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.
It means you add up all the net gain you see downstream, starting from that slot (inclusive). Add in power amp, and account for speaker sensitivity. The 99’s tubes come after volume attenuation, so that doesn’t help. The right-hand slots in the 99 account for the 1st stage of amplification, so they are more sensitive to noisy tubes. The left-hand slots come after (mu-followers), so their noise issues don’t get subjected to the amplification of the 1st tubes. I have experience with this unit. It was not RFI / EMI, at least in my case, though it's quite possible that was OP's problem.
At 23dB, it is a high gain preamp. You add a high gain amp (which many of Rogue’s own amps are), add sensitive speakers (like my Tannoys), and you will have problems with old 6SN7GT’s in those slots, guaranteed. The 99 mostly shipped with cheap Chinese Shuguang 6SN7, which honestly I wouldn’t trust to be low noise either. New (screened) Tung-Sols or vintage GTB’s are the best bet.
I’ve said for a while Rogue needs another reference rig with high sensitivity speakers, so they can hear some of these things for themselves.
Yes, I agree in general most of the old "best sounding" 6SN7GT’s from the 1940s are not usable in line stage slots due to noise / microphony.
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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.
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(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.
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Did OP ever find a solution to this problem? I’m curious because I’ve been dealing with a similar issue. Thanks - Josh
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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
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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.
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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.
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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. |
