I want to build a Gibson BR9 amp..

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Tom Lennon, Jun 12, 2018 at 10:03 PM.


  1. Tom Lennon

    Tom Lennon Member

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    I want to build a copy of a Gibson BR9 (or possibly BR6) I really like the sound of the ones I've heard on Youtube. I think I want to try a 6SNL twin triode for the input. The schematic I have uses a transformer to couple the preamp to the output stages. The other schematic I have shows a 6SJ7 instead of the 6SN7. It's RC coupled and push-pull whereas the 6SN7 version has the 6V6s in parallel. My questions are: how do I replace the coupling transformer with a RC coupling setup and how do I calculate the specs for the output transformer when the output tubes are running parallel instead of push-pull? I believe the output transformer would not have a center tap and the plates of the 6V6 would tie to the same end of the winding, correct?
    Thanks for any help,
    Tom
     
  2. HotBluePlates

    HotBluePlates Member

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    Are you really sure you want to build a BR-9? You could very likely buy one for less than the cost of parts to build one.
     
  3. Tom Lennon

    Tom Lennon Member

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    I really like reproducing something by following a recipe that is proven. I enjoy making my own classes and utilizing as many on hand parts as I can.
     
  4. pdf64

    pdf64 Member

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    The output stage and OT looks to be p-p http://el34world.com/charts/Schematics/files/Gibson/Gibson_br9.pdf
    How could a build that uses a different method of phase splitting (ie to the BR9's very unusual interstage transformer) be considered a copy / reproduction?

    Maybe you're thinking of the GA9, which uses 2x6V6 in parallel SE (but has no interstage transformer)? http://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_9.pdf
     
  5. dkevin

    dkevin Member

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    There are at least two BR-9 designs. The most commonly available schematic shows a 6SN7 preamp driving a pair of 6V6's, push-pull ,and utilizing a transformer phase inverter to do so. In my experience, this version is more rare/hard to find. I have seen many more BR-9's that use a 6SJ7 preamp tube and a pair of 6V6's in parallel, single-ended configuration. This version has no phase inverter because the amp is single-ended. There is no schematic labelled "BR-9" for the second version. 90% of the BR-9 amps you will see are the 6SJ7 version. I purchased one of each example to do a side-by-side comparison (chassis only). They sound similar but have different characteristics as they are driven hard.
     
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  6. Tom Lennon

    Tom Lennon Member

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    That should have read "chasses" not classes.
     
  7. Tom Lennon

    Tom Lennon Member

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  8. Tom Lennon

    Tom Lennon Member

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    I guess what I'm hoping to build is a combination of the two versions: My version would have a 6SN7 as the input tube in place of the 6SJ7 but be RC coupled to the 6V6 parallel out put stage. Mine would utilize both sides of the dual triode 6SN7. No phase inversion needed. so how do I match the impedance of the 6SN7 output to the 2-6V6 input (parallel). I'm not as technically schooled as I'd like to be in this impedance-matching area.
    Will the output transformer see ½ the impedance coming from plate of the 6V6s, say, 4-5000 ohms, since they're in parallel and be non center-tapped?
     
  9. HotBluePlates

    HotBluePlates Member

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    Your build might be wimpy on gain. The phase inversion transformer very likely also offered a step-up ratio, or "gain". The single 6SJ7 is probably higher gain than the cascaded 6SN7 sections.

    Regardless, you'd do what is shown in each schematic:
    Each 6SN7 section gets a 100kΩ plate load resistor. Then copy the 6SJ7 plan of a coupling cap (C3 in the GA-9 schematic) from the 6SN7 plate to the top of a Volume control (R7, 500kΩ audio in the GA-9 schematic).

    The 500kΩ volume pot handles that for you. It is the grid reference for the 6V6s, and also a load of 5x to the 6SN7 stage (so as not to reduce the 6SN7 gain unduly).

    It don't work like that. While you'll likely land near "usual answers" because of limits on how we can use particular tubes, the primary impedance is all about relationships between voltage, current and power.
    _____________________

    You're gonna have to puzzle over finding a suitable power transformer first. The GA-9 schematic shows ~350vdc at the first filter cap. 275v * 1.414 = ~389vdc, so you're probably looking for a 275-0-275v high voltage winding for your power transformer (maybe Hammond 270DX?). Assuming the 5Y3 rectifier drops the supply voltage down to ~350v, you're looking at 14w/350v = 40mA plate current per 6V6. This matches the voltages on the GA-9 schematic (16v across the 200Ω cathode resistor = 16v / 200 = 80mA). Assume your cathode resistor will be 250Ω rather than 200Ω, because you almost certainly won't have the field coil speaker dropping supply voltage.

    Add another 4mA per 6V6 for the screens & a couple mA's for the preamp 6SN7's, and you'll want your high voltage winding rated at least 90mA. If you go too much higher than that, the supply voltage will rise due to lack of loading.

    A Class A amp can make power output of 50% total plate dissipation, at best, so assume 14w power output. 1st approximation of ideal loading is supply voltage divided by idle current, so 350v / 0.08A = 4375Ω. You can go lower on primary impedance to leave some supply voltage across the tube, so call it 4kΩ @ 15w. The OT needs to be rated for no less than 80mA d.c. in the primary (important!). If you don't find a better match, you could use the 125ESE or 125FSE transformers from Hammond and accept a little less output power with a 5kΩ primary.
     
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