VOX AC4HW1 hum.........................fixed!!!

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RonnieHotdogsRonnieHotdogs Frets: 140
These amps hum from start up, regardless of volume, makes them almost impossible to record with.
Vox ignored the problem whilst owners moaned all over the internet for years, VOX said "single end amps make noise"
They don't seem to make them anymore. Maybe due to this

Bought a brand new one last month, took it to amp magician David Lane in Gravesend, he moved and turned the transformers, improved some dodgy wiring and one dodgy valve socket, now totally silent on start up. Put in some decent old valves and it now it sounds fabulous. 
As it bloody should be for £700
I could hug him.

VOX's attitude sucks.



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  • ecc83ecc83 Frets: 1031
    Well, low power, say around 4W SE amps are really just the arse end of a radio or TV sound stage and you rarely had trouble with hum, at least for the first 20 years or so!
    Mind you, radios and TVs rarely had speakers bigger than 6 inches but then there was the advent of the thumpin' Blue Spot radiogram and they also did not hum. Poor design in guitar amps?

    Dave.
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  • scovell001scovell001 Frets: 41
    edited August 1
    RonnieHotdogs, WOW ! - who is your amp tech ? - bought one of these a few years ago & had to send it back cos of that dreadful hum. Lovely little amp, if only the hum wasn’t there. Soooo jealous ! 

    Ps (you’ve probably done this already) put an alnico gold in it, ‘not’ a blue 
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  • ICBMICBM Frets: 48626
    I haven’t worked on one of these yet but that’s useful to know.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • ecc83ecc83 Frets: 1031
    ICBM said:
    I haven’t worked on one of these yet but that’s useful to know.

    ICBM said:
    I haven’t worked on one of these yet but that’s useful to know.

    I have found some Blaupunkt schematics online and they are all single ended, ECL86 or EL84 but they used a choke/output transformer with a tapped secondary in a fairly complex feedback path.
    It seems to me my hunch was right, the gitamp manfctrs don't want to spend the money EVEN at the price these things sell for, to design out the hum!

    There was also a 'hi fi' record player, the Pye Black Box but that used push pull ECL83s. That's called "doing it right"!

    Dave.
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  • ICBMICBM Frets: 48626
    ecc83 said:

    I have found some Blaupunkt schematics online and they are all single ended, ECL86 or EL84 but they used a choke/output transformer with a tapped secondary in a fairly complex feedback path. 
    It seems to me my hunch was right, the gitamp manfctrs don't want to spend the money EVEN at the price these things sell for, to design out the hum!
    Indeed, although you may find that doing it ‘too well’ takes away some or all of what guitarists like about small basic amps... it can be a difficult circle to square, and you sometimes can’t win. The things we like about guitar amps (and their speakers/cabs) are often the direct results of doing things as cheaply and crudely as possible :).

    However, simply not putting the transformers in the wrong place so they induce hum isn’t going to ruin the ‘raw’ sound of a simple circuit, so there’s really no excuse for Vox not getting that right.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • RonnieHotdogsRonnieHotdogs Frets: 140
    RonnieHotdogs, WOW ! - who is your amp tech ? - bought one of these a few years ago & had to send it back cos of that dreadful hum. Lovely little amp, if only the hum wasn’t there. Soooo jealous ! 

    Ps (you’ve probably done this already) put an alnico gold in it, ‘not’ a blue 
    https://www.facebook.com/people/David-Amplification/100013096985615
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  • ecc83ecc83 Frets: 1031
    ICBM said:
    ecc83 said:

    I have found some Blaupunkt schematics online and they are all single ended, ECL86 or EL84 but they used a choke/output transformer with a tapped secondary in a fairly complex feedback path. 
    It seems to me my hunch was right, the gitamp manfctrs don't want to spend the money EVEN at the price these things sell for, to design out the hum!
    Indeed, although you may find that doing it ‘too well’ takes away some or all of what guitarists like about small basic amps... it can be a difficult circle to square, and you sometimes can’t win. The things we like about guitar amps (and their speakers/cabs) are often the direct results of doing things as cheaply and crudely as possible :).

    However, simply not putting the transformers in the wrong place so they induce hum isn’t going to ruin the ‘raw’ sound of a simple circuit, so there’s really no excuse for Vox not getting that right.

    Yeah, we have had this discussion before IC about the 'Non-hi finess' of guitar amps! I just could not let a design out of my grasp unless it had a reasonable signal to hum ratio (why I modded son's HT-20!) .

    All the other stuff, poor PI balance, whacky biasing causing distortion, flimsy ply cabinets that 'talk' I sort of get (now) but an amp that produces audible hum at a mtr away? That is just shit design to me.

    Dave.
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  • ICBMICBM Frets: 48626
    ecc83 said:

    All the other stuff, poor PI balance, whacky biasing causing distortion, flimsy ply cabinets that 'talk' I sort of get (now) but an amp that produces audible hum at a mtr away? That is just shit design to me.
    Yes, agreed. Especially in this case when it's nothing to do with the circuit but clearly just incompetent layout design.

    This is not the first modern guitar amp I've come across with mistakes that an old-school designer from the 50s or 60s would simply have never made because the correct solution would have been second nature to them - even if they were trying to save money, it costs no more to orient the OT one way than another...

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • RonnieHotdogsRonnieHotdogs Frets: 140
    IMHO guitars have been getting better over the past 60 years, and amps get worse with every decade. 
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