restlessly evolving amp saga: introducing the ProHarvard

Discussion in 'Amp Central Station' started by alexwilds, Aug 4, 2019.

  1. alexwilds

    alexwilds Tele-Meister

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    My first amp build was a 5F2-A Princeton. It was fun, but produced a rather worthless amp = a glorified Champ useful only for practice at home. So I built a 5E5-A Pro for gigging, leaving me with the Princeton as a work bench for modding.

    I added, well, everything: 5E3 circuit, OT, tubes, and 12" Jensen alnico P12Q speaker (read: a single channel Deluxe), producing a bassy distortion machine, not to my liking at all. I messed around with many different resistors and capacitor values trying to clean it up with little success. Eventually I put it back to stock except for V1, which I kept as 12AX7 run in parallel with 1K cathode resistor and 10uf capacitor (amplification of the 12AX7 but fatter and more headroom).

    So I added a 5F11 Vibrolux power transformer (bolts right into the 5F2-A chassis!), fixed bias, a choke, and NFB (meaning a 5F10 Harvard with a choke). That sounded MUCH better, particularly with RCA blackplate 6V6GT's. My only concern now was an overdose of B+; I had 406v where 300v would be more suitable.

    I was looking for ways to drop the B+. I already had 5Y3, so no lower rectifier available. Maybe a big power resistor between the rectifier and the filter capacitors?... Then the simple solution hit me - use the extra power! I checked the specs on the 5F11 transformer against Weber's Prolux amp schematics: it would work just fine. This morning I put in old Sylvania 6L6GC's I had hanging around. WOW! Perfection!

    The result is small and lightweight like a 5F2-A Princeton with the power of a 5E5-A Pro and the warmth of a 5E3 but cleaner, tighter and lots of chime. Plenty of power for gigging. I can run it wide open and control breakup, tone and volume with my pinky on the Tele. This recipe makes for a really good working amp.

    I will post images, but warn that this was a prototyping hack job - aint gonna get the super clean build award.
     
  2. Bob Arbogast

    Bob Arbogast Tele-Afflicted

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    What chassis did you use to fit 3 octals?
     
  3. printer2

    printer2 Poster Extraordinaire

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    How about a quickie schematic, a picture of a hand drawn scribble would be fine. A prototype never looks neat (well at least in my house).
     
    W.L.Weller likes this.
  4. Intubator

    Intubator Tele-Meister

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    Love the story of your Princeton evolution/ de-evolution. I happen to be in the planing stages right now of doing a 5E5/5E3 hybrid with 6V6/6L6 switching capabilities.. Anyway, I am somewhat obsessed with original spec B+ voltages myself and vintage tubes as well. I have found that you can take the Power Trans former center tap lead (grounded) and put in series 5watt Zener diodes to drop voltage. They will step it down pretty much to where you want it to be, and those vintage 6V6's will appreciate it. AC wall voltage is 10+V higher nowadays than it was in the 50's when these amps were engineered and that can amount to quite bump in secondary PT voltage... Here's a link with a much better explanation of how to do this by Rob Robinette:

    https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage

    scroll down to "zener diode stack"
     
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  5. alexwilds

    alexwilds Tele-Meister

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    All fits easily into the 5F2A Princeton Chassis, including stock 5E3 board and the adjustable fixed bias circuit. I just drilled holes for two more tubes.

    Schematic is just stock 5F10 with a few minor changes:
    1. 6V6 style choke between node 1 and node 2 rather than resistor
    2. V1 is 12AX7 run both sides in parallel, with 1K cathode resistor and 10uf cap.
    3. pots are 250K linear wired like 5F2-A. I find 1M pots too on/off for me.
    4. 5E3 OT, 5F11 PT, JJ5Y3, old Sylvania 6L6GCs, GE 12AX7s.

    I do not know why the B+ is so high. Thanks for the zener diode stack tip! I might do that and go back to RCA 6V6 someday wilds Pro Havard schematic.jpg pro harvard chassis.jpg , but I REALLY like it with the 6L6.... I think I will just declare victory and go play it.
     

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  6. printer2

    printer2 Poster Extraordinaire

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    Nice job on the stuffing.
     
  7. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Awesome thread! +1 on the zener’d center tap trick.

    The second amp I built was a 5e3 modded for 6l6’s with reduced coupling caps for tighter low end response. Completely sterilized the amp, but back then I didn’t really know what I was doing. The journey is the destination!
     
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