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Discussion Starter #1
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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Discussion Starter #3
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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Try the power tubes anyway, takes a few minutes.
 

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Discussion Starter #5
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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Discussion Starter #6
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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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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Discussion Starter #8
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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Discussion Starter #9 (Edited)
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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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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Discussion Starter #11
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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Discussion Starter #12
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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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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Discussion Starter #18
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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