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I've got one of these old ones in for repair, reverb not working. I'm pretty sure it's the tank(It's the long one at the bottom of the cab, 4FB2B1B). Before I order the correct tank(I'm also taking recommendations on the best one for this amp), I wanted to check with the minds here, as the reverb control acts more like a tone pot. The amp gets really bright and a little hissy as the reverb knob is turned up. Even with no tank connected, it acts the same way.
I'm not seeing anything in the schematic that would make it act this way.
Is this normal operation for this amp?
 

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It's not normal to run it with the tank disconnected. ;) So there's no saying what is normal sound or not when operating with a broken or disconnected tank. Seeing how it does the same thing whether connected or not, sounds like the tank output may be open. Have you checked the resistance at the tank's in and out jacks with tank disconnected and have you checked the reverb cables?
A 4FB type tank should measure approx. DC resistance of 200ohm at the input jack and a bit more at the output jack, around 215 ohms.
The little wires inside the tanks that go from the jacks to the little coils often break and can usually be repaired, although modern tanks are very inexpensive so people don't often bother repairing the tanks.
 
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