Princeton Reverb Lite

Discussion in 'Shock Brother's DIY Amps' started by Snfoilhat, Nov 13, 2019.

  1. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Forward! The cab thread https://www.tdpri.com/threads/curbside-shelf-to-princeton-reverb-style-combo-cabinet.984670/ is done and old enough that it no longer publishes to the TDPRI home page, so it's time for the amp build thread to take over.

    IMG_3208.JPG

    My local hardware store stopped carrying the epoxy that turns 'metal' colored and replaced it with this stuff that dries translucent. All the extra holes are filled. Open octal slot is for a 50/50uF-500V caps and the larger slot will hold the PR-type 40/20/20/20-500V caps. Main circuit board and rectifier daughter board and drilled and waiting for eyelets.

    I tested the power supply and the heaters are functioning properly. I'm seeing -345-0-345V unloaded on the high tension circuit, That is more voltage than I'm happy to see, and so I'm not sure what my power supply is going to look like yet. Maybe a 5E2 Princeton / 5F4 Super -style choke smoothing stage before the OT primary, and maybe a dropping resistor in that place to get the B+ lower.

    Single-ended_Reverb_Amp_III_choke.png
    5E2 / 5F4 choke


    Single-ended_Reverb_Amp_III_dropping_resistor.png
    Dropping resistor

    Single-ended_Reverb_Amp_III_presence.png
    Possible presence circuit, slightly different power supply

    ***
    I also would like to implement a 5F4-style Presence control, but am still reading up on how to implement that. If you have any thoughts, please do share! Parts order submitted!

    Edit: delete that extra 220k resistor on the reverb recovery/ v3b grid leak -- that's a holdover from a schematic that needed channel mixing emulated like Robrob's blackvibe. I just realized it's not on the PR and doesn't belong here.
     
    Last edited: Nov 13, 2019
  2. King Fan

    King Fan Friend of Leo's

    Posts:
    4,222
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Cool stuff, always love your design/build threads. What kind of "PR Lite" are you going for? Less Filling? Tastes Great? Simplified PR? Hot-rodded PR? Highly-modded PR? PRINO (PR in name only)? In any case, I like it.
     
    Snfoilhat likes this.
  3. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    PRINO:lol::lol::lol:
     
  4. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Good question @King Fan, I should explain the theory of operation as far as I worked it out so far.

    Take a AA1164 Princeton Reverb and delete the tremolo and one power tube.

    Substitute a medium-high mu duo-triode, the 6SL7GT, for a high mu duo-triode 12AX7. I hope it sounds smoother than my first Vibrochamp build from a few years back, which had a rattiness. I'm not sure if the theory really bears this out, so it's more of an experiment.

    Substitute a 6K6GT pentode (like the 6G15 Reverb Unit) for the 12AT7 reverb driver.

    All mid-1950s octals!
     
    Last edited: Nov 13, 2019
    alathIN, Nickfl and King Fan like this.
  5. King Fan

    King Fan Friend of Leo's

    Posts:
    4,222
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Aha, cool, "the '55 PR."

    Having recently restored a 1955 Gibby with parallel-SE 6V6s, I gotta wonder what'd happen if you did that here?
     
    Snfoilhat likes this.
  6. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    While the rest of the parts travel from AZ and the chassis is uncluttered, here's a shot of the input jacks. 2-conductor cable with a braided shield and an insulation jacket. A pair of grid stopper resistors will connect the terminal strip directly to the grid pin on V1. The input jack sleeve to chassis connections at the input jacks are my circuit ground for the whole amp. No split ground, no chassis bolt connection for the power supply -- not that that method doesn't work great for a number of builders, just that I haven't needed to do it to solve any noise issue.

    IMG_3211.JPG

    Note to self: check those heater connections on V1. It's supposed to be pins 7 and 8 for a 6SL7GT, but the photo makes me wonder if I accidentally got pins 6 and 7. End-of-the-day blunder
     
    Last edited: Nov 14, 2019
    King Fan likes this.
  7. Nickfl

    Nickfl Tele-Afflicted

    Posts:
    1,642
    Joined:
    May 24, 2016
    Location:
    Florida
    Since you are going with all octal preamp tubes, have you thought about going with larger plate and cathode resistor values? A lot of the amps from the octal era used larger resistor values than the "modern" Fender standard 100K/1.5K pair. A lot of valcos used 220K/2.2K (sometimes 3.5K for the cathode) and some of the octal Fenders did the same. I suppose it should result in a similar bias point for the tube, but there are still some subtle differences along with a lower plate voltage that may be responsible for the "warm, sweet" tone that is sometimes associated with octal preamp tubes.
     
    Snfoilhat likes this.
  8. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Slow going

    IMG_3213.JPG
    Inputs 1 & 2 measuring good resistances both tip to ground and tip to grid.

    Found a couple brass standoffs for the circuit board.
     
  9. Peegoo

    Peegoo Tele-Meister

    Age:
    59
    Posts:
    478
    Joined:
    Oct 11, 2019
    Location:
    USA
    Snfoilhat and Junior Little like this.
  10. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Parts!

    IMG_3216.JPG

    It looks like just about everything associated with V1 - V3 fit on this board, which follows the PR layout pretty closely. V4 is the power tube and will be wired in more of a point-to-point style right to the filter caps that feed it. I forgot to order the 10 pF cap across the 3.3M. I wonder if I can sub a 47 pF, the smallest I think I have, w/o tipping things over.


    IMG_3218.JPG
    Power supply shaping up. I like this spot for the rectifier and leaving a layer of fiberboard under the pilot light to insulate it from the chassis. Power switch moved to the front panel. I run both leads to the fuse holder back toward the chassis and heatshrink the whole thing. It runs all the primary side of the power supply back to the chassis walls out of the way. Safety ground soldered to a ring terminal and bolted to it's own hole in the chassis -- so many to choose from:p


    IMG_3219.JPG
    Going to try the choke that filters the entire power supply first, and see what kind of B+ I get. Still gotta add leads to my old transformers that were cut short.
     
    Last edited: Nov 17, 2019
  11. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    King Fan, D'tar and That Cal Webway like this.
  12. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    So the amp runs, but...

    :D

    The reverb has a buzz. I used all that vintage cloth-covered wire you can see in the post above. Next step will be to look for any mistakes in the reverb driver and reverb recovery, and probably swap in some shielded cable.

    On the up side, that old Oaktron AlNiCo speaker sounded pretty good yesterday before I put everything down.

    I'll be back with a revised schematic and voltages and whatnot when I get it a little farther along.
     
  13. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Reverb troubleshooting:

    A buzz that increases in volume w/ the reverb level control, from silent to way too loud. Disappears with the reverb return pulled out, disappears when the reverb recovery tube is pulled out. Not a high frequency noise like Wifi/cell/RF/etc, but a low frequency buzz.

    Changing RCA cables and reverb pans didn't help. Pulling the reverb driver tube didn't help. This got me thinking the problem lay between the reverb return RCA jack and the reverb return grid.

    After much chopsticking, much lead-shielding, and being frustrated at every turn, I decided to test if there was a ground loop at the reverb return by way of the chassis connection with non-isolated jacks.

    I floated (mechanically) the jack away from the chassis and grounded the jack (electrically) to the star ground, and the buzz disappeared :D:D:D You can see the molded plug (red) holding the jack away from the chassis.

    So now I have to drill out the 3/8" hole to fit an isolation washer and go forward from there. I'm getting some really nice cleans at 1-5ish and some interesting overdrive sounds above 5, but I've hardly played the amp as I've been chasing this reverb problem. Still haven't even tried to wire in the presence control.

    IMG_3228.JPG
    IMG_3229.JPG

    Revised schematic on the way, then I can ask y'all for some opinions about the really granular tone-shaping choices, how to maximize the good sounds from a little Fendery amp like this, Thanks!
     
    Last edited: Nov 25, 2019
  14. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Reverb is cavernous, enormous!

    IMG_3231.JPG
    Here it is with the back door removed to show all the big tubes.


    IMG_3232.JPG
    Here's the front panel without any graphics. I put an hour into trying different schemes to make a presence circuit work, and then scrapped it. Still thinking about what to do with that switch. I suppose a mid pot like in some of the AB763s wouldn't be the worst idea either.

    Overdrive sounds excellent. It's a pain to try to record but I'll see about making a clip.
     
    King Fan, BigDaddy23 and D'tar like this.
  15. BigDaddy23

    BigDaddy23 Tele-Meister

    Age:
    45
    Posts:
    274
    Joined:
    Jan 4, 2019
    Location:
    Australia
    You could use the switch position for a variable V1 bias (Phat switch) with a couple 1meg anti pop resistors in series to ground across both poles - you might need another eyelet there though to separate the cathode bypass caps.
     
    Snfoilhat likes this.
  16. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Single-ended_Reverb_Amp_III_25NOV2019.png
    This is pretty close to how the amp sits with voltages. I A/Bed with a 47 pF bright cap and no bright cap across that 3.3M resistor, and the no cap sounded better. Didn't have a 10 pF on hand. The amp is plenty bright without it.
     
    Last edited: Nov 26, 2019
    shortcircuit likes this.
  17. King Fan

    King Fan Friend of Leo's

    Posts:
    4,222
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    That is one good-lookin' amp. Nice...
     
  18. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Thank you, that means a lot!

    I'm really happy with this project so far. One of my two old Sylvania 6SL7GTs is microphonic and producing a really cool (and annoying) sound like dragging a fingertip across a speaker cone on just a few notes. If I damp the tube's glass envelope w/ a chopstick while playing the noise is reduced. I'm liking this amp enough that I will probably have to get some new 6SL7s. Like a lot of my builds, it may have started in an exercise in re-using old stuff but I'm getting invested :eek:
     
  19. Snfoilhat

    Snfoilhat Tele-Afflicted

    Age:
    38
    Posts:
    1,152
    Joined:
    Apr 8, 2016
    Location:
    Oakland, CA
    Enjoying a pot of coffee before I commit to the holiday. Here's my contribution to the TDPRI content mill. Happy Thanksgiving!

    ***

    The 6SL7 in V1 was more microphonic than the 6SL7 in V3, and so first thing I tried was just swapping them. That seems to have done the trick.

    EQ stuff:

    I somehow mixed up the mids capacitor value for the Princeton Reverb and almost every other Fender with a standard tone stack (47 nF) with the value from the Super Reverb (22 nF) when I built my circuit board.


    PR/DR/TR (mid control at '10' or fixed resistor at 10k).
    Screenshot from 2019-11-26 16-21-09.png


    SR and my amp, currently (mid control at '10' or fixed resistor at 10k).
    Screenshot from 2019-11-26 16-22-57.png


    And here's the mid boost I threw in based on the Rivera-era mid boosts like in PRII.
    Screenshot from 2019-11-26 16-24-58.png


    I'm really liking the raw steel look of the chassis but if I make a face plate for it, here's a very traditional take on that.
    F_faceplate_lines_NOV2019.png


    Last, here's something exciting for the new year (stock photo). 6G6 chassis that fits a big PT I've been holding on to for a push-pull EL84 amp. So many possibilities. EL84 Bassman? EL84 brown Vibrolux? AC15? Lightning?
    s-l1600.jpg

    Happy holiday all
     
    King Fan likes this.
  20. Jimclarke100

    Jimclarke100 Tele-Afflicted

    Posts:
    1,610
    Joined:
    Sep 8, 2009
    Location:
    Horsham, UK
    My latest reworking is a Princeton less the tremolo, but using the spare valve slot to put in a Deluxe inverter circuit. Oh and adding a mid control to the mix as well
     
    Snfoilhat likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.