Why does..

[leadfootdriver]

my OT primary's measure at 404VDC, but also measure about 900VAC?

[Jazz P Bass]

What you posted , makes no sense.
As well as being vague.
I guess that is why the question, huh?
Is this a flying wire measurement or is the secondary of the transformer connected to anything?
I think you need to sort out your measuring technique.
Vac readings are valid across windings.
Vdc is a rectified Vac.
You need to figure out the circuit to determine the common terminal to properly read the Vdc.
Once you are reading Vdc correctly, now you can switch your meter to read Vac (ripple).

[leadfootdriver]

I F'ed up. I meant primary's! But yeah, the plate voltage on my power tubes are at 404VDC, but also 900VAC.

[g1]

Sounds like a meter issue. Some meters won't like trying to read AC on top of that kind of DC. Even if your filter caps were shot and you had massive hum from the amp, I don't think the AC reading would be that high.

[leadfootdriver]

Sounds like a meter issue. Some meters won't like trying to read AC on top of that kind of DC. Even if your filter caps were shot and you had massive hum from the amp, I don't think the AC reading would be that high.

Are you questioning the quality of my grocery store bought $10 meter?

[g1]

Well, yes, but check the battery anyway just for fun . (unfortunately this will probably require another meter)

[leadfootdriver]

I need to buy a real one anyways. One of the lcd segments is fried. So in one spot my 7's and 9's read as 1's and 4's. Super...

[Steve Conner]

Maybe the amp has an oscillation problem. At least when the meter probe is connected to a power tube plate.

[Enzo]

That could be. Does it measure the 900VAC on all the power tube plates? Or just on one?

[leadfootdriver]

I connected my clips to the IEC, and it read 123VAC which is right on.

Could it just be the meter? Because all I did was click the meter to read AC from DC, but the ground clip and the B+ clips were never touched. Would AC current flow thru the chassis ground as if it were the neutral wire in an AC circuit?

[pdf64]

'Could it just be the meter? Because all I did was click the meter to read AC from DC, but the ground clip and the B+ clips were never touched'
See post #4.
Cheap DMMs can get confused by AC offset with DC and produce wacky readings.
If not, then probably post #8.

[Dave H]

It sounds like the meter is half wave rectified, average sensing calbrated RMS for a sine wave and it doesn't have a DC blocking capacitor so it will read 404 x 2 x 1.11 = 897V when measuring 404V DC on the AC range.

Dave H.

[leadfootdriver]

It sounds like the meter is half wave rectified, average sensing calbrated RMS for a sine wave and it doesn't have a DC blocking capacitor so it will read 404 x 2 x 1.11 = 897V when measuring 404V DC on the AC range.

Dave H.

You are the Man.

[Enzo]

SImple test: Set your meter to AC volts and then measure a 9v battery. It SHOULD settle to zero volts on the AC scale. If it lacks a blocking cap, then it will measure a voltage on AC.

[J M Fahey]

It sounds like the meter is half wave rectified, average sensing calbrated RMS for a sine wave and it doesn't have a DC blocking capacitor so it will read 404 x 2 x 1.11 = 897V when measuring 404V DC on the AC range.

Dave H.

Itīs exactly that.
I got in to answer when I read the title but you had already done it.
Cool.
It happens with all the cheap multimeters which have just 2 AC scales: 200V and typically 750V (may also be 500 or 1000).
Only meant for tyros to measure Wall voltage ot transformer secondaries, neither of which has a DC component.
It also happened on old needle multimeters, which to read AC on plates, had an extra input labelled, ironically, "Output" which had a .1x400V capacitor in series.
Now, if you have a multimeter which can typically read down to 200mV AC, they have added an amplification stage or at least an active "perfect rectifier" so they *can* afford an input capacitor.
But in a $10 multimeter, forget it.