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Line Stage vs. Phase Inverter - 12A*7

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.
 
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.
 
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.
 
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.
 
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.
 
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A 12AX7 is almost an ECC83; good gain, the highest is always the second anode section, and noise characteristics but a fairly high Anode (plate) impedance.
A 12AU7 is almost an ECC82. Less gain than an ECC83 but larger Anodes so more current can pass therefore for driving EL34 or 6L6GC valves quite an advantage as a Phase Splitter and a lower impedance.
A 12AT7 is almost an ECC81. Higher frequency use with slightly higher noise figures and a reasonably Mu.
 
The 3rd edition has been available as a PDF for ages on (apparently?) legit sites. (so is Douglas Self's Design of Active Crossovers.)

No, it's not. Those are pirated. I would say the same for your books in the same situation. If you could use the email button here and let me know where there are posted, I can inform Newnes.
 
A 12AX7 is almost an ECC83; good gain, the highest is always the second anode section, and noise characteristics but a fairly high Anode (plate) impedance.
A 12AU7 is almost an ECC82. Less gain than an ECC83 but larger Anodes so more current can pass therefore for driving EL34 or 6L6GC valves quite an advantage as a Phase Splitter and a lower impedance.
A 12AT7 is almost an ECC81. Higher frequency use with slightly higher noise figures and a reasonably Mu.

Just to clarify. There is a reference to a Mullard Lond Tailed Pair, earlier in this thread. That is a perfect example of the British doing what comes best to us, designing and fresh inovasion.
The ECC83 is the original valve. With all due respect, the RCA Corporation produced the 12AX7 as a cloned copy. A very good copy and it was with our permission, but not as good as the ECC83.
I will climb down from my soap box now. Thank you for your patience.
 
If one looks at the data of the Phillips ECC83, the internal capacitance values differ between pins 1 + 3 and 6 + 7 respectivly Anode Grid values. Also the plate current differs which is why when used as a phase splitter the anode load is different (100k on pin 1 and 82k on pin 6, with 320V HT).
Reference is at
http://www.r-type.org/pdfs/ecc83.pdf
 
If one looks at the data of the Phillips ECC83, the internal capacitance values differ between pins 1 + 3 and 6 + 7 respectivly

I would hope so. Pins 1-3 is anode to cathode of section 1, pins 6-7 is anode to grid of section 2. The grid to plate and grid to cathode capacitance between sections is the same. Plate to cathode vary less than 0.1pF. If you measure the characteristics or disassemble the tube to examine the innards physically, the sections are the same. It's a twin. The only minor differences are the capacitances to the structural elements and getter because of the physical placement in the envelope. And I mean minor- seeing any electrical difference between sections (other than production tolerances) is a challenge.
 
Yes, ECC83/12AX7 are twin triodes, as are ECC81/2/12AT/U7. There may be minor differences in capacitance and hum pickup, but other characteristics are identical (within manufacturling tolerances).

Harleyjon said:
The ECC83 is the original valve. With all due respect, the RCA Corporation produced the 12AX7 as a cloned copy. A very good copy and it was with our permission, but not as good as the ECC83.
My understanding is that 12AX7 came first in the US. Then Philips/Mullard in Europe improved it by using a spiral heater for low noise and called their improved version ECC83. The US then brought out a version of ECC83 called 7025. Later on, many 12AX7 also had the improved heater.

Most, but not all, 1950's European miniature valves were copies of American designs. By the later 50's/early 60's the two sides of the Atlantic somewhat departed e.g. Philips invented frame-grid pentodes for RF which were never quite so popular in the US.