Amp trouble


2016-03-14 3:39 am
Hello everyone, I'm having troubles with my amp (Vox AC30CC2) and posted about it on my board of choice (mylespauls forum) and the folks there recommend I also post here. Here are the links to my posts on the other forum, they said you guys have more technical expertise with amps so I'm extending my call for help here. Thanks guys

Vox AC30 Trouble -

Troubleshooting a PCB -

hopefully we can get this trouble fixed!

Thanks guys


2004-05-23 6:54 pm
You wrote: The tech said he changed a few tubes and a couple of minor things within the amp." Do you know what he has done?
You also said that you compared an AC30 in the shop and yours.
Are both the same amps? AC30CC2's?
Do you used the same guitar?
Is the top boost channel OK?
There isn't much in the normal channel.
Is the brilliance switch working?
Is the volume pot of the normal channel working?
I assume the 12AX7, V1 and V2 have either been replaced or swapped.
Did this do anything to your sound?
Last option; Attach another speaker cabinet to your amp.
If that doesn't help; sell the amp.
Welcome to the forums and congratulations on your purchase. The AC30 line of amps has been in production for decades, and has gone through many revisions. The model you cite is the "deluxe" version of the 2 AC30's that are made in China. Build quality is understandably not the same as a hand-made domestic model. It is also used, who knows what the amp has been through before you owned it.

Your symptom sounds like it is only present when the guitar is connected to the normal input. How does the sound compare to the top boost input? The normal input is naturally light on treble response comparatively. You also stated that the symptom occurred while you were playing through the amp. Can you elaborate?

The ultimate result you are seeking (stock functionality or better) can be had in many ways. If you are not willing to get intimate with your amp and how it works, as well as dismantle it, and/or remove and replace components without breaking something, you will need to seek help from someone who is. Your recent service sounds like the tech performed routine maintenance on the unit, much like a tune-up on a car. If you are not confident or eager to fiddle with your amp, you might want to take it back in again. Expect to pay more labor for a tech to diagnose your custom symptom. To find a local repair tech who you can speak directly with to solve the issue is probably going to be more difficult, time consuming, and expensive, but it all depends on which way you decide to go at this problem.
Hi Guys

You don't say how old the amp is, just that it is second hand. Most problems with modern electronic items can be fixed by resoldering the entire PCB. Since the amp is mass produced, its board is wave soldered and then touched up at the end of the line. With wave soldering, every component lead gets the same amount of heat andthe same amount of solder. This is quite dreadful since many leads are much thicker and need more heat and more solder for a reliable connection.

A friend of mine operates the largest service centre in the country and his crew routinely resolder every board that passes through their shop. This fixes 90% of first symptons (customer complaints of why they brought the product in) and eliminates call-backs due to new problems. I would ne surprised if this did not make your combo amp much more reliable if not also fixing the treble loss.

if the tubes are old then they are likely broken-in and will not change their tone for decades. people who want to sell you tubes will tell you to change them as soon as the tone changes, but this is folly and a waste of your money - and a waste of tubes. Tubes will last for decades if not mechanically upset. once they pass their infant tone they are on the "tone plateau" and do not change. I would not change the tubes unless they have actually failed, or if you want to try alternate types.

Despite the AC30 being in production for many decades and built by many different companies, the heart of the design has stayed the same, which also means that the original flaws of the circuit are still intact. There are some things to change if you wish to keep this amp for a long time.

The two main things to change in any Vox are the screen resistors and the biasing. The screen-stops (if present) will be a joke value of 100R. This is entirely too low to protect the tubes in guitar amp use and especially in a combo - the worst environment for tubes. Each tube should be given its own 1k-1W flame-proof (metal-oxide) screen-stop. This will keep the amp from eating tubes.

The shared bias resistor for four EL-84s is ghastly for reliability. If one tube fails the others will be damaged. As TUT3 shows in the AC-30 chapter, split biasing is more reliable and allows pulling tubes if you wish. This just requires adding extra bias resistors and caps, with proper values for the split biasing.

Have fun