The Mildly Interesting Vox AC30 Reverb Problem

Mark’s like-new Vox AC30 worked well, except nothing came through the reverb signal path.  The foot switch checked out, so we loaded it up and worked on it at the Unbroken String lab.


A gorgeous amp, built like a tank, sounds sweet, but no reverb.  Slapping the tank didn’t bring forth any spring sound, so it’s time to get it on the bench for some minor surgery and some amp tech porn pics!

This amp was purchased new just a few months ago from a major retail chain.  Even a Chinese amp should be pretty solid for a few months.  Now I’m really curious what is going on here.

10 LoudspeakerSome amp tech porn.  Wharfedale supplied loudspeakers for AC30s for one year, Sure enough, here they are!

The chassis slides out of the back of the amplifier.  Time to check the preamp tubes, seen mounted horizontally behind the final amp tubes in the foreground.

Well, that was easy to find.  The current through this tube barely moved the needle.

06 OverheatedComparing the bad tube to its neighbor, the getter material is a little darker.  Nothing else out of the ordinary here.

07 SovTek GoodnessThankfully, economic sanctions against the Russians haven’t pinched off the free flow of SovTek 12AX7 tubes. Yet.

08 New TubeSome techs cuss at the rubber shock mounts around the tubes as you see here because they interfere with inserting the tube into the socket.  But the tube gods smiled this afternoon.

09 First TestWe are ON but nothing is coming through the reverb circuit.  Again.  Uh oh.  Have I failed?

11 EMI Choke1A visual check sometimes yields the quickest results.  Nothing was out of the ordinary.  But this clamp-on ferrite choke does not appear on the schematic.  The red and white RCA cables go to and from the reverb tank.  What’s going on here?

13 Bottom ConnectionsThe other end of those red and white cables terminate in the bottom panel of the amplifier cabinet. The cables from the reverb tank plug in here.  The signal passes through the amplifier cabinet through these bulkhead RCA connectors.  Interestingly, the loudspeaker wiring goes through this quarter-inch jack, greatly simplifying service. Thank You VOX!

12 Broken WireThis is a view from the other side of the black panel in the previous picture.  This open white wire goes to the RCA plugs that actually plug into the amplifier circuit.  Now we’re onto something!

14 Test Run1Space is cramped, so I took some time to set up this portion of today’s program.  I removed some solder from the RCA bulkhead connector, so I can make a new solder joint.  I’m using a pair of tweezers to get the wire lined up because my big fat fingers won’t go inside the amp here.  And I don’t want to be anywhere near the hot soldering iron when I’m fixing this.

15 FixedAh, that’s much better.  The original Chinese solder was that high-melting-point lead-free stuff, containing lots of tin.  In my experience, the lead-free solder is a bit brittle, so this particular solder joint was at-risk since the day it came from the factory.

16 Strain ReliefWhile I’m on a rant about lead-free solder, let me rant about this ferrite choke.  This part has nothing to do with sound reproduction or tone, but was likely added in an effort to comply with FCC or UL regulations.  The only support for this choke was the wires from which it was dangling, and you saw the damage it caused.  Here, I’m using a tie strap to hold the ferrite choke away from anything delicate.

17 Outta My WayWhile I’m at it, I’ll add a second tie strap.  That should keep this from happening again.

18 HookupNow everything is hooked up again and the reverb tank really, well, reverbs!  I think we’ve got it fixed now.  This is a nice shot of how the signal path moves from one section of the amp enclosure to another.

19 Chassis ScrewsI had forgotten how many screws had been removed to take this guy apart.

20 ReassemblyHere’s more screws.  Are we there yet?

21 Final TestMark has his amp back and he is pleased!  He mentioned that this amp is for sale, so if you are interested in this unit, drop me a message from the “Contact” page and I’ll hook you up.


Thanks for reading all the way to the bottom!


David Latchaw

Cell : 281-636-8626