Schade Common Gate (SCG) Preamp

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#1,582
Rahul,

Working with PSUD it does look like you can get rid of lots of excess voltage with larger resistors in the filter.

What do you think of using enough resistance to do this?

Now that the recommended voltage has come down - even with a choke input on my 90 volts secondary there is too much voltage.

What would be the harm of using the following: assuming all caps are the specified 150 uF : 51R - 150R - 250R - 560R?

You do get a predicted very quiet supply - still using the LCLC before the CRCRCRCRC.

Will this strangle the current delivery of the supply? The chokes are 5H/250mA and 300 mH/600mA - plenty of margin there.

Your counsel would be appreciated.
 
#1,583
Higher value resistors are fine but you have to increase the power capability then too. Current draw per channel is about 30 mA conservatively. So, at 500 R, there is I^R or .03 x .03 x 500 or 0.45W, which means at least a 2W resistor. As long as you cover that you are good. For two channels on the same supply, the power goes up to 1.8W dissipated, or at least a 5W 500R resistor.
 
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#1,584
I tend to use higher rated resistors as a rule for that kind of thing. I am using one supply per SCG channel - the one transformer supplies two boards and that is where the input choke which branhes to two smaller chokes and then to each SCG supply - which is what I did before.

I did not remember/know the current draw so I will be using smaller resistors than those I listed above. Nonetheless I will probably use five watt OHMITE WH wirewounds.

Did not want to buy and then replace a transformer.

Thanks for the assurance.

Take care,
 
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#1,585
Got the PS boards.

Looking at El Arte's list of parts I see he has specified a mosfet you have never mentioned on the thread.

I had been using the isolated version of the original mosfet.

Is this one your latest recommendation, Rahul? Is it one of those cases where almost anything will work?

Realized I could use a SIGNAL isolation transformer before my SCG raw supply to bring down the voltage so I will not be using large resistors for the filter.

I had conceptualized in my fanciful brain that varying the time constants would be better than all being the same - yet when I sim in PSUD the resistors all being the same value gives less ripple for the same voltage. I was starting small and getting larger with each subsequent stage. Going in the other direction results in about the same numbers.

I will be using four 51R resistors.

Take care,
 
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#1,586
Yeah, the STF3LN80K5 is rated for high voltage and has a plastic case. Most N-ch mosfets in a TO-220 package rated for at least 100V should work here. The STF3LN80K5 was a part I had on hand and it works fine. Others here also seem to use that part. It is totally fine to use whatever you have on hand. The plastic case makes it easy to mount to the heatsink.
 
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#1,587
ra7 said:
Here it is folks. Schematic, gerbers, and BOM attached.

Few notes:
1. Testing jig schematic is attached to this post. Use that to measure Vp (JFET pinchoff or the voltage at which almost no current flows through the drain and source of the JFET) across the 1Meg resistor and for testing JFETs for other positions.

2. A lot of 10 J111s, 20 J175s, and 20 J113s are needed at a minimum to select JFETs for various positions. Don't worry too much about channel matching. If you get two exact matches, great; if not, no problem. The circuit is immune to small changes in device properties. Bigger lots will give you a higher chance of matching between channels, but this is not strictly necessary.

Note below that J175 and J113 are used twice in the circuit. Each circuit position of the same JFET requires unique selection. Pay close attention to the JFET selection procedure below and put the correct JFET device in the correct position on the PCB after you perform the selection.

J111: Select the two with Vp closest to zero, one for each channel.
Q101 J175: Select the two with Vp farthest from zero, one for each channel.
Q106 J175: Select two out of the batch that measure 10-11 mA at 130 ohms source resistor. Replace the 1Meg resistor in the test jig with 130 ohms and measure the voltage across the 130 ohm resistor. At 10 mA, it would read 1.3V.
Q103 J113: Select two out of the batch that measure 15-20 mA at 10R. Replace the 1Meg resistor in the test jig with 10R and measure across 10R.
Q105 J113: Select two out of the batch that measure 10 mA at 47R. Replace the 1Meg resistor in the test jig with 47R and measure across 47R.

3. Note the orientation of the JFETs on the board. The flat side goes with the flat side of the JFET. The J175 is immune to orientation, i.e., it can go in reverse and still work fine. For the Q101 position, I have a slight preference for J175 going in reverse relative to the PCB orientation. YMMV.

4. Power supply: I suggest anything from 50-70V. Don't go above 70V otherwise you are pushing the dissipation limit of the little guys.

5. Jumper JP1 conveniently provides a way to either use the buffer stage or bypass it. If you are driving an F4, say, which has an input buffer, feel free to skip the output buffer of this preamp. Now, on the other hand, if you want to drive a big MOSFET or SIT directly, like in a MOFO or the TDV, then I'd recommend using the output buffer. On JP1, the pin closer to the JFETs is connected to the non-buffered output.

6. The only setting on the circuit is RV1. Turn the pot to achieve the midpoint of the supply at the output before C102. Most easily measured at the leg of C102 closest to JP1. For example, for a 50V supply, turn RV1 to 25V at the output.

7. As usual, stuff the smallest parts first. Start with resistors, then JFETs, then the caps and other parts.

Hope you have fun!
Click to expand...

I am having some difficulties with the selection process.

Do the quoted mA values depend on the voltage supplied to the jig? (I do know that Vp varies if I use 12V instead of 9V.)

For instance, for Q105 J113, the lowest I can do is 11 mA. I suspect my DMM is not precise enough for the job, but I am not sure.
 
#1,590
Well, the conclusion for tonight is that I need to work on those jigs. My breadboard versions suffer from bad wire connections and other issues (when I touch the wire leading to the gate of a J113, it conducts more, for instance). Hence, I lost all confidence in those breadboard jigs.

I will also order more J111 and J113. I have fiddled a little too much. The work continues.
 
#1,591
IMG_3958.jpeg


I also have one of those “cheaper” transistor tester; it gives pinch-off and Idss.

Could I use the square law formula to calculate current for a given source resistor?

I would rather get the rigs reliable, but…
 
#1,592
Yeah, those breadboard connections deserve to be checked. I have used some of the ones from Amazon and it can get frustrating.

It might be that Vp or Idss could predict Id at a certain point, but it is so easy to test.

There may be other ways to hardwire that circuit. Amazon sells some solderable breadboards, like this one.
 
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#1,598
rickmcinnis said:
I am getting some small clips to attach to the legs of the FETs - like the ones that came on my tester.

These could be smaller but this is the easiest way to do it. Cut them in half, of course.
https://www.amazon.com/Goupchn-Silicone-Multimeter-Minigrabber-Electronic/dp/B0852KRNPP/ref=sr_1_2_sspa?dib=eyJ2IjoiMSJ9.K_fhOMIO0U-QczwBc9AX6260R3taVEPzJM4Alv9co_d-XMnm71kyLl3Xkz3ev0WcSEWPvMZTtMfXwpqf1Tp3BfXILhWcc9Q3iTWabD_iuEdDPq7LzMCnQnFwrl97uZ-LfO1Qvw416b2zaKhgTjtfRY091XBy-3YYMh-V6OLUKvq85GtuMzjXWgTQIGuGHJevmrZBbib0MNwkP6ia18GJscaW9Yr0w08Rb3sGMAaXoz0.0uVUtwgIMt7vWCk9s3rkbT-ZJsZCjPjOX2mU5D53NDU&dib_tag=se&keywords=Test+Clips&qid=1737388893&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&th=1
Click to expand...

The soldering breadboard is proving to be too cumbersome.

Yes, it makes contacts between supply, resistor and board really solid, but contact with the JFET legs become doubtful, even with those clips.

So, back to a new breadboard, where I have replaced all Dupont breadboard wires with solid core wire that fits tightly in the breadboard pin sockets. I also use the shortest clip-on leads I have for my DMM.

The resistor leads are still too thin to fit tightly in the breadboard. 1/2W parts would probably solve that, but I do not have them.

Also, increasing the supply to 12V as this seems to give more stable results.
 
#1,599
I thought you were having trouble getting good contact with the FETs.

My tester uses clips and the connection has never been a problem with those. Maybe your clips are tired? Or need cleaning?

I am getting those clips to make my tester. My initial attempt was a failure, too, trying to be too clever and cheap.

I am going to use about 15 volts regulated with a LM317 since it is laying about.

Perfboard with solid soldering joints - I have never cared for breadboard - never works for me.
 
#1,600
rickmcinnis said:
I thought you were having trouble getting good contact with the FETs.

My tester uses clips and the connection has never been a problem with those. Maybe your clips are tired? Or need cleaning?

I am getting those clips to make my tester. My initial attempt was a failure, too, trying to be too clever and cheap.
Click to expand...

In fact, I believe I bought the very same clips. The gauge of those wires is very low. I am guessing that is why simply holding the gate lead would drastically change the measurements.

If I could find a good socketing solution for the FETs, I would use those veroboards I have. Having to switch 3 clips when you have 200 FETs to measure is more than i can handle.

Measuring such small quantities is challenging for sure.

I was trying to expedite the process, but it’s now clear it can’t be expedited.
1 position per session, it’ll probably take half a day.

Good luck, Rick. Gotta enjoy a nice Scotch when I finally listen to this new build!
 
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