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Trouble shooting new PR clone build

Discussion in 'Shock Brother's DIY Amps' started by bryansbizaar, Mar 17, 2017.

  1. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    I just finished a Princeton Reverb clone and wondered if anyone has any ideas about next steps regarding the following...

    I plugged the power cord in to a home-made limiter (light bulb) and it lit up when I switched on the power. I checked several other working amps and none of them lit up.

    However when I went ahead and plugged in a guitar (still through the limiter), it sounded pretty good. All the pots were responsive, plus the reverb and tremolo worked fine.

    I went through and triple-checked the circuit and hit all the solder points again. The tubes are all pretty new, but I still plan on swapping each out one at a time.

    As for voltages, I only really checked the plate voltage at pin 3, which came up at around 300v, which seems low. I wasn't sure if this was because of the limiter, and I didn't want to plug it straight into the wall outlet until I got clear what is going on or if it's even safe at this point.

    Any thoughts on how to proceed? Thanks!
     
    wmsimpson likes this.

  2. rocksmoot

    rocksmoot Tele-Meister

    124
    May 6, 2012
    Rogers, Arkansas
    I just finished a DR clone that did the same thing. I had some low plate voltages and it turns out that a couple of the tone stack caps were shorting. They were bad because of some wiring mistakes I had made earlier but fixed, but the caps fried as a result. Had the same symptoms, sounded good but the limiter lit up because of some current flowing when it shouldn't. All the filter cap node voltages were lower than they should be, but the B node voltage was way lower. Found that a tone cap in the V2 circuit was bad. One clue is that while things sound good, the amp's volume overall is lower than it should be. The limiter don't lie.

    So check your filter cap node voltages and write out all the voltages on each of the tubes. Any particular node voltage that seems lower than it should be (relative to the other nodes, if there's a short they'll all be somewhat low) points you to the circuit that it feeds. I found the problem by desoldering the caps in the relevant circuit and testing them with my capacitance meter.
     

  3. kleydejong

    kleydejong Tele-Meister

    391
    Aug 30, 2010
    Orange City, IA
    Did the current limiter flash right upon flicking the switch, but then die down? I would describe that as normal operation. Like this:



    I believe a current limiter will affect your voltages, so I wouldn't put too much stock in that reading.

    You may not have a problem.
     

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  5. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    The bulb stays on.

    Unfortunately I haven't invested in a meter that will measure capacitance yet. I do have a few extra caps, though. I'll try swapping one at a time.

    It looks like the voltage of pins 1 & 6, V1 & V3 is 130v. The diagrams have it at 160v.
     

  6. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Low voltages are normal when using a light bulb limiter. What watt bulb are you using?
     

  7. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    It's a 200w bulb.
     
    wmsimpson likes this.

  8. dan40

    dan40 Tele-Holic

    585
    Aug 19, 2015
    Richmond Va
    Stays on dimly or does it light up really bright and stay that way? Dimly lit is normal as the amp does draw a little bit of current at idle. Like Rob mentioned, voltages will be low when checking with the limiter.
     

  9. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    Here's a pic. I wouldn't call it dim, but it's not 'really bright' either. The filament just barely glows with two other amps that I tried.

    Bulb.jpg
     

  10. King Fan

    King Fan Tele-Afflicted

    Jan 1, 2013
    Salt Lake City
    If a 200w bulb glows brightly, your amp is drawing too much power. (Many experienced builders use 100w -- even that size won't glow brightly after the caps are charged for a second.)

    But sadly, the bright glow just means there's a short of some sort at some point -- any point.

    Luckily, Rob has written up a sequence that can help you use the limiter to diagnose what sector of the amp has the problem. (If you weren't using the limiter, that current draw would be blowing fuses):

    https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Blows_Fuses
     
    robrob likes this.

  11. beezerboy

    beezerboy Tele-Meister

    Age:
    66
    308
    Sep 7, 2016
    AK
    if you have trouble shooting it... try a shotgun. hope this helps
     

  12. dan40

    dan40 Tele-Holic

    585
    Aug 19, 2015
    Richmond Va
    Did you modify the build to include a bias pot? If so, set the pot for the highest negative DC voltage possible at pin 5 of one of the power tube sockets. This will ensure the lowest current draw possible for the particular set of power tubes you are using. See if the light bulb is glowing any dimmer now.
     
    robrob likes this.

  13. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    I followed the steps that applied from the link. Since I don't have a standby switch, I mostly just removed the tubes, first one at a time and then all together. The bulb stayed constant with each step.

    I did include a single bias pot. The bulb didn't seem to be affected when turning the pot to either extreme.

    The transformers are the Classic Tone 40-18112 PT (5-15w) and the 40-18031 OT (5-15w). There were a lot of wires since it has universal voltage options, but I got a confirmation regarding the correct wiring from the company before I started (PT: black/brown for one connection lead, black-white/brown-white for the other).

    I also used a CE cap can, which appears to be wired correctly. I've seen pictures of old PRs where they ground two free tabs to the chassis, but I just ran one ground wire from a pre-drilled free tab to a ring terminal, which is bolted to the chassis.

    Since pulling each tube didn't make any difference, does that isolate things to the transformers or the power filters? Once again, I am getting a decent guitar signal that seems to sound as it should. Perhaps I should just try it without the limiter and see if it blows a fuse?
     

  14. Phrygian77

    Phrygian77 Tele-Meister

    412
    Apr 30, 2016
    Crawfordville, FL
    You said you pulled ALL of the tubes. Does that mean you also tried it without the rectifier?
     

  15. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    I did briefly try it without the rectifier. I misread the suggestions. It didn't appear to have any affect.
     

  16. dan40

    dan40 Tele-Holic

    585
    Aug 19, 2015
    Richmond Va
    If the bulb stayed lit with no rectifier tube installed, you must have a wiring mistake or short that is drawing current before the rectifier. It may be best to disconnect the all of the filament leads and all of the high voltage secondary leads, insulate the ends of all wires and then power up again. The bulb should not light with everything disconnected. If it does, there is a problem with the PT or possibly the power switch wiring. If the bulb does not light, it would be wise to get some AC voltage readings from all of the secondary wires. This will involve removing the insulation from the wire ends and taking live voltage readings. Be very careful with the bare wires.
     
    robrob likes this.

  17. dan40

    dan40 Tele-Holic

    585
    Aug 19, 2015
    Richmond Va
    I also wonder if the bias circuit is wired properly. Were you ever able to check for negative DC bias voltage at pin 5 of the 6v6's? Posting some good pics of everything will help us spot a problem also.
     

  18. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR
    I double-checked the bias wiring and it correctly follows the example from the 1st 'modified Princeton bias circuit' from http://el34world.com/charts/Biascircuits.htm.
    I didn't actually check for the voltage at pin 5, but I did dial the pot to each extreme for comparison.

    Yesterday, I disconnected all of the stranded wires, which were mostly from the PT and OT, re-stripped/tinned and reconnected them.

    I'll keep going over the wiring to make sure it all adds up.

    I'm in the process of uploading some pictures and will put them up shortly.
     

  19. bryansbizaar

    bryansbizaar TDPRI Member

    Age:
    46
    30
    Jan 19, 2017
    Portland, OR

  20. aerhed

    aerhed Tele-Meister

    Age:
    58
    398
    Dec 24, 2016
    Boulder, WY
    Bias board is not right. Missing a wire at least. Where does the yellow wire on bias upper corner connect? Is it spiced to the PT blue?
     

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