Peavey 6505 Head problem

[Trollmakt]

Tried to change all tubes .

No change

Still the loud schreeching and crackling when the red connector is connected.

I guess this is the feedback loop
When I use the clean channel it is worse. when I use overdrive I have to turn it up and the higher it gets, then the shcreeching gets louder

The sound is not crackling when it is disconnected, but the sound is unclear and really high pitched

Can mention that it is just as bad through the effects return

Any ideas?

Been checking the schematics really close .
https://www.thetubestore.com/lib/the...-Schematic.pdf

And what I can make out of it is this:

There is a signal sent from the speakerjacks through J101/98 connector on the FEEDBACK that goes to J71/70 on the Tube breakaway board . Through R56 then to NEXT3 over to Main board
Then to connector J16 - J34 . This is connected to the Resonance potmeter and feeded back to the signal coming from Presence pot meter to NEXT2 going to Tube breakaway board and then into the Phase Inverter again

This crackling sound I get is only there when the J101/98 to J71/70 is connected with the white wire with red connectors.
So it works with a more high pitched sound when the feedback loop via the resonance potmeter is disconnected

Could this be a bad potmeter
or maybe the capacitor C12 ?
R56 is measuring ok

The weird thing is that I get the same bad sound through the effects loop . And this bypasses the Presence and resonance part.. going past it and into V3 preamptube and over to the phase inverter
The potmeters have no effekt on presence and resonance on the guitar sound , but the crackling is affected when turning them both. Less crackling when Presence is all the way up

This is the sound I get from the amp when it is connected
https://drive.google.com/open?id=1gm...RSCtoVs2WFouQ8

[Valvehead]

if you swapped ALL tubes and it does it...have you tried playing it while poking around / moving wires / pressing on the boards ? with a pencil or chopstick of course .

Might be a loose connection or cracked solder joint? I would pull the pcb's out and inspect if possible

[doombass]

The weird thing is that I get the same bad sound through the effects loop . And this bypasses the Presence and resonance part.. going past it and into V3 preamptube and over to the phase inverter

I think you might have made a misinterpretation of the schematics. The effects loop (through R85) and presence/resonace circuits share the same grounding point, that's all, there's no bypass. It would have been more obvious if presence/resonance had been drawn on the same sheet as V3 cathodes.

[Trollmakt]

Still havent figured this one out.

If I turn Presence on full it is better on lower volumes . But problem gets worse when I turn the amp further up

[doombass]

if you swapped ALL tubes and it does it...have you tried playing it while poking around / moving wires / pressing on the boards ? with a pencil or chopstick of course .

Might be a loose connection or cracked solder joint? I would pull the pcb's out and inspect if possible

Have you tried this ^^^^ yet? You have narrowed down the problem to the feedback loop/presence area so I think you have a pretty good chance to find the cause. Also if you have tried the resonance pot fully cw/ccw you have only about five components left to check.

[strato56]

Have you checked the ground connections between amp and speaker output PCB ?
Does using another position of the impedance switch changes anything ?

[Trollmakt]

Have you checked the ground connections between amp and speaker output PCB ?
Does using another position of the impedance switch changes anything ?

No change when I switch impedance

[DFuller]

That red ended wire is the negative feedback connection. If you're getting oscillation it means the output transformer primary is backwards.

[Trollmakt]

And what does that mean? A connection cable that is reversed?



Which is primary and secondary on this schematic?

[g1]

Primary is shown as left side of T2. To reverse primary swap wires at J1 & J3.

[Enzo]

The brown, blue, and red wires are usually on a molex plug at the end of the power tube board, and can often be rotated 180 degrees if it is backwards.

[Trollmakt]

That worked!

[Trollmakt]

The brown, blue, and red wires are usually on a molex plug at the end of the power tube board, and can often be rotated 180 degrees if it is backwards.

That worked!!

[Enzo]

Great!

Dfuller actually suggested it first.

[Trollmakt]

Great!

Dfuller actually suggested it first.

indeed , but not as understandable for noobs