why do i get more B+ voltage.....

[iknowjohnny]

....with a 180R sag resistor after the rectifier than without it? I would expect the resistor to drop the B+, and i seem to recall that when i tried this a while back. but now i'm getting 350v without the resistor and 400v with. can someone tell me why this is?

[Zippy]

You're right, I'd expect voltage to drop as well.

Did you put it in backwards?

[Structo]

How are you measuring the voltage?

Does the rectifier have a load on it?

[iknowjohnny]

Sorry....i didn't mean that i was measuring B+ when i typed those voltages, but i was measuring plate voltage. But if i measure the B+ at the first can right after the rectifier the voltages are higher but still the resistor yields the high voltage. I think measuring there was resistor in=430v, resistor out=375v.

I have started it down for long periods, but the wiring is so simple i cannot see what i could have done wrong. It's like this....

rectifier output goes to common (center) of a DPDT switch.

one side of the switch goes to the R and the other side of the R goes to the first cap can.

The other side of the switch goes to the cap can.

Now i know what you are thinking, but i checked the switch with my meter on continuity on the unused side of the DPDT switch shows the center is indeed common, plus i checked and re-checked many times to insure the center was connecting to the wire that goes to the input side of the R when the switch is in the position i labeled "sag". I keep thinking i must be spacing, but i don't know how many more times i can check it before i think i'm ready to be institutionalized !

[iknowjohnny]

Man, my face is red ! I don't know how i mangaed this, but i somehow did indeed space when i metered the switch....the position labeled sag is on the wrong side of the toggle ! Go ahead....i deserve it.

[Jana]

Go ahead....i deserve it.

Okay.

Good Job Homer!

At least the switch wasn't for "Condition Normal" and "Self-Destruct."

[iknowjohnny]

Doh !

[Jana]

I just have to ask, 50 volts drop across a 180R resistor is 277ma current. subtract about 10ma for screens and maybe 7 ma for ax7's and that leaves roughly 260ma for a pair of EL34's. Right? Is anything glowing really bright red or getting really hot? Just wondering.

[iknowjohnny]

No, but i didn't realize this would happen. So when i use a resistor there it raises the current in the power tubes? If so, what size R do you suggest? my PT is 250ma by the way, and the amp has SS rectifier and is a cathode biased pair of el34's.

EDIT: while the difference in voltage at the tubes is a lot more between the resistor in or out of circuit, i just measured across the resistor and got 22v.

[Structo]

So the resistor is dropping 22v.

[iknowjohnny]

Can someone explain the math to me and exactly what Jana meant? I'm not even sure what he meant. Was he saying that he did the math and came to the conclusion my amp is drawing 277ma from the PT? Wouldn't my PT go into meltdown since it is a 250ma PT? It doesn't seem to get hot at all. I'd just like some clarification if someone could help me.

[stoo]

Can someone explain the math to me and exactly what Jana meant? I'm not even sure what he meant. Was he saying that he did the math and came to the conclusion my amp is drawing 277ma from the PT? Wouldn't my PT go into meltdown since it is a 250ma PT? It doesn't seem to get hot at all. I'd just like some clarification if someone could help me.

I=E/R...=50/180=0.277 amps or 270ma.
Stew

[iknowjohnny]

Sorry for my total ineptness when it comes to numbers/math, but can i assume that since the voltage drop i measured across the resistor is 22v that my PT is putting out 122ma? (22v/180=.122)

Why i measured 50v difference between the resistor and w/o the resistor when measuring the plates i don't know. But measuring the DCV across the resistor i got 22v.

[mhuss]

The voltage you are measuring across the resistor is rectified AC/pulsing DC, so the meter will not give you a correct reading. There's no easy way to measure this value unless you have a 'scope, or a meter that reads "true RMS" value on AC measurements.

--mark

[iknowjohnny]

I asked this elsewhere too and was told the resistor won't change the current that is being pulled from the PT. So unless someone tells me different i will go back to the thinking that you can estimate the demand simply by using the tube specs and adding them all up.