The amplifier was in a sorry state when I received it (broken faceplate, broken headshell, bad soldering on the replaced filters caps, someone had removed some but luckily not all of the original mustard caps, the two potentiometers could not be saved by cleaning, etc.).
The crucial parts, however, namely the huge Partridge transformers were intact and in good working order.
This amplifier begged for some attention and I decided to restore it to a state better than its former glory - a proper full Supergroup.
The first challenge was fitting the whole preamp. The headshell of these units is shorter than of a normal Supergroup and only two preamp tube holes were in line with the power tubes. So, the first challenge was choosing where to position the extra preamp tube and its board. The best solution was deemed to be the one in the picture. The advantage is that the first preamp tube is very close to the inputs and far away from the transformers. The disadvantage is that the length of the heater wires is unavoidably longer than desired, which can lead to noise if not implemented right. To avoid noise, the tightly twisted heater wires are running along the chassis corner and as far away from the signal wires as possible. The wire used between the input grid stoppers and the 1st tube grids, the coupling caps and the gain pots (wiper and end lug) is of the shielded variety to help protect against inflicted noise. The distance between the input jacks and the grid stoppers is so small that there was no need for shielded wire.
The amp was fitted with Alpha pots, F+T filter capacitors, Xicon 1W resistors, Ohmite resistors (for the higher wattage positions), vintage mustard caps and styroflex caps. The faceplate was designed from scratch to match the original as much as possible. The headshell was repaired and retolexed by Chris Uff in the UK.
The original was using bus grounding, so i decided to implement that scheme as well but with better consideration regarding the order and position of the grounds. Indeed, the amplifier is very quiet even when fully dimed. The bias circuit was made adjustable and the toughest current production tubes were installed - a quad of Shuguang EL34BHT. The amplifier puts out a whopping 600V on the plates! The problem with most current production tubes though is with the screen voltage. Hopefully those tubes will survive in that amp but to be on the safe side, the screen resistors were upped to 1K/5W from 470R/5W.
The amplifier sounds phenomenal and quite unlike other amps of that era. The notes have great clarity (the quality of the Partridges certainly contributes to that) and the amp stays clean up to seven on the gain knob - which is LOUD. After that point, the amp distorts but in a different way than say a Plexi or a Hiwatt. I can definitely see the connection with the early Iommi recordings.
All in all, this was a very satisfying build and the result is a unique looking amp - I actually prefer the looks of this to the original Supergroup.
