Intermittent poping sound on tube amp when inputing a loud signal - Page 1
Forum section: electronics / repair

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: December 15, 2020, 11:14:54 am
content: so I am building this guitar tube amp, it's not super high gain, having only 2 gain stages and an LTP PI.it works ok but after like 20 seconds of it on, it starts making like short poping squealing sounds when I play loud.I've narrowed it down to 1 output tube, I tried changing the tube between 3 other tubes, to no avail, tried changing the coupling capacitor from the PI, still the same, changed the bias adjustment range resistors, resoldered all the pins on the tube socket, still the same issue, only parts left that I haven't changed are the grid leak, grid blocker, and bias pot resistors, but I doubt they could cause this problem.I don't have an oscilloscope, but I used a multimeter to measure the DC voltage on the grid of that tube, when I have to tube out, everything seems normal, when I have it in tho when the poppy-squealing noise happens, the DC voltage jumps up to sometimes even (positive) 60Volts, which would let me to believe that the tube is arcing from G2 to G1, (it's fixed bias amp so the cathode is grounded),and if I remove that particular tube and use the amp then, then everything is ok, grid DC voltage is normal, so is the sound.output tubes are EL34's, B+ is 680V, the screen voltage is 480V, I have biased them at 75% max dissipation. which puts the grid at around -35volts DC at idleit seems it could be the grid leak resistor going open, but idk, would it cause the G1 voltage to jump up to 60Volts?!?, I measured it when the amp was cold and off and of then it measures ok, tried stabing it with a stick when the amp was on and no pops or anything

author: Refrigerator
author_rank: Super Contributor
timestamp: December 15, 2020, 11:27:22 am
content: I wouldn't rule out the resistors yet, might be worth measuring them to confirm that their resistance hasn't drifted if they're old.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 15, 2020, 12:06:51 pm
content: You might want to borrow a scope to rule out (or in) ultrasonic oscillations.

author: CaptDon
author_rank: Super Contributor
timestamp: December 15, 2020, 02:39:30 pm
content: If the output is class B push pull you have the bias way to high.Even in single ended class A the bias is too high. If it were arcingin the tube you would surely see it!! You may be causing the ampto go into self oscillation. I have seen that happen at frequenciesas high as 100khz and it will surely destroy the output tubes bymelting the G1 grid wires from excess grid current. What is thecircuit layout? Transformer interstage coupling? Capacitor Coupling?What is the output stage, single ended class A or push/pull AB2 or B?You may need to change the gain vs. frequency negative feedbackloop as they often caused problems at certain load impedances.Did you design it yourself or copy from somewhere else??

author: Refrigerator
author_rank: Super Contributor
timestamp: December 15, 2020, 03:35:45 pm
content: Quote from: ELS122 on December 15, 2020, 03:30:53 pmFilm capacitors have this self healing feature where they burn the metal layer off the film when they arc unitl the spark can't jump the gap anymore.I was about to mention arcing caps but you said you checked capacitors so i decided not to because i thought it would be redundant.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 21, 2020, 07:23:48 pm
content: Wiring layout/lead dress is very important in valve amps.I once had an old head that would oscillate, turned out it was because some of the wiring routing had changed, and was no longer running close enough to the chassis.You need a scope, as ultrasonic oscillations can start with small but very visible artifacts, then can suddenly go Pop at full power, causing the heavy 50Hz/60Hz ripple you can hear.You can poke at the wiring, change the layout while observing the effect on the onset of catastrophic oscillation.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 21, 2020, 08:13:45 pm
content: You need a scope.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 21, 2020, 08:53:32 pm
content: Quote from: ELS122 on December 21, 2020, 08:28:49 pmYou mean you tried moving the signal wire around and you didn't SEE any change.You need a scope.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: December 21, 2020, 09:06:50 pm
content: Look,you can have hf oscillations in a valve amp without ever knowing they are there, unless you can see them with a scope.They can appear at the top and bottom of the waveform for just a short part of the cycle.Eventually, given certain conditions, they can evolve into full blown full power hf feedback, which you then will notice.You can observe these beginings of castastrophic oscillations with a scope, and see what makes it better or worse.You won't hear or measure any of this with a multimeter.You need a scope.