screen grid glowing red when output tube overdriven. - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: February 16, 2022, 03:53:39 pm
content: so I get that this is kind of a dumb question being like: "no I wont put large grid stopper resistors, but why do my grids glow red?"but I have 330ohm screen stopper resistors, and 220 ohm supplying both, and they dont get warm at all under operation (yes I made sure they are wired right)but when overdriven I see a red glow from inside the tube, behind the plate, the plates arent red plating.could it maybe be the control grid? I only have a capacitively coupled Long Tail Pair phase inverter... I dont see it delivering that much current...could this just be a normal thing? the tube didnt instantly die so idk...could it be the cathode heating up when a lot of current is passed trough it?tubes are fixed biased 6P3S russian 6L6 equivalents, plate voltage is 430 volts at idle, about the same for screens.

author: jonpaul
author_rank: Super Contributor
timestamp: February 16, 2022, 04:02:04 pm
content: Photos? Power output, input? topology eg cl A,,B,?Try genuine USA made vintage valves instead of USSR knockoffsJon

author: TimFox
author_rank: Super Contributor
timestamp: February 16, 2022, 04:31:41 pm
content: Have you measured the screen grid currents under these conditions? Carefully connect a voltmeter across the screen stopper resistors, with due consideration to safety. Also, use an oscilloscope to look for signs of high-frequency oscillation during these conditions.

author: TimFox
author_rank: Super Contributor
timestamp: February 16, 2022, 05:44:18 pm
content: If the screens are red, then they are drawing a large current. For a 6L6GC (the highest-power US 6L6), the screen dissipation is rated for 5 W maximum. At 250 V, that is 20 mA. 20 mA in a single 330 ohm stopper is 132 mW, which might not heat it up much. These questions are quantitative.

author: TimFox
author_rank: Super Contributor
timestamp: February 17, 2022, 02:43:52 pm
content: Almost all US tubes with 6.3 V, 0.9 A heaters are 6L6s in some sort of package, including my favorite: the 6BG6 with a plate cap.By the way, the higher-voltage heater tubes such as 25L6GT, 35L6GT, and 50L6GT were designed for series-heater operation and are not as strong as the 6L6.Original 6L6 is 19 W plate dissipation, but the 50L6GT is only 10W.The first card-punch machines I used in college (ca. 1967) used 25L6GT tubes as solenoid drivers for the hole-punch mechanism. When I needed to purchase spares for my E H Scott WWII-vintage merchant marine receiver, the ones I found were branded "IBM".Originally, the "W" suffix denotes an American tube "ruggedized" for mechanical stress and vibration in military use. A "Y" suffix denotes having a low-loss octal base (e.g., 6AG7Y and 6SB7Y). Current manufacturers of the 6L6 series do not necessarily follow the original American nomenclature. "A", "B", and "C" denote evolution from the original suffixless model. In small-signal tubes, often the "A" suffix denotes having a warmup-time controlled heater structure. The original 6L6 without suffix was built in a metal envelope, while the "G" denotes a shouldered glass envelope. The GB and GC models have a larger cylindrical envelope, while the GT is a smaller glass tubular envelope.Evolution of plate dissipation: 6L6 and 6L6G at 19 W, 6L6GB at 23 W, 5881 is military ruggedized 6L6WGB at 23 W, leading up to the 6L6GC at 30 W.