EL84 BF Princeton?

Discussion in 'Amp Tech Center' started by schmee, Sep 17, 2019.

  1. schmee

    schmee Poster Extraordinaire

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    Would I have to change anything really to try EL84's in a BF Princeton circuit? I'm modding my Mono15 to a Princeton and thought "hey, why not just try it with EL84's before I punch those power tube holes bigger?"
    Thoughts?
     
  2. radiocaster

    radiocaster Poster Extraordinaire

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    You'd have to bring B+ down to a lower voltage.
     
  3. schmee

    schmee Poster Extraordinaire

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    You think so? My Carmen Ghia ran at 425-450 plate volts! Which, I agree, is way out of spec for EL84's.
     
  4. radiocaster

    radiocaster Poster Extraordinaire

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    I'm not an expert, maybe it's in conjunction with the mA reading that is the real test, the power dissipation rating.
     
  5. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    If you're sticking with the Monoprice transformers ( 006093 and 006094 ), you'll be fine. Those transformers are used in many Laney 15 Watters including the Cub 10, LC15 and VC15. If you're going cathode biased, you'll need a different value resistor, but that's about it to go between EL84 and 6V6.

    If you're using a different power transformer and you're worried about frying EL84s, don't use EL84s. Use 6p14p-EV instead. The Cub only puts around 300V to the plates. My Jet City JCA20H gives the EL84s a good workout with ~370V

    How far down the Princeton route are you going?
     
  6. David Barnett

    David Barnett Doctor of Teleocity

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  7. 1300 E Valencia

    1300 E Valencia Friend of Leo's

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    Yellowjackets?
     
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  8. schmee

    schmee Poster Extraordinaire

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    I just thought I'd start out with the Princeton layout and use the Mono trannies. Not a fan of cathode bias anyway. But I do have a Princeton Power Trans laying around here. I have some of those Russian replacement tubes for EL 84's I've had laying around for years. But I was surprised how well JJ's lasted in my Ghia at high voltage.
    I'll probably just try it with EL84's for giggles before I change out the tube sockets....
     
  9. Phrygian77

    Phrygian77 Tele-Afflicted

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    EL84s don't need as much input grid signal for one.
     
  10. uriah1

    uriah1 Telefied Ad Free Member

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    I tried those yellowjackets in a couple of amps both 6v6 and 6l6.
    They all sounded thin to me..
    imho
     
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  11. BobbyZ

    BobbyZ Doctor of Teleocity

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    I got caught up in the 18 watt Marshall clone craze a few years ago and built a 18 VIB Weber kit. It did nothing for me so I rewired the normal channel like a 6g2 Princeton without trem, that's on the other channel. That helped a lot I don't use that amp much but with that rewire and some more tweaking it'd probably do the trick.
     
  12. schmee

    schmee Poster Extraordinaire

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    Posted this in the wrong thread yesterday. Not sure I can use that mono power transformer. Not sure I even understand it!

    Orange to Orange: 48 ACV ....Bias tap?
    Grey to Ground: 35 ACV
    Grey to Grey: 7.5 ACV Heater tap? .....pretty high...
    Red to Ground: 36 ACV
    Other Red to ground: 60 ACV
    Red to Red 530 VAC ! Uh oh!
     
  13. schmee

    schmee Poster Extraordinaire

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    OK the Mono is stripped of it's guts. I've decided to go for a non reverb BF Princeton to save weight for this amp. I have a PR already. (with bigger trannies)
    -The board is complete and loaded. It's going to be a tight fit.
    -I have a Princeton style cap can so may have to use that to save room.
    -I'll be using a typical Fender mid control as the Mono15 has a labeled hole for it on the face.
    -I have an old PR power tranny I'm using for now. I may not leave it in there but until I see how this works out avoiding buying a new tranny.
    -I'll try the Mono15 OT as I only want a low wattage amp.
    -I'm going to try EL84's first just for giggles. Not sure what trouble that's going to cause along the way.

    The one remaining question is what to do with the extra hole in the face that was "gain"?
    I'm thinking of doing a Presence control, or an adjustable NFB in that spot.
    QUESTION: What range of adjustment would you use for a NFB adjustment?
     
    Last edited: Sep 24, 2019
  14. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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  15. schmee

    schmee Poster Extraordinaire

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    Yeah, think I'll just do Rob Robinettes' 2 values NFB and off with a 3 way I guess.. Thanks for the link.
     
    Last edited: Sep 24, 2019
  16. schmee

    schmee Poster Extraordinaire

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    Well this is getting done fast, but it is a challenge. The Mono15 chassis really is small inside, seems smaller than a 5E3. One thing that is helpful is the L shaped chassis is two pieces that bolt together, so I could do some things prior to putting the halves together.
    Awaiting some parts right now which should be here tomorrow or Friday.
    I have my fingers crossed on some of the components being pretty close together... hopefully this will not be noisy...
    [​IMG]
    [​IMG]
     
    Last edited: Sep 25, 2019
  17. schmee

    schmee Poster Extraordinaire

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    Well, I'm about ready to power this thing up. As mentioned previously I left the 9 pin sockets and for giggles will try it with the Princeton circuit but EL84's first.
    It struck me yesterday that I have no way to measure bias on EL84's. I normally use a Bias Rite with octal adaptors. Unfortunately, My old VOM that had ma measurement is no longer functioning.
    So I'm contemplating just flipping it on and looking for red plating, or having to come up with an old school scheme to measure bias. Can't even remember that scheme, but it involves resistors. There's so damn little room inside this chassis now it may be difficult.
     
  18. timfred

    timfred TDPRI Member

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    Cathode bias, right? Shared bias resistor? What value?

    For 350-400V at the plates, 150 Ohm is about right. Measure the voltage drop across the resistor, divide by the resistance, divide by 2.
     
  19. schmee

    schmee Poster Extraordinaire

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    No not cathode, fixed bias. BF Princeton style.
    I spent a few hours redoing some of this Monday. I had a filter cap can mounted inside the chassis, but it was just unworkable room wise. So I punched a hole and mounted it outside. That is a bit better. Still very tight room inside.
    I flipped it on and it makes some sound but I may have to swap the OT primaries as it kind of made a garbled squeal. I was cold and tired so stopped for now. Will get back to it soon.
     
  20. timfred

    timfred TDPRI Member

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    Oh right, duh. In that case, when amp off measure the resistance from the OT center tap to one of the plates (like 200 ohm). Then at idle measure the voltage drop across the same points (like 5 V). Bias current = 5/200 = 0.025 A = 25 ma.

    More or less.
     
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