Motorola 12AX7's

[rtcook]

Hi all,

I picked up four of these NOS Motorola 12AX7 tubes today. They are in original boxes and say U.S.A. on the tubes. Can anybody give me some info in these? Are they good sounding tubes? Who actually made them?

Thanks,

Roger

[VelvetGeorge]

Cool. They might be GE, Sylvania, RCA ect. I don't know what tubes were branded as Motorola. Regardless thay should sound pretty good.

George

[rtcook]

Thanks, George.

Upon closer examination, three of the tubes are identical in construction, are silkscreened Motorola, have the markings "78-39" and "188-5" on the side of them. They all have halo getters. I believe they are GE tubes. The fourth one though has a faint Motorola silkscreen on it but it has 12AX7A on it. It's plate constuction is slightly different and has a slightly smaller halo getter. It's still marked U.S.A. What's the difference between a 12AX7 and a 12AX7A?

Thanks again,

Roger

[VelvetGeorge]

For US tubes, the "A" postfix indicates the lower noise revision of the tube.

George

[RattleCat]

I cant seem to find information to verify, but doesn't the A suffix indicate that it can use either AC or DC on the filament? Or was it a series/parallel thing?? Ok this was a bad idea

The duncan site indicates it means controlled heater warmup time, or some other "better" rating... e.g. low noise.

http://www.duncanamps.com/technical/tubenumber.html

All the Motorola's I have are short plate GE's

[RattleCat]

I knew I read this somewhere before....

12AX7: the original version of this tube. This can only be used in parallel filament circuits.

12AX7A: This version can be used in series or parallel filament circuits.

[dynaman]

The numbers "1885" found on the tube indicate GE manufacture.