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Thread: For Enzo PV Classic 30 repair dilemma

  1. #1
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    For Enzo PV Classic 30 repair dilemma

    Hi Enzo--I got a C30 that I've had for about 12 years. Bought used. The entire time I've had it there has been a significant hum that stays constant regardless of whether the reverb is turned up or not. If I plug a dummy plug into the effects return the hum goes away. I've retubed--no difference. There is a loud pop about a half second after I turn it off.

    My question is this--This sound like the kind of problem that can eat up a lot of bench time. My dilemma is to invest in the repair at 75 bucks an hour or just find another C30 and have this as a backup. It works great other than the hum.

    thanks
    Randy

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    Supporting Member tubeswell's Avatar
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    Filter caps?
    Building a better world (one tube amp at a time)

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    That was my first thought, but then I was confused by the fact that it goes away when plugging in a cable to the effects return. My somewhat marginally informed thinking was that if it was a filter cap problem it would be there in the output power stage too. I've heard of people doing filter cap upgrades to these amps. Is there a particular brand you would suggest?

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    Supporting Member tubeswell's Avatar
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    If its in the pre-amp section (suggested to me by your isolation trick with the FX jacks), it could be the pre-amp filter/decoupling cap C43 - This is the cap that decouples the B+ from the B++ on that schematic. It is a 22uF 450V cap. These decoupling caps isolate one part of the power rail from other parts of the power rail and absorb voltage fluctuations (that would otherwise occur in the power rail from normal operation of the relevant gain stages' plate resistors). When these caps start to go, it can induce hum, because these voltage fluctuations are not being removed thoroughly enough to keep the power rail constant.

    However if one filter cap is gone, then the chances are the others may not be far behind. Since the guts of the C30 is a little bit of a PITA to get at, I'd be inclined to replace all the electrolytics in the amp when it is opened up, for longer term peace of mind. JM2CW
    Building a better world (one tube amp at a time)

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    Thanks for your suggestions, Tubeswell. I guess I should just bite the bullet and do the whole series of mods I've been thinking about if I'm going to pull it apart. You sent me the schematic of your Soldano mods earlier and I've been thinking about that but I don't have a point of reference--I've never played through a Soldano. Here's where I'm aty currently with the amp. I've installed a cannibis rex hempcone and retubed with JJs except for V1 which is now a Tungsol. I think the amp sounds very sweet but I wish the OD channel sounds more like the clean channel for it's basic tone. There has to be some kind of voicing going on the OD channel because It's like a bandpass filter with a midrange bump. Chops of some lows and highs. Maybe this is necessary to get a smoother sounding OD--I dont know. How does the Soldano mod affect this characteristic? The SteveA tonestack mod is a given.

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    Lifetime Member Enzo's Avatar
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    You can do all the mods in the world, but if it is humming now, it will probably be humming after you are done modding too.


    Isolate the problem. Is it 60Hz hum or 120Hz hum? 120Hz hum is power supply ripple. 60Hz hum is everything else, like grounding problems, radiated hum, etc.

    I might look at C45,C46, they filter the -36VDC that the heaters run on.

    But here is my ace in the hole. Look on the schematic just above the power supply and right below the power tubes. See the two resistors grounded at each end? R61,62. Make sure one of those is not open. They are located just off the + end of C46. Here is the complete schematic that includes layoout in case your schematic lacks it.

    In circuit they will often read shorted, since the grounds they connect are bonded elsewhere.

    You mentioned the reverb control, but do any of the other controls have ANY effect on the hum? If they do, they are after its source.
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    Quote Originally Posted by Enzo View Post
    You can do all the mods in the world, but if it is humming now, it will probably be humming after you are done modding too.


    Isolate the problem. Is it 60Hz hum or 120Hz hum? 120Hz hum is power supply ripple. 60Hz hum is everything else, like grounding problems, radiated hum, etc.

    I might look at C45,C46, they filter the -36VDC that the heaters run on.

    But here is my ace in the hole. Look on the schematic just above the power supply and right below the power tubes. See the two resistors grounded at each end? R61,62. Make sure one of those is not open. They are located just off the + end of C46. Here is the complete schematic that includes layoout in case your schematic lacks it.

    In circuit they will often read shorted, since the grounds they connect are bonded elsewhere.

    You mentioned the reverb control, but do any of the other controls have ANY effect on the hum? If they do, they are after its source.

    Yea, old thread, I know, but have to ask. Concerning R61-62, what's the difference between ground, circled ground, and "E"? Those two resistors are just hanging out there, what's their purpose?

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    Supporting Member Jazz P Bass's Avatar
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    The resistors are not 'hanging' anywhere.

    The different "ground" symbols are used on the schematic because that is how the grounding scheme of the amp is wired.

    I read them as Circuit ground, Chassis ground & Earth ground. (ground, circled ground, and "E"? )

    As to your question :" Concerning R61-62, what's the difference between ground, circled ground, and "E"? "
    The difference is the individual value of the resistors: 47 ohms.

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    Quote Originally Posted by Jazz P Bass View Post
    The resistors are not 'hanging' anywhere.

    The different "ground" symbols are used on the schematic because that is how the grounding scheme of the amp is wired.

    I read them as Circuit ground, Chassis ground & Earth ground. (ground, circled ground, and "E"? )

    As to your question :" Concerning R61-62, what's the difference between ground, circled ground, and "E"? "
    The difference is the individual value of the resistors: 47 ohms.
    Got it, thanks

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    I believe it was explained to me (haven't searched for the thread, sorry) that 'ground' and 'circle ground' are shorted out by chassis fasteners while the unit is in service, but when disassembled for repair, the 47R resistor allows the input section to maintain its relationship with the signal ground. I'd wager a similar thing for the ground-to-earth connection. The 47R resistors are there to inhibit ground loop currents when the unit is fully assembled.

    It's one of those things that "eyes on" would explain in a heartbeat, but the schematic doesn't without appropriate engineering notes.
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    Could the hum be from a bad effects jack (J3) or cracked solder joints to that jack? If those jacks don't properly ground out there will be hum.

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