Modded Princeton Reverb Layout, Help Review Please!

Discussion in 'Shock Brother's DIY Amps' started by djbDJB, May 30, 2021.

  1. djbDJB

    djbDJB TDPRI Member

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    Hello,

    I have a few posts on here already relating to certain aspects of a modified PR build. I have attached Rev1 of a chassis layout to facilitate any help I can get.

    I have deleted the tremolo, added a MIDS pot, an NFB selection switch, 1 ohm resistors for bias measurement, bias trimmer, and a reverb dwell and tone control. Plus a variety of other mods found and recommended by RobRob on his website.

    My modded arrangement has seven pots on the chassis face rather than the typical six. One issue I hope to solve with the below layout/board is the lack of space near the typical bias board location that would make it hard to install seven pots on the face, thus why I moved the bias circuit to the main board.

    A few questions: would a seventh pot fit if I move the bias circuit back to its original location? On the attached layout, should I move the bias circuit to the other side of the power dropping/filtering circuit? My reverb dwell pot has a fairly long lead (yellow/black-line wire), could this become an issue?

    Thank you in advance!
     

    Attached Files:

  2. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Tough, inter-related questions. Just a couple rough, guesstimated thoughts; others will know more.

    Bias current is very small, and it's DC after the bias diode — though the AC *voltage* from the rectifier to the first bias resistor is large. Could we leave the bias board in place? How close would that get AC to the pots? And in any case, could we just use Rob's shielded mic cable for the leads that run to those last two pots? He shows folks running sensitive MV wiring all the way down to ground/standby holes in his 5e3 mods.

    BTW, should the bias tap from the rectifier come from pin 4 or pin 3? Do we want to pass it through two (opposite) diodes? And as a detail, do we need to ground the *heater* CT on the cap can? I did put my HT CT there hoping to reduce ripple return current at the ground bolt, tho Rob agreed with me that may or may not matter with a cap can. But AFAIK the heater CT can go straight to the power ground.
     
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  3. djbDJB

    djbDJB TDPRI Member

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    Thank you @King Fan!

    I may use the RG174 cable for the yellow/black wiring to the Reverb Dwell pot. I should also consider using a shielded mic cable for the runs to Reverb Level lugs 2 and 3.

    I may be able to have a seventh pot with the bias board in its original location, especially if I install the bias circuit on terminal strips oriented away from the chassis face.

    I will put the Heater CT on the power ground. And you are right about the bias tap, probably should come from pin 3 before the diode.

    Thank you again.
     
  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    My pleasure; I'm a big PR fan too. You've done a lot of work in DIYLC -- I hope smarter folks will weigh in on the bias board.

    EMF/noise aside, was one of your questions if it'll even fit physically with the pots down in that corner?If it helps, here's an overhead of that corner in mine. (No, I no longer use PT bolts as ground anchors; this was my second build. But I did do star washer under, keps on top, and loctite in the middle.)

    upload_2021-5-31_12-54-35.jpeg

    So, spacing. Could you move it closer to the PT? or even squeeze a narrow bias board left of the PT? Are you custom drilling your chassis/faceplate so pots could sit closer together? I'm not quite sure about *vertical* clearance, but the front of the board has 'low' components, and with a backing board, the board is also pretty low. Do you have a chassis/holes/pots etc to do some dry-fit testing?

    To try to get more brains looking at this, let's stick in your drawing full-size. Are those pot locations real/measured? Mine go nearly that far with two fewer pots. Maybe you're custom drilling your control panel.

    Princeton_Edit_Rev1.png

    One thing I've wondered about is other options. Could you lose the low input and sneak a pot there? Not sure -- Rob does something like this with his 5E3 MV mod IIRC.

    Or... Could you stick reverb tone and dwell on the back of the amp? Sure easier, and sure keeps the front of your amp looking purty. My guess: neither dwell nor verb tone seem like things you set by eye. I have both my mids/raw and my NFB pot back there, and it's awful easy to reach over and adjust by feel and ear. There are quite a few holes already, especially if you don't need an ext. speaker, and I drilled a couple more, one for the NFB on-off-on miniswitch and (bigger) for the external-adjust bias pot. Note the mini-chickenheads -- easy to feel your setting, and I just put zero=up.



    IMG_6940.jpg

    Finally... speaking of controls in the back, I see you have a standby switch there. Dude. Completely unneeded and potentially bad. Read Merlin on standby -- your eyes will be opened, your tubes will live longer, and you'll have a spot open for a *useful* pot or switch. :):):)
     
    Last edited: May 31, 2021
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  5. djbDJB

    djbDJB TDPRI Member

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    Thank you @King Fan for the information and recommendations! I will get rid of the standby switch, thank you for the Merlin link.

    I don't have any parts yet. I plan to modify a Mojotone small parts kit/chassis. But I haven't even ordered them yet. My plan was to determine the front chassis layout then order a custom faceplate from PrecisionDesign, mark the extra hole location using the custom faceplate as a template.

    Looking at your bias board shot, I think I can use these terminal lugs to move the bias board closer to the PT, which would surely make room for the seventh pot.

    I would say the front chassis of the DIYLC file is not to scale with respect to potentiometer spacing. The seventh pot will have the same spacing as the other pots. So my custom faceplate won't really have room for any amp name callout. I know I am making my life a little harder with this. Depending on how this thread continues, I may just do as you suggest and utilize the external speaker jack for the Dwell pot.

    Out of curiosity is your BIAS jack a TRS jack that you connected each power tube's cathode resistor to the tip or ring? Pretty nifty idea.

    Appreciate your time!
     
  6. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    To each, his own of course.

    Curious as to why you are ditching the tremolo? The PR trem is one of the coolest sound things ever invented IMO.

    Not saying it is, but if your main motivation is to open up space for a mid pot in the front panel, my advice is put the mid pot in the back, where the ground switch used to live. Because, you probably won’t need to adjust the mid pot or an NFB switch all that often (YMMV- for me, having done both, I find the no-op setting ideal, for both. After *lotsa* fiddling)

    Good luck with your build!
     
  7. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    It's a Fender style 10KL bias pot -- adjusts with a bladed screwdriver. Allows you to bias the amp without dropping the chassis. That is, IF you install test points (tip jacks) for bias measurement. :)

    upload_2021-5-31_20-28-54.png
     
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  8. MatchlessMan

    MatchlessMan Tele-Holic

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    When I rebuilt my Princeton Reverb I left the bias pot where it was but moved the bias board onto the chassis end panel next to the PT:

    [​IMG]
     
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  9. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    When I converted a blues jr to a PR...
    I mounted the bias board under the pilot light... like this: 20210406_213753.jpg
    Also, can't you have more range on the mids by omitting that 6.8k resistor?
     
    Last edited: Jun 2, 2021
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  10. djbDJB

    djbDJB TDPRI Member

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    I think I will just go @MatchlessMan 's route. Looks to be plenty of space there. But I may user vertical terminal lugs instead of the board... I can probably mount the bias pot at that location too with the terminal lugs as support.

    @MrCoolGuy I want to keep the 6.8k as a minimum so I can dial the mids pot to zero and have the stock circuit. Pretty awesome conversion on your end!
     
  11. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    Make sense to me. In general, I think that mids pot is the best mod for a Princeton Reverb. It's almost like a gain pot... look forward to seeing this amp come together. And thanks, the conversion was fun.
     
  12. D'tar

    D'tar Friend of Leo's

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    As @King Fan mentioned earlier... You will need to tap your bias feed from pin 3 before the backup diode. Illustrated clearly on @MatchlessMan photo above.
     
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  13. Greg70

    Greg70 Tele-Meister

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    I've always wondered why Fender used a separate board for the bias. Didn't they do this on other amps too?

    It looks like the OP is using the extra space from deleting the trem circuit to move the bias board onto the main circuit board. If the bias board is out of the way, there is definitely room to mount another pot on the faceplate.

    Something I did on my PR build that you may consider: put the power switch in the spot where Fender originally put the ground phase switch. This puts the power switch right between the fuse and the power cord so there is less internal wiring. You now have the original power switch location free and clear to use without any wires running across or near it. In my case I used it for a NFB switch.

    upload_2021-6-3_15-22-40.png
     
  14. djbDJB

    djbDJB TDPRI Member

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    Oh! I just noticed @MrCoolGuy, you have a few pots mounted above and right next to your bias board. Any issues with noise/hiss?

    @D'tar I definitely plan to use Pin3 for the bias tap.

    @Greg70 Exactly, just trying to use available main board space, make biasing easier notice the two 1 ohm cathode resistors, and making room for extra pots on the front of the chassis. Thanks for the suggestion on moving the power switch over. I will do that. Did you use shielded wire for your NFB wiring?
     
  15. Greg70

    Greg70 Tele-Meister

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    I did not use shielded wire. The amp is still very quiet. I usually leave the NFB off. It seems like it gives a little bit of boost, about like if your guitar volume knob went to 12.

    For the power switch I used a DPST or whichever one kills the hot and neutral. Because this switch is slightly larger, I had to orient it so that it throws sideways. I also put in the 1 Ohm resistors on the cathode.
     
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  16. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    No, no issues. At first there was some hum, but increasing the first filter cap to 40uf fixed that. It ended up very quiet, surprisingly. Blues jr chassis are kinda small. I also eliminated the footswitvh for the reverb and trem. I put a kill switch on the trem. And a simple bright switch... to me it's a useful mod on a PR.
     
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