05-03-2023, 06:41 PM
I have been quiet in this thread for a while, but the truth is that I haven't touched an electronic instrument for weeks. I did however repair a Hammond chassis for a friend, which had all sorts of issues. The organ is an E100 from the late 1960s. 79-pitch tonewheel generator, plus some percussion effects etc which are solid-state, with 2-channel valve amplification (main & reverb). Most, but not all, of the audio guts apart from the percs are on a single chassis. During initial testing and evaluation after acquiring the organ, they had identified most of the problems as being on the main chassis and had started to sort some of them out. However, we knew that some of the valves were below par and wanted to crank them through the AVO, so it was more convenient simply to bring the whole chassis down and pop it on the bench here.
The main power amp uses a pair of 7591s connected tetrode-fashion with fixed-bias, driven by a concertina phase splitter using a 7247 (ECC832.) The reverb amp is smaller and uses a pair of ECL86s with cathode bias. These are fed respectively by a GZ34 and an EZ81, and our first problem (which we were already aware of) was that the GZ34 was low. But neither of us reckoned on half of it being completely dead, so a new valve was fitted forthwith. We had tested the 7591s on the AVO Mk IV and found them to be low and a poor match, with one taking barely half the standing current of the other. This was no surprise, because one of the first tests Jamie had done on receiving the organ was to check the cathode currents and found the bias control set so far out that one valve was barely conducting. We replaced them with modern ones which were a bit hot, so we had to increase the overall bias slightly to tame the current. This was exaggerated because with the rest of the organ disconnected some of the many HT rails were rather on the high side. The bias VR is only balance, not total, so we changed one divider resistor to get the cathode current down for now.
The 7247 in the power amp section turned out to be completely dud, with a near short g1-k on the phase-splitter half. Luckily we had one new 7247, and installing it got the power amp back to normal. There are two more of this valve type in the chassis, in the reverb driver and recovery amps. One was OK, the other turned out to be not a 7247; it's a Mazda with partly rubbed-off markings but 3 minutes with the AVO confirmed it to be an ECC82. A 7247 (ECC832) is half an ECC83 and half an ECC82, so the reverb gain was slightly out. Didn't have another spare 7247 so that has been ordered. During this process, I ran into an error in the AVO valve data, even in Gerry Horrox's version with many of the known errors corrected. The grid bias setting for the '83 half is paired with the anode current and gm of the '82 half and vice versa. I expect that when I have tested them before I have used the '82 and '83 settings from memory, but this time I couldn't recall which of A1 or A2 was the high-gain half so I looked them up in the book and found the error.
The ECL86 amp was not too bad, the valves are not perfect but one does not want to be chucking out ECL86s when they are only a little bit down. The voltages around these valves look a little odd until one remembers that the combined output cathode current passes through a 22V zener to generate the regulated supply for other bits of the organ. A few rogue R's and C's later and all was good, with 35W output from the main amp at 1% distortion. It simply remained for Jamie to trim the bias once the chassis was back in situ in the Hammond with the HT rails loaded as normal.
In a fortnight's time we are going to install multiple outputs for Leslie cabinets, with Jamie tracking down various crackles and niggles in the preamp in the meantime.
The main power amp uses a pair of 7591s connected tetrode-fashion with fixed-bias, driven by a concertina phase splitter using a 7247 (ECC832.) The reverb amp is smaller and uses a pair of ECL86s with cathode bias. These are fed respectively by a GZ34 and an EZ81, and our first problem (which we were already aware of) was that the GZ34 was low. But neither of us reckoned on half of it being completely dead, so a new valve was fitted forthwith. We had tested the 7591s on the AVO Mk IV and found them to be low and a poor match, with one taking barely half the standing current of the other. This was no surprise, because one of the first tests Jamie had done on receiving the organ was to check the cathode currents and found the bias control set so far out that one valve was barely conducting. We replaced them with modern ones which were a bit hot, so we had to increase the overall bias slightly to tame the current. This was exaggerated because with the rest of the organ disconnected some of the many HT rails were rather on the high side. The bias VR is only balance, not total, so we changed one divider resistor to get the cathode current down for now.
The 7247 in the power amp section turned out to be completely dud, with a near short g1-k on the phase-splitter half. Luckily we had one new 7247, and installing it got the power amp back to normal. There are two more of this valve type in the chassis, in the reverb driver and recovery amps. One was OK, the other turned out to be not a 7247; it's a Mazda with partly rubbed-off markings but 3 minutes with the AVO confirmed it to be an ECC82. A 7247 (ECC832) is half an ECC83 and half an ECC82, so the reverb gain was slightly out. Didn't have another spare 7247 so that has been ordered. During this process, I ran into an error in the AVO valve data, even in Gerry Horrox's version with many of the known errors corrected. The grid bias setting for the '83 half is paired with the anode current and gm of the '82 half and vice versa. I expect that when I have tested them before I have used the '82 and '83 settings from memory, but this time I couldn't recall which of A1 or A2 was the high-gain half so I looked them up in the book and found the error.
The ECL86 amp was not too bad, the valves are not perfect but one does not want to be chucking out ECL86s when they are only a little bit down. The voltages around these valves look a little odd until one remembers that the combined output cathode current passes through a 22V zener to generate the regulated supply for other bits of the organ. A few rogue R's and C's later and all was good, with 35W output from the main amp at 1% distortion. It simply remained for Jamie to trim the bias once the chassis was back in situ in the Hammond with the HT rails loaded as normal.
In a fortnight's time we are going to install multiple outputs for Leslie cabinets, with Jamie tracking down various crackles and niggles in the preamp in the meantime.
