Nasty hum in my '74 V4B

Discussion in 'Amps and Cabs [BG]' started by MichelD, Apr 17, 2019.


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  1. MichelD

    MichelD

    May 19, 2014
    My old Ampeg V4B hums like an a old Kelvinator fridge when I turn the dial anywhere past 1/4 of the way up. Both channels. The hum can be diminished somewhat by playing with the boost switches, particularly the bass one, but still, this isn't normal.

    And it is not just the ground in my house this happened at the practice space too.

    It actually over head and quit on me once too the last time i I had it on and played for a couple hours.
     
  2. Wasnex

    Wasnex

    Dec 25, 2011


    If the amp settles down when you turn the volume down, it could possibly be a couple of bad preamp tubes. The volume controls fall between the triodes of V1 and V2. You could try rolling a new tube into V1 and V2, but I wouldn't hold my breath if I were you.

    If the amp quit working and then started working again, you probably have an intermittent connection somewhere in the circuitry.

    Sounds like you need to take the amp to a good tech.
     
  3. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    Could be alot things.
    Possible pre tube gone bad. Or some connections need touching up.

    I'm trying to remember if the V4 had hum balance pot. Or set resistors. Either way could need new pot or resistors.

    God forbid if I mentioned capacitors. Lol cause agedhorse will freak out.
    But if you got noticible hum over time. And obvious transformer heat our hum. Well it's probably time to recap.

    Likewise shot gun a few things like bias cabs. Cathode caps and phase inverter caps.

    The good news is if the amps been surviving on old cabs for awhile. It will really really wake up and be a whole new amp. All said n done. Correctly biased and smooth running V4 is a power house.

    My first V4 benched at 90 watts and then started tossing fuses. After recap and bias it was 132 watts. Bing bang boom. Sweet
     
    bobyoung53 likes this.
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    As mentioned, there are many possibilities and the V4B in particular is a complex beast. Have a tech look at it.

    I have a '74 as well. When up to spec, it's a great amp. It had hum issues as well, there were a lot of issues that needed to be addressed.
     
    BassmanPaul and jastacey like this.
  5. Basslice

    Basslice Supporting Member

    May 11, 2008
    Western Massachusetts
    A couple of years ago I completely rebuilt and recapped my V4B from the ground up. Lots of fun, lots of work. Many successes and a few failures, but nothing tragic. I have oscilloscopes, a tone generator and dummy loads - in other words I have an idea what I am poking around in. Although I know so little about your problem from your description or the frequency of the hum, I would guess that you have a bad connection or lifted ground somewhere in the circuit. Could def be a tube, but if I had to bet I'd say bad solder joint somewhere. You could open it up and try chopsticking it, but I am highly against telling anyone to do that if the person isn't skilled or comfortable working around high voltage electronics (B+ in this thing is over 500v). It sounds like you may still have an ungrounded amp that hasn't been safety fixed (does it have a two prong or three prong power plug. if not you should have it replaced?) it sounds like this is above a simple solution like spraying deoxit on stuff. Ask yourself this, do you know the visual difference between a canned capacitor, paper or foil cap, resistor and a diode? Can you follow a basic schematic? Do you know how to safely discharge a power cap? If any of those answers is no, then don't mess with it.

    Probably your best bet is to go to a guitar store and ask for an amp tech. It isn't as expensive as you might think. At least not much more than a DIY.

    As mentioned above, the V$b has a hum pot, but this is not your problem (unless it is poorly grounded). Previous poster also said that one clue is that the problem is volume knob related so can probably be found before the main power tube part of the amp. Preamp tube, tone board, probably not your large caps. I had a poorly grounded 1/4 input that vexed me for a while and had a similar effect. Could even be a dirty volume or tone pot. Good news is that it doesn't sound like a major near death problem. It sounds fixable. Isolating the bad thing in the chain is always a pain and like solving a good Suduko puzzle.
     
  6. MichelD

    MichelD

    May 19, 2014
    Thanks. Going to jam with my amp-building/repairing buddy tonight. We'll discuss this.
     
    Basslice likes this.
  7. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    I have the same exact problem with my 75 v4b it hum quit a lot even with all volumes off , my tech checked if has been recapped retubed by me with new tubes but nothing is changed..midrange switch make the hum and the buzz even worse..i ve to say the tech said that is a problem in the pre amp exactly in th v1 but he tried a lot of things to rid if the humming buzz without success..I dont know what to do.
     
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    There are so many possibilities. But let's start with some basic servicing details for a vintage V4B when it comes to hum.

    - The input jacks are the main ground point. Check that the contact between the jacks and chassis are clean with no oxidation present.
    - The output jacks need to have isolation washers installed both on the outside and the inside of the chassis. Without them, there is ground loop hum.
    - The shielded wires should all be checked. There are often issues with these wires.
    - Sometimes there is gunk accumulated under the tube socket between the socket pins. This has to be carefully cleaned.
    - All the wires to the boards need to be checked, sometimes there are solder joint issues.
    - In general, with the V4B running so hot, all the solder joints should be carefully examined.

    The next step is to use a signal generator and scope or listening device. First try to isolate the preamp from the power amp to determine where the hum is. If it is in the preamp, check every component along the signal pathway, starting at the input and looking on each side of every component to track down the source of the hum. This meticulous work but the details are important.
     
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Find a real, qualified tech that knows how to troubleshoot, since randomly replacing parts obviously hasn't worked out well for you, has it?

    This is why I discourage "recapping", as it's rarely the caps. Sometimes, but not often.
     
    xj98jeep, BassmanPaul and 12BitSlab like this.
  10. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    probably you right .it make sense on a 45 years old amp because there will be always something that has not been replaced because left original and in that case, it will be difficult for any tech to find the fault. or simply the problem will go in other components, it probably makes sense a full restoration..all new components apart switches and tranny..I don t know. my tech as checked all the joint and test almost everything but he also said the having 4 circuit board in an amp instead of only 1, it make worse these noise problem and difficult troubleshooting..
     
  11. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
     
  12. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    I am just curious how a transformer with max output 120rms on a circuit that has been made for 100w can reach 132w RMS I always hear these things in a lot of forums,but all the time I ask to a tech they always say that is impossible due to amp design.
     
  13. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Generally I find way more amps that have problems due to poor quality work and mistakes made during repair and "restoration" attempts. Probably in the order of 5-10x more amps screwed up this way versus real component failure

    My recommendation is to leave things alone unless they are VERIFIED (by analysis, testing, not by internet suggestions) to be drifting off spec or failing.

    There are exceptions, especially with amps that are 50 years old, but too many amps are ruined by parts swappers who profess to understand how the amp's circuits work yet in actuality do not.
     
    slagheap likes this.
  14. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    do you generally suggest full restoration in cases where it is difficult to find the fault?? because as you said in the previous message it could be anything, probably a very stupid thing but the difficulty in finding the problem dur component with different age and wear..
     
  15. MichelD

    MichelD

    May 19, 2014
    Original poster here.

    I finally chose a repair shop in April. The tech flushed pots, resoldered tube sockets retubed it, replaced one input jack and said it was good to go. Brought it home and it still hummed like crazy. Brought it back and he replaced the hum control pot and now it works fine, still with some residual buzz when I turn it up past 6 or 7 though.

    He didn't recap it and said caps were fine.
     
    pcake likes this.
  16. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    No, I recommend thorough and correct troubleshooting and analysis. It could be anything, but replacing "everything" is not how to repair an amp. It is a great way however to create additional problems.

    So, he did a bunch of work but didn't correctly identify the faulty component. If the tubes were the cause, that would have resolved the symptoms. Therefore, it's likely that the original tubes were perfectly good. Same applies to the resoldering of the tube sockets... etc. "Flushing" the pots makes me cringe, because I see more pots ruined (or greatly reducing their life) this way.

    Had the tech correctly troubleshot and identified the faulty component, replacing of the hum balance control likely would have resolved your symptoms with an hour of labor and a $10 part.

    Does the amp buzz with nothing plugged into the input? If not (or it's much lower still), than it's likely that the noise is coming from your bass itself. Again, proper troubleshooting is used to rule out what might be the cause so that the actual cause is narrowed down to eventually a few possible components. This is how a qualified professional will approach amplifier repair.
     
    mc900ftj and pcake like this.
  17. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    my amp buzz with nothing plugged in
     
  18. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Then additional troubleshooting and diagnostics will be necessary to identify the source of the noise.
    My next step would to use a shorting plug into the input to see its effect on noise level.
     
  19. FunkHead

    FunkHead Supporting Member

    Mar 10, 2007
    This right here is why so many people try to repair amps themselves. You trust a shop and they just swap parts and screw up your pots, steal your functioning tubes(sometimes) and then send you home with the same hum.
    Ridiculous is an understatement.
    I would out the offending shop here on TB and find another one.
     
  20. seamonkey

    seamonkey

    Aug 6, 2004
    Could be from the power transformer. Take a cell phone video and move it around the amp and see where the hum mostly comes out.

    Bless a good tech's heart, but a cell phone or oscilloscope can often hear better than a tech in the back of a music store.
     
    Sunnampeg likes this.
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