Hello all, I have some few problems with my tube amp. The most important one is those nasty high frequency parasites (more than 100Khz) after my EF86 first stage. You can see it on the picture (5mv/div, 5ms/div) (the 100hz noise is clearly audible, I dont ear the 50hz one)
I guess it's because of the current rush in the main reservoir of the power supply, it stops as soon as I stop the amp. Those parasite are not present in the output of the power supply.
What should I do to get rid of this ? Filter it after the preamp ? adding some resistor before the main reservoir (right now there is 220ohms on a 150μF cap, I fear that more could be too much) ? A small inductor before the reservoir ?
I feel like filtering is would be the better solution.
Thank you really much for your attention and your help !
Regards,
Simon
I guess it's because of the current rush in the main reservoir of the power supply, it stops as soon as I stop the amp. Those parasite are not present in the output of the power supply.
What should I do to get rid of this ? Filter it after the preamp ? adding some resistor before the main reservoir (right now there is 220ohms on a 150μF cap, I fear that more could be too much) ? A small inductor before the reservoir ?
I feel like filtering is would be the better solution.
Thank you really much for your attention and your help !
Regards,
Simon
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That looks like rectifier commutation noise. You may not see it on the supply line because of resolution limitations with the large DC, but see it on the output of your amplifier stage (because it gets amplified and the DC blocked) Power transformer might need a snubber.
Thank you really much !
But I dont see anything in DC.
(cf picture, 2V/div for the reservoir, at the bottom, 10mv/div at top, for the preamp)
Is it possible that it's just electromagnetic parasites ?
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I’ve seen it show up on the amplifier, even though I can’t see a thing on the supply. It can even come in on the ground. Try a proper snubber on the supply before you go pulling your hair out looking for oscillations in the stage. If it goes away the instant you turn the power supply off but before the charge bleeds off tells you it’s being initiated in the supply.
Also try turning off your fluorescent lights. I’ve seen that cause it too. Electronic ballasts can be chock full of commutation noise as well as SMPS artifacts. Those can even mix with commutation noise in your amp’s rectifier which might not otherwise be audible.
OMG ! It worked ! thank you really much. I added a small resistor and capacitor in serie with the secondary of the power transformer.
It was "just" that.
I noticed that the distance between the cables coming out of the transformer and the preamp tube affect the sound.
For my tests I used 12V battery to power the heaters of the tube, now, It's time to plug back the nasty SMPS… But at least now I know the nature of the problem 🙂
Thank you so much !
It was "just" that.
I noticed that the distance between the cables coming out of the transformer and the preamp tube affect the sound.
For my tests I used 12V battery to power the heaters of the tube, now, It's time to plug back the nasty SMPS… But at least now I know the nature of the problem 🙂
Thank you so much !
HT SS rectifiers can generate this when they turn off. This gets onto the heater supply through the transformer. It then does not take much pF to couple onto the grid of the EF86. Sounds like a buzz. Make sure you have the heater supply not floating and keep away from the grid.
Hello, thank you for your answer
I still a have a problem with that. I managed to have a very decent DC on the heater (with LC filter), yet I still have those 100Hz parasites.
What do you mean by keeping the heater away from the grid ?
Keep we wire away from the one going to the grid ? or setting the voltage far from grid ? (like the main reservoir ?)
Heater circuits just make great radiators. For both the line frequency and anything else that finds its way onto it. Grids are easy to couple into. Rectifier noise (which is poorly damped high frequency ringing, repeated on ever half cycle) can find its way onto virtually anything. It’s like heat sink grease - put a dab of it on the baseboard and it will be on the ceiling by morning. High frequencies can be “black magic”. Unless you fully analyze your circuit - all loops and mutual couplings, every unintentional capacitance, you may never truly understand where. Those who have experienced it kind of know where - even though they are not obvious. Attack it at its source and be done with it.
If you’re getting a “hummmmmm” at 100 Hz, and not a “bizzzzzzzz” you could have heater-cathode leakage. Get a worse if the cathode is elevated. it’s another heater related problem to watch out for, and can couple in rectifier noise quite well if that’s happening. I’ve been thru amps that I swear have no grounding issues or loops that haven’t been dealt with (because the other monoblock is clean) and found this. Seems to happen once in a while.
If you’re getting a “hummmmmm” at 100 Hz, and not a “bizzzzzzzz” you could have heater-cathode leakage. Get a worse if the cathode is elevated. it’s another heater related problem to watch out for, and can couple in rectifier noise quite well if that’s happening. I’ve been thru amps that I swear have no grounding issues or loops that haven’t been dealt with (because the other monoblock is clean) and found this. Seems to happen once in a while.