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Another Haze 40 mod thread

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  • Another Haze 40 mod thread

    Hi folks, iím A noob of course.

    Although iíve Benefited a lot from this site over the years, it has answered many google searches for me and helped me along as I struggle to find tone... as iím Sure we all do... forever.

    Iím what you call a hobby player - itís my 15-yr-old son who has the potential to really do something with an axe in his life. I started teaching him when he was 12... anyway, all my gear will probably eventually wind up in his hands.

    So iím messing with my haze. I never liked the crunch channel on this thing so iíve Never really played it. But thatís ok. Because the clean channel is unbelievable. And the combination of the tubes and my Metal Zone (yeah I know most people hate it but it works great for this application) is pretty amazing.

    But recently iíve Done a few things. Changed out the original pre-amp tubes for Tung Sols. Clipped a leg of c73 per instructions here. Tried to jumper R88, although honestly iím Not positive I even got the wired soldered in right.

    Anyway, now I want to make sure the power tube bias is properly set. However, while I know there are a bunch of master electricians on this forum iím Far from that. Start with what I know. I know that VR1 and VR2 are the pots to adjust the bias in each tube.

    What I donít know arenít the things you folks think of as the most basic things. Like how to check the bias. I have read some attempted instructions and to be honest, it seems like my particular amp is the most difficult on the planet to bias. Yay.

    I have a meter bit am not sure how to use it. 5here are 3 slots I can plug my probes into, and only 2 probes. Clueless on that. Also unsure where to set the dial. Beyond that, need to know exactly where to place the probes and what values to hit. Around 39.7 or so for stock power tubes seems to be whatís recommended, but then many folks want to talk about testing plate resistance and all kinds of other garbage that, from what I can tell, most people donít have to do with other amps.

    So what I really need is a walk-through on the process. Iím not sure thereís anyone here with both the skill AND patience or willingness to do that, but there it is.

    Anyone want to take on a small project with me? There are other things I want to do too, from what I understand itís possible to get this thing sounding very close to a JCM 800. That would be nice.

    Sorry for the length of the post and whatever happens, thank you all already for all the help this forum has given me.

  • #2
    The schematic calls for 39mV so I don't know where that extra .7mV would come from, or who would have a meter that they would think was that accurate.
    You will need a meter that at least has a mV range for DC voltage. Some very cheap meters I would not trust for this.
    Put you black probe to the middle of the 3 "slots".
    The red probe you will first put to 1st slot, then to 3rd slot.
    One bias pot will be for each side (#1 or #3 of connector)
    If you move a bias pot and the meter reading doesn't change, set it back where it was and try the other bias pot.

    The bias adjustment is done with no signal applied and amp in "play" mode (if it has a standby switch).
    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


    • #3
      Weird, I submitted a reply but it didn't show up so I apologize if I wind up with duplicate posts.

      Thank you g1. Anyone with he great Lemmy in their Avatar must be very learned in all things musical.

      I dug up the manual for the meter, and located the DC section where it does have a DC mV reading. Some of the threads and videos I've seen say I should set the dial to 200m on the DC section, correct?

      So with the negative probe in the middle slot and positive in the right one, dial set to 200m, the amp on and standby off, and me being extremely careful and probably using rubber gloves, I'd be ready to test...

      Where I'm stuck is when you say #1 or #3 connector. I know where the pots are, but not the connectors you mention. Some of the things I've read say there are 3 pins i need to find... Are those actually in the same vicinity as the pots, or somewhere else entirely?


      • #4
        The central pin is earth. The other two correspond to each cathode.
        With a multimeter set in mV (200mV full scale) you must measure the voltage between the center pin and the top pin (for a tube) and then between the center pin and the bottom pin (for the other tube).
        I do not remember the exact assignment to each adjustable resistor, but moving it slightly can be deduced without difficulty.
        Attached Files


        • #5
          Sorry, I misunderstood where you referred to "3 slots" to put your probes into. You meant on the meter itself but I thought you were talking about the connector Pedro has circled in the photo he posted.
          If you found your meter manual, it should show which of the 3 'ports' on your meter you should plug the probes into. Usually one of the three is for current measurement which you do NOT want to use here.
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


          • #6
            You guys are awesome. That pic was exactly what I needed, Pedro

            And yes, the manual told me how to use the probes g1.

            So I tested things out and one was fairly close, only off by a couple, but the other one was down at 33. So I adjusted them both. Haven’t Had a chance to turn it up to any kind of volume yet to see if there’s a tone difference, but it’s good to know I have 2 properly biased power tubes.


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