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.
The classic phono preamps of the '50s and '60s were, by and large, pretty awful. Sorry, it needs to be said.
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.
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.
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.
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.
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!
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!
It can't be used for a phono stage of the Dynaco/Marantz type without distortion and slew limiting. Current and transconductance is too low, plate resistance is too high. To make things worse, the cathodes in that circuit are degenrated, raising the plate resistance even more (with a 2k2 cathode resistor, the effective plate resistance is nearly 300k!)
There are other ways to use that tube successfully in phono amps and exploit its excellent linearity, just not that way.
Not really, as he uses it in Cascode, and it's not the ideal tube for a cascode. Too low transconductance, leading to far too high distortion.
The gain sections of a 12AX7 phono such as the Marantz 7, ARC SP3 & 6 etc are how a 12AX7 should be used, just that it needs a heafty high current, hi transconductance CF after it to drive the capacitance of the RIAA network, and any following cable capacitance etc.
Suitable tube for this job would be a minimum of a 6DJ8 at 10mA, better still a 6H30 at 20mA. A 12AX7 at 1mA, or a 12AU7 at 5mA don't cut it.
Regards, Allen
limitations of 12AX7
Well, I don't have $1000 for a preamp. I enjoyed my PAS2 for 30 years, then when volume pot supply problems developed, went onto a Sony TC250 tape recorder as a preamp, then when 100 electrolytic capacitors and the source switch needed replacing in the TC250, bought a Herald "disco" mixer with 4558 opamps driving the RIAA capacitors. I had to replace the 4558's in the disco mixer with 33078's to get the midrange to sound nearly as good as the PAS2, (remove the hiss) and the disco mixer hum still exceeds the PAS2, unless I hold my hand over the mixer faceplate or turntable headshell. (yeah, the turntable ground is hooked up, also the ground to the wall plug pin) While the PAS2 design might not be ideal, it was close enough to sound really good with good speakers (bought only recently for $600) in A-B comparison with a Steinway console ($1000) sitting between the speakers. No previous speaker has been good enough to even approach doing decent piano sound. I'm not using the PAS2 not because of the sound, but because the volume pot I bought from Stereo Cost Cutters was a reject that sounds good but requires vice-grip pliers to adjust the volume.
Well, I don't have $1000 for a preamp. I enjoyed my PAS2 for 30 years, then when volume pot supply problems developed, went onto a Sony TC250 tape recorder as a preamp, then when 100 electrolytic capacitors and the source switch needed replacing in the TC250, bought a Herald "disco" mixer with 4558 opamps driving the RIAA capacitors. I had to replace the 4558's in the disco mixer with 33078's to get the midrange to sound nearly as good as the PAS2, (remove the hiss) and the disco mixer hum still exceeds the PAS2, unless I hold my hand over the mixer faceplate or turntable headshell. (yeah, the turntable ground is hooked up, also the ground to the wall plug pin) While the PAS2 design might not be ideal, it was close enough to sound really good with good speakers (bought only recently for $600) in A-B comparison with a Steinway console ($1000) sitting between the speakers. No previous speaker has been good enough to even approach doing decent piano sound. I'm not using the PAS2 not because of the sound, but because the volume pot I bought from Stereo Cost Cutters was a reject that sounds good but requires vice-grip pliers to adjust the volume.
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I understand your position indianajo, I started my hi fi with a used PAS 3 in 1963. I like used hi fi equipment. My speakers are used, my tuners are used, my video projector is used. In fact, almost everything that I have is either used or I had a hand in the design, itself. However, what we are talking about here, is the highest quality hi fi that we can think of, and that can be made. In your price range, I would consider a passive preamp and some good relay switching.
Hello Allen
Do you have a link to the Marantz 7 circuit you are talking about , I dont no the circuit details.
Regards
Arthur
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