Fender Super Twin Reverb Power Amp Tube Order?

Discussion in 'Amps and Cabs [BG]' started by KBoom, Jun 19, 2019.

  1. KBoom


    Mar 19, 2017
    Portland, OR
    I just bought my first tube amp! Very excited. Now I'm trying to learn everything I can about it. I can't seem to fully identify the tube order. I've pieced together information from a few different sources and labeled what I found out on this picture. Can anyone confirm I have tubes V1 through V5 correct? I was told the 7025 on V4 has to be balanced. But did I get the correct slot labeled?
    What is the order of the power tubes? Since we're on the question of tubes I've read that it's possible to lower the watts by running with fewer power tubes plugged in. They need to remain in as pairs. Which pairs? ie it should be possible to run with 2,4, or 6 tubes at a time.

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  2. Don't screw around with a good working amplifier by running it with tubes pulled out; it wasn't meant to be used that way, and you run the risk of damaging it. If you want less power, buy a lower power amplifier.
    BassmanPaul likes this.
  3. bigtone23


    Dec 10, 2014
    Denver, CO
    Yeah, don't pull tubes.
    It's possible to pull tubes, but with this design, the impedance will be messed up. In a 4 tube power amp, pulling two tubes (half the tubes) effectively doubles the impedance and therefore setting the impedance switch to half the load of the cab can get you by at a gig, but it's not a great long term thing to do.
    On a Super Twin, there are 6 tubes and no output impedance switch (it's always 4 ohm), so pulling pairs doesn't leave a clean, easy way to match the impedance correctly. Don't do it!
    What I have heard about is rewiring the output tubes to triode to drop the power down.
    Ampslut and Wasnex like this.
  4. KBoom


    Mar 19, 2017
    Portland, OR
    Ah, ok. Good to know. I'm a bass player.I don't want to run lower watts or fewer tubes. Just wanted to know how if I wanted to down the road. Good to know it's not recommended.

    I would still like clarification on the correct order. And to know which power tubes are pairs. Thank you.
  5. BassmanPaul

    BassmanPaul Inactive

    There should be a tube chart on the inside wall of the head cabinet. It tells you what goes where.
  6. slagheap

    slagheap Supporting Member

    Dec 23, 2011
    Super Twins have an unorthodox preamp tube arrangement. Make sure the tube chart in the head is the original and make sure your tech checks it before doing any work!
  7. BassmanPaul

    BassmanPaul Inactive

    On the matter of running with removed output tubes the amplifiers matching to the load will be out of whack. This could lead to a catastrophic failure. Don’t do it unless you know exactly what the ramifications are.
  8. KBoom


    Mar 19, 2017
    Portland, OR
    I seem to have miss communicated. Sorry about that. @johnk_10 's posting
    Fender Super Twin Bass Head about using these as stand alone heads got me excited to try it out. When I saw a chassis only listing on eBay the next day it seemed like perfect timing. Decided to pull the trigger.

    Since purchase I’ve been reading everything I can and have started to flesh out a wiki page. Fender Studio Bass Club Wiki Page | TalkBass.com

    Since I only have the chassis and no cab I don’t have a tube chart sticker. The ebay listing did have a blurry picture of one and I’ve since looked it up and believe I have the correct chart. There are a few tube differences between the Fender Super Twin and the Super Twin Reverb. I assume for the added circuit. The tube chart stickers do not have the V number sequence on them. When I called eurotubes to order they said the 7025 in V4 needed to be balanced but the 7025 in V1 did not. So I want to triple check and get the V1 vs V4 right.

    I was able to use the schematic https://www.thetubestore.com/lib/th...r/Fender-Super-Twin-Reverb-180W-Schematic.pdf and the tube chart https://www.talkbass.com/attachments/super-twin-reverb-tubes-jpg.3448965/ to label my own chart with tube models. But I don’t have all the V’s labeled. V2,V3, and V5 I’m fairly confident in. Since those are unique tubes on the schematic. V1 and V4 I think I’ve got right but the only source on that is a picture from JohnK_10s blog post. Fender Studio Bass Which has the V4 and V5 tubes labeled on the chassis. The placement also just makes sense. The schematic groups the power tubes all up in a bunch so I don’t know for certain which one is V6 - V11. Or which of those are pairs.

    Tube Chart.jpg
  9. KBoom


    Mar 19, 2017
    Portland, OR
    And thank you. I have been sufficiently warned not to run with a reduction in power tubes. :)
    BassmanPaul likes this.
  10. BassmanPaul

    BassmanPaul Inactive

    If you look on Ebay you very well might find a suitable Fender head case for the amp. I bought a couple such to house a couple of my own amp designs.
  11. Wasnex


    Dec 25, 2011
    None of the output tubes are in pairs, they are in trios...three tubes on each side of the output transformer.

    One important difference is the Studio Bass has an 8 ohm OT and the Super Twin has a 4 Ohm OT. Other than that I believe the output sections from the Master Volume to the OT are almost identical. There are a few components that have different values, but only a few. I would be very surprised if V4 and V5 are not in pretty much the same chassis locations on both amps. It should be very easy to confirm by looking at the guts and a schematic of the amp.

    As far as V4. It's nice if the tube has matched triodes, but this is not 100% essential. V4 is the phase inverter (PI). By installing a tube with matched sections, the goal is for each side of the PI produce a symmetrical wave form.

    If your going to go to the trouble of getting a matching sections V4, you should do the same for V5. V5 is the driver tube. One half of the PI drives half of V5, and the other half of the PI drives the other half of V5. Then, one half of V5 drives three of the output tubes, and the other half of V5 drivers the other three output tubes.

    As you can see there is a lot that can happen to throw everything out of balance even though you install matched section tubes in V4 and V5.
  12. Redbrangus

    Redbrangus Supporting Member

    Nov 19, 2018
    Under The X In Texas
    Dang...good post, @Wasnex. Very informative. At least I learned something.
    Wasnex likes this.
  13. slagheap

    slagheap Supporting Member

    Dec 23, 2011
    i've done two conversions of super twin combo into bass heads. do it right, and it's killer. mojo cabs makes a twin reverb head case that's a good fit - or maybe it was their dual showman case - they'll tell you. i ran them into a 2 x 15 faital pro 400 from low down sound.
  14. okcrum

    okcrum in your chest

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    Nice. The 6C10 will be a little spendy, but all the tubes are available. This will be a nice head when finished. If you don't need the reverb or equalizer circuits you could rewire it to use a single 12A(T/U/X)7 instead of the 6C10 and 6CX8. You could reuse the 6CX8 socket for that, even. :thumbsup: