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23 Jan 2020, 10:38

Building a Valve Amplifier Part 5: Final Assembly


Building a brand new valve amplifier using the RH84 design and off-the-shelf components

Having successfully built and tested the prototype two-channel amplifier and power supply, it was time to assemble the final version in its enclosure. I decided to stick with the point-to-point soldering, but I was determined to get this a bit neater and make the components not quite so cramped. Apart from anything else, this would make it easier to trace any errors should the need arise. I also needed to add an input selector, volume control, fuse, mains switch, and indicator.

Planning the placement of parts

I started the process, as I often do, with laser cut MDF templates, this allowed me to see how everything would fit together and make any adjustments as necessary. The valves and transformers are on the top of the enclosure and all the circuitry hidden away on the underside.

Once I was happy with the layout it was time to drill the necessary holes in the enclosure (867-3634) .

Again I used laser cut templates to make sure all the holes were accurately placed.

Over 50 holes were needed, so it took some time to drill, clean up any swarf and then check that all the parts fitted OK.

Circuit build

I soldered the power supply together on tagboard (043-3703) , before bolting it in place on the underside of the enclosure. For this version I upgraded the resistor, using a 7W vitreous wire-wound resistor (139-3487) , as opposed to the 3W used in the prototype.

I bolted the valve holders (678-4094) in place and then decided to also screw the voltage regulator (714-0792) to the case using a heatsink mount kit (712-8225) . This had the extra benefit of the enclosure acting as a heat sink and tidied up my circuit nicely.

I soldered the amplifier circuit together referring to my prototype, double-checking everything on the schematic as I went.

Next, I bolted the transformers in place on top of the case, feeding the wires from the large toroidal transformer through the holes I had drilled, into which I had fitted grommets to avoid them getting damaged. I soldered the necessary wires to the other transformers and fed then through the holes in the case. I then turned the case back over — it was getting quite heavy now — and connected the two power transformers to the power circuit and the audio ones to the main amplifier circuit. Again to make things neater, I decided to add another short piece of tagboard to connect the incoming mains power to the on/off switch, indicator light and power supply transformers.

It was now time to connect the phono inputs (457-2903) , speaker outputs (464-690) and the potentiometer (790-4439) and dial/button head (330-9114) for the volume control. I also added a Double Pole Double Throw (DPDT) switch (518-5320) , so that one of two different inputs could be selected. All the parts were fitted to the case and then connected up.

The front fascia needed to be fitted at the same time as the volume control, switches and indicator lamp. I laser cut this from a piece of American Walnut veneer MDF with the grain vertical, to give it that retro feel. I applied a couple of coats of spray wax wood finish to protect it from finger marks.

It was then time to install the valves themselves before fitting the cover (868-2644) .

I cut a pair of side panels from the same material as the facia and fastening them in place using just the screws nearest the bottom of the enclosure. The cover could then be positioned and held in place with screws that went through the panels, then through the lip of the cover and then the enclosure itself to hold everything in place.

Time for testing

I made up a mains cable using a Neutrik PowerCon connector (246-8278) and two speaker cables using Neutrik speakON connectors (340-0958) . I connected the speakers, power and source, and proceeded to test the assembled unit.

It turned out that in my concern to get everything neat and tidy, I had omitted to connect the audio transformers to the power supply, but that was soon rectified and the amplifier powered up, the valves glowed and sound came out!

As you can hear, the sound quality is really good and, if my experience of these Fostex FE83En speakers from my 12V Sound System is anything to go by, they will sound even better once they have been played for a few hours.

Conclusion

It has taken a while to get here, but I now have a great sounding valve amplifier built from readily available parts (a BOM is attached to this article). Not only does it sound great, but I think it looks pretty cool too — the nice black enclosure cover and the wood effect panels definitely give it that classic, retro feel.

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I currently look after production at AB Open. I have a background in the arts, environmental conservation and IT support. In my spare time I do a bit of DJing and I like making things.

23 Jan 2020, 10:38

Comments

February 3, 2020 13:37

Hi Dave,
Congratulations on an interesting article. I though it worth passing on some experience I have had with using large toroidal mains transformers in audio equipment. Although they are supposed to have low radiated EM field, my experience of using them in proximity to other (audio) transformers is that you can get some significant coupling, which manifests itself as hum over the speakers. If you do experience this, my recommendation would be to try rotating the mains transformer to see if you can achieve a "null" position. It may not be a problem with the relatively high signal levels in a valve amplifier, but I thought it worth mentioning.
I surmised the reason for this as being due to the transformer construction. An ideal core would be made up from concentric rings, but this would be expensive to manufacture. Instead, they are made up from a single long strip of material wound into a spiral. This seems to result in a mainly low overall field, but with the penalty of a very high peak in the radiated field in one particular direction. If you are unlucky, this can couple into other inductive components, or even wiring.

Kind Regards,

Graham Booth BEng CEng MIET

0 Votes

February 7, 2020 09:00

@gbooth1959 Thanks for the comment. There is only the slightest hum which is only really audible at full volume when there is no music playing. I had thought this was down to replacing the choke with a resistor in the power supply, but it could be the toroidal transformer as you suggest. I will investigate re-positioning transformers if it does become an issue. Thanks again.