John Curl's Blowtorch preamplifier part II - Page 449 - diyAudio
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Old 30th May 2010, 02:09 AM   #4481
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12AX7 is a lousy tube for pre-amp output, even as a cathode follower. The fact that so many audio designers use it so widely doesn't make it any better.
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Old 30th May 2010, 02:11 AM   #4482
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Joshua, please study the 12ax7 further before being so negative. It was the most popular tube of the 50's and 60's, Marantz, McIntosh, and Dyna used them.
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Old 30th May 2010, 04:22 AM   #4483
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Does anyone know WHY the 12AX7 was such a popular tube?
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Old 30th May 2010, 05:27 AM   #4484
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It is good at the input.
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Old 30th May 2010, 05:54 AM   #4485
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It's extremely linear as a voltage amplifier if the plate is CCS loaded and any following loads are 1M or higher. As an output tube, it's quite poor. It can't really swing more than 0.1mA with any degree of linearity. Yet it was called on to drive the heavy capacitance of the RIAA network as well as cabling and the input capacitance of downstream gain blocks.

The classic phono preamps of the '50s and '60s were, by and large, pretty awful. Sorry, it needs to be said.
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Old 30th May 2010, 06:22 AM   #4486
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Quote:
Originally Posted by Allen Wright View Post
Do you mean you do not know of any preamp that can output 2mA? Or that you don't know of any preamp load that would need 2mA?


Regards, Allen
I mean the opposite.
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Old 30th May 2010, 06:34 AM   #4487
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The 12AX7 has a mu of 100, yet it is a triode.
What I am trying to show here is that even with good engineers over the decades, oversights can be made in even simple ways. One way was the RIAA cap loading on a 12AX7.
Another common oversight is thinking that the working audio bandwidth is ALWAYS sharply limited to 20KHz or so. There are several exceptions, including moving coil phono, high speed analog tape, and the new 24/96K and related digital. A real hi fi must consider these other sources.
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Old 30th May 2010, 01:57 PM   #4488
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I might agree with SY that "The classic phono preamps of the '50s and '60s were, by and large, pretty awful. Sorry, it needs to be said." to the extent that the 12AX7 output PAS2 is not suited for driving transistor power amps except the ST120 that had an unusually high input impedance. I bought the 1960 built Dynakit PAS2 in 1970 because the college library had a fleet of Dyna SCA35's in the listening rooms that were holding up very nicely to continuous use. Whereas the dorms of 1970 were full of packaged receivers with touchy volume and tone controls, (dropout) and blown output transistors and speakers. Thirty years of use wore out the PAS2 volume control, and I have been unable to buy one of equivalent durability. I had to replace the op amps in my HA-88A mixer to get anywhere near the sound of the PAS2. It does have silent slider pots. Pro mixers available here do not have RIAA magnetic phono inputs, and used PAT4's are available only to people with Paypal accounts. In the day of $12 TO3 transistors, there was a guy in the dorm that for a fee, saw the tops off of TO3 transistors and reweld the die to the package leads.
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Old 30th May 2010, 03:04 PM   #4489
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Problem is, they were also driving the RIAA off that second stage plate, which was a lot lower impedance at high frequencies, not to mention high capacitance. This screwed up the open loop gain enough to require positive feedback (cathode-to-cathode resistor). Marantz interposed a cathode follower, but chose one that compromised the low distortion driving ability for the RIAA network's capacitance.
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Old 30th May 2010, 07:35 PM   #4490
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As a person who lived with the Dyna Pas3 for 10 years and with the Mac C22 for 1 year, the general impression was smooth, but slightly obscure or muddy. Very forgiving.
In contrast, the output stage of the Levinson JC-2 had at least 35ma available to drive the RIAA network. Sounded less muddy? You bet!
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