Vox AC4TV Buzz Problem

[jojo123]

Greetings. I have a Vox AC4TV amp. It is great sounding amp although from my experience I believe they cut some corners in the construction and there are quality control issues. Actually I got the amp for free from Sweetwater Music when I bought a Vox Virage (great guitar, no quality control issues there!). The first AC4TV they sent me was dead in the box basically; I sent it back and they sent me new one immediately.

The problem with the new one is there is very noticeable buzz/vibration in the amp when certain notes are played with amp at 1 watt setting or more and about 70% or more volume level. Specifically, the amp buzzes when I play the 6th string (lowest), at 11, 12, 13 fret. D#, E, F. It is as if the bass sound is overwhelming the construction of the cabinet at those frequencies.

I figured there might be some loose screws. I took back cabinet off. Although speaker had 8 holes for screws there were only 4 attaching the speaker and they were not tight at all. I hand tightened and added additional screws. However problem persists.

I tried with different guitar through the Vox and the same problem Then I tried both guitars through a Crate Vintage Club 20 it was rock solid on all the notes. so it is definitely the amp.

The Vox AC4TV is a nice little amp and I'd like to get rid of the buzz. I am new to tube amps, and not sure what to try next. Rather then doing further experiments I thought I would check with some of the knowledgeable people on this forum. Thanks in advance for reading this and any input.

[Gtr_tech]

Sounds like output tube rattling to me. Try a different EL84 (the larger of the 2 tubes) and see if that doesn't stop the noise. If it still is rackety, you may have questionable solder connections on the board that are causing it.

[oc disorder]

Hi jojo123 and welcome to the forum.

As well as cheching what Gtr_tech said you could also try tapping the EL84 tube with a pen or similar and see if it rattles.
There is also a possibility a turn could be loose on the voice coil of the speaker. If so it would need replacing.
Try hooking it up as an head/speaker - somehow pull the chassis from the cabinet and extend the wires to the speaker.
If the speaker is vibrating a connection or the valve - with the chassis removed the rattle should go.
If the cabinet or speaker is at fault it will still rattle.

[Gtr_tech]

He wouldn't even need to remove the chassis. Just taking the back off the amp you can get to the spkr/wires and a external speaker cable can be made up to check with a different spkr.

[jojo123]

Gtr tech & oc disorder -
Thanks for your feedback and suggestions. You helped me narrow it down I think, although I still haven't solved the problem so I am looking for any further ideas!

- I tapped the tubes and they seemed rock solid.
- I took the back off the Vox cabinet. There is black metal plate with all the elecronics attached to the back cabinet which I left attached. so the Vox speaker was now standalone. I ran the vox tubes through the 10 inch speaker on my Crate amp and everything sounded fine. no buzz. So I assume the vox tubes are ok.
- I then ran the Crate electronics through the Vox speaker, a 10 inch celestion. I thought this would be where the problem was but actually sounded great. So I assume the Vox speaker is fine.
- I then used speaker wire and ran the vox tubes through the vox speaker but they were physically separated by a couple feet. It sounded excellent. No buzz on any frets.
- I then put the vox back together and the buzz was still there. So I assumed it was emanating from the circuit board/black metal plate. Actually the buzz seemed to have moved up a fret or two. Anyways, I examined the circuit board closely and I noticed that the light that shows power is on was not tight against the circuit board. I put black tape on it to restrict any movement. Tried again, and still a buzz, in fact if anything, a little worse.

So the good news is I think the tubes/electronics are OK and the speaker is good. But when I put everything together, there seems to be harmonic effect or something in the construction that is making a buzzing sound on certain low notes. Not sure of next steps, I thought maybe getting some acoustic tape or something?? I am willing to try more radical solutions to get a nice little amp that works all in one piece.

Finally, I am disappointed in the quality control with Vox on this amp. This was made in Vietnam. It seems hastily constructed. Aside from the irritating buzz, there are random glue marks in the cabinet and an odd streak of black paint on the speaker.

Thanks for any additional help or suggestions.

[Enzo]

Either have someone else play, or apply some steady signal to the amp. MAke the vibration happen more or less steadily. Now reach in and grasp the EL84 tubes firmly in your fists. If they are too warm, use a rag or gloves. Does that have any effect on the buzz?

If the tubes are buzzing, then playing through an outboard speaker removes the vibration from them. We'd expect no buzzing then.

[KT66]

just went through same problem here

circuits are not Faraday-caged so OPT fields interacts with some PU's to cause oscillations, esp. single coils.

circuit needs to be properly boxed, cheap and easy design to manufacture
but they overlooked a very basic requirement in tube amp design

otherwise good sounding amp after basic tweaks ...