Repair of Friend’s Arbiter AC30 after Internal Fire

A very good friend of mine phoned me one day in a bit of a panic. His prized Arbiter AC30 had just caught fire internally during a rehearsal. His worst fear being that it was now a write-off!

Having just serviced his other two TBX’s, I suggested he brought it round to the studio so I could take a look. I suspected it was just a valve base fire, which does happen occasionally, especially when these amps are played at full whack with no speaker load connected!! ….. Just don’t do it!!!!

The following is a brief description of what I found and how I repaired it.

First things first; let’s find the fault!

Arbiter AC30

First pair of EL84 valve bases look OK, although the wiring is in a bit of a sorry state. That pink wire with its insulation half melted away is connected to the anodes with high voltage AC travelling through it just millimetres away from the steel chassis!

Arbiter AC30

Ahh, the second set shows the source of the fire! Basically what has happened here is that at some stage, the amp has probably been played with no speaker load connected. This causes the anode voltage to swing very high and arc to ground over and sometimes through the valve base insulation material which leaves a small track of carbon behind. This small track of carbon is now effectively a resistor joining the high voltage AC at the anode to the chassis. As with all resistors, when you pass a current through them, they get hot. This causes more of the surrounding insulation to burn and turn to carbon, thus slowly reducing the resistance. The lower the resistance, the higher the current, the hotter it gets up until the point it actually catches fire, which is exactly what has happened here!

A cursory glance around the rest of the circuit and all seems fine, for now!....

So, to repair the amp, I first had to remove the old wiring and valve bases. My plan was to replace all four bases, as there was a good chance that other bases could well have been compromised at the same time as this one!

Arbiter AC30

Old bases out and a quick clean-up (That black scarring had burnt its way into the plating and would not come out!)

I had decided to replace the old bases with nice ceramic units with gold plated contacts. These were a little larger in diameter than the originals, so the holes needed a little fettling to get them in!

Arbiter AC30

I decided to fit the new bases using new M3 machine screws and ny-lock nuts as during the works, I found out that I needed a supply of the imperial screws! (More on that later!).

Bases in, it was time to reappraise the wiring. To be honest, it was all pretty dreadful in there. Not one wire had undamaged insulation. The yellow wire to the output transformer primary had a bad nick in the insulation that was bandaged with a scruffy piece of black insulating tape. Yes, it worked, but it wasn’t leaving my workshop looking like that, so was repaired properly with a suitable piece of yellow heat-shrink sleeving.

Arbiter AC30

So, in the end, I just ripped the whole lot out and started from scratch. New heater harness, grid and screen cables were installed along with new anode jumpers and cathode lead. I did re-use the cathode bus-bar once I had straightened it.

So, the wiring finished, a new set of Mike Hill EL84’s installed, a few pre-flight checks before power-up and she fired up first time! Great! All good in the power department!

Now, my dear friend had bought this amp from one of his customers and it had been rather un-lovingly converted to Brian May mode, via the removal of its unwanted valves and apparel! This is fine if you want to do that kind of thing, however, his lordship wanted the entire amp to be restored back to its original form. Now, this is where the fun started!!

Having installed a new set of valves, very little extra worked! No Vib-trem channel, No Bright channel!..... Hmm!

This time I had a good look round the amp and was a little shocked to find that one of the main circuit boards was simply hanging in there by its wires. That’s 320VDC flopping around in a steel chassis!! Now, these things happen in life, but two things surprised me; firstly, this amp had just been back from a service from a very reputable repair shop, so why had this not been noticed and rectified and, secondly, if the screws had fallen out after the service, why were they not in the bottom of the cabinet? Everything else was! Flies, pics, woodscrews, broken cable clips, half a tuna sandwich circa 1987!! ……..

So, moaning and bemusement aside, I re-mounted the board using the old valve base screws I’d put to one side earlier!

Board now securely fixed, I continued my quest for the rest of the amplifier functions! So, the Bright channel wasn’t working because of a very dead treble potentiometer. The track had literally corroded away under the slider brush. I like easy fixes like that! And so onto the Vib/Trem channel.

Well, for this to work, the RC Low Frequency Oscillator needs the resistors in its phase shift network to be grounded. This is normally done via the footswitch, which, in this amp, had been removed. Later amps had a jack socked on the rear panel that had normalled contacts allowing the oscillator to continue with the footswitch unplugged. So, I built a small diecast box with two jack sockets in that I screwed just inside the rear of the speaker cabinet. The first socket was a normalled Vib/Trem footswitch jack, the second was for the speaker output allowing easy connection of an attenuator.

Vib/Trem circuit now running, there was a rather excessive hum on the Vib/Trem channel. Now that took some finding! In the end, it was simply poor metal-to-metal bonding between the steel and aluminium chassis members. A new set of screws and ny-lock nuts with star washers on both sides cured that little tinker!

I also shortened the mains lead by a foot as there was another piece of insulating tape wrapped around it which when removed, revealed a little bare copper in the brown wire! Not nice!!

I have to say, once everything was reinstated, it was a beautifully sounding amp! Thinking about it, maybe I should have pronounced it a write-off at the beginning and kept it!! …..Joking!..... It was a fun little project though!

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