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Old 6th May 2012, 07:46 AM   #1
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Default tester causing bias to rise?

I have an EL84 amp that I'm not sure how to bias. I made a tester with a 1 ohm, 2 watt, 1% resistor in a plug saver tied between pin 3 and ground and when I first power it up it rises to about 18ma at the lowest adjustment on the bias pot and stays there for a few minutes but then starts to rise slowly when the tube starts to heat up and keeps climbing into dangerous territory. The voltage on pin 2 drops at the same time. When I remove the bias tool the voltage on pin 2 remains steady. It's a new amp so the caps all check out 100% on my esr meter. I don't remember my 6L6 amps ever doing this. Should I go with the cold bias setting before the tube heats up and starts rising and is there a reason why the 1 ohm resistor would cause this?

I previously posted a question about this amp in a different somewhat unrelated subject.
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Old 6th May 2012, 11:18 AM   #2
M Gregg is offline M Gregg  United Kingdom
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Its difficult to answer,

Can you post or give a link to a schematic?
Sounds like your bias is way out...

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M. Gregg
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Old 6th May 2012, 12:54 PM   #3
DF96 is offline DF96  England
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EL84 a bit gassy, and the pin extender encouraging parasitic oscillation thus overheating?
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Old 6th May 2012, 06:56 PM   #4
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Quote:
Originally Posted by M Gregg View Post
Its difficult to answer,

Can you post or give a link to a schematic?
Sounds like your bias is way out...

Regards
M. Gregg
this is supposedly the same design, just an older version.
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Old 6th May 2012, 07:05 PM   #5
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Your connection through a plug saver won't work.
You didn't remove the direct cathode connection to ground in the amplifier.
You should install a resistor between pin 3 of each output tube and ground.
After the resistors are installed, you can measure the voltage drop across each cathode resistor and calculate the cathode currents.
a 10 ohm, .5 watt resistor on each cathode works well.
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Old 6th May 2012, 07:16 PM   #6
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Quote:
Originally Posted by Frank Berry View Post
Your connection through a plug saver won't work.
You didn't remove the direct cathode connection to ground in the amplifier.
You should install a resistor between pin 3 of each output tube and ground.
After the resistors are installed, you can measure the voltage drop across each cathode resistor and calculate the cathode currents.
a 10 ohm, .5 watt resistor on each cathode works well.
I cut pin 3 on the plug and soldered the resistor in between. All the other pins are direct.
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Old 6th May 2012, 07:59 PM   #7
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So then you need to measure the voltage drop across that 10 ohm resistor installed in the socket saver, not between pin 3 and ground. Is that what you are doing?
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Old 6th May 2012, 08:23 PM   #8
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Originally Posted by Captn Dave View Post
So then you need to measure the voltage drop across that 10 ohm resistor installed in the socket saver, not between pin 3 and ground. Is that what you are doing?
I never used a 10 ohm, I used a 1 ohm when I made it and the meter is connected to either side of the resistor. My question is why is the current rising. It must be the bias tool like DF96 suggested.
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Old 6th May 2012, 08:27 PM   #9
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Quote:
Originally Posted by DF96 View Post
EL84 a bit gassy, and the pin extender encouraging parasitic oscillation thus overheating?
what would be the best way to set the bias then if the tool doesn't work. someone suggested measuring across plug 1 and 17 and plug 2 and 17 on the schematic. To me that looks like it is across the B+. That doesn't make sense to me.

I think this is a better link to the schematic in a larger size.http://www.freeinfosociety.com/elect...ew.php?id=2288

Last edited by Clemford; 6th May 2012 at 08:30 PM.
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Old 6th May 2012, 08:32 PM   #10
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Quote:
Originally Posted by M Gregg View Post
Its difficult to answer,

Can you post or give a link to a schematic?
Sounds like your bias is way out...

Regards
M. Gregg
here's a direct link to a larger size schematic. http://www.freeinfosociety.com/elect...ew.php?id=2288
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