What would cause -70vac at the -50vac st120

Is this OK?
The other measurements are in spec
Well the kt88 control grid is a few off.

Ss rectifier (2) uf5408 (1) 100ohm25w resistor
Wall voltage 120vac
Pin 4 to ground - 400 - 425 volts AC. 417
Pin 6 to ground - 400-425 volts AC. 417
Any 6550/KT88
Pin 1 to ground - approx .550 volts DC (depends on bias setting) .450
Across pins 2 and 7-6.0-6.5 volts AC. 6
Pin 3 to ground - 470 - 500 volts DC. 480
Pin 4 to ground - 470 - 500 volts DC. 480
Pin 5 to ground - minus 50 to minus 60 volts DC. -47
Pin 6 to ground - minus 50 to minus 60 volts DC. -48
Pin 8 to ground - approx .550 volts DC (depends on bias setting) .450

Quad 1 480-505 volts DC. 488
Quad2. 470 - 500 volts DC. 481
Quad 3. 470 - 500 volts DC. 481
Quad 4. 400-430 volts DC. 411

Central 12AT7 - Pin 1 or pin 6 to ground > 130 - 160 volts DC. 142v
Two outer 12AT7's - Pin 1 or pin 6 to ground > 255-285 volts DC. 256v
Any 12AT7 - across the two incoming heater wires > 6.0 - 6.5 volts AC. 6
"-50 VAC" on driver board > 48 - 55. -75
"B+" on driver board > 400 - 430. 412
"G" on driver board > 0 volts AC or DC. 0

Njrob wrote:Is this OK?

"-50 VAC" on driver board > 48 - 55.       -75

Where are you finding a specification on any VTA schematic for 50 VAC?

Could you be mistaking the target voltage after R27? On the schematics I have (pre CCS 12AT7 and 12AU7 versions) it shows ~50 VDC, the ~ symbol meaning "approximately equal" to 50 volts DC", not a negative number, and certainly not AC voltage. If that's not the spec you're referring to, and you're actually measuring 75 volts AC anywhere on your driver board, then I'm lost.

But if that is where you're taking your measurement (after R27, which ties the cathodes together), and are actually getting +75 VDC, then you might consider increasing the value of R27 to reduce that closer to 50 VDC.

By the way it's a  vta st120
Pins 5 and 6 are the cathodes that reads slightly lower than spec. The -50vac is on the driverboard. Roy says it's OK. I'm just wondering what causes it and if it's an issue.
Here's a pic of the manual and the voltage requirements.  Thank you for your response
https://www.dropbox.com/s/07tjfarzeeni7wv/vta%20st120%20voltages_Drive.jpg?dl=0

You can't have -75vac. I am guessing you are measuring it after it is rectified and filtered. I think when you rectify AC your DC voltage is higher than what you measure with AC because, on a meter, AC is usually an RMS value instead of a peak value. The RMS is about 30% less than the peak (.707 * peak). That might explain the higher voltage. Someone please correct me if this is incorrect. It has been a long time since I had my AC analysis class!

https://www.dropbox.com/s/i6m4teunovz409q/VTAdriverboardpre2012_sideways-2.jpg?dl=0
This is the board pre 2012
Circled is the -50vac.
This is after the rectification.

I think the 50 volt winding from the power transformer connects there (-50vac connection). It is rectified by the diode and filtered by C17 in a configuration that supplies negative voltage to the bias circuit. I would guess -75ish volts DC would be close enough to normal after rectification. I think there is a voltage divider circuit in there to get the bias voltages into a range good for a wide variety of tubes (KT-88, 6550, KT-120 etc). It seems that you get good bias voltages. Maybe you were measuring DC, since you measured -75V, instead of AC?

I did disconnect the 6vac from the rectifier socket just for testing and left them off for now. could that have anything to do with? Thank you for the reply. The reading at neg50 is neg75ac for sure.

But I will check again bc I'm triple thinking now.

You can't have -XX AC. If you are reading negative then your meter is set to DC.

Fluke t6 1000.  good call tho. So what gives w the -50vac? Weird. It's auto I didn't catch that. I feel silly

From the Roy, "since less current needed thru the power transformer.
Nothing wrong with having -75v of bias available instead of only -50v of bias."

I built that amp 10 years ago.  Thought the -50vac labling was interesting.  Maybe it was a typo on the silk screen that didn't get caught until the boards were already made...or maybe since it is the input to the bias section which provides negative voltage it was there as a reminder to put the diode and filter cap in reversed.  -75VDC is normal for that circuit.  Glad that you got it sorted!!

Looks like that neg bias wire comes straight from the transformer. How is it getting dc from the transformer?