DIY Work on a Vox AC30...

WetDaddy

So, I'm pretty certain one of the valves on my Vox AC30TBX needs to be replaced. From what I've read and from talking to people, I reckon it's turned microphonic.

I know the way to check this is to lightly tap the valves (after they've cooled, of course!) with a phase tester. The microphonic one will resonate / ring out more significantly than the others.

Now, I know I can take it somewhere to get it repaired, but I'd really like to (a) know my way around my own amp properly, and (b) not pay an arm and a leg for something I could, with care, do myself.

I know they need to be installed in pairs, but I don't know an awful lot else. I ain't gonna do nothin' stupid, don't worry But has anyone experience in changing valves in an AC30TBX before? Or are most valve amps quite similar in process? Also, any ideas where's the best place to get the valves / schematic for the amp?

Thanks in advance, guys!

Doctor J

I don't know too much about the subject but I've read too many good reviews about this guy, give him a call, he should be able to sort you out.

Eoin Madsen

I've changed the valves in an AC30TBX. The preamp valves and phase invertor are all 12AX7/ECC83s except the single 12AU7/ECC82 in the vibrato. You can replace any of these valves on their own. The poweramp is four EL84 valves. All four have to be replaced as a matched set.

If you lightly tap the valves with a pencil when it the amp is on you'll figure out which one is microphonic pretty easily (you'll hear the tap in the speaker, and some screeching or squeeling). Just bear in mind that all valves are very slightly microphonic - and successive amplification will make the earlier ones seem louder than the others. They should cause feedback or squeeling though unless something is wrong. Tapping them when the amp is off may reveal nothing unless the loose part is very obvious. Just be careful if the chassis is out - even when it's off, the filter caps store enough charge to **** you up, so keep your hands out of the circuits unless you've discharged it.

For the valves I'd recommend http://www.watfordvalves.com in the UK, from personal experience. They can also offer advice on what brands might best suit you. I find most schematics on http://www.schematicheaven.com, though it seems to be down for the last few days. There's also http://www.drtube.com/guitamp.htm and http://blueguitar.org/schems.htm. If you don't have luck with them try Googling. The AC30 is one of the most popular amps ever, the various schematics are easy to come by - the only tricky part is figuring out which version applies to your amp.

dabhoys

Bob at eurotubes is the man to go with. Knows his ****...

WetDaddy

Cheers for the detailed reply, Eoin!

Dabhoys & Doc, will check out Eurotubes alright.

One last question: So, ye don't think it's a big risk me doing this? I mean, provided I get the correct valves and don't sent enough ampage through my body to send me across the room (a la Marty in Back To The Future )?

When it comes to actually removing / inserting the valves, is it relatively straightforward? Light-bulb esque?

Thanks again!

Eoin Madsen

No risk really. Take the case out and place it flat down (valves and transformers up) on a table or bench. Just don't put your hand inside at any point. After that, it's all safe. Nothing on the top or outside can electrocute you (don't touch your fingers into the empty valve sockets though) and you can't easily damage the amp by accident. Changing the valves is a bit trickier than light bulbs but it's not complicated. You literally just pull them directly up once you've removed the retainers, and the exact opposite to put them in. They may require a little bit of wiggling - just be really careful not to bend the pins. A noval valve socket has 9 pins around and a blank, so the valves will only go in one way. Avoid touching the glass of the valves with your fingers, use a lint free cloth or something like that. You will have to examine the preamp valves as you remove them - one of them is an ECC82, and you need to replace it with another ECC82.

I almost forgot, there's also a 5AR4 rectifier valve in the AC30. It fits into an octal socket, which will be bigger than any of the others and only has 8 pins. If you're changing valves from stock for the first time, I'd say replace this as well. Otherwise, they last for years and don't need changing as often as the rest so you shouldn't need to.

The powerstage of the AC30 is cathode biased and fixed, so you won't need to tweak anything or make measurements unless you really really want to. It does mean you should probably take a little more care in your selection of EL84s though. Watford Valves seems to specialise in AC30s, which would be part of the reason I would recommend them.

fitz

Go for Harmas accross the board. You won't regret it.

vox

Ei' also sound sweet in this amp, probably some of the nicest EL84 and 12ax7s ive tried in my ac30. The only problem is the 12ax7 EI tubes are not as reliable as others but the tone is incredible if you manage to get a nice set. Tubestore.com are great to deal with.