Vox AC30 Rebuild

(8/29/2004)

Every once in a while a project comes across my bench that both excites and scares me. The chance to work on a piece of history in the vintage guitar amplifier world is, after all, why I'm in this business. On the other hand, sometimes the years aren't good to the amps that end up at my door step. And so it was with my latest rebuild project. A circa 1967 JMI/Vox AC30 6 input top boost amp. At least that's what the control panel implied.

Before I go too far let me mention that there are three kinds of vintage amp jobs our company, DST-Engineering does; repair, rebuild and restoration. It's obvious enough what a repair job is. However, the difference between restoration and rebuild is a bit more important to understand. Restorations are specifically intended to get the amp back into original condition. This includes acquiring period correct parts, wiring, hardware, etc. Rebuilds are intended to allow a "severely depleted or abused" amplifier (read JUNK) to be made playable again. Rebuilds duplicate the original design and circuitry but most often use new parts where necessary and may also include minor enhancements to make the amp safer, quieter or more reliable.

Welcome to the AC30 rebuild project. The rebuild was chosen after discussions with the customer about his intentions for the finished amp (play, sell, worship, etc.), his time frame and budget. Although this is considered a rebuild, I still made it a priority to keep the circuitry and wiring as authentic as possible but used some new as well as NOS components. My original thought after speaking with the owner on the phone was to re-use as many of the original parts as possible. He mentioned that there were some missing components and the amp was a bit rough but his regular tech had checked the amp out and thought it would "fire up". I'm not sure what that meant. I had him send it in for evaluation and cost estimate. I was quite shocked at what I received. The photos below give a better description than I could write.

The two most obvious issues are that there are an awful lot of parts missing from the two terminal boards and that the output stage is badly burnt out. Some less obvious but still important issues are:

1. The power transformer was replaced with much later unit that does not support a vacuum tube rectifier. Also this transformer was wound to run from 220VAC ONLY.
2. The original vacuum tube rectifier was replaced with silicon diodes.
3. The vibrato/tremelo selector switches were replaced by pots (purpose unknown).
4. All of the vibrato/tremelo wiring was gone.
5. Numerous circuit modifications of unknown purpose were present.

After a sleepless night thinking "what have I gotten myself into now" I started in on plotting a strategy and put together a price/delivery quote for the customer. Once agreed upon I started in on tearing the amp down. It was obvious that there wouldn't be much to salvage. However, as luck would have it the original output transformer and filter choke were in very good shape and the chassis and terminal boards were salvageable. The first task was to clean and buff the chassis and transformers.

Power Section Chassis Before (outside)

Power Section Chassis Before (inside)

Power Section Chassis After Buff/Clean

Preamp Section Chassis After

After the chassis work was completed I started in on salvaging the terminal boards and any re-usable components. The terminal boards had nearly 40 years of residue built up on the surface that required quite a bit of time with a toothbrush and my secret formula cleaner. It's important on these old boards to keep them as dry as possible during cleaning to prevent warping and delaminating of the board itself (which are generally made out of pressed laminated paper byproducts). Unfortunately there wasn't a lot to save in terms of components or wire. Here's what the boards look like separated from the chassis.

I'll spare you the agony of details and just say that I did a near complete rebuild/rewire to original Vox spec but used mainly new components. However, as usual for this type of work I dipped into the stock of original Allen Bradley resistors and Sprague caps. That in itself is another story. Just for fun, I did reuse the original hardware, clips and screws.

The same goes for the power section shown below.

As is my usual practice with a job of this magnitude I work backwards form the output during the rebuild. After getting a new transformer in place (warning: the original Vox reissue transformer isn't quite the right size and its bell cover is black - shame on Vox) I rebuilt the power supply and output sections. This allows me to fully test these sections before tackling the pre-amp. I always work and test one section at a time. By getting the output done first I have it available for testing the rest of the unit. After this was the phase inverter, then the top boost, pre-amps and vibrato circuits etc. I always find that the most important aspect of a job like this is careful planning before getting started. It's to your advantage to map out the project in your head, then on paper before you start. Also, as I mentioned, if you work one section at a time you reduce the likelihood of gross errors that require ripping out large sections of already completed work. Components, wire type and layout must all be decided on before you start working them into the chassis. It's also well worth your time to research the amp as much as possible if you're not completely familiar with its design.

As with all Vox amps the wiring can be a nightmare. The design of the AC30 was an evolution over several years and ultimately created a fairly cluttered component layout. This greatly increases the possibility of stray noise pickup, hum and parasitic oscillations. You will notice in the finished wiring photos that I use a ground bus method of connecting all of my pre-amp, power amp and power supply grounds whenever possible. Experience is a great teacher when it comes to grounding issues.

This is one of those areas where you could get into never-ending discussions about function vs. authenticity. I know as well as anyone that part of the charm of old Vox amps is their noise and hum. But honestly, why go to the trouble to rebuild an amp only to have annoying and unnecessary shortcomings in your work. This same philosophy applies to my use of color coded wiring to make future troubleshooting easier. Anyone who doesn't plan for the possible failure with a Vox is sadly misinformed. These are great amps with very cool vibrato in my opinion, but their reliability has always been an issue. Keeping this in mind also explains the use of 2 watt resistors where the original used 1 watt, etc. The same goes for capacitors and the use of 600 volt ratings.

After about a month on this project, in between several other projects, new builds and repairs, the unit was completed. For this particular project I only received and worked on the electronics chassis. The customer chose to handle his own restorations of the cabinet and control panel. You can see just how bad the panel is in the photos. I'm told this amp came with the original Blue's and they are in good shape. Same goes for the Vox egg shaped footswitch. Below are photos of the unit in its complete form as well as before and after shots.

AC30 Outside Before		AC30 Outside After
AC30 Inside Before		AC30 Inside After

Have a question or want more details? Contact Bob Dettorre, VP Engineering, DST Engineering
bob@dst-engineering.com Thanks to owner Dan Papandrea for allowing us to show his amp!