Rivera 100 duo twelve 2 preamp tubes not lit

[jaymann]

I've been troubleshooting a problem with a Rivera 100 duo twelve.
V1 and half of V2 don't light up. (12ax7)
I swapped the tubes with other 12ax7's that were lit up in the amp, and they don't glow either when put in the V1 & V2 positions.
I checked a schematic and the filaments are supposed to get +18vdc on one test point and -18VDC on the other.
The +18 side was registering only a couple volts. -18 side was right on.
I checked the voltage regulating ic and it was getting 28V in and only putting out 2 V
I replaced the ic and the tubes lit up. The test point showed the correct +18V
Then the tubes went dark again. Now the new ic is only putting out 8v.
The only other thing I see on the schematic that comes before the voltage regulator is a bridge rectifier.
The schematic says it should output 24V but it's reading about 28V. Even still, the ic for the negative side is still steady at -18VDC.

Any thoughts on what to check? What killed the new IC?
This thing is driving me nuts!

[frankdrebin]

One thing that kills these IC s is heat,if there is not enough dissipation they can blow, because in general they have short protection,post a picture of the circuit

[Stevem]

I would test that recto bridge to make sure you have no open diodes and it would not be a bad idea to change out those 2 filters either unless you have a checker / tester that can spit out there leakage %.

[Tony Bones]

V1 and half of V2 don't light up. (12ax7)
I swapped the tubes with other 12ax7's that were lit up in the amp, and they don't glow either when put in the V1 & V2 positions.

The heaters are all in a series string like old Christmas lights. That means they all light up or none of them do. The fact that some light but others don't suggests to me that there's a problem with your wiring. Possibly there's a short to ground in between the region that works and the region that doesn't. From your description, that would be between pins 4 and 5 on V2. Admittedly, that's a pretty narrow place to have a short, but that's where the evidence points.

[pompeiisneaks]

Yup Tony's right,

It's all or nothing.

They're using a less commonly used series setup where each tube drops the full 12.6 volts across the voltage, so you should see voltages differing along the string as they drop across the tube. +18 at one side should drop 12 volts down to +4 a the other end of the heater side. then it should drop from +4 to -8 on the output of the next tube, then of course it drops 12 more to around -20 (18,20, ballpark here). So basically the voltages you're reading seem okay, the reason the voltage regulator is going out may be like Tony said there's a short to ground and it's sending too much current through the regulator.

Some of the older schematics I've looked at used to have a series winding for the tubes like that that would just be AC but say 60VDC ish for 5 12vac tube drops in series.

~Phil

[Tony Bones]

Looking again at the schematic, if pin 9 on V2 is accidentally grounded, then the amp would show the symptoms you describe.

Look around the tube socket for stray crap, maybe carbon...