What other tubes there are that are as oscillation prone as 6L6's? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: October 09, 2023, 05:21:14 am
content: Curious about what other tubes there may be that are as oscillation prone as 6L6's are in output stages.

author: jonpaul
author_rank: Super Contributor
timestamp: October 09, 2023, 05:30:47 am
content: Use parasitic surpressors on plate and 100 Ohm carbon comp or MF series R at grids.See RCA/Varian/Eimac transmitting tube and RCA Industrial Tube manuals 1960s.Jon

author: MathWizard
author_rank: Super Contributor
timestamp: October 09, 2023, 11:55:32 am
content: Whats better about Carbon resistors for this ?

author: Andy Watson
author_rank: Super Contributor
timestamp: October 09, 2023, 12:26:55 pm
content: Carbon resistors are usually a solid "lump" of carbon (or carbon-composite material) whereas metal film resistors are fabricated as a cylinder with a metal film covering. A spiral is cut into the film to obtain the required resistance - the spiral causes extra inductance - much like a wire-wound resistor.

author: SeanB
author_rank: Super Contributor
timestamp: October 09, 2023, 01:07:41 pm
content: If you cannot get carbon composition resistors there are special non spiral cut resistors available, most likely spec order from Vishay, who seen to have that market cornered for non inductive resistors.

author: CaptDon
author_rank: Super Contributor
timestamp: October 09, 2023, 01:21:44 pm
content: With poor construction techniques the list of oscillation prone tubes would be half of the RCA tube manual. 6L6's were indeed used by hams easily up to 7MHz as were 1625, 6DQ6, 807 and many others. With any reasonable amplifier construction and even mediocre shielding and bypass capacitors the 6L6 series are more than stable!! You want a real bugger of a self oscillator start building A.F. or R.F. amplifiers with 6146 tubes!! Some of the beam forming style tubes were prone to Barkhausen oscillation but that was usually at 200MHz and above and only across a narrow range of the operating curve. To say that 6L6's are prone to self oscillation says that the circuitry around them was poorly thought out.

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2023, 01:47:01 pm
content: The 6BG6GA is a re-packaged 6L6 with a plate cap, and is on the US military preferred-tube list from the 1950s.The plate cap helps to isolate the plate from the grid in-circuit, and the GA is smaller than an 807.I have had reasonable luck with 3W "flameproof" cermets (10 to 100 ohms) for low-inductance resistors.Also, be careful with the screen-grid bypass capacitor wiring: see ARRL handbooks from the tube era for hints.A few years back, when helping on a switching power supply design, I found a rule-of-thumb that for any three-terminal high-gain active device (MOSFET, BJT, triode, etc.), a parasitic-suppression resistor was needed on two of the three terminals (grid and plate for a 6L6), even for switching operation.

author: jonpaul
author_rank: Super Contributor
timestamp: October 09, 2023, 05:12:05 pm
content: we used 3x 6BG6 in parallel, in a 250 ma 0-400V lab PSU like the old RCA and Heathkit.No osc with any output or load , 100 ohm grid R and usual plate parasitic surpressors.Jon

author: TimFox
author_rank: Super Contributor
timestamp: October 09, 2023, 06:06:15 pm
content: Quite different construction: the EL34 is a true pentode, while the 6L6 is a beam-power tube.There is nothing obvious in the data sheets that would predict oscillation or not:6L6 4.7 mA/V and 0.6 pF anode-G1EL34 11 mA/V and 1.1 pF anode-G1The 6L6 has been around since 1936, while the EL34 is postwar (1953).However, the 6L6 istarted a large family (807, 1625, 6BG6, 5881 etc.), so many non-oscillating audio amplifiers have been built around it.See article on page 18 of the March, 1939 issue ofElectronicshttps://www.worldradiohistory.com/Archive-Electronics/30s/Electronics-1939-03.pdfIn my distant youth (60 years ago), Burstein-Applebee (of fond memory) sold surplus 1625s for only $0.50 each and the weird tube socket was still available from other surplus dealers. For 3.5 and 7 MHz RF, I used the then-popular recipe of a 2 W A-B carbon resistor with a single-layer winding of AWG18 magnet wire around the resistor length, soldered directly to the plate-cap connector to suppress parasitics.

author: floobydust
author_rank: Super Contributor
timestamp: October 09, 2023, 06:11:48 pm
content: "prone to oscillation" also depends a lot on the circuit and component types and grounding.Having some cathode inductance, say from a wirewound resistor also tames any RF. I measure a few uH on a 5W part, they have steel endcaps.I imagine the same goes for screen resistors, you'll see carbon or WW used and their inductance likely has an effect.I find more important is capacitive coupling from the heater-cathode. This is a backdoor path for mains RF and any oscillations to get in and cause problems between multiple stages.Hams use feed-through caps and inductors to filter RF in the heater string- but audiophools seem to never poke a scope there.6550 has high transconductance, close heater-control grid spacing and thus lower bandwidth and I don't find them wild.