Lamington III – construction

Design  |  Construction  |  Components

A goal of all of my amp designs is to keep the costs of construction to a minimum. The parts cost for an amp build can rapidly run away, and finding low cost solutions to an amp build assists in keeping the total low.  Commercial rack cases and cabinets can be rather costly.

As in the original Lamington amp design, the Lamington III was built in a readily available, low cost lamington cake tin from Big W or K-Mart. An inverted cake tin provides an attractive, teflon coated chassis which is ideal for the amp.

In addition, I was aware that most people constructing the Lamington III will have limited metal working experience. A Lamington cake tin is easy to work with, and if it gets damaged during preparation, another can be purchased for $6!

For details on chassis preparation and drilling, refer to the information on the original Lamington amp here

The Lamington III was constructed in the same way as the original Lamington amp, with the large capacitors fixed to the chassis with silicone sealant. Components can then be wired to the valve sockets and front panel controls as you can see in this photo:

Lamington III under

Always make sure that all of the mains wiring connected to the primaries of the power transformers is properly insulated and out of harm’s way. I always use heatshrink tubing to thoroughly insulate all mains wiring.

If you are building your amp using my layout, you will need to modify the mounting of the M1130 output transformer. It was discovered when building the prototype that a significant amount of hum was being induced into the transformer from the power transformers behind it. This was not an issue with the original Lamington because the M1115 used in that design had the core mounted vertically. The M1130 has its core mounted horizontally. If you look closely at the photo of the Lamington III, you can see that I removed the core from its mounting and rotated it 90 degrees and mounted it with that orientation. This completely removed any mains hum from being coupled into it.

Before you first turn on your amp, set RV1 the bias adjust trimpot to maximum resistance check it with an ohmmeter. Then briefly power up the amp, and check to make sure you see approximately +320V available at the main HT supply (HT1) with the amp idle.

You should also be seeing approx 6.3V AC across each valve heater.

To setup the correct bias current in the output valves, allow the amp to warm up fully. Then check the voltages across R22 and R23 (1 ohm). The voltage across these resistors is directly proportional to the bias current drawn by the corresponding valve. You should adjust RV1 so that you see approximately 50mv of voltage this corresponds to a bias current of 50ma per valve.

Other key voltage checks are approximately +200V at the anodes of V1a and V1b. The phase splitter (V2) should have around +200V at the anodes (pins 1 and 6) and +30V at the cathodes (pins 3 and 8).

It is also important to mention that an amp like this with a negative feedback loop can make nasty squeals/oscillation if the feedback is not phased correctly. If you wire your amp according to the schematic, all will be well. But if you swap either the anode connectors of V3 and V4, or swap the leads from V2 to the output valve grids, you will have positive feedback with attendant “howling”. If that happens, just shut down the amp and swap over the drives from V2 anodes to the output valves, and you will be fine.