Nasty hum in my '74 V4B

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


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  1. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    I ve already check few information is 12at7=ecc81 has exactly the tone I Do not LIke, is the exact opposite of 12dw7 has no clarity, not definition and not high mid frequencies, and gain wìhich is the reason I've bought the v4b so I ve asked him to Do not change the Tone of the amp to fix the Hum..and he basically did...this is irritating.especially because i ve to pay it
     
    bobyoung53 likes this.
  2. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Looks like you are going to need more troubleshooting, or if the noise floor is too high and that's the normal noise floor of the amp, you will either need to change your expectations or find a different amp that better meets your needs.
     
    BassmanPaul likes this.
  3. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    A 12DW7 is dual triode which is more or less half 12AX7 and half 12AU7. A 12AT7 falls between a 12AX7 and a 12AU7 in terms of gain. Sometimes the higher the gain, the more the noise. Current drive is another story and part of the equation here.

    No way should a V4 have a nasty hum. Something else is going on. Changing a tube type is a bandaid, the underlying problem has to be identified. I would revisit the basic servicing suggestions that I made earlier in this thread. There are a lot of possibilities that can result in hum in this amp.
     
    bobyoung53, BassmanPaul and coreyfyfe like this.
  4. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    to be honest, it's hard to say it if you do not have another 70 s Ampeg v4b to compare it in recording using at 3 of volume I can't hear any buzz in the recording, but it's noisy, and compared with my 69 sunn 2000s where i cant hear not even a quarter of that noise that the v4b does and that's weird,i mean i have SVT VR are noisy but the v4b is definitely noisier than 300w svtvr is this normal?? the tech says that is not too noisy.. soo..
     
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    It's quite possible.

    Knob position can not be used to determine relative noise between different amps. Each design has its own gain structure and control taper.
     
  6. wcriley

    wcriley

    Apr 5, 2010
    Western PA
    Wasn't there a circuit mod floating around years ago (maybe in on of the books by the Groove Tubes guy?) that changed the gain in one half of a 12AX7 to match that of a 12DW7?
     
    Sunnampeg likes this.
  7. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    That was for the made in Japan SVT.
     
    bobyoung53, Sunnampeg and wcriley like this.
  8. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    so I've collected the amp he swapped a 12dw7 with (ecc82 12au7)and it was dead silent compared to before but when I 've started to play I 've noticed, not the same angry tone that I like, it was not darker as I was expecting, but just not so open and less gain I had the feeling that ave lost some power like I've lost 1 notch or more of volume at the point where the volume at 1 I could not hear anything. and rehearsing also I noticed that at 6 volume the power was not sufficient where before at 6 volume it was glorious and loud.he also lowered the bias, I brought it back because 100w with not enough gain for bass is NO NO, the question is if I keep the mod and I use 7581A output tubes they go 35w like the old vintage 7027A each so I should also overtake the 100w like 110 so i may recover a notch of power lost in the gain , section?? the tech said there is nothing he can do to the amp for the humm..
     
    Last edited: Jun 29, 2020
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    Turn the amp up 1 notch, all's good.
     
  10. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    but still has a softer sound..
     
  11. bobyoung53

    bobyoung53 Supporting Member

    What he did was a bandaid, The older Ampegs have very high gain preamps in them. It does sound like your problem is in the preamp though not the power amp and before the tube that got subbed in, and your power tubes looked good to me, they weren't glowing. Do you have another 12DW7 to put in in place of the one that was taken out? Even a bad preamp tube can cause hum.
     
    Sunnampeg likes this.
  12. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    yes I 've tried with 3 different 12dw7 and it still does it output were at 85 % probably a bit too much, Bytheway Yes my problem is preamp but you do also have a similar problem with v4b?? if yes How did you fix it?
     
  13. bobyoung53

    bobyoung53 Supporting Member


    Have you checked all the tubes in the preamp, especially the one before that 12DW7? If I were you, I'd write down the symptoms, bring it to a good tech who knows tube amps and tell him that it seems to be coming from an early point in the preamp. It could be a lot of things, bad ground, bad tube socket etc.
     
  14. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    I ve already brought the amp 3 times to the first tech and once at the last tech today.. both with same result..the first is a HIWATT assistence tech the second worked for Mattsowball biggest gear hiring in UK.
     
  15. bobyoung53

    bobyoung53 Supporting Member


    You need to find some old timer who lives and breathes tubes or some youngster that loves them, i know there are a lot of SVT's in the UK, find someone who services them.
     
  16. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    the only place I've found that restores vintage Ampeg is far from London I don't have a car It will take 50gbo only to carry it until there
    Ampeg Repairs UK - Surrey Amps - Pro Audio Repair
     
  17. bobyoung53

    bobyoung53 Supporting Member


    What's a gbo?:laugh: It would be worth it though to have someone go through it that knows them. I had one years ago that had been all mickey moused and had a friend of mine who was a tech go right through it and it sounded great by the time he got done, was very quiet.
     
  18. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    sorry GBP great British pound
     
  19. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    How much has it cost you already?
     
  20. Sunnampeg

    Sunnampeg

    Nov 11, 2019
    UK
    150 gbp
     
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    Primary TB Assistant

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