headphone mod for dynaco pas-3

You know, in the long "John Curl Blowtorch" thread under analog line level, somebody said that the 12AX7 tube with its 250000 ohm source impedance (?) doesn't even have enough current to properly drive 100000 ohm input modern transistor amps. Dynaco's ST120 transistor amp (1966) had a special high gain input transistor to not load down the PA series preamps too much. So you really need to add something for an 8 ohm headphone. Crystal headsets from the 1930's era radios have the high impedance, but sound like ****.
I would recommend an external power amp, perhaps using op amps. Look at the mixer threads on analog line level. http://www.diyaudio.com/forums/analog-line-level/177393-diy-dj-mixer.html
If you are really wedded to the idea of using the PAS3 chassis, you could use a bit of the 12 VDC tube heater current to drive an op amp, using 2 each 100k resistor divider to center the signal after capacitance coupling, and then use the op amp to drive the headphone jack. I would try a gain of about fifteen to start with. You can look up op amp on wikipedia for a clear explanation of how the resistor ratio sets the gain. I like ST33078's op amps myself, although Peavey likes JRC4560's in their low noise premium devices. 4558 is kind of the industry standard op amp for mixers, but they hiss a bit. For full volume on the headphones, going on to a "chip amp" like on the chip amp section of amplifiers, drives the headphone better. But I wouldn't put a chip amp inside the PAS3. With no vents, its heat management is marginal anyway as is.
If you are short of cash and need a chassis and heat sinks for a chip amp, the trash can of the local PC repair shop is full of AT style switching power supplies that are dead. Besides the box, power cord, and input power conditioning devices (lightning and hash arrestors) they are full of resistors and capacitors that you can reuse. Above 47k ohm you would want to buy metal film resistors for low noise, but below that carbon comps are hard to hear hiss. I'm retired, elegant packages are not necessary to my listening hi-fi hobby.
 
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