Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Captain chunkulus on March 23, 2020, 10:56:37 am

Title: Bias drift
Post by: Captain chunkulus on March 23, 2020, 10:56:37 am

What would cause bias drift on one tube only? They are el34s and one tube continues to creep up and the other one does not.

Title: Re: Bias drift
Post by: sluckey on March 23, 2020, 11:04:29 am

Could be as simple as a bad tube. Change it.

Could be the grid bias voltage is creeping up. Monitor it to see. Check the coupling cap for leakage.

Title: Re: Bias drift
Post by: jjasilli on March 23, 2020, 11:09:23 am

Enlarging on sluckey's post:

1.  Swap tubes.  What result?  Could be a bad tube. Always suspect tubes first.
2.  Test bias supply voltage at ea power tube grid.  If different, a bias resistor may be bad; or some other component in the bias circuit.  Do not trust a DMM to give true R reading.  At full voltage an R may be failing.
3.  Leaky coupling caps from the PI, maybe.


BTW: What is creeping up: the bias voltage, or the tube current???

Title: Re: Bias drift
Post by: Latole on March 24, 2020, 06:14:17 am

Quote from: jjasilli on March 23, 2020, 11:09:23 am

Enlarging on sluckey's post:

1.  Swap tubes.  What result?  Could be a bad tube. Always suspect tubes first.
2.  Test bias supply voltage at ea power tube grid.  If different, a bias resistor may be bad; or some other component in the bias circuit.  Do not trust a DMM to give true R reading.  At full voltage an R may be failing.
3.  Leaky coupling caps from the PI, maybe.


BTW: What is creeping up: the bias voltage, or the tube current???

100 % right

Title: Re: Bias drift
Post by: tubeswell on March 24, 2020, 10:01:43 am

Some tubes are more prone to it than others.


In particular, EL34 g1 is wound close to the cathode to increase transconductance and gain, and so tends to heat up more anyway, and requires lower grid leak resistance to keep the tube stable. (This is why you need to take care matching sets in a Superlead plexi)


But if one in a set is creeping up at idle, it could be either g1 is too close to the cathode, and/or the heater in that tube runs overly hot causing more cathode emission and more g1 heat, or the particular length of wire used to wind g1 was imperfect to begin with and so heats up more easily etc, and that could be the way that tube came out of the factory (if its a 'new' tube), or it could be the tube has previously been damaged from heat (like red-plating) and grid support rods have sagged/warped resulting in unpredictable operating conditions, or if it's aged, g1 could've accumulated some of the emissive coating that has migrated from the cathode, and therefore is emitting more electrons than it should, causing excessive grid current which mucks up bias stability etc - who knows?