Ampeg SVP-Pro troubleshooting Need Help

[fuzzman72]

Hello

My Ampeg Preamp SVP-Pro started to distort heavily, so I opened it up I take some voltage measurement.
They are out especially on V2a and V4(12AU7), but I can't understand why.
I've already changed the tubes with good known ones with no results, checked resistors and capacitors in the circuits but I didn't found any faulty components.
I've added the schematic with original Ampeg voltages in BLACK and my voltages in RED.
I Hope that someone could help me, since I'm running out of ideas.

[Jazz P Bass]

That scan is impossible to read the original values.

Here is a clear one: https://music-electronics-forum.com/...3&d=1501961567

V2 pin 3 is way off.

All of the other voltages are off but not by much.

Did you check the power supply nodes for excess ripple (Vac)?

[fuzzman72]

Thank you for your reply.
Here is a more clear schematic with original voltages added.

I haven't measured the ripple at voltage nodes yet.
Do you suspect a bad filter capacitor?
I had the same suspect too, but the B+ voltage is in the ballpark.

P.S.
I've realized I posted the thread in the wrong section.
I'm trying to move it to the troubleshooting section.

[andy3055]

V4 Pin 1 is down by 50Volts? Check R5. May have gone high. Also V4 Pin 8 is way off.

[nickb]

I don't there is anything wrong with your measured voltages. The "Ampeg" numbers are patently wrong. According to them the current through the 221k plate resistor is (275-124)/221 mA = 0.68mA. That would make the drop across the 3.9k cathode resistor 2.7V. not the claimed 6.5V. Eyeballing the curves for a 12AU7 tells me your measured numbers are OK.

So, what's causing your problem? Can you feed the tuner out to a different amp? Repeat using the FX send. Now try plugging your guitar into the FX return. Let us know the results.