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    SOS! Clean Channel is the only working channel. Help!

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    Darkshines

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    Join date : 2016-12-10

    SOS! Clean Channel is the only working channel. Help!

    Post by Darkshines on Fri May 05, 2017 8:38 pm

    Hi all! I think this is my first post, but I've had my GM36 since October 2014. I love this amp so much, it is basically everything I've ever wanted from an amp.

    But!
    My clean channel is currently the only channel that is working at full volume. the 3 drive channels still have some sound, but barely audible, at all attenuation levels and at full 36 watts, even at 10 for master and volume. It was working well just less than a week ago, on Sunday night, and now this. I'm want to believe it's just tube related, but I'm not very hopeful. I have had some crackling and slight buzzes/hums for several months now and I've been meaning to replace the tubes, but money has been tight. I've really started digging into my Schecter Omen 8 the in month or so (got it late last november) and I've been starting to develop potential recording material. Well, this has put a damper on things. Do you guys have any advice for me? Where should I start? Anyone have the same problem?
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    bordonbert

    Posts : 745
    Join date : 2015-01-28
    Age : 97
    Location : Southern England

    Re: SOS! Clean Channel is the only working channel. Help!

    Post by bordonbert on Sat May 06, 2017 4:32 pm

    Hi Darkshines, welcome to the forum.

    This points towards V1b being faulty, or perhaps something around the V1b stage dropping the gain.  All channels share the first triode in V1 as the first valve stage but the Clean channel gets routed past the second triode in V1. All the other channels use V1b for extra gain.  (V1 is the one with the can over it.)  Do you have a spare 12AX7 you can swap with V1 to check this out?

    If you don't have a spare then you could try swapping it with one of the other 12AX7s but that wouldn't fix the amp, just move the problem elsewhere if it is the valve.  You could at least see if things change and know it follows the valve.  Swapping it with V3 in the Phase Splitter stage would be best as it would mean that one side of the output signal would be lower gain or maybe distorted in comparison with the other but it would apply to all channels equally. The PS is the one at the other end from V1 of course.

    Remember where your valves came from originally and make sure to put them back there after you are finished swapping.
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    HwyStar
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    Re: SOS! Clean Channel is the only working channel. Help!

    Post by HwyStar on Sat May 06, 2017 7:15 pm

    Sometimes BB, you just amaze the sh%t out of me with your amp knowledge and background! Thanks Mate!



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    bordonbert

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    Re: SOS! Clean Channel is the only working channel. Help!

    Post by bordonbert on Sun May 07, 2017 7:53 am

    Hahaha!  You give me too much credit Hwystar.  It's nothing more than decent applied engineering, which is what amp design and maintenance really is.  You know my views on the reliance on the "mojo" approach, "what people call mojo is only engineering that they don't understand yet".  (quote from The Thoughts of Chairman Bert).  The saddest thing is to see the new generation being sucked into the belief that it is more important to look wise just quoting the old adages told to them by people who themselves didn't understand than it is to apply their God given intelligence and question everything they are told in order to really learn.  If you will only question, the truth will out and you will gain wisdom in the process.  Quoting old worn out mantras is not wisdom!

    It also saddens me to see some of those gurus using the title "Techs" insisting that their level of knowledge and understanding represents the pinnacle of amp design and maintenance and that questioning and analysis or suggestion of a different solution is an insult to their position as "conveyors of the word".  I've said it before many times, the job of the tech and the engineer are different and they should respect each other for it.  While there is an overlap of the two, their areas of expertise and knowledge are basically different.  The tech in our guitar sense has to work with a huge number of amps and gear as individual units on a day to day basis.  His job is to fix them when they go wrong and maintain them working in top condition and to do this he needs to gain a background of knowing those units and their common weaknesses and the best solutions.  He does not need to understand the intricacies of how each stage is designed and put together and why, that is the job of the engineer whose job is more or less done when the amp goes on sale and the tech takes over.  The engineer has to understand each amp type as a common design not as individuals, how their shared circuit configuration works in principle down to a high degree of detail, why that was decided on, and where it's potential weaknesses might lie.  Of course, there are great techs and sound engineers who do spend time gaining knowledge in both camps but there isn't time to become an ultimate expert in both!  It is better to have respect for the other side and trust their word in their own area of expertise, (or at least the word of a few you have faith in based on their past results.)

    Really, my own advice here and elsewhere is always based on a long understanding (with limitations of course, none of us knows everything) of the engineering side.  I could not tell you what would be the most likely area of breakdown of a 1997 Peavey Classic 50 with Gain problems, only what the schematic suggests or digging in the unit shows is the most likely cause.  But a decent tech probably could point you more or less at it from his previous experience with that model, (at least one who specialises in Peaveys).  And he may know that the commonly occuring problem is shared with other amp models. I wouldn't expect them (though I know some do) to understand the relevance of load lines in a common cathode/DC coupled cathode follower pair and why this configuration is sometimes flawed. It's enough that they know that a particular amp can have a weakness in this area and why and what to do when it happens to get it up and running again.  A really good one may even be able to put in place a workaround to prevent it happening again.

    And why do I get into this often belligerent area and come across as a trouble maker?  Because every so often I learn something I was not aware of and become a little wiser myself.  As an example, I had this clarified to me by someone more conversant with valves than I am who could describe the issue in terms which instantly removed a nagging doubt as to my own explanation.  Everyone "knows" that you can remove a pair of valves in a 100W 4 valve output stage and run it at 50W.  Everyone "knows" that it should really be done with a change of output impedance setting.  There is argument online about whether this should be set to 2x or 1/2x the speaker impedance, I have seen both cases argued.  Now, I have always known that the amp should be set to 1/2x the impedance of the speaker you are driving and why.  But my way of explaining the process to myself left a niggle in my brain which the other guy's way of looking at it removed.  The modern answer with too many techs/engineers is "who cares, I just do it and rock out my 100W in smaller venues".  So how do you know it is right and you aren't risking damage?  "My tech / expert buddy / milkman's sister says it is".  That's not enough understanding for me if I'm to work on my gear with reliability, I need to know why.

    Everyone is capable of learning a little of this approach, it is the backbone of what we all do. If you don't search out real understandung the area turns into a pseudo religion with priests and holy books and sayings from on high which are the whole unquestionable truth and it absolutely pisses off an old atheist (musically speaking) like me.  I'm the one to turn over the tables of the money lenders who spread this approach in order for them to make a great living out of it.  Asking the wrong questions and pointing out the wrong facts regularly gets me pilloried, scourged, crucified and burned at the stake on sites.  (Witness my latest post in another thread re Beam Blockers, they are an out and out marketing con! Wink )
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    HwyStar
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    Re: SOS! Clean Channel is the only working channel. Help!

    Post by HwyStar on Sun May 07, 2017 8:25 am

    Well the gents around this part of the woods (and a couple gals) rely on your background and well founded knowledge to answer some of these technical questions. And, if they don't always like your solutions then so be it! It is a free country... ah world!

    It takes time and energy to research these topics. With a strong background in electronics that can't be taught in just one book. I studied electronics back in 76-78 and it wasn't for me. But the knowledge I gained from back then has helped me in many other ways that I would never have thought of. I don't have those brain cells anymore so I will timidly remain quiet.

    Knowledge is king. It separates good players from great players. Thanks again!


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