DC Imbalance with the LTP

[SY]

It's the mu that needs to be matched to optimize CMR (CMR is proportional to mu1*mu2/(mu1-mu2)). Higher mu also gives higher CMR.

[piano3]

Ah yes, of course, whereas it is mainly mismatch in gm that causes the difference in DC between the outputs, is that right? But how does one easily match mu?

[SY]

No, you can have a gross gm mismatch and still have perfect AC balance- the CCS guarantees that. DC mismatch can be caused by anything from slightly different cathode emissivities to variations in physical alignments.

You can try to match mu, but it's generally not worth it for the extra couple of dB of CMR. To measure it, set up a jig that applies a constant DC voltage between cathode and grid (I use an LED from cathode to ground), load the plate with a CCS, then measure the gain.

Mu is generally pretty consistent from section to section of dual triodes; 10% is pretty much the worst you'll see with any reasonable quality tubes.

[piano3]

Thank you Sy, I understand much more clearly now.

[DF96]

DC imbalance in the LTP can be caused by grid current, if the second grid is fed from the first grid by an RC network (e.g. Mullard 5-10). This can of course happen with any valve if it is a bit gassy. For the ECC83 it can also happen of the anode voltage is too low, as the grid-cathode voltage then wanders into the grid current region.

[Miles Prower]

Originally Posted by rlaury

Thanks Miles.
I'll be running some test at 10 - 20 Khz this week. BTW any advantage to have some small R's between the cathodes and the CCS junction?? ARC seems to do this but PSPICE shows no difference in distortion or balance

This is more of a solid state practice when using BJTs. BJTs have gm to burn, and so all that is required is to increase the current a bit for no loss of gain when including emitter degeneration. Triodes don't have all that much gain to begin with, and, unlike transistors, cathode degeneration not only results in a loss of gain, and gm -- important since LTP balance is proportional to: 1/(RtailGm) and you need all the gm you can get. It also causes an increase in effective plate resistance, and therefore makes the plate resistance a larger percentage of external plate load, and could increase distortion and/or change the harmonic distribution to something less sonically desirable since triodes produce more distortion the more heavily they are loaded.

If PSPICE is showing no difference, then it's likely your cathode degeneration resistors aren't big enough. For hollow state design, I'd forget about it. The only reason to include any resistance in LTP triode cathodes is to balance out DC offset. It's already been established here that it's not necessary unless you're DC coupling and/or making an instrumentation amp that needs to go down to very low frequencies.