Slowgear Transistors...equivilents?

[Paul Marossy]

So, has anyone anything to report on FET/zener diode combos that work? I have a bunch of different FETs and zeners to try. I'm thinking that an NTE459 with a NTE 5014 6.8V zener diode may work. I will try this out in the next day or two and report back.

[Paul Marossy]

OK, here's an update. I'm a little frustrated because nothing seems to work.

Here's what I have tried so far:

FETs - 2N5457, NTE 459, 2N3819, J201, BS170
Zeners - 4.3V, 4.7V, 5.1V, 5.6V, 6.2V, 6.8V and 7.5V

None of the combinations I have tried so far has worked. Obviously, the VGS(off) for the J201 is very low, around -1.5V max. and the 2N3819 is very high at around -8.0V max. I was expecting the 2N5457 or the NTE 459 to possibly work, but they either let the guitar thru completely or almost not at all - the settings on the attack and sensitivity seem to have no effect whatsoever. The opamp has the correct voltages on it and the transistors are all oriented correctly. I have made sure that all the pinouts were correct on the FETs, tried increasing the supply voltage to 10.5V - nothing works. One thing I thought was quirky is that when I ground the gate of the FET, signal comes thru load and clear.  :shock:

EDIT: The Toshiba 2SK30ATM data sheet I found shows VGS(off) at -6V max.

So, am I an idiot or is there a mistake on the PCB layout or something?! (please be nice now  :wink: )

I really would like to get this thing working.  

[Zero the hero]

Check tension with DMM at gate of the FET.
Then play some chords or notes on your guitar and measure he gate while playing. The tension should get lower as you play louder.

When the FET gate is grounded (0 volts applyed) the resistance between drain and source lowers to 0, so it's normal to hear your guitar load.
Post you gate values!

The circuit board is correct, I succesfully built a unit with this layout. Anyway, you can try to check all connections on the layout to see if everything is correct... it should be a good practise before building anything. I NEVER trust anything found on the internet.
Putting errors in projects is very easy, and accurate checking should be the first thing to do before beginning a project.

[R.G.]

It might be worthwhile measuring the Vp of the actual JFET you're using ala the recent threads on matching. The more-improved JFET matcher, both versions, measures Vp.

The whole trick to this is that the zener voltage has to be slightly bigger than Vp so the JFET will turn off.

2SK30A is usually the "gr" for green selection set, and that has Vp of 3.5-4.5V, I think.

Good subs are 2N5292 and 2N5485. J201 is not a panacea, as it's Vp is as low as 0.2V, making it relatively hair triggered for gain modification.

[Paul Marossy]

The circuit board is correct, I succesfully built a unit with this layout

Thanks Zero, I was just trying to eliminate that as a possibility.  I was pretty sure it was fine, but thought I'd ask... :wink:

The whole trick to this is that the zener voltage has to be slightly bigger than Vp so the JFET will turn off.

When you say slightly, how much is slightly? Also, is Vp the same as VGSoff? The Toshiba 2SK30ATM data sheet that I have does not list a Vp parameter...

2SK30A is usually the "gr" for green selection set, and that has Vp of 3.5-4.5V, I think.

My data sheet shows R 0.30~0.75, O 0.60~1.40, Y1.20~3.00 and GR 2.60~6.50...

Good subs are 2N5292 and 2N5485. J201 is not a panacea, as it's Vp is as low as 0.2V, making it relatively hair triggered for gain modification.

Hmm... that's some other ones to try. I have already concluded that the J201 is not a good candidate for this circuit. They worked just fine in my Phase 45, though!

Anyhow, this morning I was measuring voltages on a few different FETs, looking for a voltage of ~5.22 on the gate. The closest I could get to that is about 5.5V on the gate of a BS170 with a 5.6V zener. The drain and source also measured somewhere around 5.5-5.6V, IIRC. At this point, I am not sure what to think - maybe I have a bad cap or a faulty solder joint somewhere?

[R.G.]

BS170 is a MOSFET, not a JFET. You need a JFET.

[Paul Marossy]

BS170 is a MOSFET, not a JFET. You need a JFET.

Oh, now I feel stoopid because I know that!  Too early in the morning I guess. :oops:

Actually, that brings up a question I have been wondering about for a while: can you use a MOSFET as a variable resistor like you can with a JFET?

[Paul Marossy]

OK, one last question:

Using a 5.6V zener diode, on the FET, if I measure 2.8V on the gate, the source/drain will measure around 5.5 volts. This has been pretty consistent with all the FETs I have tried so far - the gate voltage is consistently roughly 50% of the voltage measured at the drain and source.

So, how am I to interpret that? Does that mean that the FET is stuck in the "off" position? If it were working like it was supposed to, what kind of voltages should I be seeing on the drain/source?

[Paul Marossy]

Yes, I am digging this up again.  

I got my hands on some 2N5485s and that doesn't work, either. I'm
beginning to think that perhaps I have a bad cap or something. Any ideas on what I should look at?

[Paul Marossy]

OK, so I decided to try and get this stupid Slow Gear working again after like 7 years have passed when I first built it. It's the only thing I have ever built from a verified PCB layout that I have not been able to get to work, so I want to get it working just because. So I got some 2SK30s and the $#*@^)! thing STILL does not work. I've double checked everything, no mistakes in parts used, no bad solder joints, etc. All it will do is pass a guitar signal with the 2SK30 out of the circuit. As soon as I put any kind of FET in the circuit, it remains completely silent whether or not the diode is in place. So apparently the FET never turns on, no matter what kind I've tried using.

As I understand it, the voltage on the gate of the FET is supposed to sit at 5.22V and go down to 2.88V when guitar signal is applied. So I measured the voltages at the place which FET would be installed, but with no FET and it measured: S=7.97V G=3.94V and D=5.48V. When I put the FET in circuit with a 5.6V zener diode in place with no guitar signal in place it measures: S=5.47 G=2.70 and D=5.46V.

Can anyone tell me how I should interpret these voltages and what I might try to get this working? I'm just about ready to take this circuit board out into the desert and use it for target practice. Very frustrating.

[Paul Marossy]

Never mind, I found the source of my problem. I got the polarity wrong on C10 (1uF electro cap) which is connected to the opamp & sensitivity control. So it appears to me that the actual value of the zener isn't that critical. It looks like that diode controls the decay on the signal. It works with a 6.2V zener as well, but the 5.6V zener seems to work the best with a 2SK30 as it makes the decay the smoothest. It only took 7 years for me get it right.  OK well it's been sitting in my circuit boneyard for that long. 

[rousejeremy]

A 2 year and 7 month debug. This is inspirational. I have an MXR Blue Box that never worked from about 3 years ago. Maybe I should get it back out.

[Paul Marossy]

A 2 year and 7 month debug. This is inspirational. I have an MXR Blue Box that never worked from about 3 years ago. Maybe I should get it back out.

Ha ha, I feel dumb going over the same mistake over and over and over and never finding it. None of the polarities were marked on the PCB layout, so I simply just got one of them wrong. Live and learn...

[rousejeremy]

Don't feel bad man, my weakness is mixing up 1k and 10k resistors.

[Paul Marossy]

Don't feel bad man, my weakness is mixing up 1k and 10k resistors.

Yeah this kind of stuff happens to everyone once in a while.