12AT7 Tube Review
Very little attention has been paid to the important role that a certain tube plays in most guitar amps. I'm talking about the phase inverter or PI tube. The PI is the tube that creates the push-pull waveform and it's the last tube the guitar signal sees before hitting the output tubes. Most amps use either a 12AX7 or 12AT7 for this purpose. Fender in particular has historically chosen the 12AT7 (AKA ECC81) for its unique capabilities and tone as a PI tube. The signal levels in this stage of the amp are fairly high and when the amp is driven hard the PI tube can be driven to clipping so that's how we tested them, with the amp driven to a fairly distorted level and then reduced to clean for confirmation that the tone remained consistent when not clipped. It's worth noting that although the 12AT7 can be used as a preamp tube in other positions in the amp, it generally does not produce very good tone for applications other than PIs and reverb drivers.
For this review, we'll be comparing several NOS (New Old Stock) and current production tubes in a Fender Blackface Deluxe Reverb and Silverface Vibrolux Reverb. I'll be listing the tubes in order of bright and harsh to warm and fat with some additional description for each tube.
JJ ECC81 – This tube was a big surprise to me when compared to the other on the list. I must say that it was the most shrill and thin of the group. JJ makes a decent 12AX7 (ECC83S) but I will be moving away from JJ ECC81 for PI circuits from now on.
Tung-Sol 12AT7 – This one was just a tiny bit smoother on the top end but lacked definition and fullness. It carries a higher price too. Not a good buy in my opinion.
Shuguang 12AT7 – This Chinese tube is marketed under various brand names. Most commonly, they show no brand and just say (China). It sounds very similar to the Tung-Sol but with slightly better note definition. It's a good choice when low price is of high importance. If I was building low-priced mass produced amps, I would probably choose this tube for it's balance of acceptable tone and low price.
JAN Philips 12AT7WC – This is a military grade NOS tube. Most available today were made in late 70s and early 80s. It's a rugged tube and not too expensive for an NOS tube. However, the tone is not that impressive in Fender PI circuits. It does have a bit more fatness and a bit less harshness than the above tubes but lacks real warmth and definition. There are better choices. This tube does make a good reverb driver where tone is less important than ruggedness.
GE 12AT7 – Another NOS tube that is still available – This one will give you a little more upper mids and clarity while being slightly smoother in the treble frequencies. It's a useful tube when you want a little more ability to cut through the mix. However it has a bit less bottom end fullness roundness than the JAN Philips.
Electro-Harmonix 12AT7 – The other big surprise to me – I've never been a big fan of the EH 12AX7 but their 12AT7 is impressive! It has a nice rich full tone with great definition, punchy bottom and smooth treble. I suspect the big plate structure contributes to the big sound. The tubes at the other end of this list tended to have smaller plates. Small plates can be an advantage in preamp tubes where low noise and microphonics are important but large plates seem to generally work better in PI circuits. This one is the winner for current production tubes and beats most NOS options!
Note - because of the large plate structure, this tube tends to be too microphonic to be useful as a preamp tube. It's best utilized as a phase inverter or reverb driver.