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Tubes comparison - 12AX7 vs 12AX7A
- Thread starter arcorob
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analog addict
Glory or Death!
I dunno if the slow filament warm up has anything to do with the noise. 7025s and 5751s are low noise versions, as are some of the premium Euro versions. I think it has more to do with construction and brand.
arcorob
Addicted Member
Actually ...
what I have been able to find is slower warm up (no flash pop) and lower noise version
"The first description of the 'A stated that it was audio-oriented due to its controlled hum and microphonic characteristics. The original 'AX7, introduced in 1947, was considered an all purpose dual triode to be used in AF amplification, multivibrator (clock) and oscillator circuitry. When the Hi-Fi manufacturers adopted it as their quintessential voltage amp. tube in the '50s, RCA changed the heater design to lower the hum and noise (it runs at a lower temperature) and taunted it as the audio 12AX7 in 1960. When they discontinued the 'AX7 a couple of years later, the 'A was referred to as strictly an audio tube, and the 7025 (introduced at the same time as the 'A) became the upgrade of the 'A. "
what I have been able to find is slower warm up (no flash pop) and lower noise version
"The first description of the 'A stated that it was audio-oriented due to its controlled hum and microphonic characteristics. The original 'AX7, introduced in 1947, was considered an all purpose dual triode to be used in AF amplification, multivibrator (clock) and oscillator circuitry. When the Hi-Fi manufacturers adopted it as their quintessential voltage amp. tube in the '50s, RCA changed the heater design to lower the hum and noise (it runs at a lower temperature) and taunted it as the audio 12AX7 in 1960. When they discontinued the 'AX7 a couple of years later, the 'A was referred to as strictly an audio tube, and the 7025 (introduced at the same time as the 'A) became the upgrade of the 'A. "
electroking
a- v- karma member
IIRC, the 1962 RCA Receiving Tube Manual gives the same noise voltage values
to the 12AX7A and 7025, even though the latter is specified as a low noise tube for
preamp service. Strange...
to the 12AX7A and 7025, even though the latter is specified as a low noise tube for
preamp service. Strange...
jaymanaa
RIP 1961-2018
I've always been of the understanding that A stands for 11 second controlled warm up, and W or WA indicated vibration resistant. The 7025 was the spiral filament, which was low noise, and the 5751 was just a lower gain tube. FWIW, the Mullard flash is not indicative of "fast warm up", it is simply sections of the filament protruding beyond the bottom of the cathode that glow white hot for just a second until the filament warms, and resistance stabilizes. I think the average Mullard is near 11 seconds warm up too. Oh, and filament voltage also has an effect on tube noise. I can't remember right off, but one voltage is quieter (6.3 or 12.6). DC is so cheap to make these days that I use it for most 12A&7 tubes except for inverter duty.
