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In Reply to: Practical upper limit of grid-stopper resistance on pentode amps? - Can large stopper 'soften' clipping? posted by freddyi on November 25, 2000 at 21:59:51:
Why would the plate section of a "phase splitter" inverter start amplifying when the output stage is driven to clipping (grid positive)?
There are two assumptions here.
1)That a "phase splitter" inverter can even provide enough signal current through a coupling capacitor to do this with any conviction
2) That the direction this assymetrical clipping would take would cause the appropriate output tube to draw current in such a way to make the plate phase of the splitter to amplify.No "phase splitter" inverter is going to clip asymmetrically anyways, at least not at frequencies either in or just above the audio band. Splitters are inherently self-balancing. That and its low tube count & simplicity account for its popularity. It's its ultrasonic performance that stinks, and that's where it becomes rather asymmetric.
The plate phase has the higher output impedance, so the problem in practice that I've run into is not that the plate phase starts to amplify (it already does actually, just not very much!) much more, but that it can't keep up with the cathode phase!
It's all a question of output impedance, nothing more. Relax, freddyi.
This isn't something to worry about. There is only one mechanism at work here, and clipping is not its biggest problem. High frequency stability is.
Oh, also went to Uncle Ned's and grokked the HF-87 schematic. EXACTLY as I thought. I still think either Engineer is just trying to have fun scrambling our brains with pure hooey or that he really might be a candidate for brain surgery...
Convert the EL34's to triode & check the time constant for the power supply to the 6SN7. It may need way more capacitance there than it already has, not on the output stage per se. Also splitting the power supply up on the 6SN7, giving one resistor/capacitor network for each triode may also help mitigate nasty clipping behaviour.
The amp is a tetrode, Eico didn't believe in ultra-linear or there were licensing fees they were too cheap to pay. Tetrodes suck, and they clip badly as a matter of course. You need to regulate the screens, or take the power cut & go triode. That sounds WAY better, anyway. I'm sure the 6SN7 long-tailed pair inverter symmetry is pretty good even with unmatched tubes (assuming that one half wasn't damaged!). I've heard this inverter before & it works mint. Good symmetry to well above 100kc/s is the rule. Nice amp, just needs a massage.
Get those magic fingers working, freddyi boy!
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- Re: Practical upper limit of grid-stopper resistance on pentode amps? - Can large stopper 'soften' clipping? - Joe Rosen 12/2/0005:10:50 12/2/00 (0)
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