Nov 13, 2022

GEC DA41 SE A2 amplifier part 2

Power supply

My long time favorite for smooth filtering is a LC supply. Not only the mains residual is almost a perfect sine wave, but transients response is better than with a capacitor input. With a large choke value, ripple can be very low. However, to keep first cell time constant around 15mS, the total series resistance must remains low. A well wound power transformer and equally good choke are mandatory. For that purpose, I use top quality Hashimoto’s irons for more than a decade with total satisfaction.

A first cell (25H/50µF) will reduce ripple to Vripple = Vout / 6π2 f2 LC √2, a second one (180 ohm/100µF) will floor down residual to negligible value.

Some voltage adjustment being necessary on CF anode to precisely set DA41 grid current, I splitted supply in two after the second cell. One arm to the 6K6/6V6, the second to the 6CG7.

Making of

Aluminum CNC chassis, epoxy coated, plus a few components

DA41 filaments supply on sub chassis

Amplifier completed, ready for measurements and listening test. It's a beefy unit weighing almost 40 pounds.

DA41 bright light is a pure enjoyment in the dark

Test setup. Max power, 100Hz_10KHz squares and THD

7,8 Vrms/8 ohm at clipping, 7,6W as expected

Despite a 18/20Kohm internal resistance the DA41 adapts quite well to the output transformer and low end roll off is not so obvious during listening test

Short reminder: transformer primary inductance has a direct effect on the low frequency response. The -3dB low frequency cutoff is determined by

FLow = Z/2πL

Where Z is the primary impedance (reflected impedance in parallel with the tube internal resistance) and L the primary inductance. For a 5Kohm reflected load and 23H@70mA inductance (Hashimoto H20-7U)

FLow = 28Hz

THD: 2,76% @ 1Watt, 6,06% @ 5 Watt. Nice harmonics distribution, mainly H2

I tried a close loop feedback between DA41 anode and 6K6/6V6 grid to get a flatter 100Hz square signal and it was disappointing. Sound became muddy, loosing all the life music could bring. Regardless of apparent poor scope results or distortion I prefer, by far, the amplifier without feedback. We must not lose sight that there is little to none relation between music, which is more in the transients domain, and squares.

Reason why it’s of prime importance to calculate accurately all time constants including power supply to respect signal ADSR.

Listening report

Garrard 301, FR24, Entré EC30, C3g phono preamplifier, 6J5 line preamplifier

On my homebrewed Klipsch Altec system the 6V6 gives the best balance with a great sense of refinement and a silky smooth sound. Each record I spin on my Garrard plays with ease and life. Dynamic is excellent, nervous or soft depending upon music. I love the way a trumpet or a sax is rendered and human voices are really addictive. Low end is firm, ample with a convincing restitution even if it doesn't extend deeply. Music is played without the somewhat arshness heard with transmitting triodes like 809 or 811. It was easy to make a comparison, the PT260 transformer can supply 6,3 or 7,5V. Is it the fact the DA41 was developed as a pure audio tube or the materials used to build it, I couldn’t say. Anyway the GEC sounds much more natural and involving than its counterparts and it blends very well with a 6V6 (Mazda or Westinghouse). The 6K6 is surprisingly more punchy with deeper bass extension, but restitution appears less natural with a forward presentation and could be a better choice on Rock music.

I am somewhat confused and surprised. To this day I don’t know which of the Visseaux A710 or the GEC DA41 is the best of my amplifiers. In any case I listen to both with great pleasure and that’s the most important. Music, music toujours…