Are 12AY7s Sweeter-Sounding Than 12AX7s?

[The New Steve H]

A noted amp guru says there is no point in using a 12AY7 tube in a 5F6A preamp, because the only difference is that the gain is lower. If I understand him correctly, he thinks you can get a 12AY7 sound just by turning down the volume on a 12AX7.

My question: is this true? It doesn't match my experience. I built my 5F6A and switched V1 in and out to see what I liked. I found the 12AX7 to be harsh and grainy-sounding, not just loud. I built a Herzog clone and switched a 12AY7 in, and the gain was reduced, but it sure seems like the sound is also sweeter and smoother. The difference is not subtle at all.

I believe both of my 12AY7s are plain old Electro Harmonix.

[Cliff Schecht]

If somebody is giving you advice like that, I would not call them a guru.

When you run a 12AY7 instead of a 12AX7, more happens than just a difference in how the volume knob works. With any system involving gain there is a defined gain-bandwidth product that dictates how much gain you can have and at what frequencies. This has to do with how the internal capacitances and resistances of each stages output interact. So going to a higher gain tube will give you a different frequency response entirely as well as a different gain number. This is true whether you switch a 12AX7 to a 12AY7 or even if you are just swapping around different AX7's. Since every device is different, every one will give a different frequency response (hence why we tube guys like to roll tubes). Since we only use a little bit of negative feedback (cathode degeneration) in our preamps the gain-bandwidth product is especially obvious; the more gain you have the less bandwidth you have (typically at least, the relationship isn't always linear!!!)

On top of the frequency-gain relationship there is also how the 12AY7 biases itself compared to a 12AX7. Once again going to a different plate impedance will affect how the entire system reacts to stimulus. Whether that is for better or for worse is up to you to decide. Different bias points mean a different value for "max signal before clipping" as well as where and how that clipping does occur.

To put this into terms guitar players understand, the 12AY7 will affect how the entire amp sounds. This is the very first thing in the chain and this stage tends to have a very dramatic effect on the rest of the amp. It determines how hard you have to play before you clip (in conjunction with volume/eq controls and such) but also determines what harmonics appear where. What you are hearing when you swap to a 12AY7 is actually more bandwidth because the plate impedance is lower than a 12AX7. Now how this plays into being smoother or sweeter may just be more how the tube is clipping compared to a 12AX7 as well as how the internal impedances of the tube interact with the external circuitry.

[statorvane]

It doesn't match my experience. I built my 5F6A and switched V1 in and out to see what I liked. I found the 12AX7 to be harsh and grainy-sounding, not just loud.

Ditto here. I tried the same thing. First amp I built was a 5F6-A; 12AY7 in V1, 5881s in the power section. Marvelous sounding but really loud (through 2 x 10's) before the tweed growl showed up. So, I popped in a 12AX7, thinking that might sound a little JTM45 like. Didn't sound good at all - harsh and grainy per your observations - both EH and a RCA NOS. Didn't sound much like a JTM45 either. The gain knob did not remove the harshness either. Quieted it down a little, but it was still there.

Funny thing how the JTM45 sounds good w/12AX7 there, but not the Bassman. They are practically the same amp.

[tubeswell]

A good article from old tele man on this very subject...

('Rp' is plate resistance, 'mu' is amplification factor)

[The New Steve H]

Interesting stuff. Thanks for the article. The Herzog is actually useful now, because of the 12AY7. Crazy tones coming out of it. Up the neck on the bass strings, it sounds like a trumpet.

It's not like any of my solid state distortion pedals. If I can tweak the sound and make it a little less brassy, it will be an extremely cool effect for blues.