Modern Balanced Tube Amp Build

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  1. coinmaster
    One per channel. The schematic is of one channel.
    "IN" is the input signal. For RCA you'd want to ensure the ground connection between source and amp is connected yes.
    I'm not sure what you mean by this.
    No schems that I know of but it's essentially a current source fed into a gyrator to buffer the signal from the heater PS.
    If you are going to be messing with DHTs and don't want to design your own heater supply just buy his regs, they are well regarded among the DIY community.
    For better impedance matching it should depend on the load or you could have different taps off the transformer for different load impedances.
  2. Sodacose
    I wouldn't hesitate to try any of Rod Coleman's designs.

    You'll need a gyrator for each channel. The "In" is the "positive" of an RCA and the "negative" should be grounded. I haven't seen how Rod built this amp, but I'm certain that all the grounds are tied together in some way. There are lots of ways to do this, but "star ground" is one of the keywords. I typically use a combination of a star ground and a bus bar when building, keeping any high current returns (eg output tube cathodes, PSU grounds) closer to the common point than grid references or input stage cathodes. This has always worked well for me.

    The output transformer can use any secondary appropriate for what you're connecting (eg a 300 ohm secondary tap for HD600s). You'll want to be sure that the impedance reflected at the primary stays in the same ballpark as designed though (3,000 ohms).
    Maxx134 likes this.
  3. gug42

    Noted ! Thank to you !

    About the design itself : I'm surpise to see so few caps and seems to be low caps isnt it ?

  4. coinmaster
    There's a cap in the gyrator and a cap between the stages, both about .22uf.
    If you really wanted, the interstage cap could be removed but it would make the design more complex and expensive, requiring higher voltages. Not worth it unless you really hate caps.
    The gyrator cap must stay.
    Last edited: Jul 28, 2017
  5. gug42

    No I'm really ok with caps. Indeed I'm suprise to see so few (and little) in this design ...
  6. coinmaster
    It's not really uncommon. You seem to spend a lot of time over at the MK9/8 mod thread and that amp only have 2 small caps per channel as well in SE mode.
  7. gug42
    Indeed you'r right.
  8. MrCurwen
    The schematic is of one channel.


    Electrically it will function no matter what as long as all the grounds are connected together. This can cause hum or noise problems, or it can be completely silent.

    In a balanced amp where the PSU is not in the picture this is much much less critical for obvious reasons. In a SE amp you have to think in terms of current loops.

    A useful and practical approach is using local grounds as I believe somebody already pointed out. For example make each gain stage or buffer stage have a local star ground, and then connect all these star grounds to the last PSU cap ground (never to the first cap ground, it's the noisiest ground in the whole circuit). This is then your master star ground.

    You could also go for a ground plane approach, and just connect every ground straight to the chassis. This works really well in guitar amps and also in high frequency circuits like CRT circuits, radios etc.

    I myself use the local ground method.

    Well he sells it, so no. Other systems that work on the exact same principle have schems floating around the technical DIY forums. Basically it's a voltage control (gyrator) on one leg, and constant current source on the other leg. High impedance on both legs; keeps the audio signal out of the PSU.

    Depends completely on which kind of topology you go for.

    The design that this thread is about uses a 10k : 4R OT that can drive any speakers or headphones no second thoughts about it. I don't see any point in matching impedances, I'm not in the output power game at all. Overkill low impedance drive is where I'm at.

    You can ask Sonic if this gross impedance mismatch causes any problems for his headphones.

    Now that is a super overkill gyrator right there. First class work. In my experience and opinion it will however not bring any audio benefits.

    There has been a pretty large consensus for years that 50x rp impedance for the plate load is about the upper limit where any audio benefits are had. Many would argue it's much lower, 20x or 30x.

    This 50x is easily had with a single FET gyrator for almost all useful audio tubes.
    Maxx134 likes this.
  9. MrCurwen
    General comments about the schematic you linked gug42:

    1) I would not parallel tubes when looking for best results, especially for headphones. For power, yes, it's a smart thing to do, but for fidelity, no. It smears the microdetails quite noticeably when using headphones. Also there is absolutely no other need for it besides output power.

    2) I would not operate 4P1L without proper grid drive. I found out by extensive experiments that 4P1L does in fact draw quite a lot of grid current on transients. This ruins the dynamics of the amp unless the grid is properly driven. This can be easily accomodated with a simple source follower, look at my Red Star schems. You can read Sonics comments on this grid drive subject in this thread also.

    3) I would not operate an OT in series feed mode. This both ruins the output tube's load line, and introduces many problems regarding the OT quality. Using the topology described in this thread you can achieve top results using only a cheap OT.

    What I mean by that is that you can get better sound quality on all metrics using a 25€ or whatever it is nowadays Hammond 125D OT in the topology described in this thread (or the Red Star) than you can using that Lundahl quality iron proposed in the schematic you linked.
    Maxx134 likes this.
  10. baronbeehive
    Hi Guys! It makes sense thinking about it. In simplistic terms amplifying a distorted signal with a component of idiosyncratic construction, ie. different tubes manufactured differently, would introduce a third factor ie, sound differences. Be that as it may, since I got my first tube amp with the Tungsol tube added, I've been addicted to the sound it produces. I'm not really a roller, I just know the sound I like and use tubes to get it, once it's there it's sorted!

    What I'm saying is that I look at tubes as a tone control, In my Miniwatt I have 2 different Tungsol types, one produces the typical liquid, immersive sound that flows so naturally, the other has more dynamics and punch, perfect! However if the amp in Sonic's build would produce that sound I like, probably better even, then no need for the tube rolling! I would happily leave it behind.

    BTW Sonic your comment that before you started your build you didn't even understand circuit diagrams has given me much encouragement lol.
    Last edited: Jul 29, 2017
    Maxx134 likes this.
  11. MrCurwen
    My version of tube rolling is looking for tubes that perform really well but nobody has heard of, or that people think are "lower quality" and then buying a lot of them. Or, think a lot about buying a lot of them...

    I now have over 500 tubes sitting in my lab. I simply cannot justify buying any more. I still browse datasheets and eBay sellers though, for fun.

    TV sweep tubes were practically free up until recently. I bought a TV repairman's box (with advertising graphics and all, cardboard box with a handle) full of 6BQ6 / 6CU6 beam tetrodes. 50 pieces, cost 14 euros + 52 euros shipping from USA! They perform pretty good in my circuit, better than expected. EL36 / 6P31S are of course top notch and pretty cheap still.

    For big show, 6P36S is quite cheap still. I've got around 30 of them.

    Too bad the american sweep tubes have gotten expensive. I like the fat bottles. Russian bottles are thin, because of the communism and all.

    We corresponded a lot, many many messages for a long time. He's really smart and a fast learner, very easy to work with. Most of this stuff is just a lot of small things to take into account, so you got to put in the hours.

    Anybody can learn this stuff, just start building. I recommend building a simpler amp first, like the SE version. Then you can get the additional satisfaction of upgrading when you later build the balanced version. Also you have less frustration because simpler circuits are easier to troubleshoot.
  12. baronbeehive
    I wondered where all those tubes went....

    Thanks. Look forward to that sometime.... atm most of my time is taken up with work on the house after the move, you might remember from the other thread.
  13. SonicTrance
    Get to building, baron! It's really fun and super rewarding! I'm also working on the house after my move. Haven't listened to music for about a week but I have things on order for my next build already! I'm really excited. Next build is gonna be 6C8G input and #47 output, so DHT magic! I'll update this thread with pics and whatnot when I start building.
  14. baronbeehive
    OMG !!

    I will sometime..... but your house is obviously in better condition than mine lol.
  15. coinmaster
    How much current did you measure on the grids? Bartola's gyrators in that schematic are intended to be used as mu followers so they are essentially a low impedance source follower anyway.
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