Line Stage vs. Phase Inverter - 12A*7

[cogitech]

So, my amp came with 12AX7 as line stage and 12AU7 as phase inverter.

I have been reading a lot about different P-P amps and it seems that much more often than not, they are configured the other way around; 12AU7 as line stage and 12AX7 as phase inverters.

Can anyone guess why my amp seems to have these tubes reversed? Does it all just depend on the circuit design?

The power tubes are EL34, by the way.

[JMFahey]

Does it all just depend on the circuit design?

Yes, of course.

[DF96]

By 'line stage' do you mean voltage amplifier? They are not the same thing by any means.

Part of circuit design is to use the appropriate valve for the task. Two different designers can arrive at quite different but equally valid designs. These could even use the same valves but in a different order. On DIY sites people sometimes choose valves on the basis of fashion; this is easier than thinking.

[cogitech]

Originally Posted by DF96

By 'line stage' do you mean voltage amplifier? They are not the same thing by any means.

To be honest, I do not know what I mean.

What I know is that the 12AX7 is in the V1 position of my amp (and v5, for the other channel), closest to the line input. I have seen this referred to as the line stage, where the initial gain is applied to the input signal. It is a P-P amp, so it only follows that the V2 tube (12AU7) is a phase inverter.

It seems to me that I usually see 12AU7 used as the initial gain stage in other amp designs, and then 12AX7 used as a phase inverter.

I am just curious as to why the designer of my amp would have chosen to do this.

[DF96]

The first stage in a power amp is normally the voltage amplifier. A line stage, if present, is normally a separate item. Nowadays there is little need for line stages, as modern digital sources have fairly high output voltages.

12AX7 has a stage gain around 50-80. 12AU7 around 10-15. Either can be used as a phase inverter/splitter. Depending on the circuit used, the PS/PI can have a gain slightly less than one or about half the gain of a normal voltage amplifier stage.

Guitar amps often start with 12AX7 as they need lots of gain so they can throw it away in clipping or passive tone controls. Hi-fi needs less gain.

Mullard popularised a long-tail pair phase splitter using the 12AX7/ECC83. Very linear and well-balanced, but may struggle to drive a heavy capacitive load such as a triode-connected output stage. 12AU7 can drive better, but has more distortion and poorer phase balance unless a CCS tail is used. Partly horses for courses; partly personal designer preference; partly fashion.

[cogitech]

[Zoooom]

That was the sound of most of what you wrote going straight over my head.

However, I do appreciate you trying to help me

I am curious more than anything, and at least you have inspired me to continue learning!

[SY]

Broken record time: run right out and buy yourself a copy of "Valve Amplifiers." You'll be stunned at how much there is to learn and how straightforward it is to do so.

[cogitech]

Originally Posted by SY

Broken record time: run right out and buy yourself a copy of "Valve Amplifiers." You'll be stunned at how much there is to learn and how straightforward it is to do so.

Accessible to someone with virtually no electronics knowledge (and what little knowledge there is has been obtained by taking things apart, rather than building things)?

EDIT: Is it the M. Jones one? I found a PDF... Will see how it goes.

[SY]

If it's a PDF, it's pirated and you're essentially stealing. No, make that "you are stealing." No "essentially" about it.

[Merlinb]

Originally Posted by SY

If it's a PDF, it's pirated and you're essentially stealing. No, make that "you are stealing." No "essentially" about it.

The 3rd edition has been available as a PDF for ages on (apparently?) legit sites. (so is Douglas Self's Design of Active Crossovers.)