Oct 22, 2019

LIE Belin line preamp revisited

Back on the air.

In an older post, I detailed a line preamplifier using outstanding LIE Belin transformers. The study was made around the C3g tube, triode wired, but despite the good results in terms of bandwidth, gain, distortion, the music restitution was disappointing. Sound was dry, thin and lifeless. I was really disappointed.

I let it apart, being involved with the LCR phono calculation and construction.
Now it is done I have time to put the LIE back to the bench.
Obviously, the tube used was the problem. The G3g prove to be an excellent tube as a pentode but just average when triode strapped, at least in this design.
Once again confronted to the transformer voltage ratio (0,1) I was on a very narrow path to find a suitable candidate. My previous tests showed that if the EC8010, E86C, 6C3Pi, 6C45Pi could be good contenders in terms of µ and ρ, they mostly bias around -1,2/-1,5V which is incompatible with the usual 2Vrms (± 2,8Vp) out of my DAC or my phono.

Is there karma in DIYer life? During a speleological journey in my stock I accidentally dropped a box of weird and bizarre miniature tubes. The kind of 9 pins ones I got for cheap and intended to stay unused whatever could happen. Among these obscure and useless tubes, one caught my attention. Not saying that I had immediately the solution to my problem. At that moment I was just curious and took time to read datasheet because the construction was really uncommon, two different units in a single envelope. This is how I discovered the 6DE7. Full data here

For sure not a champion at first glance, but a careful study of the Ip/Up curves revealed that one part of this tube was very similar to half a 12BH7A and the other one resembled a scaled down 12B4A.
The great linearity seemed to be very promising.
It encouraged me to quickly calculate gain and distortion. After a few trial and error to determine the best load for the two triodes, I found it could make a nice two stages amplifier. Of course, I was primarily looking for a single tube to match my transformers but it was a purely arbitrary choice.
Do a preamp with a single triode sound better than one with two?
I had to verify that.

Several drawings later left on paper a two-stage direct coupling preamp with a gain of 60. This is pretty high but the use of an attenuator behind transformer, like in the E80CC line preamplifier, will halve gain and for that purpose I have a Tech Lab 150/150 on hand. Very good looking and mint.
This kind of balanced attenuator, alike Daven and Langevin, was common in professional gears. Extremely sturdy, made with cost no object parts, it was intended to last a lifetime if not more. Resistors where sorted to insure less than 2% imbalance.

Contacts are pure silver, coated with a special grease to prevent oxidation.
These were the days of superior construction.

Overall gain will be 3 and this is interesting to drive the rather low sensitivity VT25/VT25A push pull amplifier.

Calculating a direct coupled stage is easy.
I begun by the medium µ stage operating point. 100K in anode, 3.5K in cathode and 350V HV are best suited for low distortion.
Gain is lowered by un-bypassing the cathode. This increases internal resistance but helps to achieve a even lower distortion figure. Idle current is 2 mA, anode voltage about 150V.

Second stage operating point was chosen from Ip/Up curves too. Anode load 15K (transformer reflected impedance with 150 ohm load), Ia 22mA, Vg -20V.
Directly coupled to the preceding stage, grid is at the anode voltage which imposes a cathode 20V higher at +170V.

The tube drawing 22mA, cathode resistor for proper biasing will be 170V/22mA or 7.7Kohm. A 325V HV supply will set anode to cathode voltage at 150v.
This resistor must be oversized to dissipate more than 3W. I used paralleled 15K/5W Beyschlag carbon film ones. Long time discontinued but worth the effort to source some NOS. Clear and clean sound.

New circuit...

and new construction and wiring

This update is powered by my LCR phono supply till I build a dedicated one.

Listening report... subjective and personal

I listened to the GE's (Canadian made) I have in stock and they sound much better than the C3g in this setup. Vivid, airy, wide, natural. Like the 6J5 line preamp, very involving and alive but smoother and faster at the same time. Great on vocals. Excellent instruments separation on orchestral or complex music giving a crisp restitution. Maybe the best on Cello (what a thrill listening Fournier or Starker playing Bach suites) with a rock solid low end. IMHO the 6DE7's are underrated and should be seriously considered by the tube lovers community as a premium choice for line or driver stages.

a few new records for this test