VOX AC4TV

[Emiel Visser]

Hi Charlie,

I recently purchased a VOX AC 4TV.
Must say very nice.
If I want less overdrive, can I change the pre tube 12ax7 for a ecc81 without any modification of the amplifier?

Thanks and regards,
Emiel

[Charlie Hall]

Hi Emiel,
You could try that, but I suspect that turning the volume down would give much the same result.
Regards,
Charlie

[Emiel Visser]

Hi Charlie,

Thank you.
I tried that, but I think that the input is very sensitive, even with my 57/62's Fenders.
Perhaps I can try to connect a resistor of 1 M Ohm at the input, like the "low" input of a AC30?

Regards,
Emiel

[Charlie Hall]

Hi Emiel,
If you have an accurate meter, check the anode and cathode resistors of the first stage. If either are a long way from the correct spec it could lower the headroom of the first stage. For example, a larger anode resistor, or a smaller cathode resistor, would increase gain and lower the headroom.
It should not be necessary to change the input circuit.
Regards,
Charlie

[Emiel Visser]

Hi Charlie,

Thank you.

Regards,
Emiel

[Emiel Visser]

Hi Charlie,

Turning down the volume on the guitar helps, but causes hum. I know that you explained once why that is.
Is there a remedy to prevent that?
The thing is that the amp comes alive when the volume is set at about 70%.

Thank you.

Regards,
Emiel

[Charlie Hall]

Hi Emiel,
Does the hum actually increase as the guitar volume is turned down?
The hum might be due to the heaters not having a balanced earth, the early AC15 had this problem, probably also the AC10. It can be modified by remnoving the earth connection from one of the heaters and adding two equal value resistors from each heater terminal to earth, but should be done by an experienced person.
Regards,
Charlie

[Emiel Visser]

Hi Charlie,

With volume at 10 there is the normal noise. At about 7 there is more noise, but doesn't increase by lowering to 3 when it's gone, but still signal left.
I find this amp very sensitive, though I like it very much, it's very lifely, it responses very good on my playing.

Regards,
Emiel

[Emiel Visser]

Hi Charlie,
By volume I mean the pot on the guitar.
Regards,
Emiel

[rogercook]

Hi Emiel

Do you have a circuit diagram for the AC4? As far as I know it uses an ECC83/12AX7 as the preamp valve. These are double triodes and my guess would be that each half of the triode is arranged as a preamp with one half feeding the other in series. So if you have high enough gain from the first triode this will overdrive the second triode into distortion which is what the designers probably intended. If you need a clean sound the answer is probably to reduce the gain of the first stage or switch it out completely.

If you have a look at this link for an Epiphone Valve Junior you will see what I mean

forums.epiphone.com/default.aspx?g=posts&t=2713

Roger

[Deleted]

Hi Emiel, best thing to do is fit a Philips JAN5751 ECC83 in the pre-amp stage. They are the best valves to get plenty of clean headroom and also clean up noise. Here is a link to Watford valves where they can be bought for £20. I use them in my Blues Junior and also in my Vox Night Train. Amazing difference in warmth and clarity.

Jim

www.watfordvalves.com/product_detail.asp?id=1784

[Charlie Hall]

Hi Emiel,
There are two circuits for the AC4 on this site:
blueguitar.org/
Select Schematics and Links on the left side, then under Guitar Amps and Mods select Vox, then select ac 4.gif or ac 4.jpg, the second one appears to be an original Vox diagram but the first may be clearer to read.
These diagrams show an EF86 as the first stage preamp. If the sound is clean with the amp volume set low, and if the tremolo works properly, and the sound is only distorted with the amp volume set higher, then the EL84 output valve may need changing or there may be another problem with the output stage, or C3 may be leaking DC voltage. I would check C3 first. This can be done by setting the amp volume to zero and measuring for any DC voltage on the top leg of the volume pot (that connects from C3). If there is the slightest amount of voltage there even on the lowest DC volts setting of the meter (it must read zero volts), change C3 for a new 0.047uF 400 volt capacitor. There are a choice of capacitor types, some people may hear differences between different types of capacitors. I would use metallised polyester or polypropelene.
The hum from your guitar with the volume set lower might be caused by a problem with the EF86 if the heater circuit is properly balanced. If your amp has an EF86 and you have a spare one, try replacing it.
The heaters show a balanced feed around earth (earth is connected to a centre tap of the transformer heater output) so unless your heater circuit does not match this, the heater circuit should be OK.
Unless you are competent with working on amps, do not do the work yourself. There can be high voltages even if the amp is switched off and unplugged from the wall.
I cannot be certain that the circuit diagram matches what is in your amp, mainly because I cannot see the date on the diagram.
Regards,
Charlie

[Emiel Visser]

Hi Charlie,

I think that there is a misunderstanding about this topic.
I am talking about the new AC4TV, while you are talking about the vintage AC4 with EF86.
I am sorry if I caused that! You did a lot of work to explain.
Thank you Jim and Roger.

Regards,
Emiel

[Charlie Hall]

Hi Emiel,
Thanks for clarifying the model. I had misunderstood, especially because I didn't know or remember about the new model and the faults you described seemed to be typical of an old amp.
If you can find the correct circuit diagram for your amp (and then scan it if necessary) and email it to me, I'll have a look.
Regards,
Charlie

[Emiel Visser]

Hi Charlie,

Thank you.
As far as I know there no diagram available yet.
As soon as I find one, I'll let you know.

Regards,
Emiel