12AU6 versus 6AU6

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Reeek, Nov 27, 2005.


  1. Reeek

    Reeek Member

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    I have three 1960's Custom Kraft tube amps. I love all of them but one of my favorites, a Model 400T came with a 6AU6 in the preamp and one in the trem socket. I've been in touch with Terry Dobbs (Mr. Valco) who doesn't recall preamps being 6AU6 from Valco ever but he stated that there were so many jobbers, it would be hard to say 100%. This amp is driven by a single EL84 and has a EZ80/6V4 rectifier.

    I have the little sister model from the same era based on the grill cloth and vinyl covering and cardboard like cabinet which is a single 50C5 amp (isolation trannie) a 35W4 rectifier and a 12AU6 in the preamp socket.

    So I'm wondering if this Model 400T is really supposed to be a 12AU6 in the preamp socket too.

    How can I tell if it supposed to be?
    What is the fidifference between these two tubes? Heater only?
    What would happen if I tried a 12AU6 in this socket? Anything bad?

    Thanks!
     
  2. Reeek

    Reeek Member

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    Nevermincd. The difference is only the heater. The 6AU6 sounded much better in it and it seems like that's what this amp was designed around.
     
  3. hasserl

    hasserl Member

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    You got it Reeek! The larger amp probably uses a 6.3 volt parallel heater circuit, the smaller amp uses a series circuit. Add the numbers of all the tubes together (50 + 35 + 12) and the sum is the amount of voltage drawn by all the tubes, the rest is scrubbed off by a large resistor wired in series with the filaments.

    Putting the 12AU6 into the 6AU6 slot it won't generate enough heat to boil the electrons off the cathode and it won't operate properly. But you allready figured that out! ;)

    BTW, those little pentodes are great sounding tubes! And plentiful and inexpensive. They are great tubes to use for fabricating great sounding circuits.
     
  4. Reeek

    Reeek Member

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    Thanks for the reply, hasserl :) And as always, it's great to hear from you! Hope things are well.

    Sometimes I just go for it and often learn the most that way but of course, I did as much googling as I could before I went for it :eek:

    Your friend,

    Rick
     
  5. mc5nrg

    mc5nrg Supporting Member

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    I'm no expert but the tube complement you mention sounds like a series heater aka widowmaker/acdc/transformerless amp where the tubes are directly powered by the line voltage without powertranformers.Since the chassis can be "hot" depending on how the plug is inserted using these amps can apparently present a electrocution hazard depending on what else you may touch.Some of these amps have built in isolation transformers and folks generally recommend them for these little amps if they do not have them.If this info is new to you do some research on google.alt.guitaramps using the above terms especially series heater/acdc/widowmaker.

    I've got a few of these Harmony and Kay etc. amps that use 50C5 and such tubes where the numbers add to approaching 115.They can be great fun for lower volume distortion tones.My Harmony 303B has been quite nice,complete w/UL sticker?!? and the big flange for a power tube socket which the amp don't got that I found floating around inside it!
    I've managed to use these amps wo/electrocuting myself but I am pretty careful about not grabbing other guitar's strings,metal parts,mikes,and sundry while also touching my guitar strings,metal parts,etc.Oh also don't play barefoot on basement concrete floors,outside,grab metal pipes either.

    Edit:I just reread and saw the reference to the isolation transformer in the first post.Still the "series heater" hazards were new info to me at one time and some folks might benefit from this info.
     
  6. Reeek

    Reeek Member

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    Agreed. It always bears repeating. I actually have one of those amps. An Avanti by Thorcraft, a widowmaker 50C5 circuit with a 6x9 Weber speaker in it. Not only is it a widowmaker but if I'm correct, I had to wet down the inside of the back panel, wear a dust mask and scrape what I think was asbestos off of the the panel. She sounds great but I certainly treat her with caution when I play it.
     
  7. Igneous

    Igneous Supporting Member

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    Hey there- I happen to have like 20-30 6au6's I pulled from an old oscilliscope that I pillaged. However, I have no use for many of the tubes. The thing was barely used and well maintained.

    Most tubes test good-new

    Various brands: Tung -sol , many GE, RCA, Sylvania

    I could send them to ya if you want to pay S+S.

    Later.
     
  8. hasserl

    hasserl Member

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    email sent!!
     
  9. Reeek

    Reeek Member

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    LOL@hasserl!

    Igneous, hasserl and I go way back and I have a decent stash already and hasserl will use them properly as well. He beat me to it so fair is fair ;)

    hasserl,

    I also have a whole bunch of other radio tubes. A couple handfuls of 12AV6 (Genelex) and a 5 pack of 6AU6's and a whole big box of other odd radio tubes. I can send you a picture of the box tops if you want. Same offer applies. S+S. They're all NIB tubes
     
  10. alexage

    alexage Member

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