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Old 6th July 2011, 12:17 PM   #1
diyAudio Member
Join Date: Dec 2010
Default Advice on 6418 design requested

I'm doing a build-it-youself valve amp project for the local electronics club based on the JAN6418 valve. I've looked at pretty much every 6418 circuit out there on the web but have come up with something slightly different for the simple reason that I want to be able to explain every detail of the circuit. Sometimes copying a circuit leaves you in a position where you don't know why the designer did something that particular way. Here's the circuit so far (without the power supplies):
Click the image to open in full size.

Some questions:
1. My understanding is that for directly heated cathodes, the voltages given on the datasheet are all referenced to the most negative point of the cathode. Is that correct?
2. I've made provision for a separate grid bias supply which would bias the grid to -1.0V relative to Pin 5. Also, I put in an electrolytic from Pin 5 to ground to decouple the power supply close to the valve and keep Pin 5 at AC ground. Does this makes sense?
3. How can I find out the grid capacitance? The 6418 datasheet does not provide it but a similar valve - the CK512AX - has a grid capacitance of 2.3pF. Would it be safe to use, say, 3-4pF for the 6418? (The reason I want to know this is so that I can model the frequency response of the input stage)
4. Should I put a grid stopper resistor in? If yes, is a value of 100K suitable?
5. I'm not sure whether to leave the gain controls where they are or put them between the valve output coupling capacitor and the Maxim MAX4410 headphone driver. What are the pros and cons? What would you recommend?

Thanking you in advance for any advice

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