Vox AC4hw hum problem (structural) lay out needed

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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

Hope you will excuse me, for all I can say is the amp tech really did use a 'tropical fish' cap to 'unleash' the bass potential. The bass potential has become awesome. I removed the backpanel (tubes are still protected) so kinda 2/3 is open.

Note: I personally do not use a lot of bass +, my guitar has bridge p.u. only. Like that. I do not compensate by turning up bas. I keep bass control very low and treble around 12'o clock.
Still the low end has improved a lot with this mod.
All with an 1962 SG junior and the standard Celestion Greenback 25 w. 16 ohm 75 hz 98 Db spl.

Do not have the technical knowledge to give a proper tech. response.
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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

probably, yes. might be some bad shielding too. we arte trying to figure this out from the bottom.
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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

Phil_S wrote:To be fair, the outsourced work was probably done to the spec Vox provided. They build it to spec, good, bad, mediocre. They don't care. They get paid to build to spec. If there's a design flaw it's not on the builder.
Totally agree.

KORG bought the legendary name of 'VOX' The hand wired amps of their Flagship series, this is one of them, are built in Vietnam: well, I thinks this is not the poin-to-point point:-)

It is all about good designs, good Q.C. and not about marketing. Guess KORG turned this around big time.
pdf64
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by pdf64 »

I don't know of any tropical fish cap that was rated beyond 250V
Certainly the 250V ones were much more common than those of 400V.
But with an AC4, it may be that 250V is adequate anyway, as their VB+ is rather lower than, eg an AC50.
OK, if eg the heaters in both the pre and power tubes failed, then it would rise towards 350V, but compounded failures tend to be outside of what's reasonably foreseeable.
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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

Thank u.

Btw: this is not a VOX AC4 like the older model with the same name. It is a new design with an old feature like 'Top Boost.'

Confusing marketing strategy i.m.h.o. as the name reminds of a vintage model.
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tony hunt
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by tony hunt »

Luuc, could you post a clear image of the two preamp sockets from the inside without giving yourself an electric shock?

Judging by some blurred online pictures on the web, there could be some improvement to the filament wiring at those sockets which we could suggest.
It looks like they took a really small filament pair from the EL84 to the preamp, twisted them tighter than a virgin's daughter, and then lost the plot at each socket. I'm thinking that the filament wire to pin 9 may be acting as a hum antenna.

Going back to stevem's post, I think you said that you have you already ruled out bad tubes, is that correct?
I could not see how the filament winding is referenced. Maybe some clear photos of the insides at the other end where the mains cable gets plugged in would help us here.

By the way, I have one of those Juniors, never sure if it is '61 or '62. It is a five digit number with the Les Paul signature before he had them stop using it on the SG. These are not exactly quiet guitars regarding 50Hz noise!
If all else fails, I have heard of people mixing the recorded track with an out of phase 50Hz track to get the hum to cancel out. Never done this myself.
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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

Pictures attached. I took it apart, well labeled and documented.
Note: Overview is of non modded amp. You can see a second ground connecting board w. chassis in the rich lower corner.

Bad tubes = ruled out.

These SG 'Les Paul' Juniors were built in 1961/'62 and a past of '63. Numbering by Gibson is rather inconsistent. Mine is # 73675 - must be spring of 1962. Some people take a loo at the pots. The stepped compensating wrap arounds stop tailpiece is an indicator as well. To narrow it down: she is a 1962 and if not 1961 (or very unlikely maybe a 1963) Several serious websites give information on numbering.
Considering mine is all original -apart from frets and maybe another nut and a Sprague Black Beauty tone cap: 'may be Vintage, is a very dear, still a tool as well.' So: I use a Lindy Fralin hum cancelling P-90 style pick up. Works well for me. (The original P-90 is in a vault:-)
Personally I would not (even) try to use the mo described. Of course it -as always- this might work well, in some situations. I do not believe it at all for me. Would find another solution.
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billyz
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by billyz »

Try elevating the filament supply., if cathode bias then ground the filaments to the power tube cathode. Reworking the ground scheme might also improve the hum. I like to have all the grounds of the preamp at the input jack or close. Screen supply and before closer to the power transformer. Etc.
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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

Thank you, still: I am working on schematics myself now. Then thinking, then tinkering:-)
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