Tube amp topologies

Discussion in 'Tube Audio' started by Squidward, May 8, 2008.

  1. atmasphere

    atmasphere New Member

    Not sure I agree with this but of course I'm known as a proponent of OTLs. So maybe I can offer some information:

    OTLs can be SE or P-P. They can be class A, A2, AB1, AB2 or B. They can be made with or without global negative feedback. They may occur in triode, tetrode or pentode embodiments (although I don't actually know of any tetrode examples right now).

    OTLs can have adjustable bias, self bias or autobias.

    So they can cover a similar range of topologies as transformer-coupled amps, only there is no transformer. To do this typically an OTL will have lower plate voltages (usually not over 175Volts) and higher currents. This means that the power tubes have to have capabilities of linearity at lower voltages and higher currents. There are 3 tubes currently (pardon the pun) in common use: the 6AS7, the 6C33 and the PL509 or variants thereof.

    Quite often OTLs have a higher output impedance and so often need higher impedance speakers but this is not set in stone. The OTLs I use drive 8 ohms with ease, and I know of one OTL that makes its maximum power into 3 ohms. If enough feedback is applied, OTLs can behave like a Voltage Source, like any other amp. However, OTLs tend to have a lot of bandwidth so feedback has to be applied carefully. Too much and the amp might become unstable. There was enough of this in the old days that many people are convinced that OTLs are inherently unstable or unreliable, and neither is true- they are just as stable and reliable as any other tube amp if built correctly.

    The advantage of OTLs is that without a transformer, they have more bandwidth and less distortion and so are more transparent. The OTLs I play have 20 Hz squarewave tilt that is unmeasurable. No other tube amp topology can make that claim. This means that OTLs can be considerably more transparent than conventional amps, something that is brought out in reviews on a very consistent basis. Contrary to popular myth, the larger number of power tubes does not seem to contribute to distortion, in fact it appears that as you add power tubes the distortion actually goes down. So then it becomes a matter of managing the tubes, but IME this is not particularly difficult- that amps I use don't even use matched tubes and have automatic bias.

    Another often-cited concern is speaker safety. OTLs can be built to be as safe with speakers as any decent transistor amplifier. IOW they are not any more dangerous to a speaker than a transistor amplifier, assuming once again that it is built correctly, but you would have to assume that about a transistor amp too. The reason I chose to compare to a transistor amp is that OTLs use either an output coupling cap or they are direct-coupled. So it makes more sense to compare them to transistors in this regard.

    In short, OTLs should be on table with all other tube amp types as an option for the newbe that is considering a tube amp. They are about the most tube you can get :)
  2. jeffhamman

    jeffhamman Active Member

    Ok with this said,more tubes the better, just how many output tubes can be tied together in a push pull parallel. I use 6-6v6 tied together for 30 watts.Magnavox designed it. Can you do this for all types of output tubes.
  3. shelly_d

    shelly_d Not An Audiophool

    Alameda CA
    In theory, yes. Any number of tubes can be set up in parallel in any of the configurations, however there are always practical imitations in the about of current drawn by the output stage, the heat produced, the power output, the tube type and many more that others could speak to with much more authority then I. I would point out however that some tubes want and need loads of several thousand ohms. To use a tube that wants a 2000 ohm output stage in push pull down to 200 ohms you would need to use 10 tubes. If it was push pull or totem output that would be 10 pairs.

  4. Hyfi

    Hyfi Super Member

    Bucks County, PA
    Great thread and I am glad I read it first. I use a Tube Pre and a Hybrid amp but have been looking into alternatives for when I have to downsize in the future. Some folks have been raving about Flea Watt SET amps and single driver speakers so I auditioned a setup last week.

    They did not have a flea watt so I had them setup a JAS Bravo 2.3 15wpc integrated and a pair of Gemme folded horns. I was not impressed with the speakers but the Bravo was very nice. They also setup a pair of Sq-10s which are Steve Monte's (Quest For Sound) own speaker with a horn and 10" driver and very good efficiency. I also was not overwhelmed by this setup. The $800 KEF bookshelf speakers actually sounded best. None of it sounded better than the setup in my signature.

    I want to go back and listen to some Raysonic Push Pull units with other speakers.

    It was briefly touched on in the original post but I was wondering if there is a simple description of each topology to it's expected sound.

    Such as:

    SET usually sounds like X
    Push Pull has characteristics of Y
    OTL will sound like Z

    Can anyone touch on that and other that power (wpc) why do you want one over the other?

    Thanks for the education!
  5. shelly_d

    shelly_d Not An Audiophool

    Alameda CA
    Providing those types of comparisons is really hard because the idea of "sounds like" is purely subjective. Given different tastes of listeners, you will inevitably end up with good and bad adjectives being applied to the same characteristics depending upon the tastes of who is posting.

  6. floatingboy

    floatingboy New Member

    Quick question: in a stereo 300B SET design, are you able to use one power transformer for both channels, or must you use one for each?

    Also (and this may be related to my first question), in the version of a WE91A pictured here, what is the function of the two toroidal transformers? Are they for the heaters?
  7. floatingboy

    floatingboy New Member

    Oh, wait, never mind. I see that he used the toroids to handle the 230V/50Hz euro mains. So, if I understand that particular design correctly, he is using one power transformer and two OPTs, correct?

    If that's so, I would like help understanding what function the transformers are performing in the WE91A shown here. It looks like T2 is its own separate transformer providing heater voltage, and T3 is...what? It looks like it's going to the cathode on the 300B.

    Any responses are much appreciated, and since I'm an obvious noob, please feel free to give any background info that you think might help fill in the gaps.
  8. MFeco

    MFeco New Member

    Tube amp powered with regulated switching power supply would be the best. IMO
    It is difficult to properly filtering the secondary High Voltage side just on the Anode.
    Much easier to have a two staged power supply with primary side puffer & energy storage. I mean much less low frequency ripple on the secondary side!
  9. shelly_d

    shelly_d Not An Audiophool

    Alameda CA
    Have not played around with switching power supplies so can't say anything with certainty. They do use higher frequencies to transformer the voltage.

    Advantages include smaller, lighter, less expensive transformers for the power required.

    Possible disadvantages: Possible high frequency inductance into the circuit, possible noise on the DC out, Possible objectionable ripple frequency in the output.

    Properly designed for audio equipment any power supply will be simply an appropriately quiet source of ripple free DC. Getting to that point is the trick.

  10. Edriz

    Edriz AK Member Subscriber

    lovin this thread. Just what us newbie tubers need. Clear, presice explainations.
  11. 4seatPilot

    4seatPilot "Broker"

    London, Ontario

  12. fulljam

    fulljam Member

    Thanks! I used to build tube stuff,when I was a kid,back in the early 70's;between your post,and a book called Radio Tube Fundamentals,by someone named Christ,I'm slowly remembering things I've there a good book on soldering/chassis layout/assembly?
  13. nj pheonix

    nj pheonix AK Subscriber Subscriber

    New Jersey
    Ok guys followed concepts. at the end of the day can anybody tell me why nobody makes vacum tubes today that sound like old tubes?
  14. dnewma04

    dnewma04 The Healer Super Mod Subscriber

    New tubes haven't had time to age.
  15. gunpoet

    gunpoet Chris Hewitt

    Austin, Pennsylvania
    Love the info! Making the dive into tubes in a few weeks and need all the info I can get! Awesome post!
  16. Brinkman

    Brinkman Analog loyalist

    Economy of scale.

    Tubes are obviously not currently produced in the quantities they were when they were a common electrical component.

    This has a couple major ramifications:
    1) Smaller pool of requisitely skilled technicians to draw from for QC and maintenance of tooling. :dunno:
    2) Undocumented methods/accrued expertise/industry secrets of the past have since retired and died with the workforce. :angel:

    Reconcile that with the fact that:
    A) Most of the technology we currently rely on and consume was developed by the defense and aerospace industry, which accomplished most of this with funding from the American taxpayer. During the build up to WWII through the beginning of the Cold War, a significant amount of these subsidies were shoring up the manufacturing base of vacuum tubes via government contract. :deal:
    B) Current industry standards are dictated by the largest market share, which is tube instrument amplifier manufacturers, not HI-FI amp manufacturers. :guitar:
    C) Scarcity of minerals has driven up the cost of metallurgical purity. :sigh:
  17. eddiegnz1

    eddiegnz1 New Member

    if anyone ever finds or posts such diagrams (suggested above by Shelly_d) please please let me know it is exactly what I need to take me to the next step of learning, that'd diagram would be priceless. Especially if it was like a picture of the inside of an actual simple tube amp with arrows pointing to each part in a sort of diagram format.

    How awesome would that be for newbs like me
  18. liko

    liko New Member

    thank you so much for the write up,this is exactly what sort of information that newbs(like me ) to the tube amp game really appreciate,thanks!
  19. IC1(SW)

    IC1(SW) New Member

    This is the Navy Electricity and Electronics Training Series (NEETS) module on Electron tubes. It is written on a level for junior technicians and does a pretty good job of explaining basic tubes.
  20. rdiiorio

    rdiiorio New Member

    Transconductance is a measure of the change in plate current to a change in grid voltage, with plate voltage held constant.

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