Sovtek 12AX7WA - Never again??

[alexc]

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.

----

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.

 

[CJ]

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

 

[Matt Nolan]

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.

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?

 

[Deepdark]

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.

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?

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

 

[alexc]

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!