WOT tube line-stage – update

Some of the appealing things about Output Transformer coupling are :

  • In a line-preamp, only 1 tube stage is required – simplicity / transparency.
  • No second stage ‘cathode-follower’ output is required.
  • No DC-blocking, output coupling capacitor is required – less colouration.
  • Much better circuit ‘efficiency’ – much lower energy loss in the tube plate circuit (compared with resistor plate loading).
  • Thus, a smaller, more efficient PSU is possible.

(See previous post – WOT tube line-stage redux.)

Now that I am satisfied that this concept sounds excellent, I have rebuilt the preamp properly, replacing temporary wiring and improving the cosmetics!

The 6SN7 WOT line-stage now looks a little more attractive :

IMG_6362

 

This involved stripping everything from the previous WOT line-stage, a temporary conversion from the earlier JE Labs line-stage :

IMG_6309

Re-cutting the 9-pin socket hole for the larger 6SN7 octal socket…

(I am using a single 6SN7 instead of 2 x (half) 6SN7s (or 6J5 single triode equivalents) used previously.) :

IMG_6310

Mounting the Octal socket on resilient rubber o-rings, to give some protection from any microphony :

IMG_6314

 

Underneath, the Tango OPTs installed, with silver (solid 0.8mm diameter) signal wiring, cotton-sleeved.

Sowter TVCs (Transformer Volume Controls) have been re-installed.

The 2 black capacitors are the final PSU ‘decoupling’ caps for each channel (47uF F&T).

6SN7 cathode capacitors are Elna Silmic II, bypassing Mills wirewound resistors.

IMG_6345

 

Updated schematic :

6SN7-WOT-010220

The ‘signal’ part of this preamp is very simple – just a vol control + one tube + an OPT.

The Tango NP-126 output winding has now been loaded with a 681Ω Holco metal film resistor (replacing a Kiwame 1.2kΩ carbon film R) – thus resulting in 11.4kΩ  impedance across the primary.

As can be seen in the Tango datasheet below, LF performance should be improved a little.

(These resistor types and values are just what I happened to have on hand here, to best match the Tango datasheet specified values.)

Tango NP-126 778x1100

The PSU is as previously built:

  • Vacuum tube rectifier, type 6X5.
  • ‘Choke input’ high voltage supply filtering ( I have found this to have more dynamically ‘ease’ and ‘power’ (than cap-input filters), the audible effect of better voltage ‘regulation’ I think, less ‘sagging’ under dynamic conditions).
  • DC heater supply is ‘elevated’ to minimise heater-cathode potential (and PSU noise pickup).

IMG_6404

The outboard PSU (above, normally enclosed), could be described ‘rat’s nest’ :

  • PTX is a vintage NOS Partridge – which usefully has primary taps 10-0-200-220-240, allowing the fine-tuning of output voltage(s).
  • The old open-framed choke on the bott left is 20H/80mA from an old Mullard TV ‘line selector’.
  • The smaller chokes are made in China 25H/50mA.
  • The PSU final ‘decoupling’ stage (47uF cap / 40H choke) are in the signal chassis (as close as poss to the 6SN7).

(Transformer orientation here is not perfect and will be the subject of an exercise soon, to minimise magnetic coupling (EMF/hum pickup) throughout all my amplifiers. As this technician shows below.)

 

Sound :

Initially, there is much more upper-midrange ‘sharpness’ and ‘sheen’ evident (due to the silver wire or the shorter wiring paths probably), highlighting some peakiness in my loudspeakers and also slight microphony in the 6SN7 tube.

I initially installed a Pearl Tube Cooler onto the 6SN7, which effectively ‘tightens’ things up.

However, after a few hours of’running-in’, the brightness has subsided

IMG_6451

(With Pearl Tube Coolers, tube bulb temperature is reduced and tube life can be extended, but usually I prefer not to use these on the tubes, because there is slightly reduced low level detail and ‘air’, together with slightly ‘damped’ dynamic energy.)

IMG_6452

Some bass ‘energy’ has been added too – eg. noticeable in string bass power, ‘tautness’.

Overall, there is more ‘electricity’, drama, considerably better dynamic ‘contrasts’ and more tonal ‘colour’.

Next:

As mentioned, the outboard PSU needs work to optimise transformer & choke orientation.

Secondly, the PSU needs to be housed in a more attractive floor-standing chassis – perhaps wood encased.

(Another project, two DC PSUs for Field Coil Loudspeaker drivers, is currently underway, with floor-standing, oak-wood cases – more on this to come soon.)

5 thoughts on “WOT tube line-stage – update

  1. Hi Owen

    Very interesting update on the linestage. Some nice old iron your using there as well!

    Thanks for posting

    Andy

  2. mesh plate 485 will bury the 6sn7 180- volts plate geat bass better then 27 no hum even on ac filments , using utc ha 133a tungsol mesh common on ebay cheap

      • 56 globe rca globe tungsol most balanced sound the rca being richer
        have every type globe 56, went alittle nuts , work well with the utc ha133
        tungsol 485 mesh plates can’t go wrong
        best george

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s