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Old 23rd November 2004, 01:42 AM   #1
Fossil is offline Fossil  Singapore
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Default Need help in circuit analysis

I came across the above circuit while searching for a CCS.

Wld appreciate it if someone can help me figure out what is the bias current into each tube.
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Old 23rd November 2004, 02:25 AM   #2
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Location: Adelaide South Oz
Default No analysis required

G'day Fossil,
No analysis required - current is set by the J FET characteristics.

Do a search for a data sheet for the 2SJ74
eg is where I found it.

Look on page 2, the static characteristics graph.
For VGS=0 (Gate and Source wired together in the schematic)Drain current = 16mA.

The tubes will share this fairly well and the 10R cathode resistors will help force them to share even better.

So 8mA each tube.

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Old 23rd November 2004, 02:48 AM   #3
SY is offline SY  United States
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Ahh, would that this were true. The sad reality is that within a part number, Idss for a FET can vary five to one. Without severe selection, that circuit will not work.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 23rd November 2004, 03:01 AM   #4
Fossil is offline Fossil  Singapore
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from the table of the 2sj74, Idss = 2.6mA (min) and 20mA(max) but from the graph, Idss = 16mA.

I was quite confused from there on....

Also, I put 16mA into the circuit and got about 10V for Vk. Does it mean Vg = -10V? Did I go wrong somewhr?
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Old 23rd November 2004, 03:02 AM   #5
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Join Date: Oct 2004
Location: Adelaide South Oz
Default Woops

SJ is correct - I screwed up.
O well no shame in being ignorant ONLY in remaining ignorant.
From the data sheet IDSS (the drain current for gate to source voltage of 0V) spec is 2.6 mA min to 20 mA max.

Circuit needs work.
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Old 23rd November 2004, 03:02 AM   #6
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If anyone needs to service that circuit I'd recommend selecting the 6922 for matched halves.

Should that not prove feasible for some reason it would have been a better idea to use a CCS per cathode (with the proper current per triode of course) and use individual coupling caps with the outputs combined.

Either way, as SY points out, IDDS is pretty variable in most FETs and I'd even select the devices if they were to be CCDs anyway.

Why you'd want to use a 6922 in // as a CF for a preamp is beyond me: a single half of that tube would have sufficiently low Zout to drive anything with a more or less normal Zin, be sufficiently low noise not to be audible and an awfull lot more stable in operation as well....

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Old 23rd November 2004, 07:13 AM   #7
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My Chinese skills have certainly seen better days, but I assume you are interested in this circuit because (from what I can gather from the text) this is the first and only valve circuit offered by DiyZone, and they can provide PCBs?

I agree with Frank that parallel 6922s for a preamp is overkill and will probably lead to other problems.

Replacing the FET CCS with a single bipolar may be a better idea since you can set the current easily (by either changing the curernt set resistor or the reference voltage at the base). Don't aim for an ultra-high Z current sink, since the next stage will ruin all of that with its input impedance. Anyway, at least in my part of the world, bipolars are cheaper and more readily available than FETs.

If you need help designing something, there are people on this forum willing to help You don't really need the PCB. Old-fashioned point to point would work fine, especially since designs like this may contain more components and complexity in the power supply than the signal section itself

BTW: From what I can gather, DiyZone is a bit of a dirty word around here, especially on the Pass Labs forum...
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