Modern Balanced Tube Amp Build

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  1. coinmaster

    I'm still a direct couple "purist" because even overdriven I don't trust OTs due to the complex reactive traits it inherent to it, it's just a big mess of potential imperfections I don't want to deal with, not that it isn't good sounding.

    Same with caps, I use duelund cast caps anytime I use caps at all because I still have them from my original MK6 mod. I'd rather just not wonder if the caps are making a difference by either using the best or none at all, so far in my experiences caps have had a significant effect on the sound whenever I try a cap somewhere of lesser quality, not that my results are conclusive to all circumstances, I'll eventually do some tests when I get around to it.
    The massive improvement in sound I heard when upgrading to duelunds in my MK6 is the reason I became interested in electronic design to begin with so it left quite an impression.

    As for direct drive producing more distortion then OT, it really just depends on the design.For a standard source follower, yeah the distortion would be considerable depending on the load.
    I've spent most of my time designing direct drive output stages because I'd rather have the input stage producing my distortion and after endless hours in spice I've got a handful that should in theory produce next to 0 distortion, like between -120db and -200 db peak distortion according to spice depending on the design.

    No matter what parasitics I introduce or components I use or even changing component values to a large extent in spice the results are basically the same meaning the designs are solid, some to a higher extent then others. I've only built two of these types of designs so far but they sound flawless, I would need a distortion analyzer to confirm the actual THD but even if it was 10,000 times worse then spice suggests it would still be above average performance. (one of the designs stubbornly registers as 0% distortion in spice no matter what I do to it, a good sign)
    It's all about error correction via non-negative feedback schemes, NFB is a subtractive process and depending on how it's used it can degrade the sound, not a variable I want to add into the mix.
    However feedforward and other error correction schemes can let you have your cake and eat it too, there's many ways to approach this and when done right the performance is amazing.

    At the end of the day I think distortion is a good thing and very desirable but I don't want untamed distortion to dominate my designs. After I get the time/money to test all my current distortion free output stage designs I'm going to switch things around and look into distortive output stages and distortion free input stages, I hear that's a popular concept in japan right now.
    I can see the appeal due to the rising harmonic content with power of tubes under load which is the opposite of how solid state parts function and in line with how sound through air functions, this is likely one of the big reasons why tubes sound so natural over many solid state designs.

    I'm not convinced. I too use mu followers for the outputs of my input stage most of the time, it's just sensible all around, constant current on AC, constant voltage on DC, low output impedance. Low output impedance meaning able to drive reasonable loads without loading down the tube at all. Pair this with the fact that my output stages have practically no input capacitances and no current draw and you have a tube that is purely amplifying voltage under ideal conditions, I've heard differences in sound with different tubes here in their linear regions.
    This alone is not conclusive evidence but I've read too many blogs or threads from people with amps using hybrid mu followers and different tubes that all have the similar ultra linear looking curves on the scope yet having their own distinct sound of their own. On the other hand MrCurwen is the only person I've seen claim otherwise. I'm putting my hat in with the majority so far. Now that I finally have a solid prototyping set-up I'll be able to do a lot more testing later this summer when I get the time.
  2. Sodacose
    Coinmaster, do you have a build/design posted somewhere that I can read about? Out of respect for OP and MrCurwen, we should try to avoid hijacking this thread with discussion of other designs/philosophies.
  3. coinmaster
    Yeah I guess I am bordering on hijacking, sorry I never have anyone to talk to about this stuff. No I don't have any posted designs my end goal is to create a product line, I don't want some chinese guy ripping me off before then :p.
  4. MrCurwen
    Those heater DC bias resistors in the PSU schems should be 270k and 33k of course. Using the values presented (about 300 ohms in total) would make for some smoke.
  5. SonicTrance
    I've tried TS BGRP 6F8G's, Sylvania 6SN7W metal base and even the Mullard ECC32 and 33. They don't sound any better than a russian 6N8S in this circuit. They all sound equally good! If there's a difference it's extremely small.

    I've also used different tubes in the output stage. I started with 6C5G/6J5G's and then went to EL36's and finally to the 6BG6/6P7S's. They all sound equally good too. The 6C5G/6J5G's had too much gain though. I only settled on the 6BG6's cause I like the look. The EL36's sounds just as good.

    So my listening doesn't count?
    I had a hard time believing this myself before I heard the amp. I've bought lots of audiophile tubes for the MK6. In that amp you can really hear the difference.

    Good catch! It's now fixed!

    Speaking of tubes, I've made adapters for the 7193's today. I still enjoy rolling different tubes even though the SQ doesn't change. My first time building adapters.
    Maxx134 likes this.
  6. MrCurwen
    Didn't you have 6P31S as well if I recall correctly.

    I'm constantly on the search for new tubes, at least mentally. However I now have about 500 tubes in my cabinet, almost neatly organized, so I have plenty already and won't buy any more. 6P36S is most interesting despite it's 2A heater. For SE duty most likely.

    Indeed! Your top caps look very good, glad to see something else besides the white ceramic ones. I get it they're behaving nicely now?

    There are popular forums with engineering minded people, where you will find more people who would assert without missing a beat that under certain conditions tubes will sound exactly interchangeable. But not everyone goes everywhere so no shame in that. Tubes are not magic, they do follow laws of nature. If the load line is linear enough in both tubes, and rp is played out of the equation (only voltage amplification); they will necessarily sound the same. SE stages will exhibit more differentiation because they are hard to get as clean as well designed LTPs.
  7. SonicTrance
    Correct. I bought some of those. I have not tried them since I got rid of the oscillating rectifier tubes though. They were noisy at the time but I'm sure they'd be quiet if I tried them now.

    Yes, the 7193's are behaving now! Top notch. I will say this though; if a tube is noisy/microphonic you instantly hear it in this circuit. Quiet tubes are a must.
  8. MrCurwen
    That has been my experience as well. It's not an easy circuit to get quiet, it can take a lot of debugging. Sometimes it's quiet straight away, but mostly not. The circuit simply has a lot of gm and a lot of wires in a small space, so noise is bound to happen.

    The SE version is much more forgiving in this regard.
  9. MrCurwen
    This is a reply to gug42 from another thread:

    Within retro confines, this is completely true. However it's not that difficult to engineer around this problem; an expensive OT is not at all necessary.

    Again, completely true within retro solutions.

    Well depends on the circuit and definition of 'easy'. If the amp has two tube stages already and enough PSU capability, it's not that difficult if you just go retro.

    However because the smart way to drive an OT will require such an extensive rebuild that in most (practically all?) cases it's much smarter to do a scratch build.
    Maxx134 likes this.
  10. gug42

    Thank you for thoses replies. When you speak about "retro" you means "old fashion way" ?

    I've read a little more and well agree : more simple to design and build from scratch than modify one topology to another one.
    Last edited: Jun 27, 2017
  11. MrCurwen
    Yes. Most people who build tube circuits use the circuits that were popular back when the tubes were made. Those circuits use many compromises that modern builders don't have to use.

    For example most modern DIY builders are not or at least shouldn't be very interested in efficiency or cost. The professionals making product designs back in the 1950's were very much concerned with cost, and that heavily affected their designs.

    Even if you forego cost and efficiency, and let's say you design an OTL amp using tubes. Most modern hobbyists are still mentally constrained by component choise. They want to use components that were available during the time their tubes were manufactured.

    A hobby is a hobby, I do see value in reproducing old circuits. But that is a goal in itself; if your goal is transparent sound reproduction for HIFI, then all else should flow from that main goal, and adherence to design restrictions of old times should become irrelevant.
  12. gug42
    Ok I see !
    I see your point, two main goal : building an old fashion amp or building an efficient one using tubes.
    By the way production cost is still really important for mass industry, but less for hobbies.
  13. SonicTrance
    You have successfully turned me, lol. Thanks again for the opportunity and help to build this beast of an amp! Couldn't be happier!

    Listening with my LCD-3's through the (insert a proper name for the amp here) amp. Sounds amazing, can't get enough of this amp!

  14. coinmaster
    That's how I see output transformers, an old flawed methodology . As I mentioned in the other thread, a voltage follower buffer by definition will skew the current through the transformer due to its inductance, I think if you are going to stick with a transformer output then driving it with an active current source buffer instead would perform better because it will ignore all parasitics thanks to Kirchhoff , after all a transformer is run by current, not voltage.
    Last edited: Jul 3, 2017
  15. gug42

    I've done some googling about a 4P1L amp. Well I found this :

    The block "HT MU_OUT" is the gyrator card :

    First some stupids question :
    - Did I need two blocks of thoses, one per channel ?
    - Can I assume the IN where the + of the RCA ? And I wire the - to any ground
    - The ground is unique ?
    - Rod Coleman : ... hum any schems/plan of this thing ???
    - About outpout transformer : can I choose it depends of the load ? 4Ohms / 8Ohms / 50Ohms / 300Ohms headphones ?

    An internal schem of the gyrator :

    Indeed I prefer the schems of the first page, those with the blue background, more easy to read for me :D

    Last edited: Jul 28, 2017
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