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…









7 comments:

  1. What I can say, I would be trembling having no possibility of self listening :)
    It is very interesting, at least from my side if majority of THD comes from driver or output tube itself, as figure is quite high and we are having grid current operation here.
    Don't you feel a difference between DA42 and A710, as they have a damping factor relation like 1:4? This was also bothering me when thinking about this tube.
    Do you know about any possible stock of DA41, as I am having only DA42s. Hard to distinguish the difference by heater only, not listening both.

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    1. Hi Bartek, most of distortion comes from buffer stage, I understand it appears quite high but listening test contradicts totally measures. I brought this amp to Jean Hiraga and listened on his huge system, the audience was surprised how clean and clear it could sing.
      I have no opinion about DA42, a tube I don't have. You can source some nice TZ40 which is a direct drop in. I listened to both and difference is very subtle...if there is one.
      About the A710 I considered it a reference by which others are compared, I just have to say that I play daily the DA41 and it's pure enjoyment. I didn't light up the A710 for more than 3 months.
      Hope this helps

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    2. I think you could source some DA41 from Billington England

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  2. Thank you for this tip with Billington, I forgot to check there. When arrive I will share my measurements result with you on the HW-40-5. Can't stand waiting to be honest :)

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    Replies
    1. You should have lower THD with a push pull. This setup cancels odd distortion

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    2. Yes, from simulations based on the 811A(quite similar), where I have characteristics, in PP @ 700V and 55mA of bias, with four tubes PP parallel I am having less then 1% of H3 @ 30W. This would give load plate to plate per pair @ >20k considering DC resistance of primary. Two tubes with load of 10k is giving around 2% of H3, which is not acceptable. One pair shall have input resistance of 3k like you mentioned(even 3.2k according DS), so two goes down to 1.5k, which is still within the limit of my pre. Based on your insights here about this tubes I really want to try this setup. Rig was already tested on 829A, where I have a very good distortion results, however I really doubt these tubes will ever get close to DA41. All in Billington hands now :)

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    3. Hi Bartek, I will follow your work with lots of interest. Fred

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