09-25-2026, 10:45 AM
OOpsie!!! I must be looking at the wrong schematic! Yes, this changes things!
OK, for the 37-620. If the 6F6 screen voltage is unstable, this is a major issue. The screen and output transformer are both fed by the same voltage, generated at the junction of the speaker field and 8 uF cap #60. If the voltage at the 6F6 plate (pin 3) is stable but the voltage at the screen (pin 4) fluctuates, check your wiring from the screen terminal on the socket (pin 4) back to the above stated junction. You may find a cold solder joint.
The plate voltage for the IF Amp is from the primary of the 2nd IF transformer. The B+ for that transformer also comes from the same source as that for the output tube stage. Measure the B+ to the transformer. If this is stable and the plate voltage varies by that much, then the transformer may be at issue, maybe a bad solder joint or bad socket connection. The transformer may ohm good under no current but with the tube plate current as a load to the transformer, the issue may manifest itself. However, I think that this may be a cold solder joint condition. Depending on the wiring, what is causing the screen fluctuation in the 6F6 can also cause the plate fluctuation in the 6K7.
The suppressor grid issues are likely not a big issue. I would expect a slightly negative voltage on both, with the RF being slightly more negative than the IF. The suppressors are tied not to chassis ground but to taps on the bias resistor that is between chassis ground and the center tap of the HV winding of the power supply. Note the 25 Ohm resistance between chassis ground and the next resistor, the 8 Ohm section, then the 202 Ohm portion of resistor #61. The other end is tied to the HV center tap. This is how negative grid bias is generated in many sets of this vintage.
The bottom line here is to trace through the B+ supply. Note that where the junctions are made on the schematic may not be where they actually are in the physical wiring, but the result is the same.
BTW, note that the polarization indicated on the schematic is incorrect. The upside down U is usually the can. This is definitely NOT connected to positive. However, I'm sure you wired the caps correctly. If not, you woud have heard a banf and found cap confetti all inside the radio. (Been there, done that, got the scars.)
OK, for the 37-620. If the 6F6 screen voltage is unstable, this is a major issue. The screen and output transformer are both fed by the same voltage, generated at the junction of the speaker field and 8 uF cap #60. If the voltage at the 6F6 plate (pin 3) is stable but the voltage at the screen (pin 4) fluctuates, check your wiring from the screen terminal on the socket (pin 4) back to the above stated junction. You may find a cold solder joint.
The plate voltage for the IF Amp is from the primary of the 2nd IF transformer. The B+ for that transformer also comes from the same source as that for the output tube stage. Measure the B+ to the transformer. If this is stable and the plate voltage varies by that much, then the transformer may be at issue, maybe a bad solder joint or bad socket connection. The transformer may ohm good under no current but with the tube plate current as a load to the transformer, the issue may manifest itself. However, I think that this may be a cold solder joint condition. Depending on the wiring, what is causing the screen fluctuation in the 6F6 can also cause the plate fluctuation in the 6K7.
The suppressor grid issues are likely not a big issue. I would expect a slightly negative voltage on both, with the RF being slightly more negative than the IF. The suppressors are tied not to chassis ground but to taps on the bias resistor that is between chassis ground and the center tap of the HV winding of the power supply. Note the 25 Ohm resistance between chassis ground and the next resistor, the 8 Ohm section, then the 202 Ohm portion of resistor #61. The other end is tied to the HV center tap. This is how negative grid bias is generated in many sets of this vintage.
The bottom line here is to trace through the B+ supply. Note that where the junctions are made on the schematic may not be where they actually are in the physical wiring, but the result is the same.
BTW, note that the polarization indicated on the schematic is incorrect. The upside down U is usually the can. This is definitely NOT connected to positive. However, I'm sure you wired the caps correctly. If not, you woud have heard a banf and found cap confetti all inside the radio. (Been there, done that, got the scars.)
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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