Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups darrenriley.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Power Transformer Secondary Question

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Jan 13, 2018 at 2:58 PM.

  1. kleydejong

    kleydejong Tele-Meister

    476
    Aug 30, 2010
    Orange City, IA
    Hey Guys,

    I'm working on an amp that my Cousin owns, but is currently not working properly. The amp is a 5E3 DP-III. Pretty cool little amp. PTP wiring. Has a 5F6A tone stack. Two inputs which allow for a Fender style input triode or a marshall style input triode.

    In my troubleshooting I believe the problem is stemming early on in the power supply near the rectifier. I'm only seeing about 16 vdc from the output of the rectifier to the filter caps. There was a period of time when he had it out of an enclsoure, and it seems like the rectifier tube was bumped pretty hard. It cracked off the little keyhole nobbin on the black plastic base. The socket seems kind of messed up as well, as I was not getting a consistent voltage with a replacement rectifier tube.

    I reviewed some of the options with my cousin and he was most interested in putting in diodes, using the tube socket as a turret strip. I have some 5408 diodes on hand, so that would be a pretty simple fix.

    However - here's my question - in following the output of pin 8 on the rectifier tube socket it sends a yellow wire back into the power transformer. Then a yellow / black wire comes out of the PT to the filter caps. My multimeter shows continuity from those two nodes.

    Any idea what is going on with the PT receiving input from the rectifier and then outputting to the filter caps? Can I just run an output from my two diodes to the first node of the filter caps directly?
     

  2. dan40

    dan40 Tele-Holic

    658
    Aug 19, 2015
    Richmond Va
    Your idea should work fine but it may be better if you could link a schematic so that we can see what is going on first.
     
    clintj likes this.

  3. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Just be aware that with diodes the B+ will be significantly higher. There's also a chart floating around online that states the approximate internal resistance value for each recto type if you want to add a sag resistor.
     

  4. moosie

    moosie Doctor of Teleocity Silver Supporter

    Age:
    60
    Jul 18, 2010
    Western Connecticut

  5. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    It might be easier to just replace the socket, if it's damaged. Then you could use one of the solid state tube replacements that emulate the voltage drop from a tube, so you maintain the same B+.
     
    Teleterr likes this.

  6. tubejockey

    tubejockey Tele-Meister

    211
    Nov 25, 2015
    the bozone
    your trnasformer is supplying a 5vac output for the directly heated cathode of the rectifier tube. This is where the dc gets pulled from to feed the filter caps. You can ignore that winding if you go with solid state diodes.
     

  7. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho

  8. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Just in case that doesn't fix things, I just happened to see a similar situation on a facebook group where a guy was only getting maybe 24v on the B+ on a Bassman maybe. Bought a new PT and that didn't fix anything. At some point someone suggested a blown/shorted output transformer and that was it. He disconnected the OT and checked the voltages again, and it went from 24v to 440v on B+.
     
    kleydejong likes this.

  9. kleydejong

    kleydejong Tele-Meister

    476
    Aug 30, 2010
    Orange City, IA
    Hmm, I wonder if that is the problem. He had been using the amp with his home built cab. The speaker port and wire to the speaker was pretty janky. It appeared the jack was not very heavy duty. The wire was maybe 20 gauge, and had one part where it seemed frayed so both hot and ground were revealed and contacting one another. I got the cab working, but now I'm wondering if that caused the OT to go caput. Thanks for the heads up.
     
    jsnwhite619 likes this.

  10. kleydejong

    kleydejong Tele-Meister

    476
    Aug 30, 2010
    Orange City, IA
    I'm not super worried about increasing the B+. The amp has a fixed bias / cathode bias switch. I am likely going to mod the fixed bias mode to be adjustable a la Rob's universal bias mod. I used the same concept in my Hammond Plexi conversion, and it is working great. I'm also under the impression that preamp circuits are fairly flexible to voltage from my review of various AB763 circuits. I understand this may change the tone and feel of the amp. If he's not happy then we'll cross that bridge when we get to it.
     

  11. kleydejong

    kleydejong Tele-Meister

    476
    Aug 30, 2010
    Orange City, IA
    Ran some quick tests. My good multimeter died, so I'm using my old wank Radioshack multimeter which I don't entirely trust. But I think I can confirm some ballpark stuff.

    The yellow and black yellow wires are lifted. They had been sending rectified voltage to the PT and then out of PT to the filter caps.

    340 vac pre rectification.

    99 vdc with diodes jumped straight to filter caps.

    340 vdc after diodes if I lift the jumper to the filters and the rectification is disconnected from the rest of the circuit.

    No audio signal of any kind at the speaker despite some grid probing throughout.

    Any other ways I can test the OT being fried theory?
     

  12. kleydejong

    kleydejong Tele-Meister

    476
    Aug 30, 2010
    Orange City, IA
    I do have a spare OT sitting around taken from a Hammond AO-29 conversion rated at about 15 watts for 6V6's. I guess I could try swapping that in. But it would be nice to know if there are any other indicators of a bad OT.
     

  13. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Sep 10, 2013
    Georgia
    Sorry for the crude marking out, but I don't want to post someone's name and picture without permission. Best I can do on my phone. So, here are some screenshots of the walk through on the page I was talking about. Maybe something can help?
    1515990256274.jpg
    1515990264130.jpg
    1515990269746.jpg
    1515990275213.jpg
     
    kleydejong likes this.

IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.