how about using J111 J175 Jfets in F5

For example if my JFETS are j113 & j176 have Idss about 35mA. So now How can I reduce it 8ma to 10ma bias with paralleling or in series. I read about cascoding article and honestly I don't have that much basic knowledge to process that information in that article.
thanks.
 
Official Court Jester
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majority of investment in amplifier is going to heavy hardware - xformer , caps , case with heatsinks ;

regarding how big decrease in performance and how small amount of saved greenies , if using any other JFets than prescribed , it's simply not wise to proceed with anything else than real thing

even taking in account that for someone obtaining them is complicated , not just or nothing at all matter of money

If you can't manage to have proper JFets , build some another amp , where you can substitute them much easily ....... or some other amp without JFets
 
majority of investment in amplifier is going to heavy hardware - xformer , caps , case with heatsinks ;

regarding how big decrease in performance and how small amount of saved greenies , if using any other JFets than prescribed , it's simply not wise to proceed with anything else than real thing

even taking in account that for someone obtaining them is complicated , not just or nothing at all matter of money

If you can't manage to have proper JFets , build some another amp , where you can substitute them much easily ....... or some other amp without JFets
Yeah right. But some newbies want economy build to dirt their hands for first time builds. Cost is too high after shipping and customs for linear systems jfets. May be Mr. Nelson or someone has some secret circuit for guys like me. If I ask only then Members will provide different ways. I have another project PLH in mind because of absolute parts of Toshiba. any exicon lateral or irf available replacement for irf610 to build PLH ?.
Thank you.
 
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Official Court Jester
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build Babelfish J - using J271 for input LTP and pretty much any IRFP N- channel for output

you can make it even on veroboard or PtP ...... or you can make your own pcbs from pdfs posted in first (old ) thread


instead of IRF610 you can pretty much use any of these - IRF510 , 520, 530 , 610,620,630

higher the number , higher voltage and higher capacitances
 
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J76 and J113 are the most likely candidates for this - lower Idss.

I suspect there is a typo above, and Nelson actually means the J176 P-channel JFET whose datasheet (attached here) specifies

2.0 mA <= |Idss| <= 25 mA​

The J177 has even lower Idss, but DigiKey doesn't sell it as a thru-hole part, only surface mount. Fortunately the DIY-focused vendor "Linear Systems," does sell it in thru-hole.

_
 

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I suspect there is a typo above, and Nelson actually means the J176 P-channel JFET whose datasheet (attached here) specifies
2.0 mA <= |Idss| <= 25 mA​
The J177 has even lower Idss, but DigiKey doesn't sell it as a thru-hole part, only surface mount. Fortunately the DIY-focused vendor "Linear Systems," does sell it in thru-hole.

_

J176 has higher dissipation rate than J177(260mw). J113 & J176 have around 360mw power dissipation.
 
The one and only
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I suspect there is a typo above, and Nelson actually means the J176
P-channel JFET whose datasheet (attached here) specifies
2.0 mA <= |Idss| <= 25 mA​

Actually, measuring my extensive population of these parts,

probably should have said J175.

The J113's come in at around 20-40 mA.

The J175's are 25-30, J176 are 8-15, and P1086 are 20-30.

Regardless of the spec in the old datasheet, this is what I'm seeing.

Getting matches for these is not easy, and for the F5 adjustment of Source
and Drain resistors will be necessary to get a balance.
 
Actually, measuring my extensive population of these parts,

probably should have said J175.

The J113's come in at around 20-40 mA.

The J175's are 25-30, J176 are 8-15, and P1086 are 20-30.

Regardless of the spec in the old datasheet, this is what I'm seeing.

Getting matches for these is not easy, and for the F5 adjustment of Source
and Drain resistors will be necessary to get a balance.

so Nelson the J175 and J113 are the right combination? Still I agree matching is required.

What should be the source resistance change for the F5 for these?
 
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Serious question: why don't you (rhythmsandy) modify the circuit design to accommodate the higher-gm, easier-to-buy, JFET types? Maybe it's as simple as changing the values of resistors R1-R2 and R5-R8.

You will have to buy two dozen of each type and sort them by Idss to get a pair that are in the range your redesign requires. When I test a bunch of Fairchild J113s, I routinely see a 5-to-1 spread of Idss.

It might be wise to (i) acquire the devices; (ii) plot histograms of measured IdssP and IdssN on the devices you actually own; (iii) find a pair that looks acceptable; and THEN, once you know the Idss you're actually going to use, (iv) choose the value of R1-R2 + R5-R8 which give your desired bias point and your desired transconductance.
 
The one and only
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so Nelson the J175 and J113 are the right combination? Still I agree matching is required.

In the population on my shelf, they are the most likely to have the same
Idss. They will still have slightly different transconductance, and also not
the same actual curves. This is where variable Source and Drain resistance
comes in, but nothing is ever perfect, not even with 2SJ74 and 2SK170.

For the F5 you want Ids in the region of maybe 8 to 12 mA.
 
Mark's answer is so obvious that it hardly seems necessary.
But I have to ask:
Why is someone who can't design trying to redesign this amplifier?

Accept as is, or find one that is good enough as is, until one has learned how amplifiers work and then progress to learning how to design amplifiers.

I have never progressed beyond how amplifiers work and find I still get stumped (cricket term) by unusual topologies. I will never become an amplifier designer.
 
Of course, the Toshiba or Linear System parts are ideal, but hey, it's DIY. Why not try other things I ask.


I'd go with MMBF5459 and 5462, as EUVL hinted (or 2N5459/5462 if you can find them). You can get 2 x 100 pcs each for a total of 30 Dollars at Mouser. Yes, these parts are not through hole, but SOT23 is BIG.


Drawback is the max voltage of the N Channel device, max. 25V.
If it's an issue (what Rails do F5 have?) cascoding is needed, or alternatively a zener (eg 4V7) at the drain, to eat up a bit of voltage.


It's DIY, it's fun, and if you can't get it to work, get those LSK parts.