Question: i keep burning a resistor

[surfsup]

The resistor is the 2k7 on the first RC filter. When i turn on the amp, she fries immediately. Here's my question, i hook up my dmm to it (removing the bad resistor) and measure current, and i get 11mA. That's low but about right. 3 for the 6sn7 and 2 for the 12ax7, 7mA for the el34 screens. Though i thoutght the two CFs would contribute another couple mA. Anyway...

So 2700*.011*.011 = .3 watts

So how does a 3w resistor burn up when my dmm measures 10mA? I dont get it.

[surfsup]

For giggles i soldered in a 680R 5w to lower the drop, increase the wattage, and put the secondary switch on low voltage. I get 325 rectified, 309 after the drop.

17v/680= 25mA

So is my dmm a pos? It measures less than half the current.

2700.025.025=1.7 watts

A 3w resistor should have still been fine. The 560 is holding up though. I dont know whats going on...

[The New Steve H]

Are you totally sure those capacitors are to the left are okay? I may be a mouth-breathing alien who cruises the galaxy forcing advanced species to give me technology I don't understand, but I'm pretty sure I = V/R, so if I is too big and R is fixed, something must be wrong with V.

[surfsup]

Well something is going on. With the 5W R in there, I measure voltages and get 24mA (see schematic below with voltages and current listed). Looking at the preamp nodes of roughly 3mA and 6mA, that's 10mA. And those current values look about right. That leaves ~15mA for the screens to draw. But, the voltage drop across the 1k screen resistor shows only 3mA draw at idle.

So I have three problems:

1) the DMM measures 10mA, and there's really 24mA (or is there?)
2) the 2k7 3W should have still handled 1.6W with 24mA but roasted
3) where are the other 12mA being drawn from?

1 and 2 are not huge issues, but #3 is bugging me. Can a cap draw extra current to ground?

[img877]http://chicagocadcam.com/ChrisHahn/supr ... actual.jpg[/img]

[jjman]

24ma flowing.
3ma to the screen
3.7ma to v2
6ma to v1

C9 looks like the culprit based on these figures.

[The New Steve H]

Again, we are still pretty excited about opposable thumbs on my planet, but have you measured the resistance of each capacitor? If it's not infinite (or whatever a working capacitor's resistance is supposed to be in practice), it would be a clue that it's time to change it.

[The New Steve H]

By the way SOME idiot I know installed electrolytics so the leads were touching the cases. He did not know the cases were connected to the negative lead. HA. What a BUFFOON. Who was NOT me. Oh, no.

No one can prove anything.

Anyway, that would short the capacitor out of the circuit.

[surfsup]

I tried that. All the caps read the same "0.L Mohm" on my DMM, including C9. I could guess my dmm is playing tricks with me, its a no name budget model, but the 2k7 did get roasted so i think the values i posted are correct or close.

[The New Steve H]

My brilliance is exhausted. You may have to wait for someone who actually knows what he's doing.

[diagrammatiks]

do you have the resistor before the plate connection or after the plate connection.

[surfsup]

The resistor is in the right spot, between the first two caps. Now i'm getting correct voltages but the amp is getting all staticy. I have a feeling one or more caps could be bad.

[dorrisant]

Here is something that might help... An ESR Meter...

http://members.shaw.ca/swstuff/esrmeter.html

I had to order a meter panel but had the rest of the parts on hand to build it. I suggest you change the voltage rating of C5 up to 650v, or whatever you can find that works for your situation. Works well and takes a lot of mystery out of those old caps...

Hope that helps!
Tony