At about the time I was building my 6L6 "acoustical" power amplifiers, I ran into the problem that I no longer had a standalone pre-amplifier. For my Quad IIs, I could get by using the QC22 control unit, but I would need something a bit more compatible for my new amplifiers. My solution was to build a basic SRPP line stage and equip it with its own power supply.

Circuit description

The SRPP circuit uses two triode valves: one valve, the 'lower' valve, is run as a standard voltage amplifier, but the second 'upper' valve is used as an active load. In a sense, the SRPP circuit is a cathode follower "bootstrapped" onto a standard voltage amplifier - hence the old British term, the "bootstrap follower". In this circuit, you get both voltage amplification (from the gain stage) and a low impedance output (from the cathode follower).

The picture above is of my prototype for this design: here, I'm using a pair of 6SN7s (one pair for the 'upper' valves and the other for the 'lower' ones), but adding stabilisation to a simple 6X5 power supply through the use of a stack of 0A2s. As can be seen, the SRPP circuit is very power-hungry.

Building the circuit

The actual circuit is surprisingly simple. Basically, a simple 6X5 rectifier (or 6X4 miniature) is used with a radio-style transformer to gain the 375V or so of HT the VR150s need to work of. The 6X5 is limited to a 4uF reservoir capacitor - I highly recommend a modern "motor-run" - but a small 10H choke from the Maplins catalogue and a 100uF 450V electrolytic capacitor work wonders. The stacked VR150s (again, 0A2 miniatures may be used) will stabilise this filtered supply.

The actual audio circuit itself is simple when compared to the power supply. Two 6SN7s may be used (or ECC82s if you prefer), but make sure that the two triodes in each envelope are either 'upper' or 'lower'. More careful builders may wish to float a separate heater winding for the 'upper' double-triode, but I did not notice any heater-cathode hum myself. Please note that you will need a second grid-leak resistor of size ½M from the 'Audio Out' to ground unless, like me, you already have on input to your power amplifier. 

As with all of my designs, the you can spend as little or as much on the actual components as you like. I tend to use a mixture of ancient scrap and standard catalogue parts, but more discerning builders may wish to go for high-end exotica like paper-in-oil coupling capacitors and tantalum resistors.

Listening to the finished pre-amplifier

The sound is quite clear.

The above picture shows just what can be done using an off-the-shelf chassis from the RS catalogue.

A while back, I was delighted to hear from someone on the other side of the world who'd built my design for himself, but using 6CG7s instead of 6SN7s (see below). Quite frankly, I'm impressed with the build quality and looks of his pre-amp - it does make mine look rather "British industrial"!