Need help with diagram

Discussion in 'General Electronics Chat' started by Guitarboyled, Apr 16, 2008.

  1. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    I’m new to this type of project and I will obviously need some help!

    Looking at the following diagram (guitar amp)

    http://www.schematics.ca/uploads/PDdownloads/peavey_classic20.pdf

    I don’t seem to see the on/off switch and the pilot light. And I don’t understand what is connected in the Red and Yellow identifiers

    Thanks guys
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    the two lines marked 'red' are from the high voltage secondary transformer winding.
    the two lines marked 'yellow' are from the low voltage secondary transformer winding.

    the switch and pilot light aren't shown because they are on the primary side of the transformer, which is also not shown.

    be careful. when power is applied, the high voltage side will have somewhere between 160v and 250v on it. it is a good practice to keep one hand in your back pocket, sit on a wooden chair, and wear rubber soled shoes when working on such circuits. this prevents completing a current path through your torso, which could easily kill you.
     
  3. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    Thanks

    Do I need an extra diagram? I have no idea how to do this.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    the primary side of the transformer wiring will be pretty simple. basically, the cord, the switch, the pilot light and the transformer itself. the primary side wires may simply be black.

    what seems to be the problem - no output?

    if the filaments in the tubes are not lit, you will not get any output. checking the two fuses would be a good place to start.
     
  5. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    I want to build this amp from scratch, this is my first electronics project.

    If I don't see a diagram I won't be capable of building it.

    I want to learn the basics but this is not my main job or hobby
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    oh, i see.

    i suggest this is a bit ambitious for a first project. do you really want a tube amp?

    the filament supplies won't be a problem - you can buy 12.6v transformers all day long. it's the high voltage supplies that will be the trick. you could build a flyback inverter for the high voltages, but i'm not certain what kind of current you'll need.

    what kind of power output do you expect?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    you will need an output impedance matching transformer. they can be a bit tough to find nowadays. i'll guess that the 20 in the 'classic 20' name means a 20w output. so, you'll need an impedance matching transfomer capable of handling 20w, converting the high tube output voltage at high impedance to your low impedance speakers, 4 or 8 ohms.
     
  8. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    Yes I believe it’s a 20watt amp… I wouldn’t mind if it was just 5 watt as I need something that will sound good at low volume. I now some people use attenuators or energy dumps to be able to get a good sound out of the tubes yet play at a low volume.
     
  9. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    I read something about this amp having a solid state rectifier... not sure if that as anything to do with it.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    It does show diode bridge rectifiers. And that has nothing to do with anything but making AC into DC.

    If you have never seen a tube amp before, then you don't have a clear picture of the "classic" construction technique. The enclosure was an "amplifier foundation", which essentially was a chassis with a perforated cover. The tube sockets were mounted in the top of the chassis. The passive components were mounted underneath using "point to point" wiring. They attached to terminal strips (metal loops riveted to a phenolic strip). There could be several rows of terminal strips spaced apart so the resistors and coupling caps ran between them. The wires went from these terminal strips to the tube sockets. You might have a hard time visualizing this.

    I think I have seen plate transformers with high voltage and filament windings in the mouser catalog. They may carry impedance matching transformers as well. Be aware that a preamp is going to be necessary so you can have volume and tone controls.

    This is a very advanced project.
     
  11. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    I'm getting into this to

    No 1 learn more stuff
    No 2 save money (but not sure how much such a project would cost in parts)

    There's already a master pot and volume pot on this diagram, I need a preamp on top of this? :(
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    yes, you'll need a preamp.

    the preamp will be easier to build, and will use low voltage components. many preamps use two 9v 'transistor' batteries as a supply. that is because the preamps use operational amplifiers, like a tl072, that require both a positive and a negative supply voltage.

    the preamp components will be comparatively easy to get and inexpensive. it will be a much safer first project, since everything is low voltage.
     
  13. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    Based on your experience, How much do you think it will cost in components to build both the preamp and the amp? exluding or including the speaker and the tubes
     
  14. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    I have a few pictures of the amp and probably have a good idea of how to place stuff. But yes, I will need some major assistance! Cause I’m just copying what I see on the schematics without understanding the use of each components.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    well, a good starting place for getting ideas would be to find the datasheets for the tubes that the schematic uses.

    basically, the schematic uses two 12ax7 and two el84 tubes.

    an el84 also goes by 6bq5. they are 'power pentodes'.
    the 12ax7 also goes by ecc83. they are 'hi-mu twin triodes'.

    you might look at the 12ax7/ecc83 first. it will be a bit easier to understand.
    you can find many resources by searching for 'tube datasheets' on google, or click here;
    http://www.google.com/search?hl=en&q=tube+datasheets

    actually, read the tube tutorial on this site.
    http://www.allaboutcircuits.com/vol_3/chpt_13/index.html
    triodes and pentodes are covered. you should really have a decent understanding of how things work rather than simply assembling them - otherwise, you will have a difficult time in the end trying to 'get it right', or perhaps even getting it to work at all.

    be aware that many folks refer to 'electron tubes' as 'valves' instead. don't let that confuse you.
     
  16. Guitarboyled

    Thread Starter New Member

    Apr 16, 2008
    9
    0
    I have to find a diagram for a preamp... where will it connect to the amp... is it the red and yellow connection again? Will any preamp do?

    Is this worthed If I can actually buy a used amp for $200?
     
Loading...