Ringing from the tweeter

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.

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

Did OP ever find a solution to this problem? I’m curious because I’ve been dealing with a similar issue. Thanks - Josh

(aware this is a very old topic) The Rogue 99 is a VERY high gain tube preamp. Combining this with NOS 6SN7GT tubes is bad news. No matter how well screened and low-noise selected the tube guy says they are, older GT series 6SN7 are not usable in the 99’s right-side slots (as you face the 99’s front) without noise gremlins. You can maybe get away with them in the left-side slots, but they will also have less sonic impact in those slots. The right-side slots "see" the most downstream gain.

My recommendation is to stick with modern Russian low-noise selected Tung-Sol 6SN7GTB (e.g. Upscale Audio’s Platinum grade) and/or try the later GTA and GTB series Sylvania tubes (angled black plates), which have a design that lowers their microphony (I prefer the tall bottle variants). Some other brands of vintage GTB might be quiet too (Raytheon, RCA), but I haven’t personally tried them in this unit. I know the vintage Tung-Sol GTB are generally quiet, because they used the same Sylvania GTB plates. If you really want to avoid modern tubes, you can try NOS 6SN7GT in the left slots and vintage 6SN7GTB in the right slots.

Ringing or whistling sounds just go hand-in hand with those older GT tubes in high-gain slots, unfortunately. You might think you’ve found a magic "quiet" set, but more often than not it won’t last (as you’ve found). These 6SN7GT tubes have very large plates compared to 12A*7 series tubes, and those 80 year old micas aren’t holding things down firmly enough for the gain levels involved. I do love the older 6SN7GT in power amps, where they are far enough downstream to not surface their noise issues, but still have a big impact (positive) on sound.

What’s a “high gain” slot in an all tube device? The tube supplies any gain, so gain cannot be greater than what the tube can provide. I’m guessing that Actusreus’ preamp was picking up RF and that the tubes per se were not at fault; he probably needed shielding or to move his preamp elsewhere in his room or to check any digital devices that can emit RF.

if you arrange 6SN7s in a cascode topology, you do get very high gain and very wide bandwidth capable of picking up RF ieither radiated from some outside source or dumped into the signal path. My Atmasphere MP1 uses 6SN7s. I have found that nearly all the best sounding NOS brands (RCA, Kenmar, etc) are very microphonic, unusable in the linestage gain position. One notable exception is Brimar 6SN7GT, with a brown phenolic base. That’s what I’m using now.