So far the only thing I have on the TIS58 says it's an N channel JFET and the J305 is a possible replacement.

A JFET has three terminals, Gate Source and Drain. The Gate acts like a anode of a diode (in an N channel JFET) and the Source and/or Drain will act like the cathode. In most cases the Drain and Source can be interchanged, the part will operate normally if you reverse those two terminals. Your meter should show a valid diode indication from the Gate to Drain or Gate to Source in both directions when the JFET is removed from the circuit. A damaged JFET often shows a low resistance in both directions on the Gate. Between Drain and Source your meter should see a resistance, in the case of the TIS58 (I'm guessing) 100 to 200 ohms. Some JFETs used for analog switches will measure lower between Drain and Source.

Pinout: Without a data sheet I can only guess but on most USA designed N Channel JFETs the terminal that would be the Collector of a TO-92 bipolar transistor like the 2N3904 would be the Gate. Occasionally it's the middle terminal, and that is usual on Pro Electron parts (2SK and 2SJ).

An N channel JFET acts like a triode vacuum tube. The Gate will be the most negative terminal, usually at ground potential. The channel terminal acting as the Source will be at perhaps 1V or 2V above that acting like the Cathode of a triode and the Drain will be 10V to 25V above that acting like the Plate of a triode.

It's common for the very first input JFET to be damaged. Excessively negative Gate voltage like from a static discharge will damage the gate unless the circuit has protection diodes.

I don't know of a DIY Beta preamp. There are one or two clones built into stompboxes that can be found. The biggest problem with the ones made by Sunn is that they need to be recapped. There are about a dozen 2uF non-polar electrolytics that need to be replaced to get the preamp running well.

If you can find one that is non-operational for a good price, buy it. The -15V regulator frequently fails and the amp goes completely silent, even the power amp stops working. There is a hard to find chip that manages the channel select and footswitch logic. The fix for that is a little piggyback module that will plug into the original PCB found here: https://forum.sunnstillshines.online/index.php?topic=6864.msg29374#msg29374

Edit: The Channel select LEDs on the front of the amp will stop working in both cases above. Normally if you plug into a channel, that channel's LED will come on, both will come on if you plug into the Both input. When the channel select logic goes bad, the power amp will still work if you plug into the Master Effects Return on the back of the amp but the Channel LEDs on the front won't work properly. Sometimes one channel may still work. The power amp has internal fuses that will blow if the power amp goes bad. In that case nothing will work except the light in the power switch. This usually indicates blown transistors in the power amp.

Edit: Jan 7, 2021 updated link to Sunn forum

Quote from: Roly on February 22, 2014, 12:15:48 AM
{willya lookit all those furshlugger trim pots!   :duh   }

It probably cost more for the trim pots than the JFETs. But the upside is the endless possibilities for tweeking. Nice to have one of those little plastic screwdrivers made for tweeking pots. 

Quote from: sjturbo on February 20, 2014, 04:43:40 PM
What effect would reducing the bias voltage's have on the gain stages?

This thing has lots of stages and most have some tone shaping between stages. There are three sections. The first section, Q2, Q3 should preserve a fairly clean signal and just add low amounts of distortion. 4.5V or 5V would be good for those stages. In the OD section, anything goes. This section is where the Ice Pick tone probably comes from. Try tweeking VR3 high and VR4 low, or vice versa. You can't hurt anything, just try to get a pleasing tone without the Ice Pick treble. The third section around Q7 and Q4 will have to be a compromise to get good tone in both modes. You should probably stay close to 4.5V but try anything. VR7 gives a treble rolloff, use it in moderation to get rid of fizz. I suggest you just adjust the pots by ear, take notes and then measure and write down the voltages. It may take some time to find what you like.

Thanks to phatt for posting the file here!

Some questions about your build:

1) Did you use J201s or some other JFET?
2) What is your Supply voltage? Have you tried any other?
3) How did you adjust all the trim pots? Have you tried tweeking the pots slightly, maybe a volt high or low from the "ideal"?
4) The slope resistor (R22) seems high, Can you install a trim pot there?
5) Regarding R32 (location F3), did you use 470 ohm or some other value?

Quote from: J M Fahey on February 16, 2014, 10:50:41 AM
Thanks for posting  :dbtu:
I would not worry at all about the displayed 2dB "loss" , considering bandwidth goes from 100Hz to 20KHz, a 2000:1 range or 8 octaves (1/4 dB / oct?) and would suspect minuscules losses added up along the signal path or even simpler, reduced oscillator output at higher frequencies.
Or all of the above.

In Reply #8 the J201 is almost prefectly flat at lower gain and bandwidth. That graph was made with the analyzer as a load. The flatness of the generator has been verified even on the JFET side of the Gate stopper. The output still isn't flat driving only a 10 Meg scope probe. It's not a measurement error or equipment malfunction.

The plot below shows various caps I tried. I built a second proto of the second stage only and run it from a 30V bench supply. I even scrounged around and found a MPF102 from a different batch and vendor, no change. The plot is the circuit driving just a 10 Meg scope probe, the AP analyzer is connected to the scope's signal output so vertical scale doesn't mean much but it's still dB. The input was 40mVrms and the output is about 8V pk-pk. You can clearly see the rolloff of 2dB from 100Hz to about 15KHz.

Quote from: J M Fahey on February 11, 2014, 02:49:17 AM
Very interesting and thanks for experimenting and posting.
I think one reason for the behaviour you noticed is not that the intput impedance "is not that high" , (by the way you don't show any input impedance graph or table), but the *output*  impedance is way too high.
The output impedance of a classic 12AX7 is around 40K , coming from a 100K load resistor in parallel with the tube's internal Rp, around 68K .
While here you have the *very* high load impedance provided by a constant current source (which in theory should be infinite, go figure), in parallel with the internal output impedance of a FET, much more similar to a pentode than a triode.
So gain is much more dependent on the complex impedance presented by the tone stack.
Which for a reason I don't understand you downscaled 10X in impedance.
I wonder why, since :
a) it still drives a 1M input impedance recovery stage
b) it's driven by a source impedance at least as high as that of the original triode stage, and probably much higher.
I'd retest with standard Fender values and see/hear what happens.
I'd also retest using TL072 gain stages, around 10X to 20X gain each, and standard Fender values, *or*  in this case, with 10X reduced impedance one if you wish, since the 1st stage will easily drive that and then some.

I have used that for ages with excellent results.
"Tubey"?
Dunno, define "tubey"

All the graphs from the Audio Precision System One were done with the tone stack disconnected before the preamp was installed in the amp chassis. The input impedance of the AP is about 100K ohms set to "unbalanced floating". The amp has the tweedish volume/tone setup shown on the schematic. It's a little different from the traditional setup so you can still get some treble boost when the volume is all the way up and there is very little interaction between Volume and Tone.

The output impedance of this configuration is set to a large degree by the voltage divider. The second stage still has the 100K voltage divider resistors. The output of that stage drops 1.2dB with 100K resistive load. For the first stage I lowered those resistors quite a bit to control the gain. This allowed a lowering of the Source resistor which increased low level even harmonic distortion. This is a big contributor to the tubey sound. You hear it at the lowest levels before the amp clips. It gives the tone a warm jangley quality that you just don't get with opamps. It's subtle and you have to train your ear to recognize it.

Another component of tube sound is compression and touch sensitivity. The primitive nature of this circuit, no feedback (almost), unregulated single rail supply and soft clipping make it FEEL more like tubes than opamps and clipping diodes. The first stage has very little compression but the second stage has plenty. I powered the preamp from one rail (decoupled) of a descrete power amp with a small power transformer. If you power the preamp with a regulated supply, the effect will be much less.

I don't have any way to actually look at the complex input impedance vs frequency. I was thinking at the time of my previous post that a non-linear Miller capacitance was the cause of the slow rolloff, but inserting a common gate JFET between the lower and upper JFETs eliminated that. Monitoring the gate with an oscilloscope confirms that the signal is flat at the input. Swapping Drain and Source terminals didn't make any difference. Small amounts (-3 to -12dB) of Source degeneration reduced the effect, but did not eliminate it.

At this point I have no explanation for why the frequency response falls off about 2dB between 100Hz and 10KHz when using MPF102's. I'd be interested to see what simulation shows.

I built up a preamp, made some changes and ran into an interesting problem. As near as I can figure the input impedance of the JFET with Soruce grounded is not as high as you would think. It causes the frequency response to slowly dribble down about 2dB from 100Hz to 10KHz before the dominate pole really kicks in. I didn't notice until I actually plotted the response. The MPF102 was the worst, a J201 gave the flattest response but less overall gain and bandwidth. I tried several JFETs that I just had laying around.

Sound is warm and somewhat tube like. I installed the preamp in an old amp I built some years ago with +/- 20V rails for the power amp. The preamp worked fine with no adjustments, I had used a 30V rail to get it running.There are better sounding preamps, but they are more complicated.

I just slapped the first stage together on a solderless breadboard. The gain measured about 35dB into the 100K input impedance of my analyzer. I suggest using lower value pots in the tone stack, 25K/25K/1K and 100K for the Volume. The circuit works quite well assuming the JFETs you use are reasonably matched, 5% or 10% should be close enough. The gain changes slightly as the power supply varies. Run it from a decoupled power amp supply rail to get some of the tube amp compression and touch sensitivity.

The circuit clips softly and generates mostly 2nd and 3rd order distortion until it starts to clip really hard, much like a 12AX7. I like to look at the X-Y transfer curve to set the gain, see the attachments below. The big resistor in the source makes the curve slightly more linear than a 12AX7 and I didn't try to tweek the DC operating point by adjusting R4/R5. Reducing them overall would also lower the gain but preserve some of the non-linearity. The source resistor can be bypassed to peak the treble, but install a resistor in series with the cap to limit the peak. Start with 3.3K.

As noted above the input stage has too much gain. But it's not the gain that is important, it is the clipping level refered to the input. Because the preamp runs off of a low voltage, the gain needs to be about 5 if operating off of a 30V rail, even less if the rail is lower. Once the signal passes through a Volume control, gain is not so critical, you can just turn the signal down. The only problem you might run into is the volume may be hard to adjust if the position of the volume control is too close to zero.

As described in the webpage linked above, the input starts to distort when the signal is about 150mV or 200mV RMS. A 12AX7 can handle about 1V RMS before it starts to distort. A resistor in the Source of Q1 will reduce the gain if the input stage to a level that will allow a bigger signal before distortion. Note that the polarity of C1 needs to be reversed. In the original circuit C1 and C7 would have zero volts across them if the JFETs were all matched. The circuit calls for 2N3819 JFETs, the MPF102 is almost an exact match except for the pinout. It won't sound exactly like a Blackface amp, but it's a start.

Without a bypass capacitor on the first stage, it's voltage gain has to be something less than 1/2. This is because the voltage across the Source resistor is more than twice as much as the Drain resistor. The current is the same so the resistance is proportional to the voltage drop. You would be better off just using a source follower. The advantage of the circuit is that the Source resistor sets the Drain current fairly accurately so you don't need a tweek.

With only 12V rails and the first stage only able to swing from the plus rail to a couple of volts, you want a gain of about 2 to mimic the headroom of a tube stage. A bypass cap with a series resistor would increase and control the gain.

The word clone is often misused. A clone should have the same DNA as the original. So for a Fender Blackface amp, the clone should use tubes. So what word would describe something that sounds like a Fender Blackface but uses FETs? I think Impostor (sometimes spelled imposter) fits. Definition: "one that assumes the identity or title not one's own in order to deceive"

Can anyone suggest another word?

22-3055 is the Sunn part number for the 2N3055. In the Concert Series power amp, the four transistors should be matched for lowest DC offset at the output. One of the transistors has a grounded collector so it doesn't need an insulator. A frequent failure on these amps is that a short develops in the driver transformer. The easiest way to find it is to remove the four output transisitors, power the amp up and measure the output voltage with no load. This will also confirm that the bias resistors have not drifted in value. Before you install the transistors, verify the base to emitter voltage at each transistor socket and that the emitter resistors are ok.

An attempt is being made to restore the old Sunn forum. At least I'm not getting the Page Not Found error when I go there. I can't login, but it's something 

Link: http://sunnforum.ampage.org/index.php