Hi friends, a while back I built this amp for a friend(with some changes). He's been using it for several years without any issues. A few weeks ago, he told me he was hearing a noise on the right channel. I thought it was the 6v6s running out. I set up the oscilloscope and detected a noise/oscillation in the negative part of the sine wave that I can't detect. Can you give me any clues on how to locate the fault? Could it be the OPT? I've already tried another 12ax7 and re-soldered it.
The red marks are where I placed the oscilloscope and found the noise.
The other triode of the 12ax7 and the other 6v6 work ok, to eliminate the potentiometer.
I inverted the 6v6 too.
thanx a lot guys!!
https://diyaudioprojects.com/Schematics/DIY-Push-Pull-PP-6V6-Tube-Amplifier/
The red marks are where I placed the oscilloscope and found the noise.
The other triode of the 12ax7 and the other 6v6 work ok, to eliminate the potentiometer.
I inverted the 6v6 too.
thanx a lot guys!!
https://diyaudioprojects.com/Schematics/DIY-Push-Pull-PP-6V6-Tube-Amplifier/
Your schematic has fatal flaws.
Eg; cathodes of 6SL7 is wrong, causing incorrect bias.
A 6SN7 makes for more drive as a 6SL7 is a low power pre amp valve.
At least it is drawn as an Ultra Linear output stage. That is better than nothing.
Please post the schematic you are using.
Eg; cathodes of 6SL7 is wrong, causing incorrect bias.
A 6SN7 makes for more drive as a 6SL7 is a low power pre amp valve.
At least it is drawn as an Ultra Linear output stage. That is better than nothing.
Please post the schematic you are using.
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I'm also working on a 6v6pp amp. Hammond iron everywhere. I was thinking of using the schematic on a Dynaco amp, like the schematic above? Is that a bad idea?
The circuit is pretty popular, but I'm not a fan of the praraphase splitter. 6V6s are very easy to drive, I'd go for the simplest setup, voltage amplifier stage + cathodine splitter. Similar to Tubelab's SPP, just using 6V6s.
It's not My schematic, it's a proven circuito that works very well, in My case with 12ax7.
It's not the purpose of the post, only the puntual problem.
Works very well
The frequency I put in the function generator is 1kHz, I don't know why the failing triode marks 5kHz, I just noticed it when I was uploading the photos to the forum.
the capacitor is not the problem
the capacitor is not the problem
The oscillation frequency of the negative part is 5khz. Can you identify the 6 and 1 pin on the schematic?
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Obviously the oscillation frequency is much higher than 5kHz. Looks more like >50kHz.
Is the 1nF lead compensation cap across the NFB resistor good and well connected?
This cap is essential for stability.
Is the 1nF lead compensation cap across the NFB resistor good and well connected?
This cap is essential for stability.
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I don't know if this is helpful, but the 1000pF phase-lead cap in the schematic was tailored to the Dynaco output transformer, which was of much higher quality than the Hammond, of course. I think what you need to do is look at a 10kHz square wave at 1 watt and check for signs of instability. Then experiment with the phase-lead cap value to see if you can tame the oscillation. It may be appearing now because the tubes are aging or some other component has drifted.
I appreciate that info, Hooman. However, my amp was working fine as it was. The other channel was working fine, the other triode on the 12ax7 was working fine, and the other 6v6 was working fine. I want to know what's happening to it now, what is this fault I'm showing you. I'm going to investigate the post you sent me.
I found the problem. The cables that run from the transformer to the chassis were marked. The copper was visible. They seemed to be touching and inducting each other. I had to remove them to examine them. This is strange because they have rubber grommets. Thanks everyone for the help and the good info and links.