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How to test an OT

Discussion in 'Shock Brother's DIY Amps' started by Mark the Moose, Jan 24, 2021.

  1. Mark the Moose

    Mark the Moose Tele-Afflicted

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    Last year I built a Weber 6a14HP 35watt Princeton Reverb. The 35watts was too much and a bad idea. So today I converted it to a standard 14 watt PR. The steps were...

    1. change the PT leads
    2. swap the OT for a smaller one from Weber
    3. remove a filter cap which was soldered to the cap can
    4. change the bias resistors
    5. switch from 6L6 to 6v6 tubes.

    Not a big job, but I have no sound. B+ is sitting at 360v, power tubes look perfect, but I checked any other voltages yet.

    Is it possible that Weber sent me a bad OT? How would I go about testing it? I've attached layouts from the two kits in case that's helpful. 6a14_layout.jpg 6a14hp_layout.jpg
     
  2. NTC

    NTC Tele-Holic

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    First, check continuity of the primary and secondary. Power off and caps drained, of course. Check solder connections, output hack connections, and if you have a speaker plugged in.
     
  3. Mark the Moose

    Mark the Moose Tele-Afflicted

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    Tested the speaker cable, checked solder joints. Hunted for wires/connections I may have accidentally disconnected along the way, nothing.

    With the green and black output leads disconnected from the output jack, I measure a dead short between them. Is that normal?
     
  4. dogmeat

    dogmeat Friend of Leo's

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    Mark the Moose likes this.
  5. D'tar

    D'tar Friend of Leo's

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    The ot secondary will read very low resistance.
     
  6. drf64

    drf64 Poster Extraordinaire

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    I'm no expert but I have built a 6a14. Make sure you've wired the speaker jack appropriately. I've accidentally shorted the jack and had the exact scenario you describe.
     
  7. NTC

    NTC Tele-Holic

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    Off topic but why did you want to change to "non HP?" I have a 5F11ish thing that I can change from 6L6 to 6V6 (different rectifiers) and the volume really isn't much different for a given speaker.
     
  8. Mark the Moose

    Mark the Moose Tele-Afflicted

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    With a different PT tap and a smaller OT, there will be a more significant change than just swapping tubes and a rebias. The vibrolux output level was blistering in my small space. Getting normal Princeton levels makes more sense for me. If I don’t like it, I’m out $35.
     
  9. Mark the Moose

    Mark the Moose Tele-Afflicted

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    A little more info. Fired it up to take voltages and the tremolo started thumping hard. With it, a little guitar signal came through with the thumps.
     
  10. NTC

    NTC Tele-Holic

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    My B+ is 420 to 425V - which is typical Princeton, iirc. If your b+ is 360, it should be much quieter.

    Thumping tremolo implies the output works somewhat. Check bias along with voltages.
     
  11. Nickfl

    Nickfl Friend of Leo's

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    Is there a lightbulb limiter involved in this process? Because you really should be using one to check for shorts after major surgery like this.
     
  12. Mark the Moose

    Mark the Moose Tele-Afflicted

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    Agreed, but no I don't have one. On the list of things I intended to build one day but haven't yet. I suppose I didn't see this as major surgery going in, but here we are.
     
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