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Old 25th July 2011, 01:40 AM   #1
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Default 6EZ8 (and other triple triodes) help please!

I recently stumbled upon a box of old tubes, many of them replacements for tube tvs. In it was a pair of 6EZ8's, and after looking this tube up, I decided it would be an interesting tube to replace with a 12AX7 or any other dual triode 9 pin.

I have a Fender Champ 5F1 that I built myself, so I decided to mod the preamp socket to use a 6EZ8. I have general knowledge about tubes and some specific when it comes to guitar amps, but as far as the 6EZ8 I just have no idea. I believe the heater coupled cathode is going to be a problem (and probably already is). I get terrible noises right now. I don't really know how to bias the two triodes whose cathodes are also tied to pin 4 and the heater. My heater supply is 6.3 at 2A with one side of the coil grounded and the other going to half of the heater pins of the tubes.

My theory is that the 6EZ8 requires an ungrounded heater supply, but it could be anything else since I have no experience with this particular tube. I would really like a 3 gain stage Champ, so any help with this would be amazing. The terrible noises i refer to is a loud hum that increases as the tube warms up. Thanks for the help.
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Old 25th July 2011, 09:36 AM   #2
DF96 is offline DF96  England
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The 6EZ8 has three triodes, which look to be vaguely similar to ECC81/12AT7. As you say, two of the triodes have a common cathode connection and also share one side of the heater. Two options:
ground this pin - this means an unbalanced heater supply (so risk of hum), and difficult biassing for these two triodes but they can be used separately from each other.
use cathode coupling - this means some form of LTP (phase splitter?) with the heater supply biased from this cathode point.

In either case you will be constrained in your design so not a good idea unless you really want to use these valves.
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Old 26th July 2011, 10:30 PM   #3
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Thank you, I actually came to a similar conclusion before I read this. They're just odd tubes. I figured I could beef my champ up but it just requires too much effort. I just ended up re-wiring the heaters so I could run a 6BK7B or a 7AU7 instead. It has a great tone now so I'm happy with it!
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