Fender Pro Reverb UltraLinear (70w)

[gui_tarzan]

Ah, of course you are right. I'll disconnect that and see what it does.

[edit] I get a decently loud hum in the speakers when I put AC on the leads. Time to start tracing backwards...

I also noticed when touching the meter lead to pin 3 of the socket I haven't replaced yet the wire arcs to the chassis. I'll be swapping that socket as well.

[gui_tarzan]

Replaced the second power tube socket. When I touch my meter lead to pins 5 I hear a little static now. But when I touch it to pins 3 and 6 (OT secondaries) I don't hear anything but I am still seeing 518-519v. I should hear a buzz like I did when I put AC on them when the amp was off.

[martin manning]

I don't think you should get much noise probing the plate or screen. Plate voltage looks good.

What is the voltage on pins 5? Can you measure plate current, say by measuring voltage from OT CT to each plate lead, and dividing by the DC resistance of the primary halves?

[gui_tarzan]

V7 -

Pin 5: -59v
OT CT to Plate: 1.2v
Pin 8, 1 ohm to grnd: 29v
Main Sec (blue) to CT: 44.8 ohm
UL Sec to CT: 6.5 ohm

V8 -
Pin 5: -59v
OT CT to Plate: 1.4v
Pin 8, 1 ohm to grnd: 28.3v
Main Sec (brown) to CT: 51.4 ohm
UL Sec to CT: 6.3 ohm

[martin manning]

The pin 8 voltage is 0.029V and 0.0282V. That doesn't look bad though- about 29mA each side, about 15W at 519V Va, or 50% of the 30W max. Seems like the power stage (post PI) is ok. You need to put signal into it and trace it.

[Stevem]

If that wiring was messed up than check the master volume wiring also!

[gui_tarzan]

So I took my 1k probe in there and I have a very quiet signal at pins one and six of the PI connections, and at the input end of the 1.5K resistors on pins 5 of the power tubes, but also a loud hum through the speaker at the same time. That's down-river from all volume controls but even so I checked and it doesn't change no matter where the volumes are set so that should rule out the entire pre-amp section.

Once I get to the input side (2 & 7) of the PI I get nothing, even after trying three different known good tubes. I have 6.4v AC across the heaters.

[gui_tarzan]

The pin 8 voltage is 0.029V and 0.0282V.

Yes, millivolt. Sorry.

[Richie]

Can you post some PI plate voltages? A common problem in fender amps is having a bad PI plate resistor.

[gui_tarzan]

There must be something else in the wrong place, I'm getting a negative reading on the connection to the PI plates and on the 22uf caps positive end. I quadruple checked my filter cap installation and they're all correct polarity.

There is a 2k7 10 watt resistor on one of the terminal strips (remember, this is an ultralinear). One end goes to the center tap of the OT, the other end goes to the cap can and to the PI plates. Believe it or not, there is -49v - yes, negative - on the end that goes to the PI.

I have not removed or changed any connections other than the filter and bias caps, the cathode caps on V5 and the power tube screen and grid resistors. I haven't touched any supply wires, diodes or anything else.

[jelle]

Ha! Someone messed with it and put the bias supply on the PI plates as well.

You got the short end of the stick. This one needs to be checked against the schematic...100%.

Yeah... all of it.

[gui_tarzan]

Yep, that's what I thought after I got those readings and that was my next step. Whoopie.

[gui_tarzan]

I've been pouring over this schematic working from the PT to the PI. The voltages at the "matching" pot look right, but where the 20uf filter cap is going to the PI and at the rail side of the 2k7 10w cement resistor are where things get really weird. I took the 2k7 out and measured it and it reads over 3 meg ohm, nowhere near 2k7. So I left it out and measured the OT B+ and it reads 519v. On the other end of where that resistor should be, which goes to the first of the 20uf caps, it reads -280v. I replaced that resistor with a new one and it still reads -280v.

How the @&#$ can it read positive on one end and negative on the other??? All the 20uf caps read negative voltage. If you follow the schem to the end of the rail it goes to V5, and also up to the 2nd and 3rd 20uf filter caps, and over to the reverb transformer (red lead). All of those points read negative volts and the wires are in the right places.

The ONLY thing that I can see that could allow negative DC onto the rail are the 47K and 68K bias resistors that hook onto the .1 caps on the output side of the .1 caps on the PI. I get -310v on pin 6 and -280v on pin 1 of the PI.

[martin manning]

Is it possible that the ground is missing from the three 22uF preamp filter caps?

[gui_tarzan]

There is a black wire running from the negative end of caps 3 & 4 (20uf) to chassis ground near the PT and a green wire from cap #5 to chassis ground under the normal channel bass pot.