What headphone amp to buy-Sennheiser HD 600?

Bill - Though it may sound complicated, it really is very simple. Once you've done it once or twice it'll be a snap. I don't know the Stax procedure either, but most manufactures give you easy access to the points you'll need to read and change the bias. BTW, many modern tube amps now use some kind of auto-biasing circuit eliminating the need to bias. Here's the basic procedure (trying to simplify for newbee, though it really is pretty simple): Get yourself an inexpensive digital multimeter from Radio Shack that reads in milivolts. Turn the amp on with the headphones (speakers in the case of a regular amp) connected, the volume turned all the way down, and no source connected to the amp. Let the amp warm up for 3 minutes or so. Each tube will have two contact points to take a reading. Set the multimeter to read mV and use the +/- probes to contact the two points for a single tube. This will give you a reading on the multimeter which may or may not be close to what you are aiming for. You will need to know exactly what the manufacturer reccomends for that amp for optimum performance (as Swampwalker said, a manual will be very helpful through all of this). Let's say it is .37 and your meter reads .49 - this would mean your bias for that tube is too hot and you need to lower it. To change the bias of the tube there should be either a knob, or more likely a screwdriver slot, in which case you'll need a small screwdriver. The knob/slot is connected to the pot(entiometer) Swampwalker was talking about and is there to adjust the bias. With the meter still touching the two points readin the bias of the tube, turn the screw quite gradually counterclockwise to lower the current (bias). You will see the numbers go down. Get the number to come as close as you can to .37 (or whatever number the manufacturer recommends as the bias point). Usually there is a tolerance of a few milivolts, and it will fluctuate/drift a bit, so just come as close as you can and don't worry about it. Go to the other tube (you only have two) and repeat the procedure with the proprietary pot and reading points for that tube. Once you've done the second, you may want to go back and check the first and re-adjust, and do the second once more too. After that, you're done. You should probably check/adjust the bias once a month or so. Again, once you've done the procedure once or twice it's a snap. It definitely is not time consuming for two tubes. First time will likely take all of twenty minutes perhaps since you'll be learning by doing. After you get it down you'll be able to do it in five minutes.

Marco

Marco is correct as to the procedure, however, on my 006t, I think you had to do it with the cover off and it was a bit more of a hassle than on most amps. Not difficult, just a little tedious and of course, be careful with the multi-meter leads. On my Cary, there is just a phone jack that you plug your meter into, a single pot, and an open chassis, so its a bit easier. The first time, I would do it after 3-4 min go by, and then again in about 1/2 hr. then you should be good, unless you roll the tubes, then you will need to do it for each set.

WOW! Marco and Swampwalker, you two guys are GREAT! You explained it so well that I think I actually UNDERSTAND it (we'll see of course-LOL!). Thanks to both of you for taking the time to clarify that procedure. Now it's just a matter of waiting for the headset and tubes to arrive...
-Bill

"Get yourself an inexpensive digital multimeter from Radio Shack that reads in milivolts."
-Marco

Marco:
That sounded easy enough. When I checked, Radio Shack had something like six or seven different models, and all different price ranges, and I couldn't figure out from looking at them whether the more expensive ones were more accurate or just had more features, or just how accurate they needed to be anyway, and so on....I left the store to seek additional information, and I await your (kind, patient, and always highly appreciated) assistance.

So, not to sound like even more of a doofus, but how accurate does the thing have to be? Should I get a digital one? What kind of multimeter do YOU have? Thanks in advance.
-Bill

If I may chime in, the biasing is not that precise and digital mms are pretty hard to get on the gnat's a$$ anyways, an analog would be fine, but Rat Shack makes a small digital model where the leads store right inside the fold top case. Mine is Cat # 22-810 and is cost about $20-$25, IIRC.