MT200

[MarshallJMP]

12AT7 will not draw much more current unless you start changing the anode resistors.Also tubes have a high input impedance so current isn't so important,voltage is.

A 12at7 and 12au7 are different tubes then a 12ax7 and they need to be biased different.So a good option for a lower µ preamp tube as replacement for the 12ax7 is the 5751.These are a sort of low gain 12ax7.(µ=70 vs 100 for 12ax7) and they sound good.I use these in my mp-2.

[rnolan]

Yeah but for a phase spliter current is king!! not gain, did you read the article SC posted ??, if you want poweramp distortion (not that we do...), you need current in your phase spliter, not gain, and a 12AT7 has 10 times the current of a 12AX7 (albeit less gain). Basically (well from what I've researched and discussed with tube nuts), is a 12AX7 is the worst phase spliter tube as it's got lots of gain and bugger all current. Not that it's terrible just the wrong way to go for a phase spliter ... and gain structure wise it make sense to me. All that said, my Carvin TS100 has 12X7s' (buffer and phase spliters) and 6L6s' and it works just fine.. (well better than fine, it's shit hot as a poweramp!!

[MarshallJMP]

Where is that article?

[rnolan]

This one IIRC http://adadepot.com/index.php?topic=543.15, Reply #15 on: Fri 26Sep 2014 04:22 PM

[Systematic Chaos]

http://adadepot.com/index.php?topic=543.msg5269#msg5269

[DorsetRatt]

Yeah but for a phase spliter current is king!! not gain, did you read the article SC posted ??, if you want poweramp distortion (not that we do...), you need current in your phase spliter, not gain, and a 12AT7 has 10 times the current of a 12AX7 (albeit less gain). Basically (well from what I've researched and discussed with tube nuts), is a 12AX7 is the worst phase spliter tube as it's got lots of gain and bugger all current. Not that it's terrible just the wrong way to go for a phase spliter ... and gain structure wise it make sense to me. All that said, my Carvin TS100 has 12X7s' (buffer and phase spliters) and 6L6s' and it works just fine.. (well better than fine, it's shit hot as a poweramp!!

Interesting article ... just out of curiosity has anyone on the forum done a before/after test when replacing a 12AX7 with a 12AT7 for the phase inverter? More power tube distortion? More warmth? More sag/compression?

[rnolan]

I'll probably give it a try when I retube the TS100 but that's a way off.  A consideration though is that while all the 12A? series tubes are plug/socket compatible, the 12AT7 delivers 10 times the current of 12AX7 so (while I don't think it would be a big deal for most poweramps), the power supply needs to be able to cope.

[MarshallJMP]

 Gain wise a 12at7 will not differ too much from a 12ax7 and this also depends on what type of phase splitter is used.
For instance the ADA T100S tube power amp uses a cathodyne phase splitter which has a gain of 0.97.

The most used one is the long-tailed-pair phase inverter or to be more exact the schmidt phase inverter.Let's take the marshall inverter values for a 12ax7 and a 12at7 (ECC83 and ECC81).Now these are datasheet values,in real world they can differ a bit and i presume both triodes in the tube are the same.

12ax7 inverted gain 27.8 non inverted gain 29.1
12at7  inverted gain 26.0 non inverted gain 26.9

As you see a small difference which you won't able to hear (difference is less then 1 dB)

Current wise yes a 12at7 can deliver more current but this depends on the anode resistor so replacing a 12ax7 with 12at7 which has a Ra of 100k won't give 10 times more current at all.But what you will get is a lower output impedance.

Let's do some calculations

A circuit with an Ra of 100k,Rk 2.7k bypassed by a Ck cap.No load.

12ax7   gain=61.5 output impedance of 38.5k
12at7    gain=54.0 output impedance of 9.8 k

So the 12at7 is better for driving lower impedance loads.

Note;12AT7's should have lower Ra's (and Rk) more between 20 and 50k to get it biased correct.

[MikeB]

Yeah. That's what thought.

[Systematic Chaos]

Bottom line: individual subjective preferences ;-)

I personally like Sovtek 12AX7LPS best in the PI slots of my 2:90. I find them boring in any other preamp or input slots but in the PI they´re the best sounding (subjective taste) I used so far. Also, I like them in there best when NOT balanced.

The input 12AX7 in my 2:90 (currently a Tube Town E83CC) is balanced...cuz each of the 2 triodes works one of the 2 channels of the poweramp....

...roll some glassware and stick to the setup that sounds best for you...

[MarshallJMP]

Actually yes,a 12AT7 has some better drive capability's so maybe the myth about it has some truth in it as that it sound cleaner.

So it really comes down to individual subjective preferences like SC says.But now you guys now the "real" differences and it's up to the person if he likes what he hears or not.

[DorsetRatt]

Mmmm .... I was reading the article whilst at the same time trying to diagnose a Mesa 20/20 I bought several years ago. I re-tubed it with a full set of Harma's when I first got it, but it has always sounded a bit too cold/sterile (lacks warmth). The article mentioned touch dynamics with regard to the PI tube, hence my interest. Could it cure my Mesa?

[rnolan]

Hey DR, Possibly, as MJMP says, let your ears decide. I've been discussing this with my tube nut audiophile friend, he'd never use a 12AX7 as the PI/PS, his go to tube for this is the 12AT7. If you decide to try them, let us know how you go.

[MarshallJMP]

Mesa amps are most of the time underbiased (power tubes),i would check this first.

[rnolan]

Mesa amps are most of the time underbiased (power tubes),i would check this first.

+1  , good advice