A copy of the AC80/100
"Impact" was the brand name of a range of amps marketed by Pan Musical Instruments Ltd, a small British company founded by Don Mackrill in 1963. Towards the end of the 60s the company was taken over by Dallas Arbiter, who marketed Sound City equipment. Some later Impact amps were simply Sound City units in Impact boxes. Sixten Forsen has gathered a huge amount of info about the company and its products on his website Edgar Audio.
On the left, a Dealer Support brochure illustrating various cabs and amps, including a 100W bass unit, and a 100W stack in white. For the use of white stacks by the band Dantalian's Chariot, see the photo below. On the right, Don Mackrill at the Frankfurt Fair in 1965.
Perhaps the most successful amp in the range was the Impact 60, a two channel 50-60W amp, still well regarded in terms of sound and performance, and not hard to find second hand. A good example is represented on John Chamber's excellent site: Champ Electronics. The one hundred watt version, however, is extremely scarce these days.
As Sixten indicated, the Impact 100 is effectively a three-channel Vox AC80/100. The four EL34s are run in cathode bias with 270ohm resistors; and each of the three preamp channels mirrors the unusual tonal arrangement of the Vox model - the bass control for instance adds a massive hit even when raised by a small amount from "0". Sixten expressed his concern at the red-plating of the power valves on two Plexi Palace bulletin board threads - here and here. Anyone standing behind one of these things with all valves blazing (270ohm resistors in place) will soon share his feelings.
No doubt sensing that current was the enemy in the AC80/100 circuit, the designer of the Impact 100 provided massive C-core transformers for both mains and output. But in view of the small number of survivals, it may be that his strategy came to nothing, and the amps by and large folded when pushed.
Sixten's amp is illustrated on his site: Sixten's Impact 100. Only a half dozen or so other survivors are known.
Left, a publicity photo of the band Dantalian's Chariot, with a pair of Impact 100 stacks in white. Right, Impact 100 in the background belonging to Julie Driscoll's backing band.
Track by Dantalian's Chariot - click to play - the player probably will
not appear in beta versions of Firefox though.
This is a direct link to the file
Serial number 1017-5209 - currently in the UK
A bit bashed and dented (in this instance), but still going strong, the Impact 100 is a minor masterpiece of British engineering. Note in the last picture but one the wonderful raised voltage selector. The closeness of the impedance selector to the valve base (in the last picture), however, is probably not such a good idea.
Thanks to its massive transformers, the Impact 100 is monstrously heavy - one can feel the rubber handle stretching under load.
This one arrived with a small modification - a sort of fixed bias network, which looked pretty impossible. Cathode bias resistors and capacitors were therefore reinstated in the best possible location(picture 9), but the resistors at 390ohms instead of the 270ohms envisaged in the original circuit. Even at 390ohms though, the valves are stressed relatively hard.
Serial number 1017-5138 (or 1017-5738) - currently in the UK
IMPACT 50 - currently in the UK
Some great pictures on this website.