Metro PT Bias Tap Confusion

[ViperDoc]

I need help figuring out my Metro PT bias tap.

I'm using a Metro PT designed as follows:



I wired up my PT, IEC and fuse and relay power supply that y'all kindly helped me rescue. When I wired that up initially WITHOUT the bias tap wired in, I got the following:

PT primary 121 VAC
PT HT CT GROUNDED TO CHASSIS
PT HT secondary = 185 VAC PER SIDE, BALANCED
PT HT 80% secondary = 151 VAC PER SIDE, BALANCED
BIAS LOW END = 75.5 VAC
BIAS HIGH END = 171 VAC

I noticed the Jube SCH I'm building has the low end of the Bias tap grounded along with the heater CT, and leaves the HT CT un-used. So I disconnected and wrapped the HT CT, grounded the low end of the bias tap and got this:

PT primary 121 VAC
PT HT CT NOT GROUNDED TO CHASSIS
PT HT secondary = 211 VAC ONE SIDE; 117 VAC OTHER SIDE, UNBALANCED
PT HT 80% secondary = 182 VAC ONE SIDE; 88 VAC OTHER SIDE, UNBALANCED
BIAS LOW END = GROUNDED
BIAS HIGH END = 103 VAC

What do I do? Reconnect the HT CT, wrap the high bias tap end and route the 75.5 VAC low end of the bias tap into the bias supply without a bias dropping resistor?

[Roe]

bias should be ca 100vac
HT 178/156VACx2

this is correct:

"PT HT secondary = 185 VAC PER SIDE, BALANCED
PT HT 80% secondary = 151 VAC PER SIDE, BALANCED
....
BIAS LOW END = GROUNDED
BIAS HIGH END = 103 VAC"

[ViperDoc]

The only thing I haven’t done is ground the HT CT *AND* the BIAS LOW END. Is that the answer?

[sluckey]

Show us a schematic before you connect this PT. Don't connect anything to ground yet! Remember your aux transformer CT problem?

[martin manning]

Goes like this: http://www.classictone.net/40-18053.pdf

[Roe]

The only thing I haven’t done is [a] ground the HT CT *AND* the BIAS LOW END. Is that the answer?

a - do NOT ground the CT but - b - do ground one of the bias secondaries

[Phil_S]

Oh, never mind. I see Martin is way smarter than I am.

One suggestion. If you really have a 4 wire primary where all are the same color, DCR readings should tell you what you need to know. The highest DCR is 0-240. The lowest DCR is 220-240. Then you should know the other 2 wires are 0-120.

[ViperDoc]

Here's a SCH of everything I have wired up. This is all that's connected, nothing else.

The HT secondary is red, and the 80% HT secondaries are red/blue. They're wired to opposite ends of an on-off-on standby switch so I can select high or low voltage operation. I test the secondaries at the switch terminals. The standby switch output is not connected to anything.

2555X PT TEST SCH.pdf

[martin manning]

You won't get a valid reading measuring from one side of the winding to ground without a ground reference, if that's what you're doing. You can measure across the full secondary or the 80% secondary and get the correct AC voltage.

[ViperDoc]

AHAH!!! Look, this is why I bother you guys with my ineptitude. WELL, DUH!!! That would also explain the un-grounded bias winding readings, again, with no ground reference. Martin, you are a genius. I'll check that out. THANK YOU!

[ViperDoc]

HT measures 375 VAC, HT 80% measures 309 VAC. Perfect!