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K&K Audio / Lundahl Transformers Welcome! Need support, you got it. Or share youe ideas and experiences. |
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In Reply to: Re: Here's another one ... posted by JamesD on December 14, 2003 at 12:39:27:
One to remember when working with grounded-grid circuits is the GG actually multiplies any impedance appearing at the cathode (input node) by the mu of the tube, so a source Z of 75 ohms is going to look like another 1650 ohms at the plates of an ECC99/7119, giving a total of 3300 ohms per side. This is double the normal output impedance of a ECC99/7119.Further, any *nonlinearity* in source Z (from the cathode follower) is *also* multiplied by the GG stage, which is then cross-multiplied by the nonlinearities of the GG stage itself. So the overall transfer characteristic is a composite device, and no longer really a single triode. So it's not all obvious what the distortion characteristic would be, much less a pentode/triode combination!
By the way, the pentode cathode follower will have much less output impedance if the screen is cap-coupled to the cathode - since the screen acts like a virtual plate, that improves PSRR, and also reduces distortion since the effective plate voltage is closely tracking the output voltage instead of B+. Think of it as a cascoded cathode follower and you'll get the idea.
You still have to feed the screen current, of course, but that can done with a simple screen-balancing pot like in Gary Pimm's PP47 amplifier (see R18 and R20 on the last schematic linked below).
For the all-triode version, a 220-ohm carbon-comp grid stopper needs to be added on the GG triode (right at the grid pin of the tube socket, and between the socket and gain-switch). These are very very fast circuits, and capable of oscillating at 20-50MHz given a chance. GaryP recommended 1K for pentodes and 220 ohms for triodes, which makes sense to me. Fortunately, the high/low gain switching is click-free and benign, since the grids of the GG stage are at zero DC potential (the 2X gain-switch works for your circuit too, by the way).
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Follow Ups
- Unusual behavior from grounded-grids ... - Lynn Olson 14:59:23 12/14/03 (0)
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