37-630T RF and IF grid 3 - Printable Version
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37-630T RF and IF grid 3 - fenbach - 07-21-2022
grid 3 of the RF 6K7 tube is grounded thru a resistor. but grid 3 of the IF 6K7 tube is connected to ground thru a wire with little resistance that's wrapped around the secondary of the first IF transformer. my radio is a code 125-126, schematic attached. meanwhile, in a different 37-630 [no code], grid 3 on both the RF and IF tubes is simply grounded thru a common resistor. this design seems to be more common. which leaves me wondering why the grid on mine is coupled to the IF transformer. anybody know? or know what difference it makes.
RE: 37-630T RF and IF grid 3 - RodB - 07-21-2022
Hi - you should be measuring around 11 ohms across the green and brown wires of the first IF secondary. The green wire will connect to the 6K7 IF grid cap and the brown to ground. The 6K7 RF grid cap will measure around 51K ohms to ground. The difference is that the RF amp has grid bias and the IF amp has cathode bias developed by the 400 ohm resistor in the cathode circuit.
What is the value of the IF grid resistor? The more common circuit is the one in the schematic.
RE: 37-630T RF and IF grid 3 - fenbach - 07-21-2022
boy, am i missing something. i don't even see an IF grid resistor [which grid, by the way??]. except the one on grid 2 which is labeled on the schematic as 51K ohms to ground. i do see that "the that the RF amp has grid bias and the IF amp has cathode bias developed by the 400 ohm resistor in the cathode circuit." but i read 700 ohms on the IF cathode resistor, not 400 [just a typo?]. and the schematic says 15 ohms from the grid cap to ground, not 51K. but regardless, i'd like to know why the IF grid 3 is coupled to the IF transformer, which isn't common at all on the relatively few radios i've looked at.
RE: 37-630T RF and IF grid 3 - Ron Ramirez - 07-21-2022
The OP is referring to the suppressor grids of the RF and IF tubes in his original post, not the control grids.
fenbach, the schematic below shows the circuitry as you have described it.
https://philcoradio.com/library/download/service%20info/service%20bulletins/Philco%20Service%20Bulletin%20251A.pdf
(Yes, this site has Philco schematics as well, and they are completely free with no limitations.)
You have a later version of the 37-630, Code 126. The earlier version, Code 122, does have somewhat different circuitry as well as a different band switch with more "throw" between the three positions.
https://philcoradio.com/library/download/service%20info/service%20bulletins/Philco%20Service%20Bulletin%20251.pdf
RE: 37-630T RF and IF grid 3 - RodB - 07-22-2022
I guess I was thrown off when you mentioned the IF so I honed in on that. And at first I had the earlier schematic. Yes, it is strange to have an IF wire coupling to the suppressor grid. This may be Philco doing an experiment with oscillation suppression or regeneration. I've seen 1940 models with the AVC tied to the suppressor grid but this looks like a feedback issue. Normally grid 3 is connected to the cathode.