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Thread: Sunn Concert Bass Preamp board - Build from Scratch?

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    Supporting Member Robert Technology's Avatar
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    Sunn Concert Bass Preamp board - Build from Scratch?

    Hey everyone!

    I recently acquired a Sunn Concert Bass (later version with red knobs, and a single channel) that is pretty beat up. It turns out it is missing the entire preamp board!

    I'm thinking of building up a preamp board from scratch using the pictures in the service manual to create a new PCB, but was curious if anyone already had done something similar.

    Also, although I do have that service manual PCB layout to work off of, if anyone happens to have good pictures of the top and bottom of the preamp board from this era of Concert Bass they would be appreciated!

    Thanks!

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    Old Timer Leo_Gnardo's Avatar
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    Before you embark on such a project, how's the power amp end of your prize? Not much point building the house until you know you have a sturdy foundation.

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    Supporting Member Robert Technology's Avatar
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    Hey Leo!

    Thanks for the input about the output!

    I forgot to add that I still need to check the transformers and output board... Wouldn't be surprised if they are toast given the state of this thing, but at this point I'm just investigating how much work would be needed for a full overhaul.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Robert Technology View Post
    I forgot to add that I still need to check the transformers and output board.
    IIRC there's an interstage transformer that drives the output transistors. If that's toast, you'll have the devil of a time finding a replacement. Would only be available as a pull from other wrecked Sunns.

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    Supporting Member loudthud's Avatar
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    Mercury Magnetics sells a replacement for that driver transformer. It was $300 last time I looked. Ouch!

    The second gen preamp was fairly simple, only 4 JFETs IIRC. There was a schematic on Ampix, but the site is not working for me today.

    Edit: I found the schematic in this thread: http://music-electronics-forum.com/t37060/ posted by Enzo in post 9. It only had 2 JFETs and 1 bypolar transistor.

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    Last edited by loudthud; 02-10-2016 at 09:50 PM.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by loudthud View Post
    Mercury Magnetics sells a replacement for that driver transformer. It was $300 last time I looked. Ouch!...
    Wow. That would make that little transformer a good candidate for a rewind if you had a faulty one.


    Quote Originally Posted by Robert Technology View Post
    ...if anyone happens to have good pictures of the top and bottom of the preamp board from this era of Concert Bass they would be appreciated!...
    I wouldn't necessarily say that it is a good picture but attached is a photo of the top of a Sunn Concert Bass Preamp that I saved a while back. At least it will give you an idea of what the board looked like.
    Click image for larger version. 

Name:	Sunn Concert Bass_Preamp.jpg 
Views:	727 
Size:	315.9 KB 
ID:	37748

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    Supporting Member Robert Technology's Avatar
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    Thanks for the info everyone!

    Mine is the later version, so the preamp board actually has 5 JFETS... still not too bad for an amp overhaul, and I'm thinking I could try it out in a pedal format as well if I end up with a board layout.

    I'll check out that interstage transformer to make sure that it's alright before I progress too much further... We've got some meggers at my work, so I can throw it under one of those, unless there's an easier way to test in the shop?

    If the iron all checks out I'll probably end up using this picture, along with the schematic, from the service manual to draw something up in Eagle:

    Click image for larger version. 

Name:	Sunn_Concert_Bass_Preamp_PCB_2.GIF 
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  8. #8
    Supporting Member loudthud's Avatar
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    That's the older 5 knob preamp from the 1st gen Concert Bass. The 2nd gen Concert Bass only has 4 knobs. When you say 2 channel, you must be thinking of the 1st gen Concert Lead. The biggest difference between 1st and 2nd gen Concert amps is the power transformer. The first gen power transformer has 8 wires with 3 secondaries. The 2nd gen only has 5 wires with a single center tapped secondary. There are examples of 1st gen amps with red knobs. It may be that the retail store replaced the knobs on older inventory to match newer speaker cabinets or the factory just ran out of the old knobs and used red ones.

    It looks like the PCB in post 6 was meant to be used in both Lead and Bass amps, fewer parts installed on the Bass version.

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    Supporting Member Robert Technology's Avatar
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    Thanks LoudThud!

    Can't believe I missed the difference in the preamp layout, but mine is in fact a 4-knob, 2nd generation version.

    I've got a schematic for the 4-knob version: looks like it's only 2 FETs and a BJT. I'll go off that for my layout, and use the picture Tom Phillips posted (Thanks!) as a guide.

    Of course, I still need to test the transformers!

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Robert Technology View Post
    ...I've got a schematic for the 4-knob version: looks like it's only 2 FETs and a BJT. I'll go off that for my layout...
    Since you will only be building one board and the parts count is small you may want to consider doing a perf board build. You could use standard pots connected to the board with flying leads.

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    Supporting Member Robert Technology's Avatar
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    Quote Originally Posted by Tom Phillips View Post
    Since you will only be building one board and the parts count is small you may want to consider doing a perf board build. You could use standard pots connected to the board with flying leads.
    I'm honestly more of a vero board kinda guy... I was actually hoping to use this project to get a bit more experience doing PCB layouts with Eagle Cad, but it seems like it would be way easier at this point to do something on perf or vero.

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    Old Timer J M Fahey's Avatar
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    Personally I'd build the original preamp, if yiu feel comfortable Vero is fine.

    FWIW my first succesful commercial SS amp was an Ampeg BT15 preamp, straight out of Jack Darr's book, built on Veroboard, driving a straight from Popular Electronics Tiger something designed by Daniel Meyer, with handmade PCBs, drawn with Mom's or Sis's red nail enamel on bare copperclad, the PCB design was straight from the Magazine.

    Sold dozens of them.

    So I am quite certain that if you copy the original PCB drawings, paying attention to scaling so parts fit, using waterproof Sharpie pens as etch resist, you have good possibilities of success.

    And if Power Amp PCB or driver transformer are dead but Power Transformer works, then any standard SS amp which is happy with those reils will be happy there.

    FWIW "sound" (meaning "flavour" here) does NOT come from the power amp, specially early ones, so if you get the preamp right and you use the proper cabinets, sound will be original..

    Only small problem will be that original will use lots of axials, while today radials are way more common, but you can easily redesign the PCB putting pads somewhat closer so radial leads reach them, no big deal.

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    Juan Manuel Fahey

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