Salas hotrodded blue DCB1 build - Page 32 - diyAudio
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Old 14th September 2010, 11:41 AM   #311
AndrewT is offline AndrewT  Scotland
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Hi,
variations in Tj cause this non repeatability.

The manufacturer holds Tj @ 25degC, read the datasheet.

We poorly resourced amateurs cannot replicate the test conditions.

I use comparison to a REF rather than absolute measurement.
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Old 14th September 2010, 12:30 PM   #312
Marra is offline Marra  United Kingdom
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Thanks for the tips regarding capacitors. I wasn't thinking of dropping 150-200GBP on two caps but have come across a dealer selling the Mundorf s/g caps at 60-78 per pair. A bargain??
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Old 14th September 2010, 01:26 PM   #313
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Quote:
Originally Posted by Salas View Post
Try a K75-10 bypassed by 0.1u FT2 or FT3 before you go spend 100-200 Euro for two capacitors. And there are even more expensive around.
I could absolute agree you recommendation

At the moment i use a russian K75-10 1,5F bypassed by a K72-P6 teflon 0,047F as output caps on my Tube-I-zator board. The combination is brilliant.
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Old 14th September 2010, 01:44 PM   #314
qusp is offline qusp  Australia
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Quote:
Originally Posted by Marra View Post
Thanks for the tips regarding capacitors. I wasn't thinking of dropping 150-200GBP on two caps but have come across a dealer selling the Mundorf s/g caps at 60-78 per pair. A bargain??
depends on the value? if only 0.01 or 0.1 not amazing value, pretty standard, but a larger one maybe. what is the capacitance of these you mention?
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Old 14th September 2010, 01:58 PM   #315
Marra is offline Marra  United Kingdom
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depends on the value? if only 0.01 or 0.1 not amazing value, pretty standard, but a larger one maybe. what is the capacitance of these you mention?
2.2 and 3.3uf in 800v values. he only has a couple of each
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Old 14th September 2010, 05:33 PM   #316
qusp is offline qusp  Australia
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well actually no, not so flash, PCX sells the 1200v version of same for about the same for the 2.2u and almost 20usd cheaper for the 3.3u and if you are lucky you can catch a sale at PCX and get 15% off. in fact I think theres one on at the moment.

they'll be HUGE though!! like 40x60mm or so

I actually prefer the silver/oil and silver/gold/oil versions, I would take the silver/oil over the plain silver/gold anyday
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Old 14th September 2010, 05:37 PM   #317
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I bought a few dozen Toshiba 2SK170 BL Jfets.

I've been matching them and there's something I want to discuss with whoever's done this themselves:

Bringing them to saturation, isn't quite as cut and dried as one would hope - some of them never settle, or at least not within 60 seconds. Others do settle, and it is clear what their IDSS should be rated at, but others take a long slow build up over 2 minutes or more, before you feel they are at current saturation.

When I measure one and it reads the same as another I've already done, I go, "whoopee, a match" and then check the first one again to be doubly sure - it quite often now reaches up to .5mA higher!!! Very frustrating that. Why is it?

I have tried to be consistent with my procedure with regards to ambient temperature, voltage applied, method etc.

When we say that a jfet has an IDSS of 6.72mA, for example, it may be a reading taken at 30 seconds when it settled for a bit, but I am sure that prolonged exposure to being run at IDSS in a hot DCB1 buffer for a few hours would extend that to a new rating.

Has anybody ever removed a set of "matched" k170s and measured them to find they have new higher IDSS readings, and no longer matched? I suspect this may be the case quite often.

I'd be interested to hear of your experiences.
I've matched more than 300 of 2SK170 and about 100 of 2SJ74, BL grade.
Same process, stable 9v PSU, min 2.5 mn before reading the value and... yes, measures aren't the same, variation from 0.3 to 0.4mA.
IMHO because of component and room T, batches are mixed too.
The goal is to obtain similar values, best around 7.2mA.
I don't try to desolder matched sets and tests them after...
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Old 14th September 2010, 05:43 PM   #318
qusp is offline qusp  Australia
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is 10ma idss too high? i'm ordering some LSK170 to use instead I would like to have decent current output in case of driving headphones; or should I just get the lower 7-8ma parts and parallel if needed. seems would be better to have the lower input capacitance. also may use some in an IV, so the 8ma would be fine there. 10 isnt too hot though is it?
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Old 14th September 2010, 05:52 PM   #319
Salas is offline Salas  Greece
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30% hotter at 100mW than a 7mA and a bit more capacitive. You would not drive headphones with a TO-92 JFET and 250R Zo anyway.
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Old 14th September 2010, 05:57 PM   #320
qusp is offline qusp  Australia
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yeah that would be an eventual rework, probably with some kind of follower and only with my 600R cans, like I said just an option I would like to keep. i'm getting both parts, but thought I would ask here. maybe just get them all 8ma and stack them if it comes to that. just trying to stick with getting as many of one part type/grade as possible in the order in case of some kind of discount.

Last edited by qusp; 14th September 2010 at 06:01 PM.
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