# Peavey 6505 Volume Loss



## sammyr (May 7, 2013)

I'm tinkering around with a peavey 6505 combo thats having severe volume loss issues. It's a huge PITA to get the circuit board out so I'm curious if anyone has anyone experience with peavey 5150/6505 amps. 

I'd read a bunch about issues with the effects loop jacks but I think I've got the problem isolated to the preamp. The plugging a guitar into the effects return gives plenty of volume and the power tube voltages all look in the right place. I've easter egged preamp tubes to no avail so far. There are some filter caps that are looking worse for wear but the b+ seems to measure in the right range on the preamp filters so I don't think that's the issue. 

I'm going to take voltage measurements at the preamp sockets to see if that yields any insight before I commit to going through the challenge of getting the board out. 

Thanks in advance


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## Budda (May 29, 2007)

have you tried swapping the power tubes?


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## sammyr (May 7, 2013)

I haven't swapped the power tubes yet. they seem to be biased properly and given the effects return working well I'm assuming the problem is somewhere in the preamp.


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## Budda (May 29, 2007)

Try the power tubes anyway, takes a few minutes.


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## sammyr (May 7, 2013)

no change with a new set of power tubes. I've finally got it apart so I can look at the circuit but unfortunately nothings jumping out at me. next step is going to be to look at the cathode resistors and caps and see if any are off spec and go from there.


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## sammyr (May 7, 2013)

update: wrestled the board loose and was able to give it a closer inspection. found a cold solder joint on the ext speaker jack ground tab although it had two pins and the other pins joint was good. reflowed a bunch of other suspicious looking joints. I don't know if it will cure the problem but might as well get as much done before I try to wrestle the board back into place. The last filter cap is leaky but strapping a good cap across it didn't change the symptoms but its getting replaced before it has a chance to really fail. Also noticed that there are some small electrolytic bypass caps that warrant further inspection. throwing a meter across em to get a quick and dirty read on the resistance showed some low numbers and didnt seem to charge up. so those are going to get desoldered and tested and then ill likely button it back up so i can take some voltages


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## jb welder (Sep 14, 2010)

Had you checked pre-tube cathode & plate voltages earlier or no?
And I think it has series heaters for some of the pre as well.


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## sammyr (May 7, 2013)

now that ive got it back together I'm going to try to take some cathode and plate voltages. I think you're right about the series heaters. peavey just got back to me with a schematic ill upload it in a sec


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## sammyr (May 7, 2013)

not entirely sure how I'm going to be able to get measurements as I can't really measure from the sockets and the circuit board is 3 boards strapped together in a c shape. going to have to use the ol' noodle. with all the preamp tubes out volts i checked to see if b+ was making it to th eplate pins at the socket and its definitely there. may have to make a socket adapter to take real measurements i don't think I can reach with a probe.


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## jb welder (Sep 14, 2010)

Is this the right schematic?
Sometimes I'll put wires in the socket pins and then stick the tube in there along with them. Fine wires will usually have enough room to get in there with the tube pin. Insulation stripped back a little at each end of the wire. Then you can check with the meter. Be very careful.
This way you can get voltages with the tube running, rather than having to remove the tube.

If this is the correct schematic, this one doesn't use the series heaters which makes things easier.
They are DC heaters in the preamp section though.


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## sammyr (May 7, 2013)

thanks jb thats a great idea the model im dealing with is a 6505 212 as opposed to a 6505 plus. definitely still dc heaters on the preamp. i have the schematic but I'm having trouble finding a way to attach it and not lose all the resolution. In the meantime I have some screenshots of the preamp area in the meantime


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## sammyr (May 7, 2013)

thanks again Jb I was able to get a small piece of insulated wire into the tube socket and seat the tube and get a measurement. I've only gotten to probing v1 but so far on v1a i've got 160v (only 10v off) on the plate and 1.38 volts on the cathode, on the v1b side however i've got almost 370volts on the plate and 1.8 volts on the cathode so that's way off. I'm going to continue probing down the line to see if the other tubes that use the v2+ supply are in spec or off as well. then ill hopefully be able to narrow it down to the v1b tube stage or somewhere in the v2+ line.


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## jb welder (Sep 14, 2010)

Same as this I guess?


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## sammyr (May 7, 2013)

jb welder said:


> Same as this I guess?


damn you're good. I need to up my attachment game


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## jb welder (Sep 14, 2010)

Check resistance from V1b socket pin3 to ground.


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## sammyr (May 7, 2013)

pin 3 to ground is 1.8k


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## jb welder (Sep 14, 2010)

Very odd to see 370V on that V1b plate. Shouldn't even be that much at the V1+ node of supply. I'm wondering if it's oscillating and making your meter read funny. A problem with relay K1A could make V1B oscillate. Maybe try grounding the grid and see if the plate voltage goes to normal.


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## sammyr (May 7, 2013)

I think you're right I reset the wire and the tube and this time the plate voltage was on spec. moved on to v2a and found the plate voltage was a little high and the cathode was around 4 volts. going to try to look at v2b before i call it a night


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## Guest (Sep 2, 2018)

You guys are good at this.


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## sammyr (May 7, 2013)

If i had a nickel for everytime I got advice/help from JB welder I'd certainly have a few nickels


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## jb welder (Sep 14, 2010)

I think a few is more than a couple so I'm feeling pretty good right now. 

I just re-read the 1st post and I think you still need to try the loop patch thing. Connect the fx send to fx return with a patch cord. See if the signal is still weak. Plugging guitar into the return only proved the power amp section is good. The problematic switching contact in the return jack is only in circuit when the return jack is NOT in use.
(alternately, you could run the fx send to another amp's fx return to see if the pre-out exhibits the fault)

Here's an article that explains the problem with switching jacks used in loops, inserts, and elsewhere.


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## sammyr (May 7, 2013)

Thanks again JB. I did try ruinning a short patch between the effects loop jacks and giving them a good cleaning but there was no change. I did try retensioning the jacks but was going to just go out and get some replacement jacks and shotgun them. I haven't tried plugging from the effects send into another amps loop definitely worth giving a shot before I do anything else.


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## sammyr (May 7, 2013)

I don't have any guitar amps with an fx loop but I have a keyboard wedge that i plugged the effects send from the peavey into and got the same low output distorted sound so that isolates the wonkiness to the pre-amp


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## jb welder (Sep 14, 2010)

Q1 up above the send jack on schem. is a mute fet. They are often problematic and will reduce or kill the signal when bad.
Being as difficult to work on as that amp is, I would just clip it out. Amp will work the same, just may have turn on/off thump. (or channel switching thump)
If it cures the signal issue, you can replace (or leave out if you can live with the thumps). If it does not turn out to be the problem you can replace once you are in the amp repairing the real problem.
(it could also turn out to be the control signal to the mute fet, but removing it will still tell you if it is a mute circuit fault)


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## sammyr (May 7, 2013)

another good idea jb. I had read a about issues with the mute circuit but was hesitant to clip out the transistor until i got to the store to get a replacement but ill be passing by this afternoon so I might as well go ahead


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## sammyr (May 7, 2013)

unfortunately clipping out q1 didn't have any effect. looks like ill have to get back to probin'


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## jb welder (Sep 14, 2010)

Problem is regardless of channel selected? That would isolate it to something common to both.
Reminder that the tube numbering is screwy so go by the component layout for tube numbers. PI (V4) and V3A are after the return jack so they are ok.


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## sammyr (May 7, 2013)

the problem affects both channels and yeah the tube layout is...funny. so id assume we're looking for a problem in the v1 2 or 5 sections


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## jb welder (Sep 14, 2010)

Or V3b.


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## sammyr (May 7, 2013)

got back to probing and found a dead tube or a tube i may have killed somehow during the probe. most of the preamp tubes are groove tubes so it wouldn't surprise me that at 140v on the cathode it crapped out. ironically putting a good tube in its position didn't solve the problem. probed v3b and got 140v on the cathode and 340v on the plate. v2+ is only supposed to be 300 so I'm going to continue probing down the line.

at this point I would be pretty rattled if it turned out to just be a bum preamp tube lol

update: no voltage at v5a plate. getting closer... going to do some closer probing but the lightened loading might explain why voltages seem high all around

update 2: removed r13 v5a plate resistor and measured it and its showing infinite resistance so think we may have this solved finally. cross your fingers everybody


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## sammyr (May 7, 2013)

and we're in business. put it back together and its back to being screaming loud. what a relief. now I can take it apart AGAIN and retension the socket pins and reflow the solder connections for the tube sockets to make up for all that tube wiggling and wire jamming.


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