Weird voltage behaviour.

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1,854
I was biasing my amp. I checked the plate voltages on the power tubes (2-EL34's). 1 was 400v solid, but the other was fluctuating between say 340-400 if I wiggled the alligator clip.

Would a defective power tube do this, or do I have an OT primary problem?
 

zenas

Member
Messages
8,697
You may just have a bad connection in the amp. But I'm shooting in the dark here.
What's tha amp and how old?
 

phsyconoodler

Member
Messages
4,315
You wiggling your alligator clip is likely causing the reading to fluctuate.A little bit of flux on the pin and it gets a poor contact.
 
Messages
1,854
I already made good contact. If I play the volts come up a bit, but it's not as steady as the other side.

When I connect to the funny side I get a high pitch ring or oscillation.
 

EFK

Member
Messages
1,040
Your meter or probe itself, attached to the plate, is probably causing an intermittent high frequency oscillation that is - intermittently - eating up a huge amount of current and thus the big voltage drop to 340, then back to 'normal,' etc. I'll bet if it dropped to 340 and stayed there for long, you'd see the tube start to red plate.
 
Messages
1,854
Are OT primary's susceptible to inductance or other proximity problems like if they're close to heater, or other power sources?
 

Blue Strat

Senior Member
Messages
30,151
Are OT primary's susceptible to inductance or other proximity problems like if they're close to heater, or other power sources?

About a billionth as likely as your probe either making an intermittent connection or doing what was described just above.

You're over thinking this.
 
Messages
1,854
Okay, it gets crazier. I resoldered the plate wires to the tubes, but the same thing happens. But when I move 1 end of the amp up or set it on it's side, it goes away and stabilizes. WTF??
 

WaltC

Member
Messages
2,129
(carefully so as not to electrocute yourself) retention the pins in both power tube sockets?
 

EFK

Member
Messages
1,040
Yes, and also swap tube positions and see if the problem follows the tube or is inherent to that socket.
 
Messages
1,854
Kay then, I tried the power tubes, but it's definitely the blue primary. This amp is funny tho in that if you have test leads near the board, it goes nutz. I wonder if it's just inductance.

Moreover, I pulled the tubes, and the voltage was stout. I checked the grounds too.

The amp sounds like it always has, but this voltage thing caught my eye.
 

Blue Strat

Senior Member
Messages
30,151
Test leads are antennas. You learn this through experience. It's a similar phenomena to long wires causing parasitic oscillation.
 

Keyser Soze

Member
Messages
1,476
...
Moreover, I pulled the tubes, and the voltage was stout...
Which pretty much proves this diagnosis.

Your meter or probe itself, attached to the plate, is probably causing an intermittent high frequency oscillation that is - intermittently - eating up a huge amount of current and thus the big voltage drop to 340, then back to 'normal,' etc. I'll bet if it dropped to 340 and stayed there for long, you'd see the tube start to red plate.
You guys are good.
 




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