Howdy 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 !

The 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.

Unfortunately, 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.

Hello 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.

On 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.

Which 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.

This 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.

For 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.

p.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.