Peavey 6505+ Combo Lead Channel Cutting Out

777pulse777

Hi all,

I am having a problem with my Peavey 6505+ 112 Combo.

The lead channel drops in volume really low after about 20 - 30 minutes of playing. The clean/rhythm channel works perfectly though, leading me to believe the power amp section is still fine.

I was thinking this could be a faulty preamp tube but I have replaced all the preamp tubes and it still does this.

Any advice would be greatly appreciated.

Thanks!

EDIT: If I turn the amp off and leave it for a while I can use the lead channel for about 20 - 30 minutes again. Might be heat related?

ezietsman

Could be a dirty pot maybe? Sometimes with these modern amps they kinda unsolder themselves or shake loose in places. If that's the case you'll have to take it to a tech.

777pulse777

Problem is I stay in Ladysmith, which is like almost 3 ours drive from Durban, which seems to be the nearest place with people who actually know anything about tube amps.

If it's a dirty pot, or unsoldered, got any suggestions?

Thanks again

ezietsman

I'm not going to tell you to open it up and poke around inside, they're not safe if you don't know what you're doing, even when they're off.

That said, dirty pots can be cleaned with contact cleaner, you get them in aerosol cans. Its a non-conductive thing that evaporates quickly.

If that's not the issue then I'm afraid it's tech-time.

morph

ez wrote: That said, dirty pots can be cleaned with contact cleaner,

http://www.communica.co.za/catalog/Browse?cId=744f140e7f8a46c5945dd90277b7c3bc

777pulse777

Thanks for the help guys. I will try the contact cleaner on all pots and see if the problem persists.

Just a note: If i turn the amp off and leave it to cool down, then I can use the lead channel again for about 20 - 30 minutes before it happens. Might this be heat related?

ezietsman

777pulse777 wrote: Thanks for the help guys. I will try the contact cleaner on all pots and see if the problem persists.

Just a note: If i turn the amp off and leave it to cool down, then I can use the lead channel again for about 20 - 30 minutes before it happens. Might this be heat related?

If that is what happens EVERY time, and you can reproduce it then it sounds like it could be heat related. Maybe something is loose or cracked and then it expands and loses contact.

You'll have to open up the amp to spray the pots, DO NOT TOUCH ANYTHING INSIDE. I'm not kidding. The amp can still kill you even if it is off and plugged out of the wall.

777pulse777

I have now confirmed that it definitely does happen every single time I play after 20 - 30 minutes. (I didn't really time it)

I sent the schematics to a tech in Durban and explained to him the problem, and this is what I received from him:

"My first guess would be a fault centred on MOSFET Q2 (2SJ174). This seems to be used as a muting device, probably to avoid switching thumps when changing channels.

Q1, Q2, Q3 and TR1, along with their associated resistors (R5, 6, 15, 37, 45, 60) and capacitors (C17, 18, 21) appear to control Q2 and I imagine that one of them, possibly Q2 itself has gone upside-down."

I have absolutely no clue what any of that means, so I will be looking for a tech closer to me to check it out. I did find a guy who plays guitar and works in electronics and that has, according to locals, fixed many an amp before. So I will ask him to check it out.

Will reply back with updates.

Thanks for all the help guys.

Pierre-Cronje

777pulse777 wrote: Hi all,

I am having a problem with my Peavey 6505+ 112 Combo.

The lead channel drops in volume really low after about 20 - 30 minutes of playing. The clean/rhythm channel works perfectly though, leading me to believe the power amp section is still fine.

I was thinking this could be a faulty preamp tube but I have replaced all the preamp tubes and it still does this.

Any advice would be greatly appreciated.

Thanks!

EDIT: If I turn the amp off and leave it for a while I can use the lead channel for about 20 - 30 minutes again. Might be heat related?

Just looked a the circuit diagram. It will be an easy fault to resolve, but you need a signal generator and an oscilloscope. The problem "should" lie around V3/V4. Need to inject a signal, make sure the setting is on lead, and then measure all the levels in the lead signal path. Measure all levels after about an hour and the fault will be found. Keep the master volume very, very low otherwise the tone will drive you mad. Good luck, I trust you find a competent repair man.