Serious Voltage errors...

[J-Sty]

Alrighty... Just fired up a new superbass build with a Marshall PT and pretty much metro everything else.

Started to go through testing, heater voltages look good and light stays burning, bias reading looked great and I'm getting 96 volts on the bias resistor which the instructions list as A-okay...

Then things go down hill.

V1 is negative .4 volts on pins 1,5.: Negative .25 on pins 2,6: and 12 mv on pins 3,8.
V2 is negative .55 volts on pins 1,7: Negative .4 V on pin 2: 1.2 mV on pin 3: negative .48 V on pin 6: 77 mV on pin 8
V3 is negative .48 V on pins 1,6: negative .6 on pin 2: 3 mV on pins 3,8: Negative .57 V on pin 7
V4-7 is 0 V on pin 1, 8: 3.18 V pins 2,7: negative .45 V on pins 3,4,6: Negative 47 volts on pin 5.

Only mod is a lar/mar master volume. I don't have a camera... but I built this one pretty dang clean compared to a lot of the builds I've seen on here. I've gone over every connection I can think of at least twice. I'm not sure what I'm missing...

Any help at all would be greatly appreciated.

[J-Sty]

Oh yes, I also did Larry's grounding scheme... but everything else is stock.

[J-Sty]

Pretty frustrated... But took a few more measurements a second ago. I'd put an alligator click on V1 pin 1 to drain the caps so I could work if we come up with anything, and two hours later I still have 170 volts on one mains cap and 320 volts on the mains cap with the PT CT attached.

Roe, George, Larry, calling all the other metro geniuses out there!!!!

[Flames1950]

Um, did you flip the standby switch to the On position for the DC readings? Sometimes the instructions gloss right over that...........

[J-Sty]

Took amp off standby... Same results.

[Roe]

measure AC directly from PT. then measure DC at the rectifier and at all the different filter caps

[J-Sty]

I'm getting 178 VAC from my PT secondaries, 494 VDC on the Mains Filter towards the front panel, 247 VDC on the Mains Filter with the PT CT, -.238 VDC on the Screens Filter towards the rear panel, -.47 VDC on the Screen Filter towards the front panel, I'm getting 496 VDC on the middle rectifier post that faces the board, and 247 VDC on the other four, and 0 VDC on the Rectifier Ground. Not sure if it matters, but I read -.5 VDC on the PI filter cap.

[J-Sty]

I'm getting 247 VDC on my PT Secondaries in addition to the 178 VAC.
And all my PT primaries are right on the money. 123 VAC, 242 VAC and 223 VAC.

[Roe]

I'm getting 178 VAC from my PT secondaries, 494 VDC on the Mains Filter towards the front panel, 247 VDC on the Mains Filter with the PT CT, -.238 VDC on the Screens Filter towards the rear panel, -.47 VDC on the Screen Filter towards the front panel, I'm getting 496 VDC on the middle rectifier post that faces the board, and 247 VDC on the other four, and 0 VDC on the Rectifier Ground. Not sure if it matters, but I read -.5 VDC on the PI filter cap.

double check wiring of screens and choke, including screens filtering.

what's the voltage at the pi filter cap+

[J-Sty]

I think I may have tracked it down... Would a poor connection between the HT fuse and OT, Choke connection cause these errors? And also, what's the best way to drain those High Voltage Main filters... Should I just alligator clip them to ground, or would that drain them too quickly?

[Roe]

I think I may have tracked it down... Would a poor connection between the HT fuse and OT, Choke connection cause these errors? And also, what's the best way to drain those High Voltage Main filters... Should I just alligator clip them to ground, or would that drain them too quickly?

yes, that could be it. dont clip them to ground. wire up 100k+ resistor and connect it between ground and the cap

[J-Sty]

Alrighty, getting voltages now but weird readings... Here's the strange ones.

V1 174 VDC pin 1: 1.4 VDC pins 3,8: 180 VDC pin 6
V2 160 VDC pin 1: 1 VDC pin 3: 280 VDC pin 6: 160 VDC pin 7: 161 VDC pin 8
V3 209 VDC pin 1: 23 VDC pin 2: 40 VDC pins 3,8: 200 VDC pin 6: 24 VDC pin 7
V4-7 473 VDC pin 3: 472 VDC pin 4: 472 VDC pin 6.

Bias is at -47 with no power tubes.

My Main Filters read 235 VDC and 472 VDC.
My Screen Filters read 230 VDC and 472 VDC.

The voltages for the preamp tubes correlate roughly with the expected values except for V1 which is pretty crazy. I tried 2 different V1 tubes, same result each time.

[Roe]

how are those readings weird? look good to me. try the power tubes

[J-Sty]

Put in power tubes. Filaments glow, but loud low pitch drone when I take it off standby. All knobs are turned all the way down. The noise starts high pitched then quickly gets lower and lower and eventually just drone's loudly at a low pitch.

[Roe]

Put in power tubes. Filaments glow, but loud low pitch drone when I take it off standby. All knobs are turned all the way down. The noise starts high pitched then quickly gets lower and lower and eventually just drone's loudly at a low pitch.

try disconnecting NFB (usually 47k resistor)