Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: kip.duff on December 21, 2017, 11:01:31 AM
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I have some "out-of-specs". 300B terminal IDs are mirrored, so:
A1/B4 to ground. (Is this done with tubes installed?)
*with 300Bs installed: 4.7-5.0 ohms. *without tubes: moves around, then settles at about 1.1k ohms.
A4/B1 to ground: 4.2 ohms with 300Bs. (manual says 5-6 ohms)
Also, had OL at most PT-7 mount screw terminals. Powder coated transformer tops- figured star washers would make ground (not). Need to grind away areas on bells under mounting screw heads. At least I know what's up there.
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A4/B1 goes to ground through the big bias resistors. This resistance is best measured without 300Bs in the sockets, as the 10,000uF caps can cause meter measuring issues. A1/B4 connect to the filament supply output, and you'll have a tough time getting much of a reading without a 300B installed.
I wouldn't be top worried if you're off by an ohm or two on these.
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A4/B1 goes to ground through the big bias resistors. This resistance is best measured without 300Bs in the sockets, as the 10,000uF caps can cause meter measuring issues. A1/B4 connect to the filament supply output, and you'll have a tough time getting much of a reading without a 300B installed.
I wouldn't be top worried if you're off by an ohm or two on these.
Your response agrees with my findings- my measurements are good when I use your instructions. Page 60 in the manual was misleading for me: it instructs to measure resistance at A4/B1 with 300B in socket. It does not specify whether or not to install tube when measuring at A1/B4- so it’s reasonable to assume that the tube is supposed to be left out. This is what led to me starting this thread- measurements I got where incorrect without tubes installed and I was worried.
After reading other threads, it appeared A1/B4 were to be measured with tubes in place which meant my resistance tests were OK- so I felt OK with doing voltage tests. Voltage tests were good except for deviation due to testing with an RCA NOS 6A3. Measurements at 1 and 16 were 4.0V instead of 5.0V, with 5.93V measured across filament supply circuits. Based on info from you guys in these threads, I believe 5.93V is OK, so I’m on to installing the Beequiet parts 😊. I was cautioned by Doc about plate voltage for non- 300B tubes: looks groovy at 99 and 98 VDC with mains voltage at 122.