Phase Inverters: Do You Need a Balanced Tube?

The short answer from behind the workbench: no, you don't need a balanced tube in your phase inverter (PI).

Paying extra for a "balanced triode" 12AX7 or 12AT7 for the PI slot is one of the most persistent myths in the guitar amp world. In a guitar amplifier, it makes virtually no practical difference. Here's why, from a circuit design point of view.

1. The circuit is designed to be unbalanced

Most larger Fender, Marshall and Vox amps use a long-tailed pair (LTP) phase inverter, and that circuit is inherently asymmetrical.

In a long-tailed pair, the triode driven at its grid produces slightly more output than the triode driven through the shared cathode. Amp designers compensate with unequal plate load resistors, typically 82 kΩ on the higher-gain side and 100 kΩ on the other, as in many Fender and Marshall circuits.

Putting a "perfectly balanced" valve into a circuit that's deliberately offset to correct its own asymmetry misses the point. As Merlin Blencowe (The Valve Wizard) explains in his article on the long-tailed pair phase inverter, the outputs "will not be perfectly balanced if both triodes have the same value anode resistors", which is "why the 'traditional' version of the circuit also uses mis-matched anode resistors."

Want to see a long-tailed pair in a real amp? Our 5F6-A Tweed Bassman parts list covers the amp that made it famous.

2. Guitar tone thrives on a little asymmetry

In hi-fi, you want perfect balance. Equal drive to a pair of push-pull power valves makes the even-order harmonics (2nd, 4th and so on) cancel in the output transformer, giving very clean, linear amplification.

In a guitar amp, that perfection can sound cold and sterile:

• A slight imbalance between the two sides lets some warm second-order harmonic distortion through to the output.
• That subtle asymmetry is part of what guitarists describe as "touch sensitivity", "bloom" and "musical overdrive".

3. Component tolerances undo "balance" anyway

Even if you fitted a perfectly matched valve into a perfectly symmetrical phase inverter, real-world parts would soon undo that balance:

• Typical carbon comp and carbon film resistors are ±5% to ±10% tolerance. Even 1% metal film resistors aren't perfect.
• Output transformers and screen grid resistors vary slightly from side to side.
• Power valves drift, age and shift bias at different rates as you play.

In practice, these variations in the power section usually add more imbalance than an ordinary off-the-shelf preamp valve does in the PI.

When would we recommend a balanced tube?

There are only two niche cases where a balanced PI valve is worth considering:

1. Ultra-clean, high-headroom amps: acoustic amplifiers, pedal steel rigs, or high-headroom bass amps (such as an Ampeg SVT), where you want no power-section breakup even at extreme volume.
2. Hum in amps without negative feedback: in some push-pull designs without global negative feedback, more symmetrical drive can slightly reduce the hum that gets through the output transformer. Matched power valves matter more here, though.

Our advice

Save your money. Fit a standard, reliable valve tested for microphonics, such as a JJ 12AX7, Tung-Sol 12AX7 or Electro-Harmonix 12AX7. For amps that use a 12AT7 in the PI, try the JJ 12AT7 or Tung-Sol 12AT7. As long as both halves are working properly, your amp will sound great. Ironically, a little natural imbalance will probably make it sound even better.

Further reading

For a deeper understanding of phase inverters and guitar amp design, we recommend two respected authors:

• Merlin Blencowe (The Valve Wizard): his book Designing Valve Preamps for Guitar and Bass, and the free articles on valvewizard.co.uk
• Richard Kuehnel: his Guitar Amplifier Electronics books, published by AmpBooks

Browse our full range of preamp tubes, or contact us if you're not sure which valve your amp's phase inverter takes. While you're here, learn how to set up your power valves in To Bias or Not to Bias.