I have a 2005 Marshall Valvestste 2000. the amp suffers from a certain amount of hum especially in the overdrive channel. recently I changed the standard single 12AX7 for a JJ 5751 tube which is slightly lower gain as a way of 'cleaning up' the sound. As soon as I plugged in the amp emitted a lot of hum, was unplayable so i switched off and put the old one back. Now it's OK. Either the circuit will not function with that tube or the tube is faulty. I believe the 5751 and 12AX7/ ECC83 are constructed the same .. but the 5751 has shorter plates...any ideas?

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