EL84 bias problem

[MOSFETapocalypse]

This only happens when this resistor is in circuit? How high does the current rise? Whats the plate voltage? Bias fluctuations can sometimes be attributed leaky grid cap's leaking in DC from the preceding stage.
If this is only happening while setup for biasing, assuming all connections are good, You may want to try to use a .1 ohm resistor, i typically use this value especially if this resistor has to be permanently installed.
Travis

[clemford]

Originally Posted by MOSFETapocalypse

This only happens when this resistor is in circuit? How high does the current rise? Whats the plate voltage? Bias fluctuations can sometimes be attributed leaky grid cap's leaking in DC from the preceding stage.
If this is only happening while setup for biasing, assuming all connections are good, You may want to try to use a .1 ohm resistor, i typically use this value especially if this resistor has to be permanently installed.
Travis

Thanks Travis, never thought of a smaller resistor. Are .1 ohm's easy to find and do they come in high wattage packages?
The plate Voltage is 410V and the amp is brand new condition barely used.
I checked out all the electrolytic caps with my ESR meter, all 100%.
The bias will keep rising to dangerous territory.
This only happens with the tester because the V on pin 2 stays steady whereas it drops with the bias tester.

[BJosephs]

Originally Posted by clemford

Are .1 ohm's easy to find and do they come in high wattage packages?

How much wattage do you think is necassary? If you consider Ohm's law you'll see that a .1 ohm resister will drop 1/10 the voltage across itself that a 1 ohm resisted ohm would at a given current... And therefore need to dissipate only 1/10 the power.

Brian

[gian]

When I bias, I put a 1 ohm resistor connected to ground and the other at the cathode, then I measure the voltage across the 1 ohm resistor. If it measures 0.035 volts, then its 35 mA. It doesn't need to be high voltage resistor when placed at the cathode end.

[clemford]

Originally Posted by BJosephs

How much wattage do you think is necassary? If you consider Ohm's law you'll see that a .1 ohm resister will drop 1/10 the voltage across itself that a 1 ohm resisted ohm would at a given current... And therefore need to dissipate only 1/10 the power.

Brian

I just figured it should be at least 1 watt because most bias probes are at least 1 watt. What would be a safe value?

[gian]

Originally Posted by clemford

I just figured it should be at least 1 watt because most bias probes are at least 1 watt. What would be a safe value?

1 watt should be more than fine.

What values does the bias start from and then what does it change to before you turn the amp off?

I dont know the amp or have the schematic in front of me, but, does it have a bias set pot?

[gian]

What tubes are in it?

[gian]

I had some EL84 (chelmer) tubes in a hifi amp I built, they kept running away, I replaced them and the problem went away.

See if this following thread sheds some light on things.

http://forum.metroamp.com/viewtopic....14780&p=144101

[MOSFETapocalypse]

Originally Posted by clemford

I just figured it should be at least 1 watt because most bias probes are at least 1 watt. What would be a safe value?

The resistor value shouldn't have any affect on bias runaway, however I generally use low values (.1ohm, .22ohm) when measuring a variety of circuits (including tube current draw). The lower value has less impact in circuit.
Sounds like the socket you are using may have an intermittent connect, if the bias is only running off while it's in circuit. Does it level off at any point? Could you throw some other tubes in an verify it's repeatable, maybe some
Tubes you can let run hot so you can make some more observations/measurements.
Travis

[clemford]

Originally Posted by gian

1 watt should be more than fine.

What values does the bias start from and then what does it change to before you turn the amp off?

I dont know the amp or have the schematic in front of me, but, does it have a bias set pot?

the amp doesn't have a standby switch so it's too bad that I can't let the tubes really warm up first. It starts out cold at about 18ma with the bias pot all the way down and continues rising after the tube warms up. It will redplate if I let it keep going. I have some new TAD's coming in the mail so hopefully they will be stable, unless it's the bias tool as suggested in the article link you posted. The tubes in it now are unreliable so that might be the problem although they are ok without the tester. The voltage on pin 2 remains stable. When I get the new tubes I'll make sure I clean the contacts on the bias tool before I try it in case they're causing a poor connection.

[PhilDW]

There are some EL84 tubes that are wired differently internally aren't there? I once had an amp that would go crazy if the odd NOS EL84s were used, something to do with pin 1? Just a thought...

[clemford]

turns out it wasn't necessary to use a bias probe to set the bias after all.