With the recent talk of Mark III builders, I had a spurt of inspiration yesterday. I managed to 70% wire up one of my Mk3 blocks.
I had a handful of 100 ohm carbon film resistors, which I had bought for the screen ballast, but someone recommended against using carbon film at this position (they can burn if wattages are exceeded). Instead I found some old 80 ohm and some 150 ohm carbon composition parts in my junk box. I opted for the 80 ohm resistors.
The power wiring stupefies me. It's not that I can't figure out what to do - it's that the stock layout is so darn stupid. Between the hot and the neutral, they landed one on the fuse and the other on the switch. I think safety dictates the hot be attached directly to the fuse, but then the primary of the PT is connected to hot even when the power switch is turned off. I'm not real happy about that. There's no easy attachment lug nearby to hook it up right. I guess I'll need to leave a splice hanging in the wire somewhere.
I've basically got all the transformer leads landed somewhere, except for the C-354 and the B+ to the OT. Those will go to the SDS cap board, which is already assembled but not yet mounted. I put power to the thing just to check the bias circuit. I get an adjustment range of -42 volts to -67 volts. I expect this will be appropriate for the pair of KT88. While I had it powered up, I checked the AC voltage on the filament windings. I had heard that some Mark 3 owners had problems cause by excessively high line voltage. I measured 5.0 volts and 6.5 volts on the two windings. I’ve noted before that my digital AC meter might not be giving true RMS readings, so I’m going to have to borrow a Fluke or some other decent meter and check again. I stuck my old Radio Shack analog meter on the 5v winding and it looked like roughly 5.6 VAC.
After checking the heater windings, I summoned up the courage to stick the analog meter across the PT secondary. It measured just less than 900 VAC with no load. That’s some scary stuff. I wouldn’t want to get in between it and ground. In case anyone is interested, I also measured the DC resistance of the PT’s windings. The primary side is 1.6 ohms; the high voltage secondary is 82.5 ohms (end to end). This data might be useful if anyone wants to plug it into Duncan’s PSUDII.
I left my camera at the office, so no photos this time. Maybe I’ll get a few snapshots later. The next step is to cut a bunch of short jumpers and finish the wiring to the driver board. Then I've got to install the last cap for the bias supply, and hook up the SDS board.
A couple more questions: Concerning the 6.3V heater winding, I’ve landed the PT’s green leads on pins 2 and 7 of the closest KT88 socket. I need to run extensions to the second KT88, and then to the driver board. Does the phase of this wiring make any difference? I expect I’ll simply wire pin 2 on the first socket straight through to pin 2 on the second socket, but does it really make a difference one way or another? Is it crucial that the lead going to pin 2 end up on terminal E (and pin 7 on D) at the Poseidon board? I’m thinking strictly from a “hum” perspective here.
Finally, I hadn’t realized how half-assed the ground appears on the stock Mark 3. The power supply gets grounded at one of the triple lug terminal strips, the bias supply is hanging off the shell of the rectifier socket, the output tubes’ cathodes are grounded to a lug near the can cap, and the driver board is grounded who-knows where. Apparently, it was “good enough” to use the chassis as the ground plane. Any strong advice here? By the way, I ditched the can cap and my replacement tube sockets don’t have convenient grounding lugs, so I’m going to have to come up with something…