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GB for F6 Convertible Clone boards

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Tea-Bag

Tea-Bag

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Joined 2006
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staring at a functioning board with capictors facing me, the LEDS should look like|/| on both sides.
Make sure your other jumper points are done correctly.
Here is a pic with almost all the jumper points installed (assumes SemiSouth, no degeneration.) IGNORE the LED symbol orientation on this one, the production boards switched that.

R11,12 were not jumpered, but bypassed to do even without the input JFETs in this configuration. (I did not stick with that, performed much better with them in there)

FullyJumperedF6.jpg
 
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anilva

Member
Joined 2004
F6 build - How to set the bias

Hi,

Trying to complete a long pending project during this Christmas vacation. I finished stuffing the Tea-bag's F6 boards with all the components and also jumpered the un-utilized LED and output degeneration resistor and preset slots.

I have the required Semisouths as well.

Now I need to test the boards. I do not have a variac but have a light bulb setup. To begin with, do I set the bias pots to zero resistance, and if yes, to which side? Also do I set the offset pots to centre?


Please help.

Attached is the picture. Please let me know.

Cheers.
 

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Tea-Bag

Tea-Bag

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I would install the transformers, and try to get units LEDs to light up with without the FETs installed. Should get voltage over the toshiba JFETs.

If that works, then install FETS, but get the Pots at the lower Ohm reading first, by measuring underside of unit.
 
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anilva

Member
Joined 2004
Pot is maxing out for bias on one channel

Hi,

Managed to get both the channels up and running. Adjusted the bias and offset on one channel to 1.5 amps measured on the resistors of the power supply on one rail. While the other channel biased as well, the bias preset is maxing out at 1.2 amps. Do I need to reduce the 10K resistor value (R4) in series with the bias LED, or should I change the LED?

Need some help. My understanding of the circuitry is sketchy.

I am using Semisouth PowerJFets in the output.

Cheers.
 
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aoc

Member
Joined 2006
Fellow diyers,

Just to shared my thoughts wrt resistor feeding LEDs and 5 k trimmers,

LEDs in general prefer to be well forward biased for the Vf to be well establish. For a typical blue LED, 3v forward voltage correspond to about 10 - 20mA forward current (some variation depending on make). In reality one also needs to consider 5 k pot in parallel with LED. Assuming 3v across LED, there will 2mA flowing through 10k resistor with 0.6 mA is going through pot (3v/5000) leaving 1.4mA for LED. 1.4mA is far from ideal for running any LED maybe that is the reason why there large variation in experience.

I'm using 1.2k instead with 2 blue LEDs. Reason I'm using 2 blue with in my setup is to produce about 6.2v ref voltage which gives able voltage to explore SS or IRF or any other lat fets. Pots takes 1.2mA leaving about 12 ~ 13mA for LED. Question for all, why isn't there a cap for smoothing of the Vref generated by LEDs?

Just sharing

Sent from my C6903 using Tapatalk 2
 
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aspringv

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...And all good here. Tea Bag came through with some lovely boards and Jensen was pretty swift with some transfo's. I'm fairly sure I have everything else I might need on hand, so off we go when I get some diy time.

Question though - the Jfets at the front end are just working as a buffer, so, other than noise, whats the consequence of substituting in differing jfets? Come to think of it, the answer to that is drive current to the transformers, so allow me to rephrase.

Is the reduction of drive current significant enough to impact performance? I have enough J175 and j310's on hand to find a franken-match between some...
 
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aspringv

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I will, I wish, I wouldn't want a woofer wobble. Ahem. Apologies to Dr Seuss...

Before I start the psu boards are excellent too, so thank you. Destined for an F3 build, as I might also have a couple of smps on hand to use for this guy.

I think i have a few lsk jfets here anyway, so I dont need to find sub parts but I'm always curious :)

Thanks again Tea Bag!
 
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aspringv

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...A question if I may?
There was a great document done by one of the members around matching Jfets and degeneration. They went into quite the graphical analysis to match curves between '74's and '170's. Does anyone know what that document name is and where I'd find it?
 
Tea-Bag

Tea-Bag

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Joined 2006
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aspringv said:
...A question if I may?
There was a great document done by one of the members around matching Jfets and degeneration. They went into quite the graphical analysis to match curves between '74's and '170's. Does anyone know what that document name is and where I'd find it?
Click to expand...

I think jackinnj wrote something to that affect, but this goes back into the F5 days I beleive. You could PM him about it. Between him and EVUL they recommended I think 10R on N 2SK and 15R on 2SJ if you will. Jack has a fairly nice curve tracer, and other test stuff, so was able to perform such tests.
 
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