kevin gilmore

So the transformer is going to be 4 inches in diameter. Which means that the inside of the box is going to have to be 13 x 13 inches for a single chassis thing. Heatsinks both sides.

here are some pictures, don't read too much into them. high impedance is 300 ohms, low impedance is 32 ohms, you can figure out which is which. This is before changing the CCS currents http://gilmore.chem.northwestern.edu/dsav1.jpg http://gilmore.chem.northwestern.edu/dsav2.jpg http://gilmore.chem.northwestern.edu/dsav3.jpg http://gilmore.chem.northwestern.edu/dsav4.jpg http://gilmore.chem.northwestern.edu/dsav5.jpg http://gilmore.chem.northwestern.edu/dsav6.jpg http://gilmore.chem.northwestern.edu/dsav7.jpg http://gilmore.chem.northwestern.edu/dsav8.jpg http://gilmore.chem.northwestern.edu/dsav9.jpg http://gilmore.chem.northwestern.edu/dsav10.jpg http://gilmore.chem.northwestern.edu/dsav11.jpg http://gilmore.chem.northwestern.edu/dsav12.jpg more pictures friday or saturday after i change the ccs resistors. thd measurements from above, well over 1%, IM approaching 2%

I think you actually need to measure that resistor. I know what it looks like, but i've been told that it supposed to be brown black violet which would be 100 meg, resulting in a lowering of the bias by about 9%. There is no way you can safely do 300 volt bias when you have the amplifier doing +/- 600 volts. That or someone did the modification wrong. With koss, you can never be sure. Clearly koss has some manufacturing issues.

OK, absolutely positively i should never rely on anyone who thinks they know what they are talking about. The bias on the koss headphones is absolutely close to 600 volts. Here is the proof http://gilmore.chem.northwestern.edu/kesp1.jpg So the tan resistor has absolutely no markings. The 10 meg resistor is in series with the +630 volt power supply (plus or minus a bit, on batteries, about +600) The unknown resistor is to ground in parallel with the cap. You just can't measure that resistor with it in circuit, which is the failings of everyone else. http://gilmore.chem.northwestern.edu/kesp2.jpg and this meter measures to 200 meg. http://gilmore.chem.northwestern.edu/kesp3.jpg Now usually when i see resistors like this, they are 1 gigohm. But even if that resistor is as little as 250 meg ohms, the bias is still (referenced to 600v) a minimum of 575 volts. I could bring the thing to work where i have meters that measure into the gigohms, but there is no reason. THE KOSS ESP950's ARE HIGH BIAS HEADPHONES!

Do not assume that i paid anything even close to retail for the stack of test equipment i personally own. I've been collecting stuff for 35 years. 4 oscilliscopes at home, all solid state, one portable and one transportable. signal generators, tdr's, spectrum analyzers,... The pile is probably worth more than the house that contains the stuff. 4 years at zenith during the beginning of the solidstate tv days gave me a lot of knowledge in messing with high voltage stuff without killing myself. An art that is clearly long gone.

my bad... 75 mhz these are the new ones with silicone inside them instead. TEKTRONIX|P6015A|1000X High-Voltage Passive Test | Newark.com Tektronix P6015A/2R 25 MHz High Voltage Probe - eBay (item 320511121758 end time Jul-30-10 16:09:08 PDT) eventually you will probably find it on ebay for about $500

Insulating version of 8n80c definitely a good idea. I did not even know it was available. Someone needs to verify that it can dissipate enough heat though... Definitely do not hook up both the 100 volt and 120 volt windings on the filament transformer at the same time, pick one or the other. You can hook up all the wires on the high voltage transformers if you wish. This error has been corrected on the layout so that if there is another board run, this won't happen again. Doh.

Lots of problem with the capacitive dividers too... matching over frequency very miserable. What you want is the old tektronix 1000x probe thats filled with freon. Good for 30kv. And a frequency response up to about 10mhz. Its about 1.5 inches in diameter, and two different colors of blue. I have one, i keep it locked away so that no one drops it which would trash it. The 100x scope probe should do it if you are careful. Here is what absolutely everyone including me does...when they don't want to blow up something expensive. Little pomona box with a male bnc on one end, and a female on the other. Add two series back to back zeners from center to ground. 100 volts. Insert in the line between the scope probe and the scope. Ignore the readout pin if you have a scope that uses that. I'll verify the koss bias voltage tomorrow, but mine certainly does and has worked at 600 volts... For a very long time. Need to put the L'espresso on the bench and test for distortion and rise times...

Oops, thats definitely a bug... Will figure out how i messed that one up... My transformers don't have 100 volt taps. I hate messing up the simple stuff. DEFINITELY DO NOT HOOK UP THE 100V transformer windings if your transformers have these windings!!! Thats what happens when i try to make it goof proof

Morphsci is today's winner... That went fast.

Actually you have it backwards. Because of the circuit lands on both sides of the board, chassis ground is pretty much connected to circuit ground anyway. Using a cheater cord (same as cutting the earth ground to circuit ground) is definitely not a good idea.

One person has just canceled, so there is one more set of boards/chassis available. First person to PM me gets it. But it still will be a few months and it will ship as a complete set. (figure about $1500 plus shipping)

anyone know where to get the rubber versions of these things https://sdp-si.com/eStore/PartDetail.asp?Opener=Group&PartID=1938&GroupID=545

Frank's son is evidently practiced in the ways of pabbi1. Not a good thing. Not even for a few minutes.

So i'm definitely done sans knob and inductor. I measured my chassis length as 13.035 with this http://gilmore.chem.northwestern.edu/bigger2.jpg (yep its an old picture) Which kind of makes sense when you add .125 in addition with the plastic bezels and things don't fit right. If you wanted to make it perfect with the current circuit board, then a chassis length of 12.875 makes more sense, then you notch the plastics for the board. Then everything fits flush. http://gilmore.chem.northwestern.edu/dsavitsk5.jpg you can see the grounding lug on the inside of the right screw. I kind of like it better without the cheap plastics anyway. I put in 2 x 30mm holes for the tubes. http://gilmore.chem.northwestern.edu/dsavitsk6.jpg Which gives enough room to get a tube puller in there on the base of the tubes.

I love it when a plan comes together... The whole design of the amp is to minimize the lengths of the audio traces. So the pot is in the back, the input tubes are in the back, signals go from the back to the front and then out the front hole. There are lots of amps that do it backwards, which makes no sense. The BL vs GR version thing completely makes sense. Even at 8k, there is still plenty of adjustment room, because once you get it to turn on, on that pot, .25 turn is all that should be needed.

I'm going to make another comment, and this is one of those philosophical things, some people do it one way and others do it completely differently. If you pay attention to underwriters laboratories, and the international electrical code, then for SAFETY reasons, the 3rd wire of the iec connector goes to chassis ground. Pretty much everyone agrees with this. Two wire power plugs are another thing entirely, and are real bad for audio as there is always a resistor between neutral and chassis ground (or the whole thing is double insulated) Remember this is all about safety, not good sounding audio. But for minimum noise and ground loops, it is usually (but not always) better to completely float the circuitry inside from chassis ground. The rca jacks certainly allow for this, and so does the neutrik jack. That is not possible given the current layout of the board. For smaller systems with just a source and this amplifier, it is not going to make any difference. Mix it up with a pile of other electronics, and it might cause trouble. For example on my T2 there is a toggle switch inside the amp chassis that can disconnect circuit ground from chassis ground.

And that is in fact what i ended up doing. I tried it against the smaller boss on the pot, and that did not fit right either. I'll try again tomorrow and add a very thin spacer to the pot to get the panel off the outside boss. If there is a part number for extra aluminum front and back panels, i can certainly try again. My chassis is definitely longer than the circuit board. Probably by at least .075 Its clear (at least i think) that the back panel has to be snug with the iec socket and the rca jacks. The main part of the rca jacks. This is where at least .035 of the difference comes from. If you do it the other way then the amount of ground available for the rca connectors shrinks, and a number of the rca male connectors i tested don't fit in all the way and are loose. The mounting hole for the rca jack was designed to be a self tapping screw for plastic.

the parts stuffing is on page 3 http://gilmore.chem.northwestern.edu/staxt2nc3fdh3.pdf if the active batteries are not conducting then many of the led's will not be turned on. Adjust RV2 to decrease the resistance, (i think) When things start to conduct, small changes will cause the amp to flip flop back and forth. Until both sides of the amp are balanced. Its a bit tricky, but once you figure it out, its not too bad. Yes the led's run at low currents. But from the picture i posted, they do in fact light up pretty well. Definitely not dim. Also measure the voltage across R42. If its zero, then there is a problem with the 2sk246

My unit is complete and works fine. Very quiet for a tube amp. Will swap to the inductor when it shows up. Going to add 2 more holes in the back panel and add a grounding safety wire there, and put tube holes in the top panel tomorrow.

The other trick is what to do about the top panel. Some of the tube sticks up, but not much. So there would have to be a fairly large diameter hole, otherwise there is no way to pull the tubes out. Or make reasonable holes, and slit the cover in two, so you slide the cover out to change the tubes. I'm kinda leaning towards that approach. The final parts just showed up. Gotta go... There are plastic ends available, which are one piece, and would fit perfect. And be much easier to machine. Overall, pretty good for a first round.

Yeah... About that... The board is not long enough, and the parts don't fit thru the panel right if you put the plastic piece on first. !!!! So if you assemble it correctly and push the board to the back end, then the front end parts are .126 inches off. Now for me its easy, just take .2 inches off the chassis. But i would have to do it at work because i would take 1 swipe with the top on, and i need a 4 inch long end mill for that. And a much bigger mill. height of the case measures 2.0935 (back/front panel not including the plastic surround) height of the center of the iec hole is 1.0595 from the bottom (center of ground pin) all 3 screw holes should be #4-40 body drill size. diameter of the hole for the rca jack to fit the plastic thru is .4055 inch I will add 2 more holes to mount the iec jack to the back of the back panel

If you insulate the lm338 with a thermal washer and plastic mounting washer, it should be easy to secure the heatsink to the chassis. the rest of the parts should be in today. And i got a bit sloppy with the machining as its very hard to hold stuff this thin down... http://gilmore.chem.northwestern.edu/dsavitsk1.jpg http://gilmore.chem.northwestern.edu/dsavitsk2.jpg http://gilmore.chem.northwestern.edu/dsavitsk3.jpg http://gilmore.chem.northwestern.edu/dsavitsk4.jpg Now that i know the exact dimensions it should be easy to make new panels perfectly NC'd out of .125 material that is thick enough for me to hold it in place.

these are actually great caps, available from mouser, and reasonable. And they fit too. 598-936C4W4P7K-F

No guts, no glory. Check things over 9 times and then have at it. As long as there are no shorts on any of the current sources, no parts should fly. Make sure all the pots are set in the middle, makes things much easier.