Sovtek 12AX7WA - Never again??

Another bad experience here, with the ax7 wa from sovtek, in my la2a builds. They were from 2006 or so.

Noise floor in the la2a was not usable. I use them in trem oscillator duties and even as a VU driver 

I changed to the lwp  or something like that and haven't had any probs - very quiet in the la2a.

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I do however, mostly change AX7s with EH AY7s - move the plate resistor to a value lower than the usual 100K, and the bias voltage a at around +2.0 .. +2.5V depending on the app.    In the mic, lower at +1.5V

Been a staple along with the EH 6922, another quiet and very useful tube.

Next go around with the la2a build I like to mess with, I'll change to the AY7 and change the nfb and so on.

Now, I've a couple of good ol nos AY7s and can really get the measurement party going. 

Start with some good ones, with some volunteer 'ears'. Modulate the roster of tubes-under-test with ..  say a nice red  
Note down the observations

See how they change as the night progresses  - that's real 'in-circuit' testing.

here is an octal swap out for the LA2,

http://groupdiy.com/index.php?topic=50096.msg633910#msg633910

that 12AX7a is just a 6SL7 put in the microwave and shrunk down,

you can also try 6EU7, 12AD7, 5721, 5751, 6057, 6681, 7025, 7058, 7494, 7729,

here is a VTV article on the best 12AX7's,  download pdf>

https://web.archive.org/web/20130805141923/http://www.jumpjet.info/Pioneering-Wireless/eMagazines/VTV/VTV01.pdf

Deepdark said:

When looking at a Spectrum analyser, I tried different brand at V1

JJ 12AX7: Hum peak at about -79dbfs
Bellari 12AX7 (t know the brand): -83dbfs
Sovtek 12AX7WA: about -67dbfs
EH 12AY7: about -88dbfs

Will report the test with EH 12AX7 and Tung Sol later.

Click to expand...

Have you thought about modifying for DC heaters, or at least using a trim-pot to ground instead of the centre tap, so you can set the hum to minimum?

Matt Nolan said:

Deepdark said:

When looking at a Spectrum analyser, I tried different brand at V1

JJ 12AX7: Hum peak at about -79dbfs
Bellari 12AX7 (t know the brand): -83dbfs
Sovtek 12AX7WA: about -67dbfs
EH 12AY7: about -88dbfs

Will report the test with EH 12AX7 and Tung Sol later.

Click to expand...

Have you thought about modifying for DC heaters, or at least using a trim-pot to ground instead of the centre tap, so you can set the hum to minimum?

Click to expand...

Not tested yet but will do. For now, with some of the tube, i can't hear any hum with headphones at max. JJ have a little hum, but only audible with headphones at max, so nothing to worry about, but will definitly integrat at least a trim pot for heater balancing

Definately agree with CJ regarding the 6SL7 - it can be a wonderful sounding high-mu tube.

I put them into my single 6L6 SE amp with no nfb and it went from pretty good to wow! is that realistic or what!

Always a fan of the 6SN7, and together ..  well .. that's my next SE amp ..  along with 7591a finals.

Love that LA2A build of yours CJ - it's got it all : octals and no psu traffo on board  

No psu traffo on board - happy days without the heroics!