Alamo Paragon Bass/JFET transistors?

[joshdfrazier]

I have an Alamo Paragon Bass amp (ss pre, tube poweramp) with no output. Traced the issue back to a faulty preamp. It consists of four to-92 JFET transistors (TIS58). Can't find any data about these guys. Also, I've never encountered JFET transistors, only bipolar. No idea how to test them to determine whether or not they are bad.

By the way, the Ampeg BT-15 and Jordan Entertainer are working beautifully. Thanks guys!  :dbtu:

[Enzo]

well, they all work about the same, so other than the specs, you don;t need to find specific articles on the TIS58 to learn how they work.   Just google "JFET" and a bunch of useful things come up.

You can find data sheets and other information by googling "TIS58".


do you have the schematic to post?

[Loudthud]

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.

[joshdfrazier]

No schematic available that I could find, unfortunately. Found the pinout of this particular transistors here: http://www.theremin.us/144/142transistors.htm

I made some notes when testing the transistors with the diode test function on my dmm. They all tested the same:

source to drain - .133v (both directions)
source to gate  - .733v (no reading with leads reversed)
gate to drain   - .733v (no reading with leads reversed)

Something concerns me, and I'm not sure if it's related to the problem, but the amp uses a Mallory can type capacitor with the values, (2x) 20uf, 450v, (2x) 100uf, 50v. The cap is getting extremely hot. At least as hot as the 6L6, which are running with nearly 500v on the plate. I have these values on hand so recapping will not be a problem, but wondering if these things could be related.

[J M Fahey]

JFET preamps are usually simple and designers tend to apply tube ideas to them, so hand drawing the preamp (start by drawing whay tou see of its layout) should not be very difficult.

[nashvillebill]

...source to gate  - .733v (no reading with leads reversed)
gate to drain   - .733v (no reading with leads reversed)
....

Gotta quote Enzo here from some of his earlier threads... "what do you mean by "no reading"?  There's no such thing as "no reading".  Either it's open (infinite resistance) or shorted (zero resistance) but regardless, that's a reading.  Which is it?"

I know it sounds like I'm nitpicking but over the Internet some things aren't clear.  (Not trying to bash on you, again, so don't take it personally) 

[g1]

  As far as the hot cap goes, you said you have 500V at the plates, but the cap is 450V.  This is why it is overheating.  With modern AC line voltage, original caps are often rated at less than what is needed.
  You will need higher voltage caps, or "totem-pole" arrangement to work safely.

[JHow]

My beloved Randall uses TIS -58 or also 2N5484.  The latter used to be available at mouser, but lately is listed as obsolete.  Also pinout is different:  tis-58 is D-G-S, 2n5484 is G-S-D.  I have bent the pins to make it work in a pinch.

if you were designing from scratch I guess you could use sot-23 MMBF5484, potentially you could use mmbf5484 in a breakout board and retro-fit to your circuit but that seems like way too much work.

[Roly]

I would respectfully suggest that the best place to test these FET's is in-circuit.  It would be a lot more straightforward to simply measure the Drain voltage and see if it's about half supply.  (er ... you do have supply on the preamp I s'pose?)

The TIS-58 is an N-Channel FET that looks pretty similar to a MPF102/2N3819.

http://www.datasheetarchive.com/dlmain/Datasheets-X2/DSA896000-48.pdf

You have four amplifier stages that look like the attached.  On the Drain, the junction of the FET and R3, you should have around half the supply voltage.  If it is very low then that FET is shorted, and if it's almost the supply then that FET is open, and if it's about right there is nothing wrong with that stage and you should look elsewhere for why signal isn't getting through.

Don't bother trying to measure the Gate - you won't get sensible results.  The Source, where R4 is connected, should be just a few volts, say somewhere between 1V and 5V at a guess.


Ed typo