Ringing from the tweeter

It's not uncommon for vintage 6SN7's to be microphonic, or to become microphonic. There was a post here not long ago by Kevin Deal of Upscale Audio, who is a particularly knowledgeable tube seller, attesting to that with some rather striking statistics.

A commonly used means of checking for tube microphonics is to VERY GENTLY tap each tube with a pencil eraser, while the system is powered up, and see what you hear through the speakers.

What adds to the puzzle in this case, though, is that the ringing sound is present even when no signal is being put through the system. And even when the volume control is turned all the way down. So if microphonics is the root cause of the problem, the question is what is stimulating the microphonic effect.

Assuming the preamp does not appear to be exposed to any source of low level vibration, I'm thinking that what may be occurring is that a feedback loop formed by the preamp, the power amp, the speakers, and an acoustic path from the speakers back to the preamp may be breaking into oscillation. Similar to what happens when the gain on a public address system is turned up too high and the output of the speakers feeds back into a microphone that is being used.

If that is what is occurring it could very well help to interchange + and - at the outputs of the amplifiers, or at the speaker terminals. Oscillations resulting from feedback are sensitive to phase. If you try that and it does resolve the problem, you could restore the system's overall polarity by interchanging the + and - connections to the phono cartridge, for each channel.

I note, BTW, that your preamp inverts absolute phase/polarity, while your power amps do not. If the phono stage is non-inverting, it would mean that if you already don't have + and - interchanged somewhere in the system, such as at the amp outputs, it would be preferable to do so.

A couple of other points, unrelated to all of that: Make sure that there are no wall wart power supplies, computers, or other possible sources of high frequency interference near the preamp. And try turning off any dimmer switches, fluorescent lights, and compact fluorescent lights that may be anywhere in the vicinity.

Good luck, Marek. Best regards,
-- Al

I swapped the speaker wires and the ringing stayed in the same channel so at least I know it's not the speaker, which is good news. I'm going to swap the tubes today as I do my listening on the weekends only. I guess I don't want it to be the tubes...:)

Daveyf, yes I know Bruce well. Hard to get out there for me, but may just do it this weekend and say hello. Thanks guys.

Actusreus, your problem is most likely stemming from your tube preamp. I noticed with my CAT ( tubed preamp) that depending on the tube, there can be noise from the tweeter IF the tube is at all microphonic. Just because you acquired new tubes, unfortunately does not mean they are not microphonic! I would do as Zd542 suggests, swap tubes out left for right and see if the noise changes channel.
BTW, I'm in San Diego, therefore I might suggest a visit with Bruce at Stereo Unlimited, he can probably suss the problem.

"I was hoping that some of the A-gon experts were familiar with the problem."

I'm familiar with this type of problem. In my expert opinion, I would tell you to swap the tubes from both channels.

"The only other source would be my solid state phono preamp or the cartridge, which I think is rather unlikely."

"Well, as I said in my original post, it is highly unlikely that it's the tubes as I just put in four NOS that were thoroughly tested by Andy at the VTS."

Just because you think something is highly unlikely, doesn't mean it is. If you don't explore all of your options, its highly unlikely you'll find your problem.

Well, as I said in my original post, it is highly unlikely that it's the tubes as I just put in four NOS that were thoroughly tested by Andy at the VTS. Especially given the fact that I experienced the same issue before, and the current tubes are dead quiet as far as the typical hiss associated with microphonic tubes. The same symptom with two different set of tubes worries me that something else is going on. I was hoping that some of the A-gon experts were familiar with the problem.

Agree with Zd that swapping left and right channel tubes would determine if there is a tube problem. As far as swapping speakers, just swap the speaker cables on the back of the amp. If the problem doesn't move, then it is the speaker.

Mofimadness,

Thank you for your generous offer. Alas, I live in San Diego!
I did consider the tweeter may be an issue. I'm going to swap the left and right speakers this weekend to see if the ringing moves with the speaker.

Maybe you have bad tweeters? Can you try another pair of speakers, (or even better, replace the tweeters in yours?).

If you live in northern Colorado, I have over 20 pairs of high end speakers. I would be more than happy to let you borrow a pair to try.

You could even take your speakers to a dealer or to a friends house.

Swap all the tubes in the left channel with the right ones. If the problem moves to the other speaker, you have a tube issue.