The Steel Guitar Forum

I'm considering getting an early '70s MM 410 65, which has a 12AX7 tube used as a phase inverter. I've read about how it's supposed to work in the output section governing the two power tubes. Does anyone here know how, or if, this affects tone? This configuration was abandoned in later models and even the earliest MM 65 series had solid state preamps. I have several other MM amps of the later HD 130 series so I guess eventually I'll be able to AB them but I just thought I'd ask before I buy.

All the MM have solid state preamps. All of the 65 and 130 amps use a 12AX7 as a phase inverter to drive the power amp tubes. I could find no schematics that did not include that tube circuit.

As Micheal said, the 12AX7 is the phase inverter which is part of the power amp. Other models used LM1458 op amps for phase inversion. Then the power tubes were driven by JE1692 transistors in the cathode circuit. I forget which came first.

As far as tone goes; everything affects tone. Guitar players prefer the tube phase inverter sound.

I built several Op amp phase inverters for friends in the mid nineties for use as guitar preamps. They really helped cut down feedback, especially with hollow bodies.

Totally different application from the tube amp phase inverters, but a useful tool. You see them on a lot of acoustic amps now.

Bill

Michael Brebes wrote:All the MM have solid state preamps.

That is not 100% correct. The very first ones, made in 1976 (when Leo Fender co-owned the company,)and maybe '77 had tube preamps. Apparently they only made very few of these before switching to solid state. I had one of them. Serial number something-399. I bought it used in '77.

The guy I bought it from went to work in a store where I'd buy strings and stuff. Every time I went to the store, he'd tell me that selling it to me was a mistake, and if I ever wanted to sell it, he wanted to buy it back. So of course, when I finally decided to sell it, the store had closed, and the guy disappeared.

I've been playing a MM 210 65 for years, and I love it. It's got the phase inverter tube, and I've gotten different sounds by swapping it with others of the same type (12at7, etc). Lower gain tube in that spot can give you more headroom. Careful, though: if that tube fails it can take out the output tubes and even the output transformer, from what I'm told.