Negative voltage in tube radio? - Page 1
Forum section: electronics / rf-microwave

author: ToddW
author_rank: Contributor
timestamp: January 05, 2021, 03:38:36 am
content: I recently had a little project fixing an AM tube radio. Never worked with tubes before but am learning the basics. Had bad B plus voltage but sorted it out with some new caps. Went around measuring tube pin voltages according to the schematic and all were listed as positive voltages. But there was 1 pin that was supposed to be 1.7 volts and it was -0.8 volts. Close, but negative. Also have been reading about tubes and often hear of applying negative voltages to grids for biasing. But now wondering, the power system only puts out a positive voltage, how to you get a negative voltage anywhere in the circuit?

author: drussell
author_rank: Super Contributor
timestamp: January 05, 2021, 04:22:44 am
content: You're probably looking at a grid voltage being biased negative by contact potential or grid leak bias due to the space charge around the cathode.When you put a high resistance across the grid to whatever the DC return path to the cathode is, some of the "excess" electrons floating around the hot cathode will make it to the grid, and therefore across the high resistance will form a small voltage "drop" of up to a couple volts negative...This is often used where a small-voltage self-bias is desired in low-level-signal circuits.

author: TimFox
author_rank: Super Contributor
timestamp: January 05, 2021, 04:49:12 am
content: Almost always in AM radios, the voltage from grid to cathode is negative.This can be done with "contact potential" bias, as mentioned above, where the cathode is grounded and the electrons that hit the grid develop a negative voltage across a very high grid-to-ground resistance.The other way (more common) is to put a reasonable resistor (usually bypassed by a capacitor) from cathode to ground so that the cathode is at a positive voltage with respect to ground and therefore the grid is negative with respect to the cathode.In your schematic, the usual specification for voltages is with respect to ground, which may be the chassis or the negative end of the positive supply. The pin in question that was spec'ed at +1.7 V but measured -0.8 V: what tube element (grid, cathode, etc.) is it?

author: CaptDon
author_rank: Super Contributor
timestamp: January 05, 2021, 03:34:29 pm
content: Tube oscillator stages tend to develop negative voltages on the grids.This is true of RF oscillators as well as the Vertical and Horizontal andColor oscillators in old tube type television sets.

author: ToddW
author_rank: Contributor
timestamp: January 05, 2021, 05:10:18 pm
content: Here is my schematic and voltages. Its the 6A7 tube pin G0 that is supposed to be 1.7 volts and I get -0.7 V. This pin is circled in yellow on the schematic. Also, if I remember correctly, I once read -6.9V when the local/distant switch was changed, but think that may have involved a wiring problem. Did a lot of work over several weekends and hard to remember all the details.

author: ToddW
author_rank: Contributor
timestamp: January 05, 2021, 05:17:41 pm
content: This diagram shows more of the schematic.

author: drussell
author_rank: Super Contributor
timestamp: January 05, 2021, 05:52:48 pm
content: Your measured voltages of any high impedance points aren't necessarily going to match what is listed in that chart if you're using a DMM (as I suspect you are, which has a very high input impedance) to measure them instead of the specified 1000 ohm/volt meter.

author: bob91343
author_rank: Super Contributor
timestamp: January 05, 2021, 10:50:03 pm
content: The 6A7 grid should be negative. It's an oscillator and there is rectification during the pulses of grid current, resulting in an average negative voltage.Most DMMs have input impedance of either 1 Megohm or 10 Megohms. The voltages indicated on service information often refers to meters that are 20 kOhms per volt so the input impedance depends on the range in use. However, measuring the grid of an oscillator often will quench oscillations and cause the circuit to fail. Best to use a high impedance device; even a 10 Megohm probe on an oscilloscope may be marginal, especially in short wave sets. One must take into account the input capacitance as well, and the inductance of the test lead.It's never as simple as we would like.