Building a brand new valve amplifier using the RH84 design and off-the-shelf components.
A number of recent visitors have registered surprise that we do not have any music playing in our workshop. We do have a small radio, but it seldom gets turned on. I reckoned it was about time to put that right. I have always been keen on valve or “tube” audio amplifiers, both for HiFi and musical instruments. I’ve owned various valve guitar amps in my youth and fondly remember the glow of the radiogram in my childhood home.
Valves were largely superseded by solid-state systems from the mid-1960s onwards, but have made a real comeback in the past decade or so – particularly amongst audiophiles – as people realise they produce a warmer sound, as well as having great visual appeal. These days any kind of valve amp attracts a premium price and I thought that building one for myself, daunting prospect though it was, would be a relatively economical route to take.
The RH84
After doing some research it seemed the perfect beginner's project was the RH84 single-ended amplifier. It was created by Aleksandar Kitic, who has said that “The amp was really designed for the first time builder, to provide results even with salvaged parts or the cheapest possible. At the time I designed it I was practically penniless...”
In this case, we are going to use readily available “off-the-shelf” parts, rather than salvaged ones, or indeed some of the specialist parts referred to in many of the online forums discussing the building of valve amps.
The components for a single mono amplifier circuit are:
- 1 x 3W Single Ended Output Transformer (123-7242)
- 1 x ECC81 Double Triode Valve (678-4101)
- 1 x EL84 Pentode Valve (678-4120)
- 2 x Valve base (B9A0 (678-4094)
- 1 x 20V zener Diode (654-7628)
- 1 x 22k, 2W Carbon Resistor (707-8921)
- 1 x 100k, 22W Carbon Resistor (707-8940)
- 1 x 240R, 0.6W Metal Film Resistor (014-8354)
- 1 x 1M, 0.25W Carbon Resistor (707-7903)
- 1 x 470k, 2W Carbon Resistor (707-8974)
- 1 x 27R, 2W Carbon Resistor (707-8801)
- 1 x 10k, 2W Carbon Resistor (707-8906)
- 1 x 10uF, 400V, PEG124 axial A1 Electrolytic capacitor (226-7182)
- 1 x 100uF, 50V, A1 Electrolytic Capacitor (839-6261)
- 1 x 220nF, 1kV Axial Polyprop Capacitor (011-4610)
- 1 x 1.2V to 37V, 1.5A Standard Regulator (714-0792)
There are also two transformers for the power supply: one 6.3v (050-4561) for the valve filaments and a toroidal transformer 230v transformer (117-6060) for the HT (high tension/voltage). In addition there will be a bridge rectifier and smoothing components in the power supply, and this will be covered in a future post. For now we’ll just deal with the amplifier circuit.
Layout
As well as reading online blogs and forum posts about the amplifier, and other DIY tube amp projects, I referred to “Building Tube Amplifiers” by Morgan Jones, particularly with reference to the layout of the components. Morgan emphasises that the layout should be aesthetically pleasing as well as practical, and the book has lots of advice on how to arrange components to minimise the risk of hum due to induction from transformers into each other and the valves – “This hum is generated by oscillating electric currents induced in sensitive (high gain or high impedance) audio circuitry by the alternating electromagnetic fields emanating from nearby mains-powered devices like power transformers” (Wikipedia).
Where he seemed lacking in information was in the use of a toroidal transformer, which is what I had, although on reading up about them it would seem they have a very low or negligible stray field.
Placing the valves so that they keep as cool as possible and orientated so the “heater” wire takes the shortest route was another consideration.
I imported the images for the transformers and valve bases from PDF data sheets into Inkscape, drew some of the other components and experimented with different arrangements until I was happy. I then shared my design with Alex Kitic via email, and he seemed to think the layout was OK, which was good news. I was now happy that I had a viable design at least for the audio stage of the amplifier. We would be using a pair of bench power supplies for testing, so the power supply could come later.
Speakers
Having used the Fostex FE83En full range drivers in my project to build a Portable 12V Sound System we knew they performed well with relatively small amplifiers and so decided to use them again. In fact, we decided to order cabinets from the same supplier and paint them with the same “Tuff Cab” coating. The one disadvantage with this coating is that it only seems to be available in black, red or white unless you are prepared to wait for weeks. We got in touch with our Goth side and went for the black.
Given our previous experience and the fact that these were passive speakers and therefore a lot simpler, these would be comparatively simple to put together.
Next steps
As is often the case with these blog posts I have embarked upon something completely new to me and somewhat out of my comfort zone. It is going well so far, but there is some way to go yet.
In future posts, I will be sharing the building of the prototype single amplifier, followed by the final version with two of these and a power supply. Finally, fitting this all into an attractive enclosure, with cabling and connectors etc. and, most importantly, how it sounds.
WARNING: The circuit designs are shown here for informational purposes only, this is not a tutorial and the designs are not warranted to be safe. Dangerous voltages are involved and you should not work on such designs unless competent and safe to do so!
Comments