Vox AC4hw hum problem (structural) lay out needed

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Luuc
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Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

In 2012 Vox / Korg introduced the hand wired AC4hw1 which is a new model. Not a vintage re issue. I bought this amp used. Apart from tubes and general check up, has been modded for better bass and better sound in general.
And a recap w. some Rifa caps, a tropical fish cap is used to unleash the low end.
I sawed off the lower part of the back to make it more...'open' sounding. Matter of taste. The Greenback (though Chinese maufactured) works very well for me.
Furthermore I only use 4 screws for the back panel. Korg uses 15 (!)

A stupidity where marketing meets design I can live with. As follows: Vox says everywhere that the 'hot mode' bypasses tone circuit completely. Not true, because when the cool mode is being switched to hot, the treble pot suddenly becomes a kind of attenuator. So (most of the time it may be so) if treble is around 12 h. then the hot mode will not be so hot after all. Not cool i.m.h.o. Can live with it though. What to me is unacceptable, is the hum. It is a 50 hz (Netherlands) hum. So if playing to record and / or at low volume level this amp is useless. Hum is constant. Has nothing to do with input. So now I am going to (try to) get rid of this hum. Even if the so called 'Vox sound' will be altered. Dare say this, for it is a newly designed amp. And a nice small amp should be silent. A SE as well.
Anyone w. same problem to solve? Does anyone have the lay out? Vox / Korg of course -company policy- does not help. They do know about the hum. Probably they aim at the consumer market... not @ serious musicians who are bothered by this nagging hum-tone.
Stevem
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Stevem »

Your hum issue being 50 hz can mainly be stemming from three places.
1) heater hum, by means of a bad preamp tube, poor or no center tap ground reference on the PT heater winding.
2) The flux from the PT is coupling into the OT in which case pulling out all the tubes can confirm that.
3) at least the grid lead from the input Jack to the first gain stage needs to be sheilded and may be the next grid wire also feeding into the next gain stage.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

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

Post by Luuc »

Thank U for starters,

Will look into this. Tubes are o.k. (checked, tested etc.)

If I can get this amp hum-free, without an alteration of the sound, a mistake in it's design has been revealed, agree? By the large firm Korg. In one of their Flag Ship models. They all come w. a hum. Well:

My aim is to get the amp silent and shut up and play my guitar.
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Stevem »

There is a limit to how hum free a SE amp can be as in a push pull output stage the inherent hum canceling is far greater!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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Phil_S
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Phil_S »

Show some pictures. There is a chance someone will see something.
andyfromdenver
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by andyfromdenver »

Stevem wrote:There is a limit to how hum free a SE amp can be as in a push pull output stage the inherent hum canceling is far greater!
yup.

Is there a scheme and we can see how they filtered the power supply and etc.

It would be interesting if you (OP) found and corrected a layout flaw, as I would imagine it wouldn't get past R&D.

Maybe there is a chance to up the pwr supply filtering.

I use the silver face Fender champ as a reference for quality SE amp design. When in top condition, they sound as good as one can get imo (without getting extreme). How would you rate this amount of hum to that? Assuming that is a usable reference for you. The champ has a tube rectifier and a 40mfd pwr cap.

Just curious, and good luck finding a lower noise floor! That would be good work.
ampdoc1
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Suggestion

Post by ampdoc1 »

You have probably looked for help on the internet, but if not check if other users on vox sites have had the same problems.

I bought a 15W Vox Mini 3 years ago, and noticed a growling sound as the sound faded. A search on the net revealed that a lot of people had the same problem. A number of modifications were suggested, which i tried,...but there was little change. On top of everything, the PCB was horrible and the wiring, a mess.

I gave mine away. This was a Chinese manufactured piece, and I vowed I'll never buy another amp from the orient. To be fair, I realize that oriental workers are paid very little, but in the end, Vox is soiling it's name by selling this crap. My guess is that you will wear yourself out trying to find this bug.
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Phil_S
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Phil_S »

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.
ampdoc1
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by ampdoc1 »

Tough luck to get an amp that was designed one way and built another.

Don' think I've ever run into something like this (other than a number of vintage Gibsons that are notorious for changing parts and minor circuit changes.)

No excuse for Vox, though. Gut it and rebuild it like it should be, including any changes you wish.
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Deric
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Deric »

Luuc wrote: I bought this amp used. Apart from tubes and general check up, has been modded...
So...any reason not to believe it was fine before all the mods?
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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

NO, sir, bought it 'used' - while in mint condition. I had it modded by a pro amp tech. Way better than before. Better tubes as well.

The 50 hz hum is only irritating when recording and practising, as it is not getting louder when cranked up. May users will not bother. I do.
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Deric
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Deric »

Right.

Was the hum there before it was modded?

Have you asked your Pro Amp Tech his opinion about the hum?
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Luuc
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by Luuc »

Of course, only: not so many amp techs do really, really understand SE amps.
And if it is a VOX, then it should be o.k. as it is. I.m.h.o: non sense.

This hum is a known but neglected / ignored issue.

I got in touch with Viva Analog / JC Maillet in Canada. He knows a lot about SE's. We are working on a 'hum solution.' It is not simple. Not undoable either.

Nota Bene: my only en first question was: does anybody have the schematics for the (2012) Vox (=KORG) newly designed AC4hw1.

Until now a lot of A-o.k. advice. But for me a few stations past, so to speak. With al due resopect!

Now we are drawing schematics ourselves. <to be continued>
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tony hunt
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by tony hunt »

I am not familiar with this circuit. But I always use a CLC filter set up (choke) in my SE builds.
But anyway, you say the hum is 50Hz. Isn't that just filament noise issue, not power rail?
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tony hunt
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Re: Vox AC4hw hum problem (structural) lay out needed

Post by tony hunt »

another thing, more to do with the mods: I don't know of any tropical fish cap that was rated beyond 250V.
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