DG300b Build Thread

Discussion in 'DIY' started by Jh4db536, Jan 3, 2017.

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  1. purr1n

    purr1n Finding his inner redneck

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    Mable Audio is legit. Craig and I get Chinese Pissvane tubes from Mable rather than from the sole NA distributor Grant Fidelity who upcharges a million dollars, but justifies this by describing their test methods using an Amplitrex, yet at the same time being quite nebulous about their grading process:

    All tubes that went through the Amplitrex test are then further graded according to the 5-spec test results to either A, B or Clearance grade. Top quality grade A matching will be well within 10% – in many cases less than 5%. This is usually a much closer matching than the factory matching method and all tubes with inferior specs on other parameters will be screened off.

    As many of you know, I have the Amplitrex tube tester. I have several problems with the above statement from Grant Fidelity:
    1. We have no idea if these tests are fixed or auto-bias - auto-bias tends to produce results which are more similar and arguably not as a good an indicator of tube state as fixed-bias.
    2. Even if fixed-bias, I don't think I've ever seen a new tube be off more than 10% in terms of gain or emissions.
    3. Even two different brand vintage tubes of the same type, if they have about the same amount of use, will rarely be more then 10% apart if they are low-gain power tubes.
    4. Other specs, such as the Gas or H-K leakage are rarely a factor.
    5. I'm still not exactly sure what they match. Do they match so a nominal spec or tubes to each other or on the triodes if dual?
    6. As far as I am concerned, any new tube measuring below 10% of the nominal spec for emissions is defective, knowing that most new production tubes seem to measure lower than even slightly used vintage tubes. Again, I haven't seen this myself.
    7. As far as gain, a percentage of high-gain small signal tubes will have triodes which differ significantly in terms of gain (more precisely GM). These two sides should be matched as much as possible. If two such tubes are required, then all four triodes should be close.
    Bottom line: I'm not giving Grant Fidelity the benefit of the doubt with their "testing" to pay them almost two times as much as Mable. If they do test, they better provide fixed-bias results for their high-gain small signal tubes to show that they match (I don't see a matching option during checkout).

    Too much bullshit - taking advantage of audiophile buyers who already tend to be neurotic.
     
    Last edited: Mar 24, 2019
  2. tomchr

    tomchr MOT - Neurochrome

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    One DG300B board set left.

    Tom
     
  3. Armaegis

    Armaegis Friend

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    Interesting remarks about Mable that mirror some impressions that I read recently (I think at audiocircle?).

    Curious to hear those thoughts about Grant Fidelity too. Years and years ago I had emailed them a couple questions about a product. I have utterly no recollection of what it was I was asking, but it was something technical-ish and I remember getting a very wishy washy answer in return. It didn't inspire much confidence, which is a shame since we suckers in Canada don't have that many options when it comes to finding dealers, and it might actually be cheaper to buy from them with the markup rather than risk importing ourselves.
     
  4. msommers

    msommers High on Epipens

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    Ian passed a few years ago and now it's barely run by his wife Rachel. I'm honestly surprised they're still around.

    @purr1n thanks for sharing that dude. Looking at some of the Chinese tubes, they seem about 25% cheaper than GF. Most suck anyways but I do like their KT88
     
    Last edited: Mar 24, 2019
  5. Hrodulf

    Hrodulf MOT - Sonarworks

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    Looks like I bought the last set...
     
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  6. MisterRogers

    MisterRogers Friend

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    I purchased a set too; It was an enjoyable build that I'm enjoying almost daily. As said above, this is an excellent implementation of a 300B amp; couldn't bear the thought of not being able to easily build another one in the near future.
     
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  7. ogodei

    ogodei Friend

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    Primary impedance on this is listed as 3.5KΩ 2.5KΩ, the DG300B calls for 5Ω. @tomchr, could these be used? if so, any design changes needed?

    edit spelling
     
  8. tomchr

    tomchr MOT - Neurochrome

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    In theory you get a little higher output power but also higher distortion with the lower primary impedance. Many run the 300B at 3600 ohm, 60 mA, 360 V for a few watt output at quite high distortion. I've tried that. It's not my cup of tea. I much prefer the 400 V, 85 mA, 5000 ohm operating point. The power is negligibly lower but the THD is much lower too.

    Tom
     
  9. m17xr2b

    m17xr2b Friend

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    Really kicking myself for missing out. Would have loved to build a 300B with high impedance output.
     
  10. itsikhefez

    itsikhefez Rando

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    Sorry for slight OT but relevant to this post-
    I'm trying to spec custom OPT's with Electra-print for a 45 based headphone amp but encountered some conflicting information.
    Is the purpose of the 12Ω to present the optimal reflected load on the 300B?
    What would happen without it?

    Aren't 2.5W and 270mW the values without the 12Ω load resistor?
     

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