Magic and Microphony (Pt 2) – 6SN7GT tubes

Back in 2015 – Magic and Microphony (Pt 1) – I suggested that the tendency to ‘microphony’ in some vacuum tubes / valves, can actually enhance the sense of ‘ambience’ or ‘live-ness’ of the reproduced sound.

The last thing I wish for, is a ‘dry’ or ‘threadbare’ sound that is lacking in ‘resonance’ or harmonic ‘decay’.

However, having moved on from a 2-stage tube preamp, back to a single-tube, ‘WOT’ (With Output Transformer) Tube Line-stage, I’ve found that sensitivity to ‘microphony’ can be a serious problem.

With a pick-up cartridge like the London Decca Super Gold, that seems to be able to reproduce so much dynamic energy, especially in the upper midrange frequencies, any system resonances can be difficult to eliminate.

(London Decca cartridge with Decapod mount.)

However the London Decca sound is so direct, ‘solid’ and ‘real’, it has been worthwhile ‘tuning’ a system around such ‘idiosyncrasies’.

Nothing in audio is perfect. Aside from dynamic energy, these cartridges probably also have some internal mechanical resonances (my speculation only).

With Decca cartridges, many users experience some .’mis-tracking’ distortion at upper mid-range dynamic peaks (eg. voice high-range, instrumental peaks). This requires careful adjustment to increase tonearm-cartridge Effective Mass and/or possibly some tonearm damping (eg. fluid-damping, if this is available), etc.

A ‘Decapod‘ mounting block for the London Decca/Decca cartridges is ‘transformational’ – replacing the out-dated and flimsy traditional Decca plastic mount and the cartridge top-plate. The Decapod is a good start in solving this problem – cartridge energy is MUCH better controlled.

Then I discovered, to my surprise, that my choice of Line-stage 6SN7GT tube was critical in matching with this cartridge source.

The popular Brimar 6SN7GT tube was an unsuitable choice here. This tube, with its ’round’ plates, has the ability to reproduce an appealingly large-scaled and forward ‘soundstage’.

However, in my situation it was VERY microphonic (eg. making noises whenever the volume attenuator was switched) and there was a constant ‘whooshing’ background tube-noise. (My horn loudspeakers are also very sensitive, 100+dB/W.)

On the other hand, this late production (1985) Philips-ECG JAN 6SN7WGTA, with it’s compact rectangular ‘T’ configuration, 6-point supported plates, has no tube-noise and produces a very ‘taut’, ‘fast’ and controlled, low distortion presentation. It is noticeably very resolving and dynamically ‘open’. (An initial slight ‘metallic’ tone, goes away after just a few hours.)

The structurally more rigid construction of this JAN tube seems to suit my current source and preamp system. The significant energy peaks of the London Decca are well-controlled with this 6SN7WGTA tube.

The ‘W’ designation, I believe, denotes a ‘ruggedised’ construction that was suitable for military use – not necessarily a sign of a good audio tube, but in this case, it works nicely.

(Another 6SN7GT, a Sylvania JAN VT-231, was also auditioned – this produced a warm-toned sound, but was less transparent or dynamically ‘open’, than the Philips-ECG WGTA. Surprisingly, ‘gain’ (amplification) also seemed to be lower.)

The learnings from this are that there are no absolute ‘best-sounding’ vacuum tubes or valves, it’s what best suits or matches your music-system at this time. And, what suits your ears and your music. Enjoy.

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