Fender 68 Custom Princeton Reverb help?

Discussion in 'Amp Tech Center' started by tah1962, Jan 27, 2020.

  1. tah1962

    tah1962 Tele-Meister

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    Hi all. I’m new here at TDPRI and this is my first post. I’m looking for some help with a problem I ran into today with my 68 Custom Princeton Reverb. I was putting in new 6v6 power tubes. I had my bias probe on the new power tube closest to the rectifier tube. As I powered up, I was showing a plate voltage of 530 and the probe was reading 0.00 mA. I started smelling something getting hot so I shut down the amp.

    I double checked to make sure I had a speaker load and I did. Then I put the old power tubes back in and fired up the amp. Everything powered up, and the rectifier tube, power tubes, and all the preamp tubes were glowing. I then replaced all of the preamp tubes to see if that was the problem. Still no sound.

    I then pulled the chassis to do an inspection and I noticed that the resistor in R58 looked burnt. Does anyone know what the function is for that resistor, and if that would cause no sound through my amp?

    Any help would be greatly appreciated. Thanks.

    Todd
     
  2. Uncle Daddy

    Uncle Daddy Tele-Afflicted

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    There's a good chance you didn't have the slot key in the right place on the tube when it went into the socket. Change the fuse, you may be lucky enough to get away with it this time, though you will need to fix the resistor.
     
  3. tah1962

    tah1962 Tele-Meister

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    Thanks. I definitely had the slots lined up. I double checked before I fired up the amp. It didn’t blow the fuse. It appears it just burned up the resistor in R58.
     
  4. Dacious

    Dacious Poster Extraordinaire

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    Ok - so your new tube on that side pulled too much current. You will need to replace that resistor. New production tubes aren't that finicky and it's possible to insert them the wrong way. R58 is a 1 watt 1k resistor.

    R58 is the faux choke - you'll need to replace it.

    If you're sure you had the tubes in right one of your new tubes likely has a short screen to plate most likely. Return to vendor - obviously not checked.
     
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  5. tah1962

    tah1962 Tele-Meister

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    Thanks for your help Dacious.
     
  6. Wally

    Wally Telefied Ad Free Member

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    R68 is a dropping resistor. Yes, it sits where a choke would sit if there a choke; and a choke in that position does drop a little voltage as does this resistor. The difference between the resistor and the choke is that the choke acts as an added AC ripple remover whereas the resistor has no effect on that AC ripple noise. If the amp is under warranty, I would suggest that Fender warranty deal with the problem. If not, then I have to ask about your experience. If you understand the safety precautions and want to do the work, proceed. If you do not understand the safety precautions, I suggest that it is tech time. hopefully, that power transformer is still functional.
    I would suggest using the old tubes as test tubes after replacing that resistor. It would also be advantageous to have a tube tester just to check for shorts in that new tube. IF the tube was installed correctly in the sockets when this occurred, one has to wonder what caused the episode.
     
  7. DennisM

    DennisM Tele-Meister

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    I had the exact same problem on my 65 PRRI a few weeks ago. Mine WAS caused by mis-installing the same 6v6 as you. I thought the tube would only go in one way. I was wrong. Burnt R58. I replaced the resistor with a new metal film 1 watt and all was well. I lucked out. Now, I don't just install tubes by 'feel'. I physically make damn sure the key is in the right spot. I did the repair from the top. Snipped the old resistor leads as close to the burnt resistor as possible which left enough leads sticking out of the PCB to solder the new resistor in. Otherwise, you have to disassemble just about everything to get to the bottom side.

    Like @Wally says, only do this if you know what you're doing. Drain the filter caps!! If you're not comfortable doing it, take it to a tech. Lethal voltage in there is not a myth. Good luck.
     
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  8. tah1962

    tah1962 Tele-Meister

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    Thanks guys. Yes I know about draining filter caps. I’ll get a new resistor and replace. Hopefully the transformer isn’t fried.
     
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  9. Wally

    Wally Telefied Ad Free Member

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    okay, Todd you mentioned R58. Dacious used that same designation..R58...with a reference to a ‘faux choke’. When I went OT the schematic to check out htings, I went to where a ‘faux choke’ resistor would be and found that 10K/1watt R68 dropping
    And assumed that was the resistor to which you were referring. R58 is a 10k potentiometer in the bias Circuit.
    https://schematicheaven.net/fenderamps/65_deluxe_reverb_manual.pdf
     
  10. tah1962

    tah1962 Tele-Meister

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    Mine is a 68 Custom not the 65 reissue.
     
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  11. tah1962

    tah1962 Tele-Meister

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    Also this is the Princeton Reverb, not the Deluxe Reverb.
     
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  12. fasteddie42

    fasteddie42 Tele-Holic

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    68/65 princeton reverb reissues are built on the same pcb board with minor component differences.
     
  13. Wally

    Wally Telefied Ad Free Member

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    My apologies for posting before waking up. R58...1K/1watt dropping resistor. My suggestions still hold.....
     
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  14. tah1962

    tah1962 Tele-Meister

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    No issue Wally. I appreciate your support. Have a good one.
     
  15. tah1962

    tah1962 Tele-Meister

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    Update: My 1k 1 watt resistor came in the mail today. I drained the filter caps and replaced the burned up resistor, put the old power tubes back in and biased them. Everything is working as normal again!!

    Thanks to all for your support in helping me get my amp up and running again. I appreciate it!
     
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