Tube Resistance Chart? - Page 1
Forum section: electronics / beginners

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 12, 2023, 06:07:40 pm
content: All,Can anyone tell me how to read the chart below? Example: Line #1, 12SK7-GT. Resistance should be 36 Ohms between pin #2 and WHERE?Thank you,- Wil

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2023, 06:32:50 pm
content: Presumably, they are all measured with respect to the pins 1 of the 12SK7s, 12SA7, and 12SQ7.Hence, the 0 values at those pins and a few others.The original 8-pin octals (no -GT suffix) were metal shell, with the shell connected to pin 1.On Item 1, this assumes pin 3 (G3) and pin 5 (cathode) are connected to the shield (pin 1), which for the -GT version is a short metal shell around the bottom of the tube.The strange values on pins 2 and 7 (heaters) with respect to that reference presumably occur with the tubes cold, and the heater resistances very low.The 12SQ7 heater is between 7 and 8, to keep it away from the grid on pin 2.If you follow V4 (8 and 7) to V3 (7 and 2) to V2 (2 and 7) to V1 (2 and 7), in that order, you see the resistances add up for the 12 V tube heaters.Go back to your wiring diagram to see how the heater string connects to the negative (sort-of-ground) end of the plate power supply.

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 12, 2023, 08:27:39 pm
content: That sort of makes sense, although, I was not able to measure any kind of resistance on any of the 12* tubes from pin 1 to any of the non-0-Ohm pins in the chart. The only thing I was able to validate is that all of the 12* tubes have continuity between the header/filament pins and each of them measures between 11 and 15 Ohms.When you say, "If you follow V4...the resistance adds up to the 12V tube heaters.", I don't know what you mean by "V" 4. Does "V" mean something? Below is the, what I believe is called "heater string?", diagram which I believe you could be referring to.I thought that the "12" at the beginning of each of these tubes' names meant that each used 12 volts... but where does resistance come into this? Clearly, I'm missing a few pieces of information.Thank you,- Wil

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 12, 2023, 08:32:33 pm
content: Maybe, this phrase was what I was missing?!

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 12, 2023, 08:34:10 pm
content: Maybe these values are only applicable when the radio is fully functional and working... not when taken apart and measured at the component level.- Wil

author: TimFox
author_rank: Super Contributor
timestamp: November 12, 2023, 09:59:00 pm
content: Since the power (DC and heater) is not connected directly to the chassis, "common negative" is the negative end of the DC supply.Tubes in these diagrams are normally given "V" numbers (V1, etc.), so I used the Item number when I could not find a reference designator.In your wiring diagram, the "ground" symbol is not the chassis, but "common negative".The series string of heaters is, indeed, the string of heaters connected in series.Yes, the "12" indicates that at specified current, 0.15 A, there should be 12.6 V across the individual heater for a tube number that starts with 12.Therefore, the resistance (when hot) should be 12.6 V/0.15 A = 84 ohms, but the cold resistance (no power applied) should be much less: apparently 12 ohms for the "12Sx" tubes.Resistances must be measured with power off, but the voltages (AC and DC) are measured with power on.An important (typical) note in the manufacturer's instructions is that you should do alignment of the RF and IF circuits with a 3 V battery connected to the AVC line (-3 V w.r.t. "common negative").

author: Wil_Bloodworth
author_rank: Regular Contributor
timestamp: November 12, 2023, 10:17:38 pm
content: Thank you! All of that makes perfect sense now.And I was not sure what the manufacturer meant by the 3-volt battery bias so thanks for clarifying that as well!- Wil