Psionic Audio 61 South (Big Pix)

Discussion in 'The Squawk Box' started by Lyle Caldwell, Dec 30, 2013.

  1. Lyle Caldwell

    Lyle Caldwell Senior Member

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    A good client wanted to know if I could build him a '55 Super. Or maybe a '54 Pro. Or maybe a '57 Deluxe. So I came up with the 61 South, that gets the exact circuitry of the Pros, Supers, and Deluxes from '53 through '57. Here is a whole lot of vintage Tweed tone in one modern high quality amplifier.

    Galactic ground scheme, decoupled stages, extremely high quality low noise long life electrolytics, Vishay/Dale 1W resistors, and a combination of Sozo and Wima caps. Vintage goodness without the vintage problems.

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    Start with the basics. Ceramic tube sockets securely mounted with stainless locking nuts to a good quality chassis. Also stainless steel standoffs for the board.

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    Rubber grommets for the transformer wires, bayonet bases in place for the preamp tubes. While I don't personally like tube shields, I use high quality ones.

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    "Bear trap" retainers for the octal tubes. This amp can use 6L6s, 6V6s, EL34s, 5881s, pretty much any octal tube out there. And because it's cathode biased, you can mix and match if you want.

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    Board in progress. Custom board, everything military spec, through hole plating, heavy duty traces, separate shield and grounding, galactic ground scheme, etc. Everything can easily be serviced - even the electrolytic caps can be replaced without removing the board. PCB with the serviceability of eyelet board construction.

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    Mercury Magnetics transformers ('59 Bassman power, Radio Spares OT). The original Fender designs used way too low a primary impedance for the 6L6s being run in cathode bias. I chose a 45W MM Radio Spares output transformer so I could offer both the Fender 4K primary impedance and a much more correct 8K primary. The amp is much louder and more dynamic at 8K.

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    PEC and CTS pots, shielded connections where necessary, everything "just so." As this was the first build, some parts of the circuit were tweaked, so there are about three wires and one "flying" cap that won't be present in future builds.

    While this build used Sozo coupling caps, due to Sozo's unpredictable availability, future builds will use Cornell Dublier WMF coupling caps (same high quality, same "correct" construction for the desired tonal response).

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    Rear panel. Note the ability to switch between 450V and 350V for the plate voltage. The old amps used 5U4 rectifier tubes and had a low plate voltage. As the amp is cathode biased with a fairly steady current draw, I skipped a tube rectifier as "sag" was not an issue and all the tube would offer was more noise and a weak link in terms of ruggedness. At the 350V setting the amp has the B+ of the 5U4 (and you can use old 6V6s safely). But it's nice to get the most out of 6L6s, so the 450V is there. You can also choose between a choke and a resistor for filtering, and between the old and a more correct primary impedance for the output transformer.

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    The channel selection and trem can be controlled either from the front panel or remotely.

    The V1 socket is wired so that if the client uses a 12DW7 tube, Channel 1 (no tone control) has more gain than Channel 2 (tone control). This makes it really easy to go from clean to dirty with the touch of the foot switch.

    V1 can be 12AU7, 12AY7, 12DW7, or 12AX7, depending on the amount of gain desired.

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    Steel inserts hold all the screws securely to the cabinet.

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    The finished amp. I chose the "carbon fiber" fabric as a nod to the tweed Fender used, but clearly this is not trying to be a "vintage amp."

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    OK, the magic switches...

    V1 bias chooses between grid leak bias (as on the early 5C Fenders which is more compressed with more low end and not as much note to note separation) and cathode bias (as found on almost all amps made since '55 or so).

    Channel Mixer chooses between the early Fender channel blend (with very interactive controls and a ton of gain) and the later mixer as used on the Bassman, which has more channel separation and a lot more clean headroom. Nice to have both.

    Inverter chooses between the early Paraphase phase inverter and the later Cathodyne, which has a little more output and a different kind of overdrive.

    The NFB (Negative Feedback) switch is a 3 way switch, which offer no nfb in the center (like the old Fenders), slight negative feedback to the left (slightly cleaner sound), and a good bit of feedback in conjunction with a (preset) Presence control (as introduced on the Bassman) which offers a gain increase with more brightness.

    And while the old Supers, Pros, and Deluxes didn't offer it, I put in a Bias Tremolo of my own design. Because they are cool. My trem circuit uses a solid state oscillating circuit to save heat and tube failure, but the solid state components are not in the audio path.

    Also note the "Voltage" knob instead of a Master Volume. This is my own version of power scaling and it works fantastically for cathode biased amps.

    The Speed LED always flashes in time with the Trem, and is slightly dimmer when the Trem is not on. So the player can set the speed visually before turning on the Trem. Nice not to have to guess.

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    The rear. This amp is using two Weber Alnico 12A125As at 20W each. A perfect match for this amp. The cab is my design and was built to my spec by Lopo Line - they did a great job. Void-free birch plywood, dado joints. Solid as they come.

    The cab has wonderful low end response and a huge, detailed sound.

    Note the rear jack on the top panel - this allows the cab to be used as a 2x12 with any amp. Plugging in here bypasses the connection from the 61 South.

    The 61 South can be had in the 2x12 version shown, a 1x12, or a head version. Silver (shown) or black carbon fiber finishes.

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    The foot switch for the 61 South, which connects to the chassis with any 5 conductor DIN (MIDI) cable.
     
  2. Lyle Caldwell

    Lyle Caldwell Senior Member

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    One of the interesting challenges was that the '53 Fenders used parallel 6CS7s for each input, and then this was briefly changed to parallel 12AY7s before going to the individual halves of a 12AX7 per input.

    I first did some modeling in spice, then actually built a test board and confirmed with my own ears, that I could get the correct sound using just the one tube.

    So while there are values in the wiring for V1 that are not Fender values, the sound is the same.

    You can use a 12AY7 for the earlier sounds (or a 12AU7 for a sound more like the old 6CS7s) or a 12AX7 for later higher gain sounds. My preference for this amp is the 12DW7, which has one half with a gain factor of 20 (12AU7) and the other half with a gain factor of 100 (12AX7). This gives the most on stage flexibility with the two channels.
     
  3. chef

    chef Senior Member

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    Sexy amp. Welcome back!
     
  4. b-squared

    b-squared Banned

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    Wow, quite an amp...very well done.

    You still in the mid-south?

    BB
     
  5. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Thanks! Yup, still here in Memphis, a short distance from the actual 61 South.
     
  6. b-squared

    b-squared Banned

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    Fantastic...I wish you much success with this amp. Did you mention a price-point?

    I'm tapped after the holidays, but would love to hear some good sound clips regarding some of the most awesome features that this amp contains.

    BB
     
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  7. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Still getting the pricing together for future 61 South builds. Depending on the version chosen (head, 1x12, or 2x12) the price will be from around $2000 to $2800. Not cheap, but considering the tonal variety the amp offers and the quality of the construction, a very good value.

    The owner of this one has promised some sound clips soon. I'm not set up to record at the moment, but four pro players have all played this amp, and wanted one (three are placing orders).

    I'm not posting this here as an advertisement, per se, but more as amp porn.

     
  8. b-squared

    b-squared Banned

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    ...And amp porn it is...like, where can I get this movie for myself? :D

    BB
     
  9. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Here's how it works for various year tweed Fenders:

    Early '53 (5C model) Pro and Supers and Deluxes had parallel 6SC7 (later 12AY7) preamp tubes with grid leak biasing into a paraphase PI with no negative feedback. The Pro had a single 15" speaker, the Super had two 10" speakers, and the Deluxe (which used 6V6s rather than 6L6s) had a single 12" speaker.

    To get the sound of these amps, use a 12AU7 or 12AY7 preamp tube for V1, set the V1 bias to position 1 (grid leak), the Inverter to position 2 (paraphase), and NFB to the middle (none). If you want it as close as possible, set the filter to use the resistor instead of the choke, set the max plate voltage to 350V, and set the OT primary impedance to the 4K setting. You can then take that very compressed relatively low volume amp and set the max plate to 450V and the OT primary to 8K and get the same feel at a much greater volume. If you want a slightly less stiff feel, set the filter to the choke position.

    The '53 5C Deluxe also had a negative feedback loop, though it had relatively little effect on the sound.

    In late '53 Fender came out with the 5D models of Pro and Super and Deluxe. They went to cathode bias for V1 (set V1 Bias to position 2), retained the parahase PI, and added the same slight negative feedback found on the 5C Deluxe (though the 5D Deluxe lost the negative feedback). Position 2 on the NFB switch gives this effect on the 61 South. Speakers and output tubes were the same as on the earlier 5C models.

    In '55 Fender introduced the 5E series, which stayed with cathode bias for the preamp and introduced the cathodyne PI. While the Pro changed to a different circuit (leading to the later tweed Fender Bassman preamp circuit), the Super and Deluxe kept the preamp section the same. To get this sound, set V1 bias to position 2 (cathode), set Inverter to position 1 (cathodyne), and NFB to the middle (off).

    All of those amps used the early mixer circuit, with the output of the preamp stage feeding the wiper of the volume pot and lug one of the volume pots ted together feeding the second stage (PI). With the Mixer switch set to position 2, this is what the 61 South gives you.

    But that's a lot of gain and a lot of channel interaction. If you set the Mixer switch to position 1, you get the later Bassman mixer, with the preceding preamp stages fed to lug 1 of the volumes, with the wipers feeding 270K resistors which then mix together at the next preamp stage (PI). This gives a good bit less gain (more clean headroom) and more channel independance. A nice option.

    I considered offering the '55 Pro topology as an option, but it's not really that big a change and that concept is better expressed in the '58 Bassman (of which there are literally thousands of examples in the world). I stuck with the really rare bird amps that most players never get to try. And I gotta tell you, a 30W 5E3 Deluxe with a 2x12" cab is a lot of fun...
     
  10. Lyle Caldwell

    Lyle Caldwell Senior Member

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    The '55 Pro also went to fixed bias. I did experiment, at one point having a switch to go between fixed and cathode bias, but this amp has a more unique character in cathode biased mode. Like I said, if you want a great sounding fixed bias Tweed Fender, they're already out there. I wanted "the lost years" represented.

    'sides, this is not the only amp I will be offering... (muah ha ha ha).
     
  11. Soul Tramp

    Soul Tramp Speaker Snob V.I.P. Member MLP Vendor

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    Great to have you back, Lyle. Beautiful amp, and I love the unique design and voicing.

    Think you have a typo there on the speakers. Looks like "A" not "F" speakers.

    I understand well the struggle of getting worthwhile demo clips. I'm struggling with it as well.


    Again, welcome back!
     
  12. bilbarstow

    bilbarstow Premium Member

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    Good to hear from you Lyle. Very sexy. Amp porn done right ! Love the retro gone modern.
     
  13. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Don, thanks, and right you are! 12A125As. Edited, thanks!

    I auditioned a lot of speakers with this amp. The second place speakers were the Eminence alnico Eric Johnsons (fantastic sounding speakers but at 50W each they were a little too stiff for this relatively low wattage amp - they are going in another amp I'm working on that's at 50W).

    And another speaker that almost made the cut is the Eminence Patriot Cannabis Rex - I was floored by how good this speaker sounds. A bit more of a Vox/Marshall vibe than I wanted for this amp, and again, at 50W each, a bit stiff for this amp, but a great sound.
     
  14. eaglewolf

    eaglewolf Senior Member

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    A first class build like every build I've ever seen you post about. One of these days I am going to have order one of your products so I can brag about them myself! (A job is going to have to come first, though - maybe soon.)

    On the minus side, I like the red end bells used on the Soul Tramp builds better than the black, but given the color of the amp it works in this case! :D

    Seriously, it isn't often that I see this much attention to detail in a product build. Great job, and I will be looking forward to hearing some clips of the amp when they are available.

    David

    P.S. Yeah, I want one too. All you builders (including Don) are giving me serious GAS I can't afford right now!
     
  15. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Thanks David! I ordered the transformers before I knew for sure what the cosmetics of the amp would be, so I went with black - it goes with everything. I might do silver bells on future 61 Souths if they are getting this silver carbon finish.
     
  16. redking

    redking Senior Member

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    Lyle, welcome back to the Squawk Box! You have made quite a splash with this offering on your return and I can't wait to hear it in action!! :naughty:
     
  17. joeyvelour

    joeyvelour Junior Member

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    That is some beautiful work there Mr. Caldwell!
    Are you hiring?
     
  18. blamo

    blamo Senior Member

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    gorgeous. any sound clips?
     
  19. eddy999

    eddy999 Senior Member

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    Stunning.

    A superb looking amp, loving the different switchable options. Great stuff, and excellently crafted as always.
     
  20. Lyle Caldwell

    Lyle Caldwell Senior Member

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    Thanks guys! The owner is receiving it tomorrow, and has promised a lot of clips.
     
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