Wierd self-oscillation issue in tube amplifier - Page 1
Forum section: electronics / beginners

author: MarcusFunt
author_rank: Contributor
timestamp: November 09, 2023, 07:27:24 pm
content: Hi guys.¨I'm making a DIY guitar vacuum tube preamplifier based on this schematicThe only thing I have changed is substituting the 12ax7 with a 12at7 tube. They have very similar characteristics aside from a small difference in gain.the problem is that the circuit self-oscillates.When coping the attached schematic exactly the circuit oscillates with a sine-ish waveform with a frequency of only 8 hertz. (see attached picture).I've experimented extensively with different circuit configurations and I think I tracked the problem down to the input side.I exchanged the whole input circuit for that of the Fender Champ.The modification didn't change the waveform much aside from raising the frequency to about 40khz(see attached picture).I'm putting an electric guitar into the input of the circuit(which consists of a fairly large inductor and a few filters). However, if I unplug the guitar, the whole circuit gets much louder and outputs about 4 kilohertz. The frequency changes if I put my hand near the circuit(only with no input). It also introduced noise when I physically move the tube in its socket.At this point, I would suspect that the tube died, but if I unplug the circuit, the oscillations stop, and the circuit seems to function until the caps dischargeI should also mention that the circuit works. if I play a note. the note is just superimposed on top of the self-oscillation(see attached file)Don't mind the voltages in the pictures being so wildly different. In reality, they are about the same but measured before and after a voltage divider.Can anyone tell me why this Is happening/tell me how to fix it? (I'm more than happy to make some more measurements. Just tell me where to probe:-))

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: November 09, 2023, 07:53:30 pm
content: Hi MarcusFunt,in the case of 4Hz your amp is motorboating.Try lowering the coupling-caps or raising the capacitance in the supply.How did you assemble the amp?40kHz and 4kHz smells like feedback.Try shielding between in- & output.Ground the center-lug.Did you try another tube, maybe go with the original (12AX7 / ECC83)?Did you use the values of the schematics?Post some pictures of your build.I hope the supply-connector is not a DIN-type?!Good luck

author: TimFox
author_rank: Super Contributor
timestamp: November 09, 2023, 07:54:47 pm
content: Actually, the 12AT7 differs substantially from the 12AX7.Both are high-mu dual triodes, but the 12AT7 is designed to operate at much higher plate current with higher transconductance.https://frank.pocnet.net/sheets/093/1/12AX7.pdfmu = 100 at 250 V, 1.2 mA gm = 1.6 mA/Vhttps://frank.pocnet.net/sheets/093/1/12AT7.pdfmu = 60 at 250 V, 10 mA gm = 5.5 mA/VThe bias requirements are therefore very different: -2 V at 1.2 mA for 12AX7 vs. -2 V at 10 mA for 12AT7Your 2.2 k cathode-bias resistor and the plate resistors are too high for the 12AT7, which will be close to cut-off.

author: Grandchuck
author_rank: Super Contributor
timestamp: November 09, 2023, 07:59:34 pm
content: Is the 33 uF bypass capacitor good?

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 09, 2023, 08:08:12 pm
content: Most likely the electrolytic bypass capacitors are open or nearly so. Parallel a similar to higher uF valued cap across each 8-uF/450V caps.Note: you can obtain/salvage a small, good as new similar electrolytc cap for replacement (commonly 10 to 33uF) rated at 450V from taking apart most common SMPS switching wallwarts (phone charger) or other obsoleted home audio equip(DVD Player, etc.).Also a 12AT7 is definitely very different than a 12AX7 and may cause your problems.The 12AX7 47uF cathode bypass in the preamp and the 33uF/200V bypass cap might also be causing this oscillation.

author: Vovk_Z
author_rank: Super Contributor
timestamp: November 10, 2023, 05:24:51 am
content: Yes, 8 Hz is motorboating, that is caused by wrong or bad bypass cap, or wrong bypass wiring design, or bad ground wire design etc.

author: floobydust
author_rank: Super Contributor
timestamp: November 10, 2023, 05:35:01 am
content: OP, tell us about your power supply. I think it is too weak.

author: MarcusFunt
author_rank: Contributor
timestamp: November 10, 2023, 12:43:04 pm
content: Hi again. I'm using a basic linear power supply to supply the board. It's feeding it about 140vdc. I omitted the 33uf electrolytic since the onboard rectifier caps are so big. I kinda just guessed the voltage since the schematic doesn't give any values. The whole circuit is boils on pertinax prototype board to give it a more "vintage" look. The tube being close to cutoff it very favorable in the guitar community. I should probably also mention that the input is only about 150mv peak

author: MarcusFunt
author_rank: Contributor
timestamp: November 10, 2023, 09:39:40 pm
content: I just guessed the voltage based on the component choices. Is 140Vdc to low?There isn't much drop or ripple

author: MarcusFunt
author_rank: Contributor
timestamp: November 10, 2023, 09:41:02 pm
content: I omitted it because I have a much larger smoothing cap in its place. is the value critical??

author: MarcusFunt
author_rank: Contributor
timestamp: November 10, 2023, 09:42:42 pm
content: How do I test if that's the case?

author: floobydust
author_rank: Super Contributor
timestamp: November 10, 2023, 09:50:41 pm
content: Put a voltmeter or scope on B+ and see if it is also moving up and down with the oscillations - it needs to be quite solid. The 33uF part is very important to keep the HV clean.Why do you have a light bulb on the board? The original schematic shows a cheezy shunt regulator with the LED, presumably a weak DC-DC converter was used.

author: MarcusFunt
author_rank: Contributor
timestamp: November 10, 2023, 10:39:19 pm
content: I don't have any bulb attached. Didn't think it was necessary. I have a 2x47uf smoothing caps and therefore didn't add the 33uf one. Attached are pictures of the 100hz ripple from when it does the 40khz oscillation (150mv vpp) and the much weirder waveform when it's doing the 8hz thing

author: MarcusFunt
author_rank: Contributor
timestamp: November 10, 2023, 10:42:15 pm
content: Sorry I'm having trouble attaching pictures

author: MarcusFunt
author_rank: Contributor
timestamp: November 10, 2023, 10:43:01 pm
content: Here is the 8hz wierdness