Brown Princeton mods

Discussion in 'Amp Central Station' started by SanAnTex, May 15, 2021.

  1. SanAnTex

    SanAnTex TDPRI Member

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    Hello tdpri,
    I recently completed a 6G2 build and I’ve been wondering about possible modifications to the 6G2 circuit to get a little bit more gain out of the amp. Does anyone have any mods in mind that could be applied to this circuit? Right now the amp has a WGS veteran 10 in it which helps break up a little early.
     
  2. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    I've heard the 6g2 can sound a bit limp due to the small mains transformer. What are you using?
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Can you post a voltage chart? The schematic shows some low voltages. I would think you have used a more powerful PT. Ime the schematic voltages may not be valid for determining a path forward.
     
  4. SanAnTex

    SanAnTex TDPRI Member

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    It’s got a Hammond 290ax PT and a 1760e OT. The B+ is at ~330V.
     
  5. SanAnTex

    SanAnTex TDPRI Member

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    I got very close voltage measurements when compared to the voltages shown on the fender layout.
     
  6. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    Which rectifier tube are you using?
     
  7. SanAnTex

    SanAnTex TDPRI Member

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    It’s an old RCA 5y3.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    OK.

    Listen to the changes as they are installed. It may change in ways you do not want.

    The voltage on the preamp tube is low for a 12A_7. Parallel the 10K power dropping resistor with another 10K. 1/2 Watt should be enough. You can clip it in temporarily. This will make that value around 5K. This will increase the voltage on the PI as well. Have a listen. No worries with the RC filter. The 5K resistor with the 30uF cap still has a low enough cutoff to keep hum at bay.

    The 1k5 bias resistor on V1b does not have a bias cap. A 2.2uF to 250uF 16V or larger cap can be installed but this will mess with the NFB. A switchable NFB might be wanted. Look at Rob's 5E3 mods site to glean info on the NFB mod. You might like the 6G2 without the NFB???

    The bias resistors on V1 could be replaced with 820R. You can temporarily clip in a 1K5 in parallel to have a listen.

    Increasing the 100K plate resistors on V1 can give an edgier sound. Try 150K. 220K to 470K you will want to increase the bias resistors to ~2K7 ish.
     
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  9. Lynxtrap

    Lynxtrap Tele-Holic

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    What do you mean by gain in this case? Are you looking for overdrive at lower volume?
    An increase of gain in the preamp will result in more volume, unless you install a master volume or some other means of lowering the volume past the output stage.
     
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  10. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    I agree, what he said. Let's make sure we're heading in the direction you want to go.

    On mine, built stock with schematic voltages, the simple idea mentioned by LLC of an NFB cut is just the ticket. It's the simplest NFB switch on any of my amps, since (on mine) a total cut gives it rich, earlier, but very musical juke-joint breakup.
     
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  11. printer2

    printer2 Poster Extraordinaire

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    I took the request is, well, a call for more gain. Increasing the voltage to the preamp stages will do the opposite, keep them cleaner without adding much gain. A cap across the second stage cathode resistor will shunt the NFB to ground as well as the localized NFB across the cathode resistor. May just want to disconnect the NFB. A BF NFB scheme could be used with a 49R resistor and the appropriate NFB resistor. That way the 1.5k resistor can be bypassed without effecting the NFB. I have used the switch arrangement as outlined on Rob's site. You could increase the plate resistor values in the first two stages also.
     
  12. Dacious

    Dacious Poster Extraordinaire

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    Increasing voltage to the PI will not increase breakup. It's a cathodyne stage which can't go past unity. Ditto increasing voltage to preamp. Ditto a bigger recto. All these things will increase headroom and maybe stop the trem from working.

    NFB and some bias resistor and bypass cap swaps might get you there.
     
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  13. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    FWIW, my NFB cut; as I say, simple:

    upload_2021-5-16_9-5-27.png
     
    Last edited: May 16, 2021
  14. Wally

    Wally Telefied Ad Free Member

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    Ime, a 6G2 has a wonderful circuit...inherited from the 5F11 Vibrolux. Turn it up somewhere past halfway and push the tone up there, as well play Humbuckers or P90s. If you are a Fender single coil player, do what has been proven to be necessary with any amp other than a very high gain amp and put a boost in front of the amp. Fender single coils do not push an input gain stage without help. If it is too loud by the time it is yielding its customary and wonderful strong ripping overdrive, then get an attenuator.
    There is not enough space in a 6G2 circuit board/chassis to create a truly high gain preamp.
    ..and imho it does not need such even if it were doable.
     
  15. Wally

    Wally Telefied Ad Free Member

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    Or....do away with the trem circuit and use those two triodes for gain stages. A Rivera Era SuperChamp is basically a BF Princeton Reverb without the trem.....and it is thus a descendant of that 5F11, too.
     
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  16. printer2

    printer2 Poster Extraordinaire

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    Or do as I have, do away with the second tube and run a fixed bias Mosfet. A small perf board and away you go. One preamp tube, two power tubes. with SS rectifier can fit in a Tweed Champ chassis.
     
  17. SanAnTex

    SanAnTex TDPRI Member

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    Sorry for the late response to all of these great suggestions. I had to leave home (and the amp) for a last minute business trip. To the question of what I was looking for, I wanted the amp to break up at lower volume with out really changing the character of the amp. I also failed to mention that this amp has the NFB permanently disconnected. I do like the idea of using an attenuator but haven’t had the chance to try one yet. I also like the idea of modding to achieve earlier gain.. so many options! I suppose I could build an attenuator for a decent price. Do any of you have a suggestion on an attenuator to buy or try to clone? Thanks for all the replies!
     
  18. Wally

    Wally Telefied Ad Free Member

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    I did a search for a certain attenuator and found this page with a brief overview of several different attenuators...purely resistive, reactive, T bridge....and plenty of links to follow.
    https://monster.partyhat.co/article/amplifier-attenuation/

    I know which attenuator I have played through that was the most impressive to my ear, but it was in an amp that had an output transformer with certain windings other than the secondary that were integral to what went on with the attenuation. That amp was a Tone King Imperial MII.
    I was looking at the Jim Kelly attenuator schematic and reading his thoughts on attenuation. That attenuator is purely resistive.
    I once owned a Jim Kelley FACS amp, which had the attenuator as part of the package as designed, back around the turn of the century. It is the amp of all others that I regret selling....and not just because its worth is somewhere between 6 and 10 times what I got for it....hardwood cab, cane grille.
    There are a lot of attenuators and approaches. This one is perhaps not so expensive and could be built if one understood.
    https://tedweber.com/mass/
    There are more expensive attenuators that very good attenuators as well as having other attributes....built in power amps in order to reamp the signal for larger venues if needed. Bad Cat builds one with a solid state power amp. The Fryette Power Station has a tube reamp...silent recording ability as well.

    Here is the schematic for the Weber Mass 100. https://tedweber.com/mass/
    I have no affiliation....
    enjoy the research.
     

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  19. Wally

    Wally Telefied Ad Free Member

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  20. Lynxtrap

    Lynxtrap Tele-Holic

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    Earlier breakup, yes that means a PPIMV or an attenuator in this case, IMO.

    One thing that might be good to clarify, that often causes confusion:
    Gain = Amplification = Louder!
    Gain is not overdrive or distortion.
    Enough gain can cause overdrive, but without a volume control downstream it will also increase output volume until one stage (usually the output stage in an amp like this) reaches its limit.
     
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