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Old 6th November 2017, 12:39 PM   #4951
AndrewT is offline AndrewT  Scotland
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Idss=4.5mA means you have used a GR rated 2sk170.

The spec requires a BL rated device.

But you can still get the necessary drive voltage to the output devices by altering the drain load resistances.

Earlier you stated
Quote:
Idss is low and measured across R3/R4 it is about 4.5mA
you cannot measure Idss in circuit.
Measuring the voltage drop across the source resistor allows one to determine the Id, i.e. the actual quiescent operating current. This is NOT Idss.
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Old 6th November 2017, 01:11 PM   #4952
JeffYoung is offline JeffYoung  Ireland
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F5 Turbo Builders Thread
Quote:
Originally Posted by Zatopek View Post
Jeff, I wasn't sure if your comment about the "ZV9 article" was adressed to Andrew or myself. I looked at that article and sure it's always delighting reading NP's papers. However, as this is a very much different device and not really relating Vgs to Idss it will not quickly help me deciding if and how I need to tweak Idss of the 2SK170/2SJ74.
Yeah, that was just in support of Andrew's idea of having a table/graph. I won't help you with the F5T.

Quote:
Originally Posted by Zatopek View Post
Do I have to do something about the Idss=4.5mA?
As Andrew stated, what you measured is Id rather than Idss. (Idss will be higher.)

In any case, the only thing you would need to do with a lower Idss is to raise the drain resistors to allow reaching the desired bias point (which you've already done).

Cheers,
Jeff.
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Old 6th November 2017, 02:39 PM   #4953
Bfpca is offline Bfpca  Canada
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When I built my F5V3 I had jfets that biased up at just less than 6ma. I had trouble getting to 250mv bias. I increased the series resistor to 1.5k and zeroed the pots to make sure there wouldn’t be any exploding mosfets. It is a bit time consuming to have to do the biasing over again. It may be useful to provide some general advice in the build guide for when you can’t reach your target bias point.
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Old 6th November 2017, 05:42 PM   #4954
Zatopek is offline Zatopek  Germany
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Default F5T biasing

Andrew, I bought a set of devices from the DIY store assuming they know which grade and brand. I was checking the data sheet of Toshiba and the BL-grade is rated at Idss=3mA there.

Refer to data sheet attached.

To Bfpca: Yes, I went exactly through this same biasing procedure and ended up with R5/R6 being 1.3k which gives sufficient bias for the Output stage.

By now we are discussing the bias of the JFETs which is said to be 8mA in NP's F5T article but my reading is 4.5mA only. Did you find a way increasing Idss of the JFET's???
Attached Files
File Type: pdf 2SK170_BL_Toshiba_datasheet.pdf (345.1 KB, 23 views)
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Old 6th November 2017, 06:34 PM   #4955
AndrewT is offline AndrewT  Scotland
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Read the datasheet again.
Bottom of page 1 shows the Idss range for each of GR, BL & V grades
BL grade runs from 6.0mA to 12mA
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Old 6th November 2017, 07:16 PM   #4956
AudioSan is offline AudioSan  Norway
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DIY Audio store never stocked 2SK170 or 2SJ74. It's LSK and LSJ. And the grading letters is not the same.
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Old 6th November 2017, 08:46 PM   #4957
JeffYoung is offline JeffYoung  Ireland
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https://www.trendsetter.com/v/vspfil...170_Series.pdf

GR -> A
BL -> B
V -> C
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Old 6th November 2017, 09:18 PM   #4958
AndrewT is offline AndrewT  Scotland
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try comparing the datasheets.
grade B = grade BL, since both are specified @ 6.0mA to 12mA.
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Old 8th November 2017, 11:45 AM   #4959
Zatopek is offline Zatopek  Germany
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Default F5T biasing

First I apologize for not having read the data sheet carefully. Looking at the graphs of page 2 of the Tosh data sheet I do see, that indeed this is different from Id which is what I measured across the 10Ohm resistor.
Thanks for teaching me my celtic friends.

If I understand Jeff correctly, the advice of increasing drain load was already done by changing R5/R6 to 1.3k, right??

Per the description on the DIY store page, they are selling the B-grade of the LSK170 which is equivalent to the BL-grade of the Tosh device. So the device probably is OK and doesn't need replacement.

As 6L6's build guide was completed in 2014 and never updated (I guess), the chance is, that the community is reinventing the wheel on such common issues regularly.
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Old 8th November 2017, 02:20 PM   #4960
AndrewT is offline AndrewT  Scotland
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Yes, increasing the value of the drain load increases the bias voltage applied between gate and source of the output mosFET.

If you start with a lower current jFET, then you need a slightly higher drain resistance/load to generate the necessary bias voltage.
Some vertical mosFETs (eg. 2sk1503/j201) and most Lateral mosFETs require a very low Vgs to turn ON.
This may require the drain load resistances to be lower than a standard F5 build.
If one used the standard F5 resistance values for the Toshiba 2sk1530/j201 output pair, you would find that most of the jFET current passes through the variable resistor. This would not suit long term reliability of the VR.
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Last edited by AndrewT; 8th November 2017 at 02:24 PM.
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