first off building this is dumb, taking advice from people on the internet that you dont know is dumb. you should probably not even build this what i describe below. but it sounds realllly good.
note: in the "schematic" above i say "omitted" regarding the caps spanning ground to v+, v-. i did use them, i jsut didnt draw them in the schematic.
nowthen:
the above is a drop-in replacement for the buffers on a millet hybrid amp, that uses no output cap. below i describe also how to use it without an "intermediate" coupling cap.
i built it on a piece of perfboard with 2 parallel strips on it (available at rat-shack, as described in tangents cmoy article) 2 dip-8 sockets can be hot-glued on, and the whole shebang will stand proudly, and drop in...
nowthen, assembely is straightforward. i kept the 2 voltages sperate on the board (left v+, right v+, left v-, and right v- from the millet) i also used 2 tle-2426's as this can drive k-1000's, and why not get the extra seperation from not sharing a grond chanel? for the purposes of driving "standard" headphones, i simply tied the 2 tle-2426's together. in the place of the 2 100k resistors, diferent combinations can be used. i employed this "voltage divider" as the lowest gain for the "drop in tubes" is about 7.5. this is simply too high for most headphones, so i droped it in half with those resistos. alternately the one closer tot he cap can be removed. be sure to keep the "apparent resistance" high, as the tubes have to drive these resistors as a load. the 12xx6 tubes dont like to drive anything anyways 200-250k ohm is about as low as i dared to go.
inital startup should go something like this: build everything. put the tubes you want to use in. put your buffers in the sockets ON THE MILELT BOARD. power up, check bias as normal. i HIGHLY recomend simply using a "bias voltage" as close to half of your rail voltage as possible. once staible, remove buffers, and install this board. install buffers (in todays buf-634 short scociety, who can spare 2 or 4 buf 634's to build this, its mostly parts you already have otherwise sockets are highly reccomended...) DO NOT HOOK UP THE HEADPHONES YET!! it would suck to blow your 'phones now from one weird conection. once the whole thing is all fired up and running with the board in place, check your output voltages. durr, you should do that with everything.
after verifying that there is not excessive voltage offset, plug in and listen. betcha a dollar that if you used a decent cap (i used a pair of orange drops i had in the parts bin) or better you will be quite pleased.
nowthen, in another thread (which i will link to here in it) it was asked how to use NO cap in the signal path.
the purpose of the cap is NOT to get rid of dc, its to get rid of DC OFFSET. BIG diference.
simply: you dont get rid of the plate voltage, you make it not have any offset compared to yoru new virtual ground.
how to do that is as follows:
but first, i will repeet: i moded what i built above to be what i describe below, and unmoded it. sound improved, but i didnt think it was worth the risk. if you think its worth the risk, and want to try it, that is yoru choice.
what you need to do is set the plate voltage to EXACTLY half (where a tle-2426 railsplitter sets to) of the rail voltage. its a very delicagte opperation, and sensative to how warm your tubes are. after doing this for a few days, i decided that i was too afraid to listen adequately: aka it wasnt worth it, and put the caps back in. what actually caused this was a bit of tube distortion/a driver crackling on a recording that i had not previously heard (the distortion) i was listening on my k-1000's and i heard this. my first gut wrenching thought was "crap, i toasted a driver" it was very real, and right there.... i realised how close to "bad stuff" i got, and took a step backwards.
build the above, and bypass the caps with either a bit of wire or a resistor.
follow general install instructions as above.
after that, my general method was to turn the amp on for a few miniutes (no headphones on it) hook my dmm upto the outputs of the amp, and then turn the bias pots untill i got a couple mv of offset. then do the other side, and repeat. it is VERY hard to get a good low offset on the output with the tinny pots that the millet uses as standard parts.
some form of active "bias driver" based on output dc offset would be nice, but i lack the skills to design one, or the motivation to steal a design. the help of others is always welcome here
i have a couple ideas for building a "self biasing" circuit for the millet, but eeh.
the amb "dc and delay device" would probably add some piece of mind for this mod
another bit of gear i lack the motivation to buy and build... it took me this long to get this from the paper napkin in the first post of this thread to a bit of perfboard...
if you are afraid to be the second person to build this, dont build it.
