Strange voltages across bridge rectifier

[defaced]

Schematic attached with voltages indicated.

I'm measuring different voltages across the "positive" diodes than the "negative" diodes in the full wave bridge rectifier I'm using in my amp. The voltage drop across the positive diodes is about 197VDC and the voltage across the negative diodes is 257VDC. This is with the tubes removed and the amp power on and standby on. If I turn the standby switch off, the voltages drop but the trend stays the same, 190VDC and 195VDC, positive and negative diodes, respectively. Does this seem strange to anyone else?

[Merlinb]

When you say "across the diode" do you actually mean between ground and the cathode of each diode? Don't forget that you will be measuring dirty AC in the case of the "negative diodes" and dirty DC in the case of the positive diodes, so unusual readings are to be expected.

[defaced]

Yes, from cathode to anode of each diode. What gets me is that I'm 99% sure I've taken these readings on a friends amp and the readings between the diode pairs were the same, eg, on his amp the positive and negative diodes had the same DC voltage drop across them. Moreover, when I read AC voltage across my diodes, I get 0 VAC.

[Jazz P Bass]

Full Wave Bridge Rectifier

I think that you are measuring your voltages incorrectly.
If you measure Vac across the tranformer windings, that is the Vac.
There will be no Vdc component.
Where you have marked "197Vdc" is the output of the bridge rectifier. Measured with respect to Ground.
There should be a minimal Vac component with the switch closed.

[defaced]

I'm measuring them correctly, I'm just not measuring what most people measure. I'm measuring the voltage drops across the individual diodes, from cathode to anode. I'm not talking about measuring AC voltage across the secondary winding or the DC B+ output voltage with respect to ground.

[Jazz P Bass]

Measuring Voltage

O/k, I'll bite.
Why?

[defaced]

Oh, same reason as always: I'm a nosy engineer. There's also the part about these measurements not computing. My B+ has also been very weird, and at times very very low. Last night under idle load it was about 530VDC, and OCV was 560VDC, but I've seen it as low as 505VDC at idle. Could be wall voltage differences, but I usually measure secondary AC voltage when I measure B+ so I can get a picture of what's going on, and the secondary AC voltage is usually pretty good.

I should crack open my Epiphone Valve Special and see what it's diodes measure.

[Enzo]

I usually use the term voltage drop to indicate a loss due to current flow. It seems to me you are measuring the result of blocked voltage or blocked current flow. perhaps I am the only one who makes that distinction, not sure. I expect the voltage drop across the rectifier to be on the order of half a volt, while the reverse voltage blocked by the part will be whatever the difference is between the two points in the circuit. To me voltage drop would be forward voltage.

It is DC voltage because the reverse is shunted by the diode, other than that half a volt.

Do the equivalent of leaving the switch open and grounding the filter caps. In other words, you have nothing but a 100k resistor to ground at the + corner of the bridge. Now what do you read.

It would seem to me that you have a DC voltage at the + corner held up by the filters, even with the switch open, the 100k being enough to get the voltage there. You don;t have that on the AC side. Remove the capacitor storage, and I would predict your numbers would even out.