Apr 14, 2016

6J5 Line preamplifier Part 1

The 6J5 is a general purpose not to say an ubiquitous small signal triode.
Its linearity is well known and appreciated by many DIYer's. It is commonly said that what goes in is what comes out. It is the successor of the glorious 27 / 56 family of tubes and has been declined in numerous shapes (G, GT, Metal) and constructions (i.e.the weird E11488) to meet industry or military requirements. The British denominations is L63.
The electrical characteristics of the 6J5 are identical to the 6SN7 with just a single triode in the glass enveloppe.
Using such a tube when seeking for high fidelity audio reproduction is a very good choice.
Complete data here

Below 6J5G / L63 GEC ST shape tube in white military box.


and fancy Marconi L63


A simple line preamp, theoretical approach.

Building a very simple but very good sounding line preamp as always been challenging.
Very simple means just a few parts of the highest quality for utmost sound
reproduction. The simplest would be nothing more than an attenuator which I tried with some disappointments. No gain, lacks of dynamic and random noise. Something was wrong in my approach of simplicity and I decided to use an active element to reach my goal ( that clearly appeared to be an impedance matcher with gain). Obviously an active element in signal path means alteration and I had to decide which tube would be up to the task.
Playing with triodes for decades taught me that the ones that could be used are not so numerous. For the indirect I would go to the 27 (76, 6C5, 6J5) family of tubes or the Bi / REN904 and PTT100, for the direct the RS242 and relatives, the 841 and some post tubes like Aa and PTT0. All these tubes share excellent linearity, medium µ and exceptional sonic qualities. Unfortunately most are expensive or too scarce for the average amateur I am and I decided to go for the 76/6J5.
These are very cute 6,3V triodes, affordable and easy to source. A good point for future sonic character comparisons of different brands.
Having a pair of Hirata's NP206 (20K/600 ohm) on hand I started to think about the best operating point for the lowest distortion.
My first choice was for the 76 but its higher ρ did not perfectly match my transformers, thus I went for the 6J5 (which I did not regret…).

Some drawings and calculations...

A line stage in my system must have a gain of 2/2,5 (+ 6/7dB) to accommodates my amps sensitivity. Loading a 6J5 with 20K will roughly give a gain of 14.5 (+ 23 dB) with a fully decoupled cathode and at operating points Va 240V_250V / Vg -8V / Ia 8mA.
With a 20 Kohm load distortion appears to be very small at usual grid input voltage. Most DAC's have a 1.5 to 2V rms output asymmetrical mode, means a maximum of +/- 2.8 Vpeak on grid. Right in the linear region.
The transformer voltage ratio is 1.73 10-1 (-15,3 dB) multiplied by 14,5 it gives me an overall gain of 2.50. Just what I need!



Line stage drawing. Very, very simple, the components choice will determine the qualities of the preamp.



Construction.

As I said previously parts must be of very high quality.

First of it, main attenuator. Might better not consider the usual potentiometer but prefer a stepped attenuator. Just 2 resistors in the signal path making a precise voltage divider.
Good rotary switch and quality resistors give better tracking and balance than the usual plastic or carbon pot.

One exception the ALPS RK40 "Black Beauty". Having both on hand choice was not easy and I picked up the stepped attenuator randomly.

Tubes. I will tell you in a next post, when preamp will be completely finished, my impress upon the different 6J5 / L63 I have on hand.

Transformer. Here a Tango, but any good transformer with 20K primary handling 15 to 20 mA will be fine. Some good ones from Hashimoto like HL20K-6 (I love Japanese trannies).


Resistors and capacitors. Plenty of choice, I used Takman metal film resistors with some vintage Sic Safco low ESR professional caps.

Tips. Short leads, star grounding plus some good oil caps just next to the transformers. Especially important to keep a good transient response when PSU is on a separate chassis. In that case I always split in two the last decoupling capacitor. Can really see the difference with an FFT analyzer.
Some feedback can help in the very low end, and just for once I prefer this preamp with a small amount (2/3dB). Better sound focus and tighter bass.


Next step power supply considerations, complete preamp in its new suit plus some listening tests ....