In Reply to: questions on a new design posted by DanK tubes on September 10, 2009 at 21:12:21:
mu on a 7788 is ~50, maybe higher. You have a 125R unbypassed resistor for bias. This adds more than 6K to the output impedance of the stage, which frankly makes using a 7788 a waste of effort and money. Might as well use a 6J5 - cheaper, easier, and will have a similar Zout. Otherwise, replace that resistor with a LED -- some blue ones are in the 3.5V range, use a bypass cap, or find another bias method (hint -- use a CCS for the heater and run the current through the bias resistor).
Second, your numbers don't add up. At 28mA with a 3.5V bias, the 7788 will have ~240V on its plate, not 190V, which means it will be dissipating close to 7W which is too much. I'd tone it down to about 15mA and a -3V bias which will put the plate close to 190V and dissipate less than 3W.
At any rate, 28mA is high here. This tube sounds better with less current, probably because it is more prone to oscillation as the current increases.
100R from G2 to the plate is enough, so long as layout is tight.
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Follow Ups
- RE: questions on a new design - drsx 23:40:37 09/10/09 (2)
- Cathode cap - John Swenson 16:19:06 09/11/09 (0)
- RE: questions on a new design - DanK tubes 06:57:52 09/11/09 (0)