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In Reply to: RE: Replace 12ax7 with 12at7 posted by Jim McShane on March 31, 2012 at 11:49:48
Jim
Some say to drop the 12at7s in the mc240s and the mc60s in place of the 12ax7 in the final stage..Its a cathode follower but its plate is loaded with the OT primary.I don't recall hearing that much difference.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
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Well, as a cathode follower it will always be less than unity gain, so I doubt the mu difference is going to be audible.
The 12AT7 has a much higher Gm though, which may well be audible. I've never understood why Mac ran the relatively beefy tubes (12BH7, 12AU7, etc. ahead of the drivers and then used such a wimpy low transconductance tube as the driver. Especially as the signal voltage drives the power tube grids close to zero volts the drivers are called on to supply a bit of current - and a 12AT7 can do that much better than a 12AX7. The lower output impedance of a 12AT7 follower would be useful in that position I'd think.
But the amp is a classic, so?????
The 12AT7 has a much higher Gm though, which may well be audible. I've never understood why Mac ran the relatively beefy tubes (12BH7, 12AU7, etc. ahead of the drivers and then used such a wimpy low transconductance tube as the driver"
thank You Jim,
I thought was just a little dense and missing something when I went over Mac schemos a while back.
Yes, the amp is a classic? the iron is classic, and the unity coupled circuit, I will give them that.
The Mind has No Firewall~ U.S. Army War College.
Jim
Thats actually what they did with mc275,they used the 12at7 in the final stage.
I've never understood why Mac ran the relatively beefy tubes (12BH7, 12AU7, etc. ahead of the drivers and then used such a wimpy low transconductance tube as the driver.
Its kind of strange but the 12uau7 of course is the phase inverter and the 12bh7 is where the voltage gain takes place.Mac does most all their voltage ampification in those primary stages and once its fed to the output stage,the kt88s produce a 2 to 1 gain but its also amplifying current with the unity coupled circuit.You can actually pull the output tubes out of a mac and drive most speakers to listenable level.You won't have any bass due to the impedance mismatch but you can drive them.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
What about that nasty 12AU7?
I change those to a 6cg7.
When problems become increasingly difficult,we can always rely on our collective knowledge and seek relief.
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