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Peavey 400BH bias

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  • TomCarlos
    replied
    The thermal breaker is mounted to the output device heatsink. It doesn't matter how you hook up the leads. The device is a simple switch. It opens when it gets hot. So that means you will not have connectivity between the two terminals.

    Click image for larger version  Name:	image_25934.jpg Views:	0 Size:	216.0 KB ID:	958242
    Last edited by TomCarlos; 04-15-2022, 01:03 AM.

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  • bubasknr
    replied
    Cool! I couldn't figure out how to look it up. Thanks a bunch

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  • Jazz P Bass
    replied
    Google L185 switch NC
    https://www.bing.com/search?q=L185+F...NC&FORM=HDRSC1

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  • Enzo
    replied
    Hi
    Do you mean the round thing with mounting tabs, and two push-on connectors up top? That is a thermostat or thermal breaker, not a thermistor. A thermistor is a resistor that varies resistance with heat. This one wired to the mains? It opens when the heat sink gets hot.

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  • bubasknr
    replied
    Hey, does anybody know where to get one of these thermistors? I have one here that cracked and broke in half. Part number r82-222 l185 79/n. Thanks a lot

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  • Enzo
    replied
    Loose pilot light socket?

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  • J M Fahey
    replied
    Add to the excellent checklist above any extension cord or cheesy wired mains outlet feeding the amp.

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  • azatplayer
    replied
    So punk bass guy came by with his cab and gear. Set his cab up and i got the head on. Asked him for his speaker cable and he handed me a pedal board patch cable.. convenient length he exclaimed.
    I grabbed my nippers and cut it in half before him. Stripped it and some figure 8 and showed him the difference.

    Thats a crap story as far as techs go. But i actually love these kinda musos. Theyre in it primarily for the craft. Not the gear or tech stuff. Was never about money or opinion. just a convenient choice. Thats not usual among guitar fiends, more so among bassists. And much moreso amongst front men
    Doesnt explain the heat or disco pilot lamp. Ill work that out. But there will be tones at satdays post lockdown gig.

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  • Enzo
    replied
    I liked the Special 130 from the start. When it came along, i decided it was the perfect "first real amp" for some guy. it was large enough to gig with, had clean and dirt, and sounded OK.

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  • azatplayer
    replied
    Ah yeah of course its made in the USA 185F, which ironically translates to a bit over 80 C! Yeah i generally ask guys for their whole rig, but he just landed at the door with the amp. He should be here soon enough with the rest of it.
    Ive had jacks fail on cabs too, so always look for them. They get treated pretty hard. Oh i just finished a restore on a 130 special. And far out thats some pretty great SS guitar tone! One of only 2 SS amps id like to recreate, ive enjoyed playing it so much. Maybe the guitar worlds not quite ready for a boutique SS experience yet tho
    Great point on the thermal. Theyre a kinda reed switch with a bi metallic strip that bends right? So has to cool to bend back. Its been 40 years for this old girl. I'll clean all the contacts in the ac path around it and try and find another to be sure.
    Thanks for the Farenheit correction, i wouldnt of thought of it that way here in the land of metric.

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  • Enzo
    replied
    I was thinking the 185 might be the temp.

    You cranked it into 4 ohms and it ran for a while, doesn't sound like it is thermalling to me. Also, once the thermal heats up and opens the circuit, it won't close the circuit back on until it cools. SO it won't flick off and on quickly when it thermals. That sounds more like a loose connection.

    Yes, always check their speakers out, and don't forget to do a serious check on his speaker cords. They need to be real speaker cords, not plain guitar cords. Plus they need to be in shape.

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  • azatplayer
    replied
    Leads are pretty solid. Thermistor is R82-222 L185 81/M I guess that means 81 degrees. It makes 135 watts or so into 7.5 ohm load and i sat it there for 20 minutes, it hit 55 degrees at the thermistor and the hottest on one output was 60 degrees, hit those temps within ten minutes and stayed there. I had it running at 200 watts yesterday into 3.75 ohm load and it was happy on the scope and ran around 65-70 degrees at the thermistor. Something like 55 degrees at the rear sink close to the outputs. Certainly passes the 5 second touch rule.
    That was for me here in controlled environment. Im getting him to drop his cab in later to see whats up with that. Its a behringer BA115, and it could have voice coil issues, wouldnt be the first behringer speaker that ive replaced.
    I reckon hes overdriving it into an inefficient 8ohm cab that may be on the way out. As the amp looks good.

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  • Enzo
    replied
    What numbers are on the thermistors? Are we actually testing temperature? Or are we assuming it is the thermistor cutting open?

    Low voltage across the emitter resistors would actually be evidence of the amp running cooler.

    For the thermostats to open, the heat sink would be really hot to the touch.

    Wiggle the wires to the thermostat, does that make it cut out? Likewise wiggle the power switch, the power cord, the fuse holder, any other mains related wiring.

    That amp should run full out into 8 ohms all day long. And it is rated down to 4 ohms anyway.

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  • azatplayer
    replied
    Oh also, anyone know what temp the thermistor kicks in?

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  • azatplayer
    started a topic Peavey 400BH bias

    Peavey 400BH bias

    Enzo. Firstly thank you for all the help youve been over the years, about anytime i look for an answer, there you are with one! Youre a legend among my electronics mates down here in Australia!

    I have a question regards a MK 3 head with 400BH PA. It makes advertised power, sounds nice but runs hot. The only thing that looks not right is the .05V at Q3-11 emitters. I have .02V at both. Im measuring that across R26 and R60 as the schem reads.The base voltage is about right, around half a volt. Bias string CR13 has the right volts either side of it, everywhere checks out. Except that .02V which i guess just means the drivers may be a tad weak. I really dont think theres much at all wrong.
    The owner is running into a single 15 at 8 ohms and it has cut out after a few hours of gig, pilot light flickering which suggests the thermistor is doing its job.
    My first reaction when he said he was running an 8ohm load was, that wont hurt the amp at all. But now im not so sure. Im feeling like hes running the thing at clip as its not loud enough with the 8 ohm cab. I reckon if he was running a 4 ohm cab, then he would have it dial'd back somewhat.
    It does sag about 5V on the 51V rails at around half output. But makes full power into a static load. Is he asking too much from it?
    Cheers! Don from Aus. Oh this was my first post of 11 years!
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