push pull 6L6 amp, oscillates even with just one output tube and nothing else. - Page 1
Forum section: electronics / repair

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: February 12, 2022, 05:28:54 pm
content: I am building a push pull 6P3S (6L6 equivalent) amp, and I cant find a way to fix what I asume is it oscllating.problem is this:the amp sounds fine with at lower volumes, but once it gets loud enough, the output tubes rapidly increase (negatively) in bias, they go into cutoff.when this happens, the audio becomes super muddy, and looses trebble, and the B+ supplyincreases, it doesnt necrease, it increases, kinda expected since the bias becomes more negative but a weird problem anyway...the bias supply is stable, at around -20V.I've probed the supply rails with a signal tracer and they have practically no interfeering signals on there,I've tried removing all the preamp tubes, and the PI tube, which didnt change it,I removed one of the 6P3S output tubes and that reduced the magnitude but it's still therewhen I remove both output tubes the oscillation stops.what Ive tried:twisting the plate wires, keeping them as far away from the grids as I couldtwisting the grid wires, keeping them as far away from anything else.adding 1.6k grid stoppers for the control gridadding 680 ohm grid stoppers for the screen gridputting snubber capacitors on the heater linesshorting the control grid directly to the bias supplyputting a shield between the transformers and output tubeshorting the NFBL to groundnothing fixed it, well the 680ohm screen resistors reduced it slightly, but I cant just go and put like 22k screen resistors on the tubes, that's just not the right way to fix it.my PC sound card input only goes up to 96khz bitrate, so the highest frequency I can scope is only ~45khz,but there seems to be a ton of spurs at around 20khz, that are only there when the amp is turned on, only when the tubes have warmed up.I've attached pictures of the FFT chart and scope. (note the file names)the probe is connected to the output of the amp.I also attached a schematic of the output section of the ampoh and one thing to note, the transformers are oriented in the same direction, one is not turned 90 degrees, I cant easilly fix that, I'm just working with the chassis that I have at the moment... but maybe that could be the cause of this? even tho the B+ rail doesnt have any of those spurs on it...and the OPT is shielded.

author: TimFox
author_rank: Super Contributor
timestamp: February 12, 2022, 05:46:30 pm
content: Have you tried plate stopper resistors, somewhere between 10 and 100 ohms, directly at the tube sockets?Have you looked for the oscillation with an oscilloscope to estimate its frequency?Have you tried disconnecting the 3.3 k feedback resistor as a test?The feedback network is suspicious: often, there is a pull-down resistor on the end opposite the output to form a voltage divider, and a small capacitor across your 3.3 k resistor to compensate the phase at high frequencies.

author: TimFox
author_rank: Super Contributor
timestamp: February 12, 2022, 06:37:02 pm
content: The 6L6 and its big brother the 807 were often used in ham radio transmitters years ago to above 20 MHz, and they were prone to "parasitic oscillation" at even higher frequencies. The plate stopper was the normal fix for this. Even though the oscillation frequency is much higher than the audio range, there is nothing to stop its amplitude and it totally screws up the DC values and the AF amplification, as you have noticed. When it occurs, the parasitic oscillation happens with the single tube, regardless of the circuit around it.If the frequency of oscillation is too high for the sample rate of your sound-card system, you will see it as an alias (q.v.).I missed the 100 ohm pull-down resistor in your drawing, but if the entire circuit is oscillating (not a parasitic in one tube), a small capacitor across the 3.3 k might still be needed. Try 470 pF as a test and see what happens (that would cause the closed-loop hf response to roll off above about 100 kHz). If your feedback-removal test did not stop the oscillation, then I suspect a hf parasitic oscillation.

author: TimFox
author_rank: Super Contributor
timestamp: February 12, 2022, 06:54:56 pm
content: There are all sorts of circuit-wiring details that may not be important at 20 kHz but cause problems at 20 MHz.Look up "parasitic oscillation". Here is a forum thread for guitar amplifiers:https://www.antiqueradios.com/forums/viewtopic.php?t=90967

author: TimFox
author_rank: Super Contributor
timestamp: February 12, 2022, 07:49:24 pm
content: A modern (somewhat idiosyncratic) book about vacuum-tube amplifiers is Morgan Jones, "Valve Amplifiers, Fourth Edition", Newnes (2012).He mentions parasitic oscillations and anode stoppers on pp 468-9. The ham trick that he cites is to wind a 10-turn coil around a 100 ohm, 2W carbon resistor.Fundamentally, this type of tube is capable of parasitic oscillation when connected with long wires to an arbitrary transformer with parasitics.If you can't get access to an oscilloscope (he suggests you need 100 MHz bandwidth for these problems), you are in the dark and can only proceed by trying individual suggestions such as adding resistors.A few years back (before retirement), we had problems using a very high frequency GaAs device (capable of frequencies up to 3 GHz, if I remember correctly) at around 20 MHz in a low-noise amplifier. Unfortunately, we did not have any oscilloscopes or spectrum analyzers that worked up to GHz frequencies. As in your amplifier, the prototypes sometimes oscillated out-of-band. The only way that I could detect the oscillation when it occurred was to connect a microwave detector (diode) package rated up to several GHz RF input (with an N connector at the input and BNC at the "video" output) to the N connector on the prototype box and look for DC at the video output.

author: TimFox
author_rank: Super Contributor
timestamp: February 12, 2022, 10:46:29 pm
content: The wire type should have no effect on your problem, and litz wire requires that the strands be individually insulated and connected together only at the end.The disappearance of the problem with 5.6 nF from grid to ground confirms to me that you have an RF parasitic oscillation in the tube itself, rather than with the entire circuit caused by the total feedback loop.I have never seen before a suggestion to wrap the plate wire around the grid: that would increase the plate-grid capacitance which is not normally a good thing.I seriously suggest trying, say, 22 ohmsdirectlyat the plate pin of the tube socket before the wire to the primary. The purpose is to decouple any hf resonances in the rest of the plate wire and transformer impedance from the tube itself.What choke did you remove? I didn't see one on your diagram.Also, I do not see any screen bypass capacitors on your diagram. I would suggest at least 10 nF directly at the tube socket, or from the other side of the stopping resistors, to ground or to the cathode. In a tetrode amplifier (as opposed to an ultra-linear with the screen to a transformer tap), the screen should be at the cathode for AC to achieve the low plate-grid capacitance that helps avoid oscillation.

author: TimFox
author_rank: Super Contributor
timestamp: February 13, 2022, 08:29:41 pm
content: Your attachment is missing.A couple of comments:I presume that the downward slope when you clip the peak is due to the low-frequency response of the output transformer. It's what you would see with a low-frequency square-wave drive to the transformer due to its finite inductance.Oscillation or ringing on the waveform is usually something worth investigating, even if it doesn't affect the sound of the guitar.Thegmof one tube doesn't make it run hot, but does change the bias voltage on that tube required to get the correct quiescent current. If the two quiescent currents are far from equal, you have a net DC in the transformer primary that is not a good thing.You might want to add 10 ohm resistors in each cathode lead, which will have negligible effect on the amplifier, to verify the bias currents with an external voltmeter.