Icon Audio ST40 MkIV 6L6 Integrated Amplifier

hfnoutstandingDesigner David Shaw has stripped the best-selling Stereo 40 tube amp back to its basics and replaced its KT88s with 6L6/KT66s for a less 'power hungry' musical experience

Who'd have believed that the world would be awash with affordable valve amps in the 2020s? Certainly not those who recall the desperation of trying to source fresh tubes during the era when solid-state ruled and valves were yet to make a comeback. But now the choice is so vast that you can look beyond nursing vintage Leaks or Quads, with the risks that entails. Arguably the doyen of affordable valve amps is Icon Audio, its latest the absurdly cost-effective integrated Stereo (ST) 40 MkIV 6L6, starting at £2200.

Be careful when ordering from Icon Audio – its catalogue is vast, as are the options. For this unit, you can add remote control for £200, and specify other valves. Although named after the 6L6 tube, our review sample arrived with its equivalent, and fondly remembered KT66, for £2400.

Lean Machine
To lower the price in a sensible manner, this iteration of the Stereo 40 does away with a headphone socket, standby mode, remote-as-standard, and triode switching to create a no-frills, line-level-only model with a 28W/ch rating. It promises to be an ideal solution for an entry-level valve system, especially appealing in this country as the unit is assembled and tested here, with mainly UK-made components.

Everything about the Stereo 40 MkIV 6L6, despite the removal of the above functions, is so right: the comprehensive manual, the inclusion of a cage, easy-peasy user-adjustable bias via a meter on the fascia and screws next to each valve – indeed, all the necessities are present to preclude any thoughts of compromise. The front, from left to right, contains all you need to control a system: a rotary to select each tube when biasing, a meter to assist in that function, a volume control, and a selector for three line inputs. The on/off rocker is on the left side, while the back invites a bit more than you might expect in a cost-cutting operation.


Point-to-point wiring features strongly in the Stereo 40 MkIV 6L6, and note separate PSU for the 6SN7/CV181 driver/phase-splitter triodes [top; and sockets, bottom]. The 6L6/KT66's transformer is topside [sockets, centre]

For openers, all the fittings – both the multi-way terminals for 4ohm and 8ohm speakers and the RCA sockets – are top quality and there are two bonuses for expansion. The first is the record-out facility for users requiring a tape loop, and the second is an earthing post despite no phono stage. That way, if you add a phono amp, you can earth the turntable and/or the phono amp to the Stereo 40 MkIV 6L6.

The amp arrives with its valves stored separately, each with a number on its base and the sockets clearly labelled, so it's just a case of matching the tubes to the sockets. The main choices are the 6L6 or KT66, and a third option is the 5881.

Ladder Logic
Designer David Shaw does not advise going up the KT ladder, nor trying 6550s, because they require higher heater power. Just stick with any of the three, however tempted you might be to respond to your inner tweaker.

Bigger than the usual input tubes, the four forming the front row of the Stereo 40 MkIV were new to me; my life seems to be full of 12AX7s or ECC-prefixed valves. Instead of the usual suspects, Icon Audio uses a quartet of CV181 double triodes, sort of an equivalent to an ECC32 (yes, '32, not '82, '83 nor '88) or a 6SN7. As does the period-look 'Easy Bias' meter, this valve serves to reinforce the vintage feel (which I'll get to) of the Stereo 40 MkIV 6L6 because the CV181 dates back to WWII.

Top Value Tubes
And there's more good news, because a complete set of replacement valves costs £120-£160 depending on whether you opt for 6L6s or the dearer KT66s. That's less than most single power tubes. Why? It's due to the fact that 6L6s and KT66s cost, according to Icon Audio, over 50% less 'than KT88/EL34 types with only a relatively small loss in power (30+30W ultralinear)'.

Up and running in 15 minutes, the Stereo 40 MkIV 6L6 needs another quarter of an hour warm-up to come on song, while it continues to reward the listener with audible gains after 30 minutes. Like many of you, I have been driven to keep an eye on my smart meter and this amp certainly strikes me as less 'hungry' than quite a few others in my arsenal.

This confirmed what Icon's David Shaw told us were the Stereo 40 MkIV 6L6's raisons d'être. Says Shaw, 'I have developed this amplifier in response to the effects of inflation and the increasing costs of amplifiers and valves, as well as customers' concerns about power consumption'.

Icon Audio
Supplied by: Icon Audio
0116 2440593