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Thread: Peavey valveking 100 head blowing the fuse for the 14V circuit

  1. #1
    Member stevesamps's Avatar
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    Peavey valveking 100 head blowing the fuse for the 14V circuit

    I have a dead short on the small power supply that feeds the reverb return
    I noticed the transistor there is a small 2n4401 , not the ska3919 stated on the schematic, and parts list, at any rate, I found 2 shorted diodes, but it still cooks the
    fuse, I have not removed the board just yet, These are VERY DanGEROUS amps
    FYI I plan to install a discharge resistor,
    un-plugged, now its holding 460V
    with no discharge path to ground, this fuse blows instantly BTW, it seems
    like something is dead shorted, I plan to remove/replace the filter caps on that
    line as well, just wanted to see if anyone had a similar issue with this amp

    and I know be careful this is not my first repair I build amps, but geez
    man this is easliy one of the most dangerous amps out there
    when the amp came it it had been off/unplugged for more than two weeks!
    and it still had 360v DC charged up.
    I will admit I got hit and shit that woke me up

  2. #2
    Member stevesamps's Avatar
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    the diodes are shorted again

    I have removed and tested all kinds of components on this line
    caps transistors, nothing is shorted ?
    could this be the transformer winding itself?
    This amp has DC heaters on this line also..

    This is the Valveking 100 two of the bridge rectifier diodes

    http://www.schematicheaven.com/newam...ng_100_212.pdf
    d207 and d208

    Why Dc heaters on the power tubes? and PI

    The fuse still cooks , glad I installed a block holder.. those ckt boards types suck

    I assume thats what HTR stands for, the DC heater supply

    I also found the reason it charges up the caps so bad
    on standy notice the two 100UF caps and the 400 ohm resistor
    with no bleeder! I installed a 270K to ground

    Steve
    Attached Files Attached Files

  3. #3
    Senior Member Enzo's Avatar
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    Why DC heaters? It has all the tubes on DC. It allows them to run the entire heater string off one lower current DC supply rather than a high current low voltage supply like we see elsewhere. This eliminates potential failures in pc board connections, because teh entire tube heater stri9ng draws 0.9 amp.


    The fuse is after the transformer, how would it blow the fuse?

    Let's look at the circuit. Thw transformer AC is rectified and filtered to make about 30VDC. That flows through 1 ohm R213 to supply the heater string. From there another 400 ohm resistor drops us to 14v for the loop drive, the reverb circuits, and the relays.

    If I short the 14v to ground, that makes the entire 30v across the 400 ohm 5 watt R212. If I am doing my Ohm's Law correctly, I have 30v over 400 ohms for about 75ma of current. (0.075 amp) That will make R212 warmer, but 75ma shouldn;t blow that fuse. So my reaction is that the problem is not on the 14v rail.

    My guts tell me a power tube has failed, or possibly the plastic peg on the base has broken off allowing it to be plugged into the socket the wrong way. Pull the power tubes amd see if it still blows fuses. Actually, just pulling V4, the powr tube closest to the phase inverter tube should answer the question. One other possibility is one of the filter caps between tubes, C7, C8, C145 could be shorted.

    If pulling the tubes makes the fuse hold, try a different set of 6L6 in the amp.


    And make sure to use a slow blow 3 amp fuse.


    And 2N4401 or the 3919 NPN small signal transistor? ANY small signal NPN with sufficient voltage rating will work there just fine.
    Education is what you're left with after you have forgotten what you have learned.

  4. #4
    Member stevesamps's Avatar
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    Smile

    First mistake was trying to fix it while the guy waited,

    now I am not quite sure the order of events

    however I think what may have happened is a shorted tube
    and also the blown and shorted diodes , so it "continued to blow the
    fuse after the tubes were removed however, this was due to the blown diodes

    I think what I want to try is to put all the caps back in all were fine
    and the little transistor was also, just fine,

    restore the fuse one more time with the all 4 new diodes to be safe
    and see what happens then, I did not think the transformer could do it
    like you said the fuse it after (duh)

    I think I was trying to be a nice guy and do this in a hurry
    which was dumb

    I will try a slower more systematic method and see if I can get my sanity and the amp
    back..

  5. #5
    Senior Member Enzo's Avatar
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    AH, we've all been there.


    My old shop teacher used to harp at us, "No one has time to do it right, but we all seem to have time to do it twice."
    Education is what you're left with after you have forgotten what you have learned.

  6. #6
    Member stevesamps's Avatar
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    Thumbs up its fixed

    not sure if it was the tubes but after I replaced all 4 diodes and put back
    all the parts I took out to test, it fired up and worked with different tubes

    I did not have my tube tester so I put the ruby's in pairs in another amp
    they did not seem to cause any trouble, but I am still
    making sure the guy buys new tubes,

    I have to admit I was actually not interested in even chancing those power tubes
    back in the amp after all the time I spent on this

    thanks Enzo

    Steve

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