pilot: Too many tubes

The Pilotuner T601 schemetic shows 5 tubes/valves. Mine has a 6th tube under the chassis. A 6C4. Not sure where to go from here. I replaced all caps, and all 6 tubes are good. Just trying to figure out why the 6C4?

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Hello Victor,

please show some pictures of your Pilotuner T601 to help determine if this is an original factory variant or a later modification.

Thank you.

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I've never seen any documentation to indicate that there was ever a version with an added 6C4. I think that this was likely a user modification, not done at the factory. It's not normal for a factory built product to have a tube mounted under the chassis. As Bernhard suggested, a picture would be very helpful.

Off the top of my head I can think of a couple of reasons for adding this tube. The most likely is to get a bit more audio output. The other possibility (less likely) is that it is part of an added AFC circuit.

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