80's Series III x100 preamp tube problem

Legacy, V3, Vintage, X100B and SX Guitars Amplifiers & Enclosures

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80's Series III x100 preamp tube problem

Postby BonesMachine » Sat Apr 14, 2018 8:21 pm

The V1 preamp tube in my amp seems to be going bad after a while. It will cause a speaker crackle type sound. Also if i tap the tube after it goes bad it will cause a pop sound. I haven't swapped out all the 12ax7 tubes after replacing the power filter caps but I'm pretty sure this is what's going on.

I'm pretty much a novice with this stuff but i'm thinking it could be:
- another cap that needs to be replaced ie coupling cap (all caps are original except the filter caps)
- bad zener diodes (i remember reading that this was a common problem?). This could possibly cause voltage regulator problems ????

I tested the zener diodes:
- Zener diodes D6-D10 tested good
- Zener diodes D11-D14 had low resistance both ways but i read that you can't always get a valid reading if the diode is soldered into the circuit.

Any thoughts? Thanks.

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