The lack of bass turned out to be caused by the too low capacitance of the output pentode's coupling capacitors (C5 and C6). These were only 0.047 uF, resulting in a rolloff frequency of 31 Hz. The volume control also sucked. After ordering new caps and an Alps 27112 (50KA) volume control I got to work and plonked everything in. The board had a most unusual pitch of 12 and 20 mm for the capacitors, so the more common pitches of 15 and 22 mm had to be accommodated by drilling extra holes. It turned out that the caps I ordered had axial leads but would still fit a 22 mm pitch. Mounting the Alps was a challenge too: not only was the thread size bigger (M8), the fixing pin was also located farther away from the axis. This meant I had to attack the front panel again. This unfortunately resulted in ugly scratches... I also replaced the C1 capacitors with second-hand Wima MKS units.
Time to repeat the listening tests. I had made the same setup with the speaker switchover box. Now the bass was full, the way it should be. No difference with my trusty DFA888. Unfortunately the high end of the audio spectrum still sounds a bit cramped compared to the openness my DFA provides. But I've had it with this amp. It goes back to its owner...
Copyleft 2 June 2012
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