Peavey eddie van halen amp.

[Jazz P Bass]

J174's are typically used by Peavey for channel switching applications.
Or they may be used as a momentary mute when switching channels.

They are a 'normally closed' device.(low resistance)
(check one on your bench. Without a gate voltage, D to S is low resistance)

Also, they are a 'P channel' device which indicates that they require a positive voltage to turn them off. (high resistance state).
That is the 'clamp' voltage.

[catstrat]

5150.

It looks to me according to the schmatic the j 174s will bleed the signal to ground if they are turned on. Am i reading the schematic right? Thanks

[Jazz P Bass]

[QUOTE=catstrat;373151]I checked the amp with my scope again and it's the preamp that has the problem and not power amp. I ran a signal into the effects return and the o/p section sounds strong and has a good sine wave. QUOTE]


There is conflicting information here.
In Post #9, it was stated that the preamp was good.
It would be good to know:
-What signal amplitude did you use to 'prove' that the power amp is good? (ie: what does it take to drive the power section to 100W/@8 ohms)
-What signal amplitude do you get at the FX out jack? (with a 100mv/ 1K signal at the input jack & all controls dimed)

[Jazz P Bass]

[QUOTE=catstrat;373151]Can someone tell me what Q1 is supposed to do. The reason i ask, after spending some time with the schematic i checked Q1 and Q2 and the source and drain test as shorted. The schematic says clamp to the gate.QUOTE]

Let's see.
No posted schematic.
No indication where the Jfet's in question are located on the (unposted) schematic.
(Coordinates)

[catstrat]

5150.

Hi Jazz P. I've been so busy the last few months i have trouble remembering what amps i work on. The o/p section was tested with only about a 1 volt signal from my generator and the volume was a lot louder than it is when using the actual amps input. So i'm assuming the o/p is fine. The signal from the pre amp is almost non existant. You're probably going to need at least 8 volt AC signal minimum to drive the o/p to say 20 watts would be my guess. But i better double check that to be sure. This is only a 60 watt rms amp. I'm pretty convinced it's a pre amp issue. I've checked most of the resistors for the grid, cathodes on each preamp tube and they test fine. I've not checked all of the phase invertor resistors yet. I've done all of the tubes and cleaned all of the jacks and tube sockets. It is almost like it's in mute. That's the reason i brought up Q 1. If i remember the schematic right, i don't have it in front of me right now, if Q 1 is shorted then want it just bleed the signal to ground?
The other question, i know what Jfets are normally used for in most applications but i'm not sure what it does in this circuit. On the schematic the only thing it says is "clamp" written right where the gate is for the transistor symbol. I'll look at the schamatic tomorrow and find out where the trigger voltage is coming from. Bed time for me. Thanks.

[catstrat]

5150

Hi Jazz. I just ran across another thread and Q 1 is exactly what Enzo was suggesting. Q 1 is used for muting when the channels are switched so you don't get a pop. That amp had low volume to. It looks like from what they've said Q 1 is controlled by Q 4. I'll pull those from the circuit and test them. I'm running out of things to check on this amp. I'll do that tomorrow. Thanks.

[Enzo]

OK, I don't have the schematic in front of me. If I recall, Q1 and Q4 are both mutes, I don't recall one controlling the other. But they are wired together, so if one fails it can make the other not work.

They are mutes, simply remove them and see if the amp now works.

They are JFETs. it is normal for a JFET to measure low resistance, as in almost shorted when they are NOT powered. You have to put a voltage on their gate to turn them OFF. If you measure them with power off, good ones will check as shorted. The trick is to turn the amp on and measure source/drain resistance while it is running. If the resistance is still low, then either the JFET is bad or it is being held on by missing gate voltage.

[catstrat]

5150 Peavey

Thanks Enzo for explaining how this works. In this particular amp, you have 3 P C boards that are connected by jumpers and they are all 3 bent into a U shape to get them into the chassis. It's a pain in the butt. If i just applied 6 or 7 volts to the gate from an external power source, a battery, would that turn the jfet on and off so i could check it. What if i just leave Q 1 out of the circuit all together? When i switch the channel with the panel switch, the channel LEDs light up like they're supposed to. I believe the Jfet control voltage comes from the channel switch doesn't it? The schematic i don't think shows anything. The only thing the schematic says is the word "clamp" written next to the symbol. Let me look at the schematic again. Thanks again.

[g1]

Could you link to or upload the schematic you are referring to? Somewhere above you said 5150 II was it but I don't see those fet's on the schematic Tedmich posted.

[tedmich]

maybe cause I posted 5150, not 5150II (6505)

this should be it
http://schems.com/manu/peavey/peavey_5150-II.pdf

[g1]

That seems to be the same file as post #3, I don't see Q1 & Q2 fets, or any "clamp" reference?

[catstrat]

5150

They're about 3 or 4 different schematics out there on this. I told Peavey to send everything they had. I believe the one i have is the same as the 6505. Anyway, it turned out to be a shorted Q1 or a Q 4. I went ahead and replaced both Q1 and Q4. and now it works like a charm. Thanks everyone for your input on this.

[catstrat]

Hi G one If you look at the proper layout, if you can locate it, it will show 3 boards jumpered together with bus wire. Q1 and Q4 will be on the middle board between the Pot board and the pre amp tube board. Q 1 and q 4 are muting transistors. Thanks everyone.

[g1]

Ok, finally found a schematic with Q1 & Q4 mute fets, 5150 combo.
"Clamp" control voltage comes out of footswitch circuit below footswitch jack.

[Enzo]

If you have a three sided board like a Classic 30 then you must have a 5150 Combo, not a 5150. Oh, duh, he just said that. The mute circuits work the same in all those models, but part numbers may differ.