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Thread: Hum (50Hz) from GP3 output stage

  1. #1
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    Default Hum (50Hz) from GP3 output stage

    Hi there,

    I'm a new proud owner of VHT GP3 Valvulator preamp and really being excited about the overall flexibility and tone quality of this unit. I have paired mine with Mesa 20/20 power amp and Mesa cabinet with two C90 Black Shadow speakers and this setup really works great for me. All my gear is European 230V/50Hz version as I live in Europe/ Czech Republic. Coming now to my problem: I have noticed some strange hum (like 50Hz) coming out of my GP3 (using AMP OUT - plug). I have also found out that amount of hum depends on MASTER VOLUME setting and not on CHANNEL VOLUMES of particular channel sections (they can be even muted to fully counter clockwise - like to 8 o'clock). The more volume on MASTER LEVEL the more hum in output signal. If MASTER LEVEL volume knob is also muted (8 o'clock) - there is no hum. So currently I'm using a work-around like I set channel levels almost to maximum and try to bring down MASTER LEVEL to some compromise setting (something between 9-10 o'clock). Here couple of hints and facts I tried already:
    - Tried LIFT/GROUND switch, currently I'm running GP3 in lifted mode as GROUNDED brings really significant hum. MESA 20/20 runs in GROUNDED mode.
    - Tried another my old preamp (Soldano SP-77), same cables, same setup - no hum at all
    - No other cables/effects are connected to GP3, neither guitar
    - Experimented to use as an alternative RECORDING OUT - plug. Same result - hum was there as well.

    So my question is: Any ideas what the problem can be? I don't think that such professional unit should produce hum. Indeed noise is there, but it's nice tube noise, I like that! What tube(s) is/are dedicated to that "low power" section which might be faulty and produce hum? Thanks for any hints, folks!

  2. #2

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    If you browse the forum for a bit, there's a dedicated section where a lot of the manuals are. You'll probably find your answer there.

    Good luck !

    Giga

  3. #3
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    If it has no hum with another amp I doubt it is the GP3.
    Try disconnecting the shield of the cable plug, amp end.

    Love mine - welcome to the club!

    V6 is the "Low power" tube but it doesn't sound like the problem.
    May also be a problem with wiring to the Mesa.
    Last edited by Mark; 11-12-2011 at 04:30 AM.

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    Hi guys!

    I'm putting here some progress to the original post I made below. After investigation and inspection done by professional technician the fault was found. Some switching relays were faulty inside my GP3. So he replaced almost all of them (10 pcs) and my baby rocks again!

    Other experience I gained relates to placement of unit in rack. Originally I had GP3 laying on top of Mesa 20/20. But definitely it is better to have them separately i.e. having 2 rack spaces between them. This significantly eliminates hum produced probably by Mesa transformer which is then induced in GP3.

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    I can understand that the mesa power amp will bleed hum into the GP3. But changing the relays makes no sense at all. They don't hum or pick up hum so that's a big question mark.

    The relays we use there are the same we use in all other amp models. They have been the most reliable and least replaced part I have seen, so having to replace them is not only highly unrelated to the hum issue, but not likely to have been bad in the first place. I don't think we have replaced 10 relays in 10 years of servicing amps here.

    More likely your tech took the unit apart to replace the relays and fixed something else accidentally that really fixed the problem - that is if the problem was more than simply proximity to the power amp - which I doubt.

    By the way, I did respond to your emails to tech support, but I never heard back from you.
    Dave Phelge
    Chief Misconception Eradicator
    support@fryette.com

    "We are what we pretend to be, so we must be careful what we pretend to be". - Kurt Vonnegut

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    Hi Dave!

    thanks a lot for your reaction! I'm kind of shocked now because any of your replies has never reached my mailbox. So I thought no one has replied to my emails. Maybe your messages were filtered-out by some anti-spam filter. Really sad situation.

    I have discussed with my tech what in detail he has done. So I can update you with more info:
    Basically your are right, I don't say that all 10 relays were totally faulty. But most of them simply were no longer reliable. When knocking on them with a screwdriver they caused fall-outs of signal. So my tech has replaced all relays in signal path of all 3 channels to make sure they are not causing some other hassle.
    I bought my GP3 used, with gain-mod on it. So during inspection my tech also has found very amateurish soldering of those resistors and capacitors, so he fixed them properly. He has found also some bigger teardrop of solder on the opposite site of PCB which might bridge some contacts.
    I hope this clarifies a bit more my problem. And it's also kind of advice for the others who might think about gain-mod by do-it-myself (like previous owner of my GP3 ). Be careful and make sure you know what are you doing and how.

    Anyway my GP3 just works great now and thank you people @ Fryette for it. You made a really great sounding preamp.

    Dave, I'd appreciate if you can resend me your email replies in case they include some helpful info for the future! I'll PM you my email. Thanks!

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    Quote Originally Posted by Petr View Post
    Hi Dave!

    When knocking on them with a screwdriver they caused fall-outs of signal. So my tech has replaced all relays in signal path of all 3 channels to make sure they are not causing some other hassle.
    These relays are very reliable, but a shock like hitting them with a screwdriver will definitely make them jump. Don't do that!

    Quote Originally Posted by Petr View Post
    Hi Dave!

    I bought my GP3 used, with gain-mod on it. So during inspection my tech also has found very amateurish soldering of those resistors and capacitors, so he fixed them properly. He has found also some bigger teardrop of solder on the opposite site of PCB which might bridge some contacts.
    I hope this clarifies a bit more my problem. And it's also kind of advice for the others who might think about gain-mod by do-it-myself (like previous owner of my GP3 ). Be careful and make sure you know what are you doing and how.

    Thanks!
    That part doesn;t suprise me at all...
    Dave Phelge
    Chief Misconception Eradicator
    support@fryette.com

    "We are what we pretend to be, so we must be careful what we pretend to be". - Kurt Vonnegut

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    Dave, there was more then obvious that at least one of those relays is faulty. There was problem with clean channel. Whatever component on PCB you touched, it caused a strange noise with signal drop out. As you know those relays are glued to PCB therefore it was difficult to identify which of those relays is jumping. And you know as well there is almost impossible to measure anything when you have GP3 decomposed. At the same time we were missing that switching scheme and doing some reverse engineering stuff wasn't our intention.
    BR,
    Petr

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