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.

[nevetslab]

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.

As you've already swapped out tubes, and the voltage measurements at each tube (with the exception of V3 & V4) is in the ballpark, do you have access to an oscilloscope & signal generator? When all else fails, I power up my test bench and go looking to see where the signal gets bent up. I've never been inside the SVP, though it looks a lot like the circuit used on Ch 1 of an SVT-VR, with the midrange circuit from the SVT-CL.

I'd look at what's happening at the plate of V2B, where it feeds V3A's grid, and on thru V4. As well as V2A, who's plate and cathode voltages are low. That's where your DC voltages are not matching the nominal published values. I'd also be looking at the bottom of the PCB for solder joint fractures thruout the PCB. If typical Ampeg mounting practices, the front side of the PCB is supported by the pot PCB solder terminals, where I always find radial fractures. If there are connectors where we see all of those symbols where the tube plate/grid/cathodes pass thru the schematic rectangles, I'd look there too. So many of the faults I find on Ampeg SVT tube gear is mechanical in nature.

[fuzzman72]

Thank you for your reply.

I've checked all the connections between the tube board and the main pcb and they're all ok.
Now I'm focusing on the first issue in the chain, V2a.
As you can see from the schematic I've posted the cathode voltage on pin 3 is completely out, showing 1,5V instead of 5.
I have isolated this stage removing C40 and C47 and tested all the components involved, but nothing is changed, the cathode is still at 1,5V.

I'm quite puzzled

[nevetslab]

Thank you for your reply.

I've checked all the connections between the tube board and the main pcb and they're all ok.
Now I'm focusing on the first issue in the chain, V2a.
As you can see from the schematic I've posted the cathode voltage on pin 3 is completely out, showing 1,5V instead of 5.
I have isolated this stage removing C40 and C47 and tested all the components involved, but nothing is changed, the cathode is still at 1,5V.

I'm quite puzzled

Does the cathode voltage change when switching in and out the Ultra High function? I see 220k plate & 3.9k cathode resistors on this stage, so the gain is near that of the input stage V1A, only it doesn't have the 22uF cathode bypass option. Your voltages on V1A are nominal. What happens if you swap V1 with V2?

[fuzzman72]

Switching the ultra high switch doesn't change the cathode voltage.
I've swapped the the tubes obtaining the same results.