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Post subject: Re: Opinions requested: My amp is developing distortion
Posted: Mon Dec 10, 2012 3:16 pm
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The 12AZ7 has less gain than the 12AX7 which would explain why it is louder now than it was then.

I think the 12AX7 has a gain factor of 100 and the 12AZ7 is around 60. They are pretty much the same as a 12AT7 but with a different heater. Word has it that they are waaay too microphonic for use in a combo amp. But don't quote me on that. 8)

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Post subject: Re: Opinions requested: My amp is developing distortion
Posted: Mon Dec 10, 2012 6:13 pm
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Art, 12AZ7 are in many TVs, as a front end signal receiver and gain stage tube. Tuner section. Note the heater current draw versus the 12AX7.

http://www.mif.pg.gda.pl/homepages/fran ... /12AZ7.pdf

Microphony in the audio range is not as critical for this app. It's amazing, but in a perfectly good-working Tektronix scope, out of nine Mullard/Philips 6DJ8 tubes --- nine will be microphonic in an amp (guitar or hi-fi).


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Post subject: Re: Opinions requested: My amp is developing distortion
Posted: Tue Dec 11, 2012 2:41 am
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Sounds like a good reason NOT to use it in an audio amp. But maybe someone put it in there as a last resort because they didn't have any 12AX7s on hand. I can't remember now, this was something like a 40 year old amp, right? So, somewhere along the line, somebody grabbed a Z7 and stuck it in there just to get it running I would suspect.

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Post subject: Re: Opinions requested: My amp is developing distortion
Posted: Tue Dec 11, 2012 7:48 am
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There are "techs" that think anything with a 12A(_)7 will work. Actually, most tubes with this designation are B9A Noval pinouts. So you won't blow-up the amp or tube by interchanging the family members.

Tone-wise, is another story. Here's a little 12AX7 history:

http://www.tutorgigpedia.com/ed/12AX7


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Post subject: Re: Opinions requested: My amp is developing distortion
Posted: Tue Dec 11, 2012 11:35 am
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Oh, I'm familiar with the 12A_7 family, escpecially the 12A_7WA variety. Back when I did my time, the only solid state equipment we had, never worked. Traditional tube gear was up 24/7. (My actual "specialty" was AN/FPN 44/45/46) But I've never really dug around inside consumer gear too much.

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