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Re: What is happening???

Okay. First, we miscommunicated about the caps. I was referring to the PS caps. Since you have a PS in place I'm sure it's fine, whatever you used will work--PIO is okay, whatever you like to get the 750VDC. For the bypasses, put 100uF across the EL34 cathode resistor and anything between 220-470uf across the 6N7 cathode resistor for now. Just single caps, don't double them up. Again, be sure they're oriented properly (you know this). The bias supply doesn't need current, whatever you rigged up should work okay as long as you're getting the negative voltage you need.

Now for the screeching--if it only happens with K-horn, I suspect you're getting some mechanical system feedback. This may be the result of excessive gain overall. Are you using a high-gain preamp, like a 6SN7? The circuit has pretty high gain intended for dynamic (as opposed to horn) speakers and a low-gain front end. If this is the case, try 1) removing the bypass cap from the 6N7 cathode OR 2) if you have one, substitute a 6J5 for the 6N7. It'll drop right in. OR 3) substitute a 6L6 for the EL34. Unbypassing the 6N7 will reduce the gain but it may not sound as good. If it turns out to be an issue of too much gain, you're better off to substitute a 6J5 or 1/2 6SN7 for the 6N7, though you may lose a bit of the magic that the 6N7 seems to provide. Replacing the EL34 with a 6L6 will give you a tad less overall gain.

Also, make sure you tied the suppressor grid of the EL34 to the cathode--meaning pins 1 and 8 should be tied together.

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