18Watt.com

Good Morning,
I am looking into the 18W I built a while back, which is a Ted Weber 18W TMB kit that currently has no mods. I was recording the voltages at each pin of the tubes and when I got to the PI tube, things got weird. The plate voltages and screen voltages do not match. For those not up on the Ted Weber version, pins 3 and 8 are jumpered together at the tube socket so they are common. The plates come off their own dedicated 100K resistors with pin one measuring 248VDC and 0 volts dropped across the 100K resistor. Pin 6 measures 163VDC with 85VDC dropped across the 100K resistor. Obviously V2A is not conducting any current.

Move to the screens and I am unbalanced there as well which leads me to believe that is where the problem lies. Pin 2 comes in at 26VDC and pin 7 is 46VDC. I have followed the wires to make sure they match the layout and make sense against the schematic. I have measured the values of the individual resistors to make sure they match the schematic and measured for DC voltage on the other side of coupling caps to make sure they are not shorted and letting DC pass them by. I have replaced the PI tube with another and have the same results.

The amps sounds fine clean, when driven either by diming the TMB channel or using a Tube Screamer, things get pretty farty and not pleasant.

Can you post the layout&schematic you are referring to?

Here are the layout and schematic. Let me know if I have done this correctly, the below links are from the Ted Weber website.

Are you trying to measure voltages on the PI grids (V2 pins 2 & 7) and are you using a regular voltmeter or multi-meter?

If so, it's important to note that those two grids are at a very high impedance, due to the way they're bootstrapped via the 470k grid reference resistors (R& & R8) to the PI tail (R19) and the relatively low impedance of your meter loads them down, giving unreliable voltage readings. The solution is just take a DC voltage reading at the top of R19, as it will always be at the same voltage as the two grids. Alternatively, you could measure using an oscilloscope with high-impedance probes.

HTH

Ok, problem solved. Embarrassingly enough, the wire that supplied one of the plates was broken near the solder joint on the tube socket. I never saw this because I covered all of the tube socket pins with heat shrink tubing to cover the solder joint and prevent pins from touching. When I measured voltage at the other end of the wire connected to the turret on the board I got voltage. I got desparate and ohm'd out from the tube socket connections back to the wire landing points on the board and found the issue.

Where it gets truly embarrassing? This amp has been this way for a year. I never owned an 18 watt before building this so just lived with some things.