WOT tube Line-stage redux – Pt 1

A Line-stage With Output Transformer (WOT) was built back in 1998, a single 5687 tube per channel with output transformer (OPT).

However I failed to get rid of noise (EM pick-up and attenuator switching noise).  I lacked the expertise to optimise Grounding and to shield and/or orientate the output transformers (OPTs) properly.

Now, I have been fortunate to acquire a pair of Tango NP-126 Line Output transformers. So, I must re-visit the WOT Line-stage.

5687 WOT

Diego Nardi-designed 5687 WOT Line-stage (with 6X5GT rectifier)

The original concept was simply one tube stage followed by the Output Transformer to drive cabling and the power amp input. A coupling capacitor is unnecessary.

Later, this was converted into a Berman/JE Labs Line-stage – with Type 76 tube + 6SN7/6J5 cathode-follower output, to drive cabling & the power amp input.

Now, a conversion back into an OPT-coupled line-stage, using the (ISO) Tango NP-126 Line Output transformers below…

IMG_4532

FLVqkiC

Tango NP-126 778x1100

(Top graph shows single-ended to single-ended connection.)

A 6SN7-WOT Line-stage (again, only one half of a 6SN7 used per channel).

(Later, I will also test a 76-WOT topology, but right now this configuration sounds wonderful!)

IMG_4660

6SN7-WOT Line-stage – a temporary installation shown, with copper top plate removed).

Transformer-coupling offers the following advantages:

  • Removal of a coupling cap (to my ear, ALL capacitors introduce a ‘sound’ flavour).
  • Reduction of plate load resistance, more efficient plate circuit and PSU.

The new circuit schematic is below:

6SN7-WOT-310818

Notes:

  • I have connected the NP-126 for 20k:1.2kΩ configuration (initially using some Kiwame secondary loading resistors that I have on hand).  Later I will test 10k:600Ω which, according to the above Tango datasheet, should give slightly better bass response.
  • The 6SN7 (or 6J5) operating point is as suggested by A Direct Heating Triode Blog – very linear and low distortion.

6J5-loadline

  • The PSU choke-input, LC cells are specified not only for adequately low ripple, but also keeping the RC time-constant not more than 15 milliseconds for the first LC cell.  Thereafter, RC time-constant increases from the first to last decoupling cell. (As recommended in the above design, to maintain a ‘punchy’ sonic response.)
  • The DC heater supply is ‘elevated’ above Ground as previously (to minimise cathode-heater potential and any hum pick-up).

Correct phase:

A single-tube stage inverts ‘phase’, as shown when the 6SN7 plate is connected to the ‘start’ end of the OPT Primary winding :

IMG_4877

Input (top), output (bottom) – 180 degrees out of phase.

However, with OPT Primary winding reverse-connected, output is in-phase with input signal:

IMG_4954

(Excuse some noise on output trace – my rough probe technique.)

Sound:

  • Transformers (coils, inductors generally) deliver signals well in the lower frequencies, where the ‘power’ in music resides.  Dynamic ‘horsepower’ is much improved – rhythm and percussion, orchestral weight.  Dynamic ‘freedom’ and headroom seems ‘open’ and effortless. This is music for the ‘heart and soul’, not just for the ‘brain’.
  • When the signal is in correct phase, stereophonic effect is maximised, unsurprisingly.  The sound changes slightly, with more stereo image precision and ‘sharpness’, more treble brilliance, image width and ‘expansiveness’.
  • Some would point out that transformers have restricted bandwidth, especially high frequencies, – however, distortion (harshness) seems audibly lower than resistance devices (eg. resistors, potentiometers).  Smoother, more ‘coherent’, natural, it seems.
  • These sonic effects are similar to what I hear with other coil devices – eg. Transformer Volume Controls, choke-input PSUs.

See also – WOT tube Line-stage – Pt 2 update

17 thoughts on “WOT tube Line-stage redux – Pt 1

    • how did it turn out ? globe 56 with ha 133 is what iam working on ,didnt work well with 6sn7 . sound is clean and sharp bass little light . What voltages did you end up with
      best george

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  3. Hello Owen, very cool design ! The french blog Direct Heating is very interesting. I use also a préamp with DHT triode 26, I love it (you should try DHT préamp, see the big thread on DIY Audio, and the Bartola site) and 300B Tango amp driving VOTT A5, I want to use a Tango NP126 trans in the output. I know that it’s also interesting to use a TVC pot. behind the NP126, Thomas Mayer write about this (he use Dave Slagle TVC). A question about phase : is it possible to wiring the phase normally at the primary of the Tango, and invert only at the secondary ? Congrats for your blog. Bruno – Paris

    • Hi Bruno – sincere apologies for replying late (I must have overlooked this when I was away overseas) – yes, I think no problem to reverse Sec connections. It may be even better, as the ‘start’ of the Prim winding will be normally connected, I don’t know.
      Thanks for looking in here – Owen

  4. Hi George – thanks, I have not reconfigured the input tube to 76 yet. I’ll check out the HA-133 specs, but encouraged to hear that your 56/HA-133 has enough gain to work in your system.
    What source & amplifier used?

  5. hello did you already try the 10k load
    i will build that line stage also but i have only 10k line output transformers
    regards Robert

  6. Hi George – sorry I have not tried this yet.
    My phono stage is undergoing upgrade work right now & the system not active!
    I will keep you updated here when I do so.
    Cheers, Owen

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  9. Tim de Paravicini employs transformers on the balanced/XLR outputs in his EAR-Yoshino 868 and 912 pre-amps. Tim is known for the quality of his transformer design and build.

    • Hi Eric bdp24 – indeed Tim d P is a legend & long may he continue to produce good work.
      SE-to-Bal conversion is nicely done with an accurately wound (interstage) transformer.
      I have not had the pleasure of owning EAR stuff, but have enjoyed EAR519 amps numerous times at a pal’s over the years.
      Cheers, Owen

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