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Old 8th September 2012, 04:23 PM   #1491
Zen Mod is offline Zen Mod  Serbia
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it's possible to damage them only if you exceed their Ugs limit ;
chance for that are small - requires idiot driving insane voltage in input
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Old 8th September 2012, 04:38 PM   #1492
lhquam is offline lhquam  United States
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Default SemiSouth R100 variability

In my previous post F6 Amplifier I show significant THD differences between 2 PC boards I built. Today, I swapped R100 output JFETs between boards (that was easier than swapping transformers). Here is THD vs Frequency of one of the boards, before and after the swap.

This indicates that there is considerable variability between the SemiSouth R100 JFETs. I do not know if it is input capacitance, transconductance, or something else.
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Old 8th September 2012, 05:04 PM   #1493
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probably different gate leakage
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Old 8th September 2012, 05:23 PM   #1494
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Quote:
Originally Posted by Zen Mod View Post
...connect top of lower bias net (JfetCCS) to GND , instead to output...
not seen this one but yes.
Two mistakes in the shematic but may be not in the circuit.
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Old 8th September 2012, 05:27 PM   #1495
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Quote:
Originally Posted by lhquam View Post

This indicates that there is considerable variability between the SemiSouth R100 JFETs.
Bad news
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Old 8th September 2012, 05:31 PM   #1496
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Quote:
Originally Posted by Zen Mod View Post
dunno
but , if made as drawn , lower Jfet can't be biased

Cap is on bottom section as well. Lower Bias network is not attached to ground at top but output. Everything biases perfectly, but i suspect ZM is right, its the tangled mess. Thats what i figured from beginning, as it was thrown together. I will rebuild and retest.
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Old 8th September 2012, 05:42 PM   #1497
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Quote:
Originally Posted by Zen Mod View Post
it's possible to damage them only if you exceed their Ugs limit ;
chance for that are small - requires idiot driving insane voltage in input
By mistage, the output voltage may reach power rails [and stick] as one JFET opens and the other simulatneously closes; like a switching or clipping situation. Will it affect [damage] the bias circuitry, and especially will this cause DC to flow through the secondaries [and primary].
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Old 8th September 2012, 06:23 PM   #1498
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Zen Mod View Post
probably different gate leakage
At 20kHz 1watt, the RMS AC gate current is about 40uA. I would expect leakage currents to be much lower than that. I seem to recall measuring the gate current as about 2uA when biased for 1.3A drain current. I did that measurement for only one R100 JFET.
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Old 8th September 2012, 06:24 PM   #1499
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Wondering if the R100A parts are showing their colors vs their non A counterparts.
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Old 8th September 2012, 06:26 PM   #1500
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Wondering if the R100A parts are showing their colors vs their non A counterparts.
Yeah, I was thinking the same. I am trying to order some non-A R100s right now from Newark.
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