zener base drive clamp

[indianajo]

I'm repairing a conventional split supply NPN-PNP bipolar output amp with NPN-PNP predrivers on another PCB from the output transistors. After the O.T overheated and the diac-triac DC protection on the output crowbar burned through the PCB land, the 95V rail burned through a driver transistor, assorted PCB lands and about 15 4148 diodes, the TO92 transistors, the 4558 input amp, some resistors, and even shorted an electrolytic capacitor. Specifically, this is a PV1.3k amp 1994 vintage, the schematic is on eserviceinfo.com search pv1.3k by Peavey.
So I replaced some jumpers on the driver PCB in the driver emiter line out to the O.T. bases, with fast blow 1a fuses, hoping these will be big enough for the drive current in the 2 ohm 22 A output load condition.
This amp is typically used driving 4x15" speaker cabinets at 4 ohms per channel, on bass guitar typically, so extreme fidelity is not required. I'm testing with 10 ohm 450 watt resistors, wirewound, with a car radio speaker across 5 ohms of the resistor for some sound check. I don't dare connect these things to my 300 watt rated SP2-XT speakers, particularly as the first test it still had one shorted O.T on the "good" channel and blew up 25 more little parts on the driver board of the "good" channel".
There is some kind of a crossover elimination bias circuit involving reading the emmiter resistors on the O.T's, running them into PNP or NPN to92 transistors, then some other stuff I don't understand, but presumeably the O.T.'s are supposed to be slightly on most of the time,
So I propose to put 3.3v zener diodes backwards on the driver output (emmiter) to the O.T. bases to ground, (across the 47 ohm driver emmiter resistor, really, some of which blew up last time) to try to limit the damage next time it happens hoping the zeners will take out the base drive fuse before all the other little parts go.
I also added 25 amp rail fuses to the O.T's assembly, replaced the crowbar triacs and diacs, and put an 18 ga wire in place of the burned PCB land on the output DC detection crowbar circuit.
I presume the bases to the O.T.'s run 0 to 0.7 V at all times, so the zeners are invisible. Is this assumption correct? Are these zeners correct?
Can I also put fuses on the OT emiter resistor read lines, and clamp the input to the sensing transistor with 8V zeners to ground? Are these emiter resistor signals supposedly 0-2v for 4amps out of each O.T. across a 0.5 ohm O.T. emiter resistor.

[nigelwright7557]

I dont think the zeners would do anything as the output transistors switch on at 0V7.
And 0V7 is all it takes to blow up the OT into a short.

[indianajo]

Not expecting the zeners to protect the O.T's into a short. Trying to prevent the 25 little parts on the driver PCB from going at the same time as the O.T's, as well as the lands on the driver PCB. It takes a microprocessor and a current transformer on the output the protect the O.T's, (like a CS800s) and I don't have one in a PV1.3k. Trying to protect the DC crowbar parts and the speaker with the 25 amp fast blow fuses on the O.T. supply rails, as the master input AC breaker obviously wasn't fast enough to protect the crowbar PCB lands last time.

[nigelwright7557]

Its a bit of a juggling act with those powerful amps, setting the breaker to catch problems but not cut out on high loads.

[AndrewT]

what size rail fuses are needed to prevent nuisance blowing?
These fuses should also allow all music signals to reach the speakers.

95Vpk into 4ohm speaker load is ~2kW.
That would need approximately F20A to F25A fuses to never blow for all valid audio signals into all valid speaker loads >=4ohm.
You might find you can get 2kW using an F16A fuse in each rail.

If your amp is <<2kW then the fuses should also be <F20A.

[indianajo]

Thanks for looking again. The PV1.3k is rated 1000W/ch 2 ohm load, 650 W/ch 4 ohm load. The rail fuses to the O.T. collectors I have installed at 25 amp 32 volt AGC to try to limit the damage when the DC detection crowbar trips because an OT is shorted. Might save an OT or two instead of all of same polarity shorting next time. Don't have it working yet, so don't know if they will nuisance trip. Don't have a 1000 W 2 ohm load, either, just 10 ohm 450W. Can buy another 10 ohm at 450 w pair of resistors for $20 + $10 shipping CA-IN.
The base drive fuses I am thinking I need 5 amp fuses to allow hfe 4 on the output 5 pairs of O.T. with the 2 ohm load . It has MJ15024/5 base drivers, so 5 is a little generous, maybe 4 amp fuses. Anyway when an O.T. goes, I expect 95 v to come careening in the base line, and hopefully blow the zener and the 5 A fuse before 25 other little parts blow out or short. Zeners are 1.3 watt this time, hope they have enough surge current to blow the 4A fuse.Last time the smoke was pretty impressive, and the plastic popping up off the to92 transistor and the 4558. Not having any expensive in circuit testers, it is time consuming lifting one lead on all these little parts and examining them with an ohmeter, although they don't cost much. All were junk PCAT power supply parts last time except the MPS8099 & MPSA56 transistors and the 0.15 uf 10% ceramic cap. But took a week of odd bits of time to find them all (most of them?) I've got junk PCAT power supply fuse holders I can drill the driver PCB for, although these little 1/2" fuses don't come in a DC rating. Don't know how fuses/zeners on the O.T. base drive line (driver emiter) will sound like, maybe nobody does this for a reason.
On the OT emiter sense resistors (2.2k 1/4 w) been thinking of downsizing to 1/8 W resistors and putting 1 amp fuses on those lines, as some of the current blasted in those lines and blew up some transistors and stuff, hopefully not the unobtainable LM3180 op amp. Then maybe the 25 A rail fuses would help limit the current to those lines next time.