sorenb Posted June 23, 2015 Report Posted June 23, 2015 (edited) Couldn't trim the 100pot to get 10mA from the CSS ...had to change the 250ohm -> 100ohm ... used JimL method of adjusting ....apart from using the LV rather than a battery and measured the drop across the 100ohm at B+ rather ...the method works fine .... had to lower the 1k5 -> 800ohm in order to offset adjust the amp again .... Sound impression: well for what it's worth, I find it sounding super tight and fast ... expected to get some hum/noise from using the 'open architectur chassie' having heater wires lying around ...but the amp is dead quiet ....no hum, no hiss, just dead quiet Don't know whether it will change sound signatur over time ... have limited expereince with tubes... Edited June 23, 2015 by sorenb
nopants Posted June 23, 2015 Report Posted June 23, 2015 in my limited experience most issues occur when you are trying to case everything
kevin gilmore Posted June 23, 2015 Author Report Posted June 23, 2015 fireproof cardboard box always works. way way way back when, i built a 1kw xenon strobe in a cardboard box...
sorenb Posted June 23, 2015 Report Posted June 23, 2015 1kw Xenon strobe in a cardbox??? ....have you seen "Glasslinger" on Youtube doing a 50kV power supply?
kevin gilmore Posted June 23, 2015 Author Report Posted June 23, 2015 (edited) playing around with the board for sorenb, i found that the right heatsink is not grounded. really should be grounded, best place is the ground on the power connector this probably means the original board has the same issue will update boards now Edited June 23, 2015 by kevin gilmore
Kerry Posted June 23, 2015 Report Posted June 23, 2015 Congrats! Enjoy while you figure out how to box it up
sorenb Posted June 23, 2015 Report Posted June 23, 2015 @Kevin: Yup...right heatsink on the boards I have is not grounded @Kerry: Thanks in rgrds to boxing the amps I have a few observations, one is that the almost square boards forces to be mounted horisontal (unless you want something bigger than 2U) that is why I asked Kevin to do a split version of the golden ref psu HV and LV, one side is less than 2U. The teflon sockets I use for the tubes in the KGST makes it impossible to cram the amp inside a 2U ...using 10mm standoffs ....the height is 950mm ....I really admire your KGST but since I am currently short of a CNC and don't have any experience in doing PCBs that is really not possible 4me to copy
gepardcv Posted June 24, 2015 Report Posted June 24, 2015 I have boards marked 0.41, and both heatsinks look grounded to me. I checked against the ground lead on the 5-position terminal block with the HV in and signal out. Anyone want to double-check?
kevin gilmore Posted June 24, 2015 Author Report Posted June 24, 2015 (edited) the original kgst boards are fine the ones with the cascode current source have the one heatsink ungrounded. how this happened, who knows. Its why i need a much bigger monitor. Edited June 24, 2015 by kevin gilmore
JimL Posted June 24, 2015 Report Posted June 24, 2015 So, for the current loads, 250 ohms for the fixed resistor may be too high to get to 10 mA as sorenb noted. Using a lower value such as 100 ohms or 150 ohms should do the trick.
sorenb Posted June 24, 2015 Report Posted June 24, 2015 @JimL: I'd go for 100ohm if you want to have room for adjustment ....mine is at 130ohms or thereabouts
kevin gilmore Posted June 24, 2015 Author Report Posted June 24, 2015 got a batch of dn2540 and they have a 20% variance, so much easier to pick the resistor on a test circuit and stuff it in when you build the board.
sorenb Posted June 24, 2015 Report Posted June 24, 2015 @kevin: wouldn't you still be able to adjust using 100ohm resistor + 100ohm trim?
kevin gilmore Posted June 24, 2015 Author Report Posted June 24, 2015 easier to test than unsolder. this applies to all the boards at all the desired current levels.
JimL Posted June 25, 2015 Report Posted June 25, 2015 kevin gilmore said: got a batch of dn2540 and they have a 20% variance, so much easier to pick the resistor on a test circuit and stuff it in when you build the board. That replicates my findings with DN2540 - it does vary from batch to batch. I'm guessing it varies more than a 10M90S because that was designed as as a current source so they probably keep closer tolerances for that purpose whereas the DN2540 was designed as more general purpose. I recall posting on this but I can't find it at the moment. Agree that using a test jig is the way to go easiest way to do that is use a transistor socket, solder a 100 ohm resistor to the gate terminal of the socket, connect positive voltage to the drain terminal, a variable pot to the source terminal, then a 10 ohm resistor to the gate resistor and the other terminal of the pot (to measure the current) and the neg voltage to the other end of the 10 ohm resistor. It's easier to see with a drawing, but basically you measure the voltage across the 10 ohm resistor while adjusting the variable pot for the desired current (e.g. 10 mA corresponds to 100 mV). Then measure the resistance of the pot to get the desired resistance for the drain resistor - I would use about 2/3rd of the measured resistance and use the trimmer pot to get the correct resistance in-circuit. Remember that in-circuit there is only a few volts across DN2540 so you don't want to crank the voltage too high when measuring.
luvdunhill Posted June 25, 2015 Report Posted June 25, 2015 Or just add a trimmer to the circuit. There is a good diagram in the "Impasse Preamp" AX article which is now online.
kevin gilmore Posted June 25, 2015 Author Report Posted June 25, 2015 there is already a pot trimmer. but some were out of range 1
JimL Posted June 25, 2015 Report Posted June 25, 2015 (edited) There already is a trimmer plus fixed resistor in the circuit, the issue is choosing a fixed resistor so that the trimmer is in range. Whoops, KG beat me to it. Edited June 25, 2015 by JimL 1
gepardcv Posted July 12, 2015 Report Posted July 12, 2015 (edited) Finished my KGST. It sounds spectacular with a 207, wish I had a 007 or 009 to try it with! With headphones plugged in and nothing plugged into the inputs, there is a buzz or hum with the volume pot turned way up. With an input plugged in, dead silence across the entire range. I must have a ground loop or some other wiring problem, but I don't understand it. I used a simple grounding scheme: IEC neutral ground, transformer, PSU, and pot all star grounded; amp boards connected to PSU ground terminals and not star grounded. Any idea what might cause the problem? Edit: Fixed typo, of course I didn't short the AC to the star ground... Edited July 12, 2015 by gepardcv
gepardcv Posted July 12, 2015 Report Posted July 12, 2015 ugh, sorry! I meant the IEC ground (the third non-hot non-neutral wire).
spritzer Posted July 12, 2015 Report Posted July 12, 2015 Are the heater wires tightly twisted and kept away from everything else?
gepardcv Posted July 13, 2015 Report Posted July 13, 2015 I did twist the heater wires pretty tightly. They're the orange and pink ones in the image below. Not sure how to keep them away from the rest of the wiring, short of running everything else all the way around the case. Suggestions much appreciated.
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