Guitar Amplifier Peavey Backstage Plus
Background (1st look)

This design uses a standard Class AB design with fixed gain controlled by an inverted output using the drivers as the floating ground symbol and inverted output as the capacitance divider from the power bridge diodes.

Most often it is the other way around but it doesn’t matter here since negative feedback uses the Op Amp +ve input with AC gain=R60/R61=~7

CR15 must be heatsunk to output drivers and are designed to run with temp compensation so the diode pair DC voltage is reduced by -2.1mV/‘C from 1.34V for the PNP’s and the NPN’s reduce the bias with a Factory Picked (F.P.) 22 Ohm R45 to match roughly leaving a 0.19V idle DC OpAmp to reduce crossover distortion and Quiescent current.

Problem


blows 2A fuse after a few minutes


due to power On cap surge current , fuse should be slow blow 

check all Vdc on schematic as it warms up and  any F.P. Resistors may need adjustment. (Report errors in DC
check/ replace caps with same or bigger low ESR caps as under power load with loud bass unequal rectification from faster decay time creates more DC offset and load current with distortion.
feel which parts are too hot to touch.


2nd look

Both Polarity drivers have active current limiting starting around 4A or 32V into 8 Ohms but speakers are usually &lt;4 Ohms DC so any speaker differences or DC offset from bad caps or bridge diodes may trigger current limiting but 4~5A secondary shud be &lt;~2A primary for a fuse. But again it must be slow blow.  Transformer would be expected to be about 4.4:1 (est) turns ratio .  (120*1.4/39=4.4) So it is protected.