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holden/wasp troubleshooting,

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  • #1

    holden/wasp troubleshooting,

    hi guys
    i won the bid on an amp up on ebay from the early 70's. its an australian made head with a matching 4x12 cab. the thing is rated at 200w from 4 6550's and has 4 12a*7, a few of those are 7025's but there may be a lower gain tube in there such as a 12at7 or something similar.

    when i went to pick it up we powered it on for a minute then turned the standby swithch off. at this point i could see that the tubes were glowing from the heaters. after a few seconds it made a slight bizzing noise then shut down. the guy thought it was a burnt out power tube. we couldnt see wether the fuse had blown (it had a wierd coil obstructing view of part of it) but it was fine (i brought the fuse home with me to test) we left without making the exchange and said that we'd keep in touch and figure out how to get it up and running.


    after it "faulted" the amp wouldnt power on (ie the light didnt go on). he said that thats what happened when the power tubes blew in the past, but i dont know why the power light wouldn't go off once a tube blew. it has also been 20 years he said since it has been used (save being used once the week before to check that it worked, which it did, as well as a party sometime a while ago).
    having the light go off when a tube blows, is this a feature that some amps would have for some reason? im not completely sure what to make of it, as the light shoulc be running on the 6.3 volts of the heater. makes it seem quite unlikely, but the guy does seem genuine, and seemed to want to make sure i was going out there with a working amp. he made no mention of taking the money or doing the transaction before it is working properly, so i cant seem why he would be lying to me. hes a nice guy who's still playing guitar, but has no need for a 200w amp with a 4x12 cab. (im looking to possibly use it as a bass amp, and use the cab for guitar)

    should i just get some new tubes and hope for it to be up and running with new ones in there? he said that the light deffinatley went out and nothing happened when a tube died. i cant find a schematic for the amp either, so i cant just read that to see if he is right.
    the thing deffinately powered on. the light went on and the tubes started to heat up (could see the tubes glowing), then a bit of a buzzing sound and then nothing. the fuse didnt blow either. (unless their is another fuse inside the unit)

    the light really throws me off. if there was a blown cap (not unlikely on an old unused amp like this) then it would be on still as well. what do you guys make of this.
    heres a thread about the amp
    http://music-electronics-forum.com/s...ead.php?t=4700
    Tags: None
  • #2
    Tube amps generally have two fuses: one for the mains and one for the internal high voltage supply. (called the HT or B+ fuse depending if you're English or American)

    If the mains fuse blows, the power light will go out. If the HT fuse blows, the power light will stay on.

    Bad tubes often short out, or pass too much current, which blows a fuse. The HT fuse should have gone first. Maybe this design doesn't have a HT fuse at all, or a previous owner replaced it with a nail. :P

    Some amps had a neon light connected to the internal HT, and failure of either fuse would make that one go out.

    Anyway, try powering it up again with a fresh fuse and the 6550s removed. If it doesn't blow, then it was probably bad tubes that did it.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    • #3
      Originally posted by Steve Conner View Post
      Tube amps generally have two fuses: one for the mains and one for the internal high voltage supply. (called the HT or B+ fuse depending if you're English or American)

      If the mains fuse blows, the power light will go out. If the HT fuse blows, the power light will stay on.

      Bad tubes often short out, or pass too much current, which blows a fuse. The HT fuse should have gone first. Maybe this design doesn't have a HT fuse at all, or a previous owner replaced it with a nail. :P

      Some amps had a neon light connected to the internal HT, and failure of either fuse would make that one go out.

      Anyway, try powering it up again with a fresh fuse and the 6550s removed. If it doesn't blow, then it was probably bad tubes that did it.
      there doesnt seem to be a second fuse, might be inside the unit poking up like a tube does. kind of annoying how the amp is with him, but its probably best to wait until its running, id hate to pay for it and then find that its blown the PT (doubt it, but its better to be safe and sorry. besides i have an exam in a weeks time and its best for me not to spend too long working on an amp.

      would you think that the fuse's value would be 1.5 amps (each power tube draws 303 ma, and 4 12ax7's drawing 10ma each adding up to 1.252 A). might pick some fuses of that value up to go into the amp if thats whats blown.

      i think ill get some new tubes for it anyway. the ones in there are quite mismatched (2 being the originals from 1970 or whenever, and 2 tubes that were bought seperately when one blew and put in. im sure the other tubes wouldnt be far out. they all had those burnished marks on the sides that people say suggests their not too good anymore.
      i think your right about the light being a neon powered by the high voltage, as it didnt go on until the stanby switch went on. that makes me feel more comfortable, because there is some evidence that backs the theory up. ill call him tomorrow and tell him what i think, confirm the splitting of the new tube costs, and then get him to see if he can see a second fuse so he can give me a value for it.

      the only problem is the amp is at his mums place, who is moving house (why he was selling the amp), so he may not be able to check the fuse right away. ill figure something out. when he/we find the fuse we should be able to run the amp on 2 tubes to make sure it is functioning properly. then i can order the new tubes and run it on the old ones until it is nessisary to use the new ones.


      thanks alot for the help steve, really is alot of help

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      • #4
        just looked at schematics and pics of similar units made by them (couldnt find this model itself). seems the second fuse is inside the chasis itself, but i think ive found the correct value, so ill get the tubes and pick up some fuses, then ill get to hear this monster.

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        • #5
          finally got it up and running with the guy today. ended up just being a blown fuse. must of just been old.

          good sounding amp, the pots deffinately need to be cleaned, they can cut out sometimes among other things.

          theres a nice cab with 4 plessey speakers (australian made speakers from the 70's or so, appear to be alnico)

          im curious about the presence control, as when the pot cuts out (needs to be cleaned) the sound cuts out. i thought it would be within the -ve feedback system, but then it wouldnt cut out. oh well. ill have a look at it.

          the amp is on a circuitboard, but the tracks are very spaced out, and it has good components on it. might take it apart and take some pics when i get around to it.

          the amp has 2 channels, one is very clean and warm, the other has some breakup happening at the preamp, but it isnt adjustable. either way its a nice sounding amp, and i dont think ill be redoing the circuit soon. there is a place for a 5th 9pin tube, so i may do something with that, but keep most of the circuit the same.

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          • #6
            Originally posted by black_labb View Post
            finally got it up and running with the guy today. ended up just being a blown fuse. must of just been old.

            good sounding amp, the pots deffinately need to be cleaned, they can cut out sometimes among other things.

            theres a nice cab with 4 plessey speakers (australian made speakers from the 70's or so, appear to be alnico)

            im curious about the presence control, as when the pot cuts out (needs to be cleaned) the sound cuts out. i thought it would be within the -ve feedback system, but then it wouldnt cut out. oh well. ill have a look at it.

            the amp is on a circuitboard, but the tracks are very spaced out, and it has good components on it. might take it apart and take some pics when i get around to it.

            the amp has 2 channels, one is very clean and warm, the other has some breakup happening at the preamp, but it isnt adjustable. either way its a nice sounding amp, and i dont think ill be redoing the circuit soon. there is a place for a 5th 9pin tube, so i may do something with that, but keep most of the circuit the same.
            Only just noticed your post.
            Holden Wasp 200 output tube filaments are wired in series, if you remove one of the 6550s the other heaters will not work.
            I am completely restoring a wasp 200 now. Have made replacement PCB!
            Heaps of info on these amps and many other NZ and Aussie made brands at ozvalveamps

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