DIY tube amp builders and B-15 owners. Enlighten me on my first trip to tubeland.

Discussion in 'Amps [BG]' started by greencow, Mar 1, 2012.


  1. greencow

    greencow

    Joined:
    Feb 7, 2008
    Long story short:
    I have an introductory course to SPICE, but I have already simulated some simple amp deisgns, filters etc. I haven't done anything with tubes though. So I looked at some of the B15 schematics available online. I was blown away by how simple the whole schematic was. I got an idea of actually building the amplifier. I have no previous experience with tubes and I feel like this could be a good first project. Also after running the simulations I could measure the characteristics of the actual amplifier and compare the results. Maybe this would even be enough to get me my B.A diploma.

    While I plan on buying the transformers from fliptops.net(Mainly becuase they also offer a PT for europe.) I could use asmuch information on the transformers as possible for running the simulations. Also tips, tricks, hints and reading material on building tube amps is welcome. The idea is not to only solder the thing together, but to understand every component and every current in the schematic.

    I need to check what parameters the software allows for transformers, haven't got a license yet.

    "Diving headfirst into the glowing glass world, thinking: "I'll just do this one build" but probably ending up a tubehead :help:
  2. basscooker

    basscooker

    Joined:
    Apr 11, 2010
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    cincy ky
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    Owner, Chopshopamps.com
    i'm not at all familiar with the circuit for the b15, but i know that the fender champ is kinda the go-to for first-tube-builds IME.

    yes you will be a tubehead.
    start your GAS fund now.

    i predict EL34's in your future. and a mild but permanent OCD concerning biasing.
  3. Passinwind

    Passinwind Charlie Escher Supporting Member

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    I'd recommend you consider using LTSpice, at least while you're getting up to speed. No license needed, and there's a great community centered on the Yahoo Groups forum.

    I'm playing with a 6SL based preamp at the moment in Spice. I will be building next week, I think. I'll post a thread on it then. It's going in the same box as my tube line driver project, detailed in the Amps FAQ DIY resources section. I've been sending .wav files through my sims for the last week or so -- if it sounds as good in real life I'll be plenty happy. BTW, I haven't bothered with any transformer models yet, but of course I would if I were doing a tube power amp stage.
  4. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    You are undertaking a great project but it is going to be a big task and will take a lot of time. I fear that the scope is probably beyond what you can accomplish in an introductory spice course. Here is an excellent reference where a similar analysis was performed on a Fender Bassman 5F6A amplifier. I have to warn you that it is a difficult read unless you have experience in network analysis. On the other hand, you don't need to understand everything in order to gain something from it. On the same web site they present an analysis of a James tone circuit which is very similar to the one found in the B-15. You might find it interesting to check out the Fender 5E3 power amp analysis.

    Buying the transformer set that fliptops offers is a good idea. You will have the proper transformer set for the job. There are good tutorials out there on how to build an eyelet or turret board to build the circuit on. There is some good practical build information and schematics here. The have been several revisions of this amplifier over the years, the B-15NC is a good schematic to adopt for your project. I think that it is the most classic version. The one I like the best anyway.

    There aren't a lot of specs for the transforms available.

    Here is what I can offer from some notes that I have:

    PT-108 (used in the early cathode biased models):
    PT spec: 750 VCT (375-0-375) secondary, 150mA, 5 VAC @ 3A with 117 VAC primary
    DCR average of one half of PT secondary winding: 55.5 ohms (measured)
    DCR PT primary winding: 2.5 ohms (measured)
    PT secondary average high voltage to center tap voltage / PT primary voltage: 372 / 117 = 3.18 (measured)
    NOTE: The 372 volts was measured with line voltage of 117 VAC.

    Total impedance presented to rectifier by power transformer = 55.5 + (3.18 * 3.18) * 2.5 = 80.8 ohms per plate
    At 372 volts, the 5AR4 tube requires limiting resistors of at least 27 ohms @ 5-10 W in series with each anode to bring the total resistance up to 112 ohms as per the Amperex 5AR4 data sheet. These are never used in these amps but might be of interest for modeling.

    The OPT for the early model B-15 is the OT-208. I can't locate my OT-208 specs but I do have something for the OT-214 which has a slightly higher output power transformer and was used in the later fixed bias models of the B-15. Very similar though and this is the OPT that Fliptops sells for all B-15 amps.

    Primary: 6.5K, DCR 290R
    Secondary: 8 and 16 ohms, DCR 1, 1.4R

    Hope that this helps.


    David
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  6. seamonkey

    seamonkey

    Joined:
    Aug 6, 2004
    Good read:
    Feedback and fidelity part 1

    The author confirms that his spice models reflect actual hardware.

    Duncan Amp has a small complete schematic of a tube amp, including transformers where you can get ideas of how this works.
    Models

    For a BS diploma, consider that other things you learn in spice will be much more valuable in modern times. Note also the models are correct, and you can send through a wave file. LTSpice is too slow for realtime, but it's not a big leap to see that companies with real time modeling have sped this up and accurately model amps in realtime. Check out Little Computers like the $35 Raspberry Pi that just may be capable of doing this on the cheap. This would get you farther in modern skills.
  7. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Good stuff David. There's also a complete model online for a Bandmaster 5E7, even including the speakers in an open-backed cab. I think I copped it from the Yahoo forum, I'll have to double check though.
  8. projectMalamute

    projectMalamute

    Joined:
    Jan 9, 2008
    Location:
    Westfield, MA, USA
    You are overthinking it.

    Build a clone and tinker with it. That's what Marshall did, and those amps sound pretty good. Your ears are about a billion times more useful than Spice, especially for something like this.

    Even better: get a Traynor YBA-1 for 200 bucks and start playing with it. They have very good transformers and are hand wired on a tag board so they are super easy to work on. Great amp to learn about this stuff on.

    Observe all safety procedures, yadda, yadda, yaddda...
  9. beans-on-toast

    beans-on-toast Supporting Member

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    I'm going to have to look for that one Charlie. It sounds very interesting.
  10. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Just checked, it's in the Yahoo LTSpice forum files section, under "Adventures With Analog." There's lots of other worthwhile stuff there too. DIYAudio.com also has a useful sub-forum dealing with Spice modeling.
  11. aborgman

    aborgman Supporting Member

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    Location:
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    Keep an eye on various tube models.

    Many perform very well in the linear region, but don't behave well under cutoff or grid-current limiting.

    The 6SL7 model I was using was ok in cutoff (although the curves were a bit odd) but had no way of dealing with grid current when Vgk went positive.
  12. greencow

    greencow

    Joined:
    Feb 7, 2008
    As for the modeling software: I have been using PSpice at home and OrCAD/Altium at work. My instructor said that MultiSim has worked best for him when working with tubes and said he'll get me a license(Yei!).

    Big thanks to David and Seamonkey. That's some good information and nice reading.

    As to buying a tube amp or just puting it together: The purpose of this project is not to have a tube amp or to build a tube amp but to understand a tube amp. To be able to debug a simple tube design after building this.

    Another question that is really running way ahead of plans: Howmuch of the B-15 sound actually comes from the cab? Should I be looking at building/buying a closed 15" aswell or will my Traveler 121H do just fine with the tweeter turned off?
  13. beans-on-toast

    beans-on-toast Supporting Member

    Joined:
    Aug 7, 2008
    Part of the sound is in the cab, more comes from the amp. If you're going to go to the trouble to build a B-15 amp from scratch, I think that you owe it to yourself to build a matching reproduction cabinet to go with it. My favorite is the double baffle design that was found in the earlier designs.

    Having said that, any cabinet will do. Many people like the new 115 portaflex cab that Ampeg is currently offering.

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