What is this component?

[Randall]

Working on a "77 Peavey Classic 212 that cuts out with a pop once in a while in both channels. It has only done it maybe 3 or 4 times, but I have confirmed that during the cut out I have signal at the emitter of the transistor just before the tone stack above the notes at lower right. (It would have been nice to have component designations). I have a dual trace scope on it now looking at that emitter, as well as the what I think is the gate of the FET looking component labled 558 ollowing the treble wiper.

My question is what exactly is it and what is it's function? I can't see what an FET would do there in this configuration.

http://adrsoundsense.co.uk/technical...B%20series.pdf

[Helmholtz]

Working on a "77 Peavey Classic 212 that cuts out with a pop once in a while in both channels. It has only done it maybe 3 or 4 times, but I have confirmed that during the cut out I have signal at the emitter of the transistor just before the tone stack above the notes at lower right. (It would have been nice to have component designations). I have a dual trace scope on it now looking at that emitter, as well as the what I think is the gate of the FET looking component labled 558 ollowing the treble wiper.

My question is what exactly is it and what is it's function? I can't see what an FET would do there in this configuration.

http://adrsoundsense.co.uk/technical...B%20series.pdf

"558" is a JFET in standard common source amplifier configuration. Advantage over BJT is high input impedance.

[Enzo]

Tone stack. To the left is a NPN BJT, type 953 (says so in notes), emitter follower to drive the tone stack. On the right is the 558 JFET. It is simply a gain stage to make up for the losses in the tone stack. The 558 amplifies the signal and feeds it to another 953 emitter follower driving the master volume control.

I hope at some point you have downloaded the Peavey semiconductor master cross reference, I know we have posted it in the past.; You can look up a generic cross for the JFET.

I dislike comparing transistors and tubes, but JFETs pretty much work like a triode. In our amps we usually see them as switching elements. They switch channel paths on and off. But they certainly have linear modes too - like triodes do. They amplify.


And just a suggestion, and especially with Peavey amps, it is good to stick with just the model name. Yours is Classic. In this case I don't think there was a Classic 212. (I know you meant it as descriptive) But in several other models, the Model and the Model 212 would be different amps. People call old Classics "Classic 50s" all the time, because they are 50 watt amps. But the "Classic 50" is an entirely different amp from the Classic.

[Randall]

Thanks Enzo, makes sense now. I will look for the Peavey cross reference.

BTW it says 212 Classic on the rear panel, fwiw.

[nosaj]

This is what i found for a classic 212 ( a 4 el84 amp)
nosaj

[vintagekiki]

http://adrsoundsense.co.uk/technical/

[tboy]

This is what i found...

I don't think that link worked as you wanted it to.

[Jon Snell]

It is an SPF-558 or 2N5951. A Jug FET type PN4117 will work as the originals are totally obsolete.

[vintagekiki]

What is this component?

PV Semi Cross Ref 2009 (page 21)

576/ 70400576 (40V P-CH) = 2N5461/ 2N5462/ SPS-3008/
558/ 70405951 (25V N-CH) = 2N5951/

https://www.freestompboxes.org/viewtopic.php?t=2361 ^{1)
_{Looking for a MPF102/2N5951 subsitute}}

[g1]

edit: not 6BQ5 version
see post #1 schematic

[Randall]

Error in the Peavey cross reference. While replacing the JFET, I figured I would replace the following SPS5953 transistor while I was in there, just in case it was the problem and not the JFET. Peavey lists MPS-8097, 2N6520 and SK3919 as replacements, Mouser had the 2N6520, so I added one to my order. But, when it got here it is a PNP, not NPN like original is drawn. I can't find any documentation on the original, but the other two replacements are NPN, so I'd say Peavey erred here.

I just ended up replacing the obsolete JFET with a J231. It has played music for 4 solid hours without cutting out, so I am going to call this one fixed.

[Jazz P Bass]

Well the MPS3097 IS an NPN transistor.
Can't find any info on the SK3919 (the Z is a mosfet).
So maybe the 2N6520 was a misprint.
The 2N6515 or 2N6517 is an NPN transistor.

[Enzo]

We have spotted this before.

[vintagekiki]

Can't find any info on the SK3919 (the Z is a mosfet).

https://html.alldatasheet.com/html-pdf/260937/KEXIN/2SK3919/150/1/2SK3919.html

https://datasheet4u.com/share_search.php?sWord=SK3919

Maybe it can be helpful.
I noticed that at some series of transistors, missing the number in front of the label (in this case 2)