DAC Tube i/v help!

[RollE2k]

I'm going to build two step differential gain-stage for a headphone amp, and the first stage will be a ~40-50mu tube, with LL1667/pp choke at the anode (2mA offset max) with a tail CCS. The signal will then go into second tube stage with anode CCS and parafeed output.

Now my question here is - the fist gain stage of tubes will probably be something like E180CC, 7062, 5965A, 12AY7 or 6072A. All these tubes will run a bit more current than those 2mA(around 10mA for many of them).
I started thinking that during startup the current may drift between the tubes depending on which triode half that gets heated up a little bit faster, also with time one of the halves may become bad faster, making the other half run higher current.

So first of all - is there any way when using common cathode CCS to get the current to be exact over both tubes (even if they are not perfectly matched)?
And also, is there any way to make amp not starting or cutting B+ to the tubes if the current difference becomes to big?
Or would i have to live with knowing that the chokes MAY become magnetized?
Any help would be appreciated!

 

[artosalo]

I don't see the problem you described. LL1667 is designed for 15 mA standing current.

But why to use choke at the anode when signal level is low ?

 

[RollE2k]

LL1667 is avalible in many versions (some not in the datasheet) there is also a 10mA, 7mA, 5mA and PushPull Version..
at least the PP version does take 80mA, but only 2mA imbalance.
And why i would like to use that one is because it is around 1600H inductance, which will be plenty enough.

So my question is still if there is any way to prevent imbalance with time, or heatup, or with not best matched tubes?

 

[yagoolar]

You can use current sinks in cathodes of differential stage to set desired current.

 

[DF96]

During warm-up you can't avoid some current mismatch. The issue is whether this will cause some temporary core saturation (no matter) or permanent core magnetisation (could matter). Ask the supplier.

 

[RollE2k]

I wouldn't want to use separate sinks for each phase, due to worse CMRR. I'll just have to shoot lundahl a email asking this specific question.

 

[artosalo]

So my question is still if there is any way to prevent imbalance with time, or heatup, or with not best matched tubes?

You can adjust bias balance by adding a small trimmer pot , say 100...220 ohms, between the cathodes and CCS.
Trimmer ends to the cathodes and wiper to CCS.

 

[RollE2k]

You can adjust bias balance by adding a small trimmer pot , say 100...220 ohms, between the cathodes and CCS.
Trimmer ends to the cathodes and wiper to CCS.

This is what i was thinking about doing. But it still doesn't cure the "main" problem that was my concern.
Lundahl pretty much summed my concern with "if you don't think 2mA is enough (which it usually is with push-pull configuration) then choose the 5mA version".. so decisions decisions... i really would like to have that extra inductance.. so i'll probably have to live with the possiblity of magnetisation of the choke if there is any trouble...