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The fuse of this guitar amplifier blows. Changing the output transistors had the amp working for a while, but then after a while the fuse blew again. The output transistors that were changed are new. The configuration based on the schematic here seems to be a Darlington style push pull B class amp output (correct me if that is wrong) and that all seems to be okay.
From what I understand this problem with the fuse blowing could also be the zener diodes, is that correct? If so while I am at...is it recommended to change the filter capacitors as well?
One other thing...for draining the filter capacitors...is the cement type 2k ohm, 10 watt resistor on the terminals necessary, or is that needed for only tube guitar amplifiers? Thanks. enter image description here

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  • \$\begingroup\$ Note : This is the output part of the schematic of The Peavey Backstage Plus Guitar Amplifier, not the entire schematic. \$\endgroup\$ – Jack Jun 13 '18 at 2:05
  • \$\begingroup\$ Welcome to EE.SE. That is a typical class AB amplifier. When the transistors were replaced maybe they were not a matched pair or the CR15 temperature sensor is not close to them like it should be. \$\endgroup\$ – Sparky256 Jun 13 '18 at 2:11
  • \$\begingroup\$ Okay, that is class AB...but that is a Darlington push pull, correct? (emitters to base, collectors connected). Thanks I will double-check transistor specs. That does seem to be okay though in that regard. \$\endgroup\$ – Jack Jun 13 '18 at 2:20
  • \$\begingroup\$ Thank you. And it is pushing and pulling as well correct? (+ sine wave one set of transistors, then - sine wave the other pair of transistors)Is the CR15 working to prven \$\endgroup\$ – Jack Jun 13 '18 at 2:58
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    \$\begingroup\$ I cannot seee how that circuit will ever work. Where is the push/pull output (it seems to connect to ground). How does it connect to the speaker (I see no connection). Maybe I'm having a bad day? \$\endgroup\$ – Andy aka Jun 13 '18 at 9:47
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That is a typical class AB amplifier. When the transistors were replaced maybe they were not a matched pair or the CR15 temperature sensor is not close to them like it should be.

Those outputs are wired as Darlington's to get the current gain. CR15 must physically touch or be mounted to the same heatsink as the output transistors, or they will overheat and burn up within minutes, short out and blow the fuse(s).

It is a push-pull amplifier with +/- 38 volt supply rails, so ground is the common return for input and speakers. The signal is symmetrical above and below the ground line. CR15 is a temperature sensor that regulates the bias current to the output transistors. The hotter they get, the less bias current CR15 will let them have.

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  • \$\begingroup\$ Thanks again Sparky. I will go over all of this. What about the draining of filter capacitors to release charge even when guitar amp is not plugged in? Is the cement type 2k ohm, 10 watt resistor fine for that, or is that only necessary for draining caps on tube amplifiers? \$\endgroup\$ – Jack Jun 13 '18 at 4:34
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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 <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 <~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.

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  • \$\begingroup\$ Thank you Tony. I will study this, and check everything. What about the draining of filter capacitors? Is the cement type 2k ohm, 10 watt resistor fine for that or is that only necessary for draining caps on tube guitar amplifiers? \$\endgroup\$ – Jack Jun 13 '18 at 4:30
  • \$\begingroup\$ Draining not needed since <50V is considered safe low voltage \$\endgroup\$ – Sunnyskyguy EE75 Jun 13 '18 at 12:22

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