Vox AC30 Speaker Issue

shanejunkin

Hi All,

Do you know what are the symptoms of a speaker issue? My Vox AC30 is crackling and humming away independant of whether I have the guitar hooked in or not. A lot of the time it is at a low level but sometimes it can get pretty loud and very annoying. I have recently had the valves changed and fully serviced so I don't think there's any issue there. The volume pots were also cleaned up as far as I know, and the crackling doesn't modulate as you adjust volume (or bass and treble). I suspect that the speakers need to be changed.

If they do, I am thinking of installing Celestion Green Backs. However, I don't know whether to stick in the 8 ohm or the 16 ohm version. Does anyone know whether the 8 or 16 ohm is suitable for the Vox AC30?

Also, would you recommend the greenbacks? Is there much of a difference between them and the blue backs?

Eoin Madsen

Hi,

If it's making odd or random noise at idle, then it's not likely to be a speaker problem - it would be something in the amp itself. Check if the crackling changes in tone when you alter the treble or cut controls - it may not be the pot causing the problem, but if the noise originates before the tone stack, the actual sound of the crackling should be affected by the tone controls. Were there any problems like this before the amp was revalved? My first guess would be that one of the new valves is a little faulty. Is it an AC30 with a tremolo circuit ("vibrato") or one of the new ones?

AC30 would normally have a pair of 8 ohm speakers wired in series for a total impedence of 16 ohms. As for differences between the blues and greens... the blue AlNiCos date to slightly earlier models and are considered a little extra "vintage" sounding by some. They are usually more expensive. Personally, I actually like the sound of the Greenbacks more overall, but it depends on what you're doing with the amp really.

shanejunkin

Eoin Madsen wrote: »

Hi,

If it's making odd or random noise at idle, then it's not likely to be a speaker problem - it would be something in the amp itself. Check if the crackling changes in tone when you alter the treble or cut controls - it may not be the pot causing the problem, but if the noise originates before the tone stack, the actual sound of the crackling should be affected by the tone controls. Were there any problems like this before the amp was revalved? My first guess would be that one of the new valves is a little faulty. Is it an AC30 with a tremolo circuit ("vibrato") or one of the new ones?

AC30 would normally have a pair of 8 ohm speakers wired in series for a total impedence of 16 ohms. As for differences between the blues and greens... the blue AlNiCos date to slightly earlier models and are considered a little extra "vintage" sounding by some. They are usually more expensive. Personally, I actually like the sound of the Greenbacks more overall, but it depends on what you're doing with the amp really.

I haven't tried moving the treble and bass controls to see if the tone of the crackling changes, however I don't think that it will, I did move the treble and bass controls and noticed nothing, however, I wasn't specifically listening for tone changes. I'll try again.

It's an old AC30 with a tremelo circuit. It's an AC30/6, with normal, brilliant and vibrato inputs.

Before the amp was re valved it was in a mess. All the controls were crackling and there was some transformer issues. On receiving it back from Alladin's amps in Tallaght it was fine, sounded great actually, but then this humming and crackling came up.

I think I'll upgrade it with Greenbacks once I've this issue resolved (if it turns out not to be a speaker issue)

Is there anything else I can do to confirm whether it is a valve issue?

Eoin Madsen

Swap out the preamp valves one by one with a spare ECC83 (you can probably use an ECC83 for the one ECC82 just to test, but be careful to replace the ECC82 to the correct socket afterwards), and test the amp between each time. If there's a bad valve in the preamp it should be obvious enough because the problem will not occur when the bad valve has been replaced.

To check the powervalves, first pull the outer two EL84 valves and check the amp. Then replace these valves and remove the inner two valves and check again. Don't run much if any signal through the amp in this configuration (the output impedence will be off so it may not be terribly healthy for the amp to do so), just listen for the problem.

A speaker cannot produce sound on its own, and a speaker fault will only manifest when signal is sent to it. Random noises including crackling or humming coming from a speaker when the amp is completely idle are not indicative of a speaker problem at all.

What you describe sounds more like a component failure, intermittent ground, or just RF interference. I'd move the amp to see if it continues to have the problem in a different place. Then I'd check valves, cathode resistors, and grounds.

What sort of transformer "issues" was the amp having previously? Was the OT or PT changed when it was serviced? How long after servicing before this new fault arose? Has the amp been moved much or knocked recently?

shanejunkin

Walton's in Dublin told me that the transformer had failed. However, on leaving it to Alladin Amps it turned out to be just a failed diode before the transformer. At this point the amp wouldn't even turn on, I thought it was finished.

However, on replacing the diode it was fine. They guy in Alladin Amps did a full service and changed all the valves. It was sounding great for around four weeks, then this issue arose.

I modulated the bass and treble and found that the tone of the crackling and humming does actually change so it looks like you are right.

Where's the best place to order those valves from?

Eoin Madsen

Well, that doesn't prove it's a valve problem yet - it just means the problem is probably in the preamp before the tone stack. The ECC83s are easy and safe to inter-change, so at this point using a spare (one of the old ones from before the revalve should do fine) to swap out the valves one by one might tell you something more useful.

The ECC83/ECC82s are the smaller valves. There should be 5 of them at one end (The one fatter valve on the other end is the rectifier, and the four taller ones in between are the EL84 power valves). It's those first 5 smaller valves that you need to check.

Walton's in Dublin told me that the transformer had failed. However, on leaving it to Alladin Amps it turned out to be just a failed diode before the transformer. At this point the amp wouldn't even turn on, I thought it was finished.

Hmm... there aren't any diodes in an AC30, except the rectifier. And the rectifier is after the power transformer. The only components prior to the PT are the mains fuse and the power switch. What year does this AC30 date from approximately?

shanejunkin

I think he said a diode anyway, I'm nearly certain. It was a simple electronic component that was blown. It wasn't the mains fuse either, as I am familiar with that componant as it was blown before (by adjusting the voltage knob while the amp was turned on).

My Vox is from circa 1978.

With an amp this old is there any end to such niggley little issues? would I be better off get this issue resolved, sell it and buy a twin reverb of something equivalent?

Eoin Madsen

shanejunkin wrote: »

I think he said a diode anyway, I'm nearly certain. It was a simple electronic component that was blown. It wasn't the mains fuse either, as I am familiar with that componant as it was blown before (by adjusting the voltage knob while the amp was turned on).

My Vox is from circa 1978.

He could have meant the rectifier valve, which is technically a diode (or pair of diodes internally). It's just an unusual use of the term - "diode" is more commonly reserved for silicon diodes. But when the company was owned by Rose Morris (mostly in the 80s afaik) they went through a phase of using solidstate rectifiers (with silicon diodes), so it's possible it has one of these... I'm just not familiar enough with the precise history to say. Has it got a standby switch? And could you look in the back of the amp and count the valves? Or better yet, take a picture?

I don't suppose you got any kind of report or diagnosis in writing from the service? In any case '78 probably is kind of an awkward year in that it might have an early printed circuit board which will make it harder to replace failing components.

shanejunkin wrote: »

With an amp this old is there any end to such niggley little issues? would I be better off get this issue resolved, sell it and buy a twin reverb of something equivalent?

I couldn't say. If you were to replace it, I'd first suggest trying a modern AC30 first anyway! The Twin Reverb has quite a different sound. The reality is that components are going to fail. If you did indeed blow a silicon diode in the rectifier, they you need all four replaced (which I'm sure was done). But it would not be surprising if a few resistors have drifted by now too and some of them may fail in the next few months or years.

If the filter capacitors have not been replaced since the amp was built then they're about two decades overdue a change by now. If it got a "full service" then this should have been done. I would query the tech on this and request a printed report of the service if available. Frankly, if they don't have a written record of the work they did, I wouldn't go near them again.

The niggley issues can of course be resolved. You could get things fixed as they fail, or you could pre-emptively replace the whole board. If it's a point-to-point, you could just replace the components most at risk on it. If it's a PCB, I'd be inclined to suggest just replacing the whole thing - a lot of soldering on a PCB can get messy.

studiorat

I don't think that amp has a Valve Rectifier btw, the early ones did the JMI brown cloth ones did anyway. The rectifier is the big one on it's own, near the transformer oddly enough.

It could be just the valves working loose after the few weeks and them heating up and cooling down a few times.

Problem you seem to be describing could also be down to the capacitors, they dry out over time and the heat of a valve amp doesn't help. Capacitor problems will cause a constant low crackling when the amp is just sitting there.