resistance test question

[kip.duff]

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.

[Paul Birkeland]

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.

[kip.duff]

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.