Aston Electronics 5W Valve Amp Kit Build – Finished
There is a big effect difference between voltage levels that break through the skin conductivity – which happens to be in the 400 to 500V DC range of our amp circuits! BE CAREFUL! Drain the secondary capacitors when the circuit is off and always check the circuit with a multimeter on both AC and DC sides before touching components or working on it – with the mains UNPLUGGED. If the Mains is plugged in, the plug socket and switch contacts are LIVE still, with the switch in the OFF position.
If you are testing the circuit after a change, make sure all your test leads, probes and croc clips are well insulated from your fingers and chassis contact points (chassis edges can be sharp too). I always connect the croc clips and meter and double check. I then plug in the fused mains side. I use the mains toggle switch to power the secondary with the indicator light connected so I can SEE power also, as well as checking the on/off position of the switch BEFORE I plug in the mains. I switch OFF and unplug the mains, checking the meter voltage or current level has dropped to 0V, (or use a drain lead on the capacitors ASWELL) before I move any croc clips to a new circuit point for another measurement.
For the sake of a few extra seconds, why risk safety by not switching off and draining the circuit?
A 240V AC shock will certainly help you not make that mistake again if you ever forget any of this – if you are lucky enough to not get away with it! My last one left my right arm aching for an hour!
A 500V DC shock may mean you don’t get to worry about any of this ever again…but now you know why.
The amp is all tested and sounding great – The End…
…well not quite…
After a few hiccups due to my own stupidity, I finally got this done, and am very pleased with the results. It looks GREAT eh!? I’ll get the chicken knobs on soon.
I made a stupid error early on and had the bulk of the transformer bodies on the inside of the case, as the winding connectors don’t pass through the space unless you TILT THEM SLIGHTLY…Duh, Homer… (I didn’t look at the pictures on Geoff’s site properly), so after realising this left a very cramped space, as the push fit mains socket pins touched the mains transformer, I had to undo both the mains and output transformers and put them in properly. Now it all made sense…
This is a pretty easy kit to build and provided you have more common sense than me (not difficult!), you can use a soldering iron, screwdriver, hacksaw, pliers and a spanner, and READ INSTRUCTIONS FIRST (as I didn’t properly), anyone should be able to do it. Even people afraid of electricity needn’t be, as provided you follow the instructions, take your time (!!!) and think before you act, there should not be a problem, as this kit also discharges its storage (smoothing) capacitors pretty quickly – like the Marshall 2060 – due to a 220k bleed resistor in parallel with them. So even if you turn off the thing after the first tests and leave it for a couple of minutes, you should be safe from dangerous voltages, even if you forget to check first (yeah right! DONT forget though!).
Once you have placed all components on the eyelet board, double checked all component values and placement, soldered them together, and made a tidier job of your wiring than me (below), tested for continuity as per the instructions test procedure, you will be ready to do the first power up test, with no valves inserted:
This just checks that the mains AC voltages from the mains transformer are correct. I got 265V AC on the secondary winding feeding the eyelet board (yellow wires to the diodes). This is 25V higher than the “ball park” figure to expect, but around 10% different so within Geoff’s limits. The valve heater windings were +/- 3.6V AC so fine also.
Now, DONT make the mistake I did, which was to mount the valve holders in the chassis before soldering the connecting wires, because the solder can run down the holder pins and block them! DOH!!! I was lucky to get away with only 1 hole being blocked when it could have been all 9 and I would have had to order a new holder and replace it.
Luckily, I managed to get most if the solder out by tilting the chassis whilst heating the pin, and poking a wire through, but I still had to get the old spare Marshall EL84 valve, and press it into the holder while holding the iron on the pin, then remove the valve with the solder still hot, and hope it had cleared/moulded the hole enough so the valve seated properly. I am a complete idiot for doing this in the first place, as Geoff’s instructions state that it is easier to solder connector wires to the holders, (sideways) – BEFORE putting them in the chassis (but doesn’t say why – now I know!).
You really don’t want that to happen just as you think you are in the test phase and find that the valve doesn’t fit in the holder!
Anyway, I got away with it, put the valves in, turned the chassis upside down for testing, by having the large 6V6 valve hanging of the edge of the table, and switched on the mains, waited for 2 mins for the valves to fully warm, then turned on the standby switch (do yourself a favour – get a decent workshop and bench environment to work in!):
I took readings from the test points 1-9 as per the instructions, and all were within 10% of Geoff’s figures.
The highest voltage on this circuit is with the standby switch on, at the DC side of the rectifier diodes – 335V DC – something you don’t want to be touching or shorting out with a test probe, so beware.
There is a Cathode bias calculation to be done at the end of the tests, which is taking a voltage reading across the 470R cathode bias resistor, and dividing it by 470 ohms. I got:
25.7V/470 Ohms = 55mA, where Geoff states that a typical value is around 48mA – close enough.
Because I got the wrong panel cover sent to me, and placed the 1M volume pot accordingly, so had to swap the wiring between the Master volume and the tone control (marked bass on this amp), Geoff kindly agreed to let me convert this to a treble and bass version, so will send me another 220k log and 220k linear pots, and other minor component changes required to do this.
I have now tested it with my guitar and it sounds crystal clear through 1 x 8ohm Richard Allen CG12 Super speaker. I will experiment by adding the 2nd speaker in series to give a 16 ohm load, which this amp has an output for also, as it certainly has the power to drive these (I think these speakers are rated at 12W but it’s difficult to get info on these rare classic speakers).
All I need now is a suitable donor cab for my speakers, and have a protective grill cover made at college – if I ask the Engineering Dept folk nicely – along the lines of the Vox L’il Night Train:
I can utilise one or both the Richard Allens as a twin (8 + 16 ohm) speaker cab maybe, and I’m rockin!
I’ll do a recording of this amp ASAP, so you can hear how incredibly clean it sounds, with its ringing harmonic presence. I haven’t been able to play it loud enough yet to see if it overdrives at full volume or not.
If you are thinking of making a kit amp – do it! – if I can, anyone can, and it is a good feeling to know that you built it yourself.
If you are really keen and practical, good at woodwork, and want to make personalised combos/cabs there are some amazing examples of home built amps and cabs at:
Check them out for real inspiration – there are some real beauties – and whacky ones too.