Mini - Plexi has a ways to go yet.

[petemoore]

  I'm trying to find info to help me wrap my head around output impedance, in particular push-pull using parallel output tubes for each side.
  I have a VOX output transformer [2 x EL84 Push Pull] and would like to use 12au7 tubes as the output tubes.
  So...it was made for 1 EL84 per side, and I figure that means 5,600ohms.
  Now if I could figure out the output impedance needs of the 12au7 I think I could find or eliminate a match for the transformer to the four 12au7 output stages in push pull [each tube providing 2 plates worth of output].
  Once I figure out the output impedance of a 12au7, then the daunting task of continuing to attempt to peice together information about what happens if two plates of a 12au7 are paralleled, what happens to the output impedance is it 1/2's?
  I really need to find that page that explains all this again.
  The TDSL Shows:
12au7:
 Class        Va       Vg1      Ia          Ra        S           Mu
 triode:      2..       -8.5      1...        7.7        2....       16....
 Hard for me to wrap around "Va  2...''  {Voltage at anode, but ''2...''? two what?
  Vg1 has to do with the negative gate bias
  Ia 1...  means one-whatever or one-etc. ? [has to do with current through the plate
  Ra   7.7    Some unit of measure to do with plate resistance I suppose.
  S  ''2..." I have no idea
  Mu 16 [a measure of gain].
  I have various power supplies for this project, anything from very low voltage up.

[JKowalski]

Maybe this would be a better topic for ampage.org?

I have minimal experience with tube electronics, as I just started researching them. However, I must ask, why are you using 12au7's for the output tubes? Isn't it not a good idea to use preamp tubes for your power amp? Why don't you just stick with EL84's?

[BAARON]

I must ask, why are you using 12au7's for the output tubes?

Low output wattage, so the user can crank the amp up to get power amp distortion at volume levels that won't cause significant hearing damage.

[petemoore]

  Exactly, I've built amps and can't seem to make one quite small enough yet.
  But I want an amp I can actually hammer with, late at night, when they're on the phone, when...I wanna hear nasty octave or Fuzz guitar and the TV is on.
  My own personal hellamp you might say, a mini-plexi.
  I tried 1x EL84 and 1x 6v6 output amps, quite nice, too loud indeed.
  I tried to gather information on my 'ol 50C5 tube amp, but I have the wrong tube chart, and no schematic, and no schematic until I redesign the power supply, that's a catch 22 that I could crack I suppose, but I can't even find a ground on that amp, and with the turntable motor running and hummin' up the PS, a possible chomp snapper, which is 1 less than a death-amp [looks like it might run on 90vdc]...I started workin on it but it's like 'strung two stories deep and tight on the top floor...I looked it over and tried schematicizing from the PS and tube as starting points, no tube info, power supply is whacky with DC motor as a component [yukk]   
  The Plexi-Mini project holds more allure at this point, if I can crack it, perhaps it holds greater flexibility to me when impedance mathching my available iron and speakers.
  I have more than 60 [sixty] 12au7's. At the time of their unexpected arrival, I could have used 1 more 12au7. I like 'em in 12ax7 type preamps.
  Maybe this would be a better topic for ampage.org?
  Is there a particular reason you'd rather not see this here ?
    What is a ballpark impedance for 12au7 ?
    Then when I parallel two tubes, the tube impedance gets halved ?
  I'm asking for a couple little tidbits of 'advanced', 'rare' information which I searched Ampage, Aikens, GEO, the internet for days for...so far I can't even get a handle on the 12au7 output impedance.
  There just isn't a lot floating around to read about matching transformers, tubes, and speaker ohmage with push-pull having more than one pusher and more than one puller onboard doing the work  of [say a 4 tube PP amp like plexi]...
  Yea...there's the plexi schematic I re-re-re-studied, and it says something like all the others I read: something like: "a32 48b6284", looking up this transformer information revealed no transformer information...it's hard for me to formulate an equation, then check it before I test it...I'm not that good at math...maybe the numbers are there...as I said, it looks like the numbers I'm seeing have too many missing for me to formulate and check an equative formula to the degree I would like to before testing it.
  The information is always already in code, or in equative form, never both at the same time. I'ts like an intermediate math book that you can never check your answers with, to me.
  I've done impedance matching many times for El84 and 6v6, and have nice amps here, but can't seem to feel solid about how to wire this 12au7 output, it feels like the one number I need to find the other numbers is missing.
  I wish I could read an impedance ballpark for the 12au7, at say 60vdc plate V or better. The only things I've found out abuot it is it;s plate resistances..having never used an input tube as output, not sure If I'm supposed to go by the plate resistances to calculate impedance matching.
  Ok...I'm a little embarassed I can't ferret out the necessary info on my own...even...maybe I have but can't register it. 12au7 sheets and data is kinda sketchy...
  And have been warned about experimenting with tubes before...I've got a nice set of various DC PS's to work this up with.
  

[frequencycentral]

Hi Pete, I'm trying to wrap my head around this stuff too. It is worth asking some questions on tube amp forums such as AX84 or ampgarage. You generally have to use load lines to work this stuff out. B+ comes into it a lot. In a Class A PP amplifier, each power tube sees half of the OT's end-to-end impedance. In Class B, one of the power tubes is always in cut-off, and each one sees 1/4 of the end-to-end impedance. This makes Class AB a real pain to work out, since for part of the cycle the tubes see 1/2 and part of the cycle they see 1/4 of the OT impedance. All I can suggest is some 'further reading':

http://www.audioxpress.com/resource/audioclass/

http://www.radau5.ch/basics_1.html

http://www.thermionic.info/

http://www.freewebs.com/valvewizard/

http://www.diyparadise.com/tubeloadline/tubeloadlines.html

[Hanglow]

The firefly uses a 12au7 output tube , have a look on ax84 for the info on that.  it's self split and uses something like a 25k primary impedance (don't quote me one that)
ECC99 is another tube you could use


I don't think that really helps you with what you have in mind though  

http://www.freewebs.com/valvewizard/index.html  - another good source of info

maybe the radiotron handbook has something on it? it's free to download


good link for more info

http://www.pmillett.com/tecnical_books_online.htm

[Jered]

  These will have all the info you need.

    http://www.londonpower.com
    http://www.tubecad.com/

[RedHouse]

  These will have all the info you need.

    http://www.londonpower.com
    http://www.tubecad.com/

Certainly take the London Power stuff with a grain of salt, he's a lot like G.Weber in his viewpoints.

[JKowalski]

  Maybe this would be a better topic for ampage.org?
  Is there a particular reason you'd rather not see this here ?

Well, no, it's just that this is a stompbox forum. Granted, we take on all kinds of projects here, but you might get a better response from a site like ampage, where most of the people have alot of experience with amplifier design and tube electronics.

Twas just a suggestion 

[petemoore]

  Yupp.. I like the adventurous spirit here...
  These links turn new pages, that's for sure.
  Whapppp....my learning curve meets load lines and plate characteristics.
  Perhaps I can sort the information out and will report back if I work something out.
  I feel my ability to ask questions and discuss these things will have improved after I start cracking some of the reads [comprehending].

[WGTP]

The Firefly is by our very own Doug Hammond and there is a seperate forum for it here.  http://ax84.com/bbs/index.php?area=17

[km-r]

id like to build a bedroom level crankable amp too... maybe a resistor-based attenuator will good for my 18-watt...
using 12au7 means a special[or uncommon] type of output transformer maybe...

[Johan]

I've buillt a push/pull 12au7 amp using a small power toroid as output transformer..its 2x115 primary and 9volt secondary, so that gives me a turnratio of 25.5:1 or about 5200 Ohm A-A with a 8 ohm load..it works perfectly, so you 5600 transformer should be just fine even if it's a bit oversized..

...and with 60 tubes in your stock, and the worst that can happen is you fry one, why just not built it and see..

..my little amp is loosely based on a peavey classic-20 but with 12au7 as powertube and a 15watt L-pad as load/volume control. It then goes to either a speaker out ( around 1.5watts) or a line out, for driving a poweramp.   my B+ is 280volts, by the way..

one thing to keep in mind thou..a triod compresses much more before it clips than a pentode does, so you might want to decrease the gain in the P.I or attenuate between P.I and output tubes. or it might not sound as you expect..but it IS cool...

j

[petemoore]

  Workin' up to wrenching something together here.
  The organ also supplies the 9 pin sockets, all I need to do is trim some off a row [2 or 4'd] be pretty easy, perhaps a modular type design to start off with..a couple or more tubes, power supplies...whatever I can make fit for output.
  ...and with 60 tubes in your stock, and the worst that can happen is you fry one, why just not built it and see..  
  I've never fried a preamp tube before...lol...first time for everything !