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'load lines for dissimilar output tubes?'
Author:tapehead ted (registered user: 6 posts )
Date: Sat, Mar 18th, 2017 @ 13:49 ( . )

Malcolm- thank you, I'm going to have to copy down your first post and study it for a while. Perhaps that's what I've already been doing... I can't think so well while looking at a computer screen- I'll have to hole up with my charts and look into that approach.

I've spent some time (hours and hours) trying to find achievable screen voltages and bias points where I6v6 and I6L6 are as similar as possible, while keeping the overall point to point swings similar and the bias voltages similar as well. I've decided that having the 6v6 clip a little sooner might be really nice, as it clips much more smoothly in my experience, so I've set the bias voltages so the 6v6 runs maybe -17.5 while the 6L6 runs -19. The closest I could get while meeting all the above criteria was I6L6/I6v6=1.25 at quiescence.

A stiff fixed bias barking overdrive is not really the best for me, so I'm making life complicated by allowing some bias shift. I tracked the ratio of the currents as the bias shifted, which would be the relevant ratio at vg1=0, and found that they diverge more and more as the voltage goes more negative. To keep the ratio from diverging more than about I6L6/I6v6 as suggested in the original post, I'm using about 7-8 volts of fixed bias, 1/3 to 1/2 of the overall bias voltage. I had an elaborate scheme with separate "fixed" bias controls for each tube derived from a back bias, so everything would shift, more moderately than cathode bias, together, but that was way more complicated than I have a real appetite for. The combination of fixed and cathode bias should keep things reasonably well balanced and allow some bias shift.

I'm definitely aware of a tendency for the 6v6 to tilt towards below the knee, hence the above precaution with a mixed bias and definitely healthy screen stoppers.

Screen voltage at design target are 285v for 6v6s and 255v for 6L6's. Allowing for some mains variations, this is the most advantageous situation I could devise.

Who knows but I may rethink all of this once I've applied Malcolm's post.

My own intuition- a very well developed and reliable design resource, in my experience- tells me that loadlines will not tilt so very far as I calculated. As long as I'm having a custom transformer built, I'm envisioning two taps at 3.3k and 4.4k for the 6L6s and 6v6s respectively, although I believe it would work fine with both with the one 4k tap, pretty much as I heard it on by buddy's amp, but a little gentler on the 6v6 screens.

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Replies:
--* load lines for dissimilar output tubes?
3/18/2017 @ 15:21--Malcolm Irving
3/18/2017 @ 15:35----tapehead ted

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