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Thread: Thunderbass Amplifier

  1. #1

    Thunderbass Amplifier

    Howdy Folks,
    I have an early Beige Thunderbass amp with a separate cab for the speaker. It has a 15 incher in there.
    The head uses 2 8417 output tubes and as far as I can tell 1 12ax7 for each channel and then 2 tubes for
    what I would say is the pre-amp section. It located between the channel 2 board and the output tubes.
    Im trying to find out what tubes actually belong in this section. One appears to be a 12ax7 but the tube
    closest to the output tubes I cannot read anything on the tube. I've been to Prowess and browsed the schematics
    but none seem to cover this particular amp. Does anyone know for sure what tubes belong in this section?
    The amp is basic. No reverb or Tremelo.

    Thanks,
    Bert

  2. #2
    I have the same head. The odd tube is a 12dw7. I'll try to get a pic for you; they've been hijacked by Photobucket, so I'll have to get them Monday when I'm in my office.
    Last edited by AcornHouse; 10-21-2017 at 01:37 PM.
    Chris
    ---
    '56 A-50, '57 CE-100, '60 X-150, '61 M-20, '62 F-20, '64 S-50 Jetstar, '64 Mark II, '65 SFIV, '71 S-90, '75 F-112, '75 Mark IVp, '81 M-80,
    '82 S-275, '86 T-250, '88 Detonator, '93 X-500, '97 Bluesbird, '99 F-30, '16 NS S-200 Thunderbird
    '54 Masteramp, '60 99j, '63 200-S, '66 Thunderbass, '69 Thunderstar, '92 G-500
    Acorn House
    '08 Parlor, '10 Butternut Deuce, '13 Rounder, '14 Kulakeiki

    '94 G&L Legacy
    '03 Elder Lute
    ---
    http://acornhouseworkshop.com/

  3. #3
    Managed to pull a couple pics out of Piratebucket. Before and after working on it. Is this your version? 3 12ax7, 1 12dw7 (7247), and 2 8417.


    Chris
    ---
    '56 A-50, '57 CE-100, '60 X-150, '61 M-20, '62 F-20, '64 S-50 Jetstar, '64 Mark II, '65 SFIV, '71 S-90, '75 F-112, '75 Mark IVp, '81 M-80,
    '82 S-275, '86 T-250, '88 Detonator, '93 X-500, '97 Bluesbird, '99 F-30, '16 NS S-200 Thunderbird
    '54 Masteramp, '60 99j, '63 200-S, '66 Thunderbass, '69 Thunderstar, '92 G-500
    Acorn House
    '08 Parlor, '10 Butternut Deuce, '13 Rounder, '14 Kulakeiki

    '94 G&L Legacy
    '03 Elder Lute
    ---
    http://acornhouseworkshop.com/

  4. #4
    Yup. That's my amp. And behind the output tubes there are 2 pots built into the chassis. Right now Im assuming there is a 12ax7 where the 12d should be but not positive 'cause I cant read anything on it. I adjusted the pots so both tubes have a equal blue glow. there is no red plating going on.

  5. #5
    Super Moderator Default's Avatar
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    Quote Originally Posted by bertnj View Post
    Yup. That's my amp. And behind the output tubes there are 2 pots built into the chassis. Right now Im assuming there is a 12ax7 where the 12d should be but not positive 'cause I cant read anything on it. I adjusted the pots so both tubes have a equal blue glow. there is no red plating going on.
    That is the wrong way to adjust bias, especially since those tubes are a hundred bucks+, IF you can find them. Schematic here. Put a multimeter on the test points in the back of the amp and adjust the DC voltage to .414 volts. You won't destroy those tubes and the amp will sound better.
    "Steve, you are a man of many goats."~ capnjuan

    "Alternatively, you could just go the ultra-relic route with it. Basically go for a look that says the amp was on fire and they put it out using belt sanders."
    (amps - I need a "99" & "50")

  6. #6
    My amp does not have that test point on the back of the chassis. Is there another way to test and set that voltage to spec.?
    It looks like on the diagram pins number 5 is where the voltage could be checked. Can I do this with the tubes out or do they have to
    be in? BTW, Thank you for your help!

  7. #7
    Super Moderator Default's Avatar
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    The tubes have to be in. Print out the schematic, take it to an amp tech and pay the bench fee to get it biased properly. Mucking around inside a tube amp is dangerous if you don't know what you are doing.
    Last edited by Default; 10-22-2017 at 06:34 PM.
    "Steve, you are a man of many goats."~ capnjuan

    "Alternatively, you could just go the ultra-relic route with it. Basically go for a look that says the amp was on fire and they put it out using belt sanders."
    (amps - I need a "99" & "50")

  8. #8
    Quote Originally Posted by bertnj View Post
    Yup. That's my amp. And behind the output tubes there are 2 pots built into the chassis. Right now Im assuming there is a 12ax7 where the 12d should be but not positive 'cause I cant read anything on it. I adjusted the pots so both tubes have a equal blue glow. there is no red plating going on.
    If you look at my sideways pic; you’ll see the three 12ax7s are closest to the pots. The 12dw7 is the taller one right in front of the 8417s.

  9. #9
    I have found some nice 12dw7's on ebay. NOS units are in the 35.00 range with shipping. So I ordered one of those up. The funny thing is I can buy a brand new JJ's for 11.95 but I am a little skeptical.
    Last nite I was going through a box of tubes and came across 3 Telefunken w, Germany ecc83's. When the 7247 arrives Im off to the amp garage for a tune up.

  10. #10
    I have a question for you Acornhouse. Are the 39 and 40 cans between the two transformers caps? And what are the cardboard units?

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