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 :
This involved stripping everything from the previous WOT line-stage, a temporary conversion from the earlier JE Labs line-stage :
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.) :
Mounting the Octal socket on resilient rubber o-rings, to give some protection from any microphony :
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.
Updated schematic :
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.)
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).
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.)
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
(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.)
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’.
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.)