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Doge 8 Clarity: 12AT7 replacement for phono stage

Hello fellow enthusiasts... My apologies in advance for the noob question.

I've put together my first all tube system, as far as preamp and amp combo is concerned. Looking to see IF replacing the phono section's 12AX7 (Mullard ECC81) tubes with lower gain 12AT7 in my Doge 8 Clarity 2017 ed. will help lower the "hiss"/ tube rush coming off with my current set up. Doge's phono setting is on MM (using Bob's Device Sky 20 SUT), Phono: 47k, tried both normal and low sensitivity, (- 8) gain reduction, but "hiss" is still audible from my seat 7' away from the speakers. 

I'm looking at replacement tubes from Upscale - Mullard 6201 which they claim to be the "quiet" ones. Not sure if it will help in lowering the hiss I've described.

Doge's line level still holds the stock 12AT7 tubes and to me - they sound nice as the reviews online says. I'm sure it can do better with tube rolling. No audible hiss from my seat. Switching between any line level onto phono input clearly augments the hiss.

To take advantage of Doge's much improved line level over the phono, I've utilized my only external phono stage - Schiit Mani. Not ideal but i can handle that better than deal with the hiss using the Doge's internal phono stage. I've contemplated on just upgrading to a nicer phono stage but that's another topic.

So, will it work to replace the 12AX7 with lower gain tubes 12AT7 to decrease hiss in my system? 

Thank you in advice for your time and responses.

Current system:
Rega P3 (2016), NEO ttpsu, DV 20X2L
Schiit Mani to line level vs Bob's Device Sky 20 to phono level
Doge 8 Clarity 2017 ed.
Mystere PA11 
KEF R3 on stands



oyie2000
lowrider57's avatar
lowrider57
5,417 posts
 
Here's a phonostage I plan to audition...

https://www.musicdirect.com/phono-preamps/ifi-micro-iphono-2-mmmc-phono-preamp?gclid=CjwKCAiAu9vwBRA...


audition__audio
1,465 posts
 
I wouldnt use a 12AX in place of a 12AT. Often 5751s DO NOT work well in phono stages and would check with the mfgr before spending anything on a 5751. There are many tube sellers that check for noise and microphonics. Having said this, this service is only valuable if the test equipment is more sensitive than your application. 
atmasphere's avatar
atmasphere
11,784 posts
 
@ atmosphere: Being the second owner of the unit, I'm not sure if I'm able to avail any warranty on the unit. Without any other matching tubes around, I may have to pony up with replacement tubes to see if the phono section's hiss issue I'm having can be remediated.
Warranty has nothing to do with it is my guess. We're a bit weird in that we have a 1 year warranty on our tubes but most manufacturers usually offer 90 days. So my guess is no tube warranty. But Doge should still be able to provide you with 12AX7s they know will be quiet in your phono section at some nominal charge.


Now one thing I haven't seen asked: Can you hear the noise when the needle is tracking a silent groove? If no, then the noise is no concern!
mulveling's avatar
mulveling
1,887 posts
 
Even if you could lower the gain in your MM phono stage with a 12AT7 tube swap (which is not advisable anyways, as others have mentioned), it wouldn't help your hissssss noise floor much because the phono stage is before the preamp's volume control - so what you REALLY need to improve is the signal-to-noise ratio, rather than any absolute noise floor.

Your tools for increasing signal-to-noise ratio in your vinyl source ecosystem are as follows:
1. Get a different cartridge with higher output level. Your Dynavector's 0.3mV output is right around where these things start getting difficult.
2. Use a SUT with a slightly high ratio - you're already doing this, and 20x is a good ratio for your cartridge. You could go even higher (say 30x) to further increase signal-to-noise, but then the flip side is you start cutting closer into the MM stage's overload margins (i.e. higher distortion).
3. Don't load the cartridge too heavily, or you will lose signal, while noise floor stays the same - hence a loss of signal-to-noise ratio. With your 20x SUT, the 47K MM input, and a 5-ohm cartridge coil, you should be OK here.
4. Get a higher quality, lower noise MM stage. Easy win, if you don't mind spending the money ;)
5. Source as low-noise tubes as you can, when your MM stage is tubed. You may or may not already be near your stage's low-noise limits, with the stock tubes - only one way to find out! But in Chinese preamp w/ onboard phono, I wouldn't be surprised if they didn't take any care selecting the stock phono stage tubes. Even the better Chinese brands are notorious for shipping with crappy tubes. 
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