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Amp Voltages

Discussion in 'Amp Central Station' started by mike60510, Jan 10, 2019.

  1. mike60510

    mike60510 TDPRI Member

    Age:
    57
    6
    Mar 19, 2016
    Chicago
    I recently built a Fender 5E3 circuit in an old PA chassis salvaging and reusing the power and output transformers. I found a schematic on the internet showing “typical voltages” in the 5E3 amp. I’ve attached the document showing the typical voltages along with the Fender schematic. I don’t know how accurate the document is as compared to other 5E3 circuits, I’m assuming it’s a good target. When I started my amp up, the B+ voltages were much higher than what is shown in the “typical voltages” document. The original PA had three 12AX7’s and two 6V6 and a 5Y3 rectifier. The 5E3 circuit has a 12AY7, a 12AX7, two 6V6 and a 5Y3 rectifier. I’m assuming my voltages were higher because of the output was designed for a different circuit with more tubes. I’ve been increasing the value of the voltage dropping resistors to reduce the voltages in the circuit. The initial resistor in the 5E3 schematic is a 5,000 ohms. I’ve been changing the resistors and doing calculations to find the right combination. Would 10% plus or minus to the typical voltages document be a good goal? Also, do the voltage have anything to do with the headroom?
     

    Attached Files:

  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    The PT is the basic determining factor as to what B+ is delivered to the circuit. Unless the PT you are using is someht8ng close to the ‘typical’ 5E3 PT, the. Your voltages will be typical of that PT...not a 5E3. Hey, a vintage 5E3 does not have ‘typical’ Vintage voltages with modern wall voltages.
     
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  3. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    pull the rectifier and measure pins 4 and 6 each to ground..the PT high tension secondaries...and let us know what you read there...in VAC. Those two readings should be nearly to each other. And/or...what DC voltage do you read from the rectifier pin 8 to ground?
     
  4. mike60510

    mike60510 TDPRI Member

    Age:
    57
    6
    Mar 19, 2016
    Chicago
    Pins 4 and 6 of the rectifier show 375 VAC. Pin 8 of the rectifier is 470 VDC.

    Thanks
     
  5. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    And...now you know that your PT is nothing at all like a 5E3’s PT. If you took these measurements with all of the tubes in circuit, then the draw of those tubes has that PT working at the voltage level it works at. That voltage is 100 VDC too high...IF one is looking for a 5E3 thing.and no....that extra 12AX7 in the original P.A. Is not why the voltage is not what you want to see. The reason is that that PT was designed to do what it does, and the designers of the P.A. chose it for its parameters. What P.A. amp was his....got a schematic?? That 470VDC is high for any 6V6 design from back in the day, ime.
    There are ways to drop some voltage, but that is quite a bit of a drop that you need. Perhaps others have some ideas......
     
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  6. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    I'm far from competent regarding PTs, but dropping 100v will be an uphill battle and you could easily introduce new problems with whatever tactic you use to lower them. 5E3 transformers aren't crazy expensive. It's better to just buy the right PT and then deal with your modern wall voltages from there.
     
    Last edited: Jan 10, 2019
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  7. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Or...buy a 'voltage adjusted' PT that yields proper voltages with the expected modern wall voltages.
     
  8. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yes, if you look at the original Fender layout diagram, all voltages are +/-20 percent. So if you are using a 370-0-370 PT with 5Y3 you should see loaded B+ of about 440v? Is that right?

    That's high, but its on the upper threshhold of acceptable. I don't own a 5e3 but I think it typically calls for 355-0-355 PT. I myself would probably aim for 325-0-325, but what do I know.

    Also, you don't mention anything about current draw. That's important too. Do you know what transformer you have? Amps? Etc.
     
  9. mike60510

    mike60510 TDPRI Member

    Age:
    57
    6
    Mar 19, 2016
    Chicago
    A proper transformer may be the best answer.
     
    SacDAve likes this.
  10. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yep. Don't throw that one away though.
     
  11. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Those stated tolerances are interesting. Then, there is the real life experience of taking voltage measurements on all-original examples...and noting there sonics. I have some voltages recorded for one of the best sounding all-original 5e3’s that I have heard. 372VDC on the plates of the 6V6’s. That type of experience makes one like what one has seen an dheard....and in other situations not like what is seen or heard. It is also interesting to use a Variac and vary the voltage while playing and listening to the differences that voltage changes can make.
     
    D'tar likes this.
  12. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Amen. Those would be good numbers that pretty much no one really trades in. Plates, screens, all that stuff. Weird. (I'm of course referring to voltages of the authentic amps in perfect working order. Seems like with Vox and others, you don't get those voltages right it doesn't sound as Voxy as it should. Component values and B+ aren't the whole story).
     
    Last edited: Jan 10, 2019
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  13. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    Schematic voltages are ONLY applicable with the parts and components stated on the schematic...use different parts and the voltages will be different.
     
  14. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Fwiw, I have the complete voltage chart for that 5E3. If I were to build one, those would be my target voltages. I would also like the OT to be as near to a clone as it could be of that OT.
     
  15. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    They were often also applicable to the 110-117 voltages from the 50s. I also wonder if those voltages are just "safe operating range" vs optimal sounding.


    Plus, you plug in to an outlet in an overtaxed apartment in a large metro city and your voltages may be wildly different than plugging in at your brand new house in the suburbs with a reliable electric company. Plug in at 5 a.m. vs 4 p.m. Plug in during a heat wave.
     
  16. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    If it were me, I'd be turning that amp into a Bassman clone, where those voltages and that tube array are closer to ideal.
     
    D'tar likes this.
  17. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    Agreed... Better than putting on a hat, sunglasses and a fake mustache and pretending to be a 5e3.

    @Wally, my 5e3 circuit sits right about 368vdc on the 6v6 plates. That is a good spot still using 250r bias resistor. Care to spill the beans on more numbers:)
     
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  18. mike60510

    mike60510 TDPRI Member

    Age:
    57
    6
    Mar 19, 2016
    Chicago
    Thanks for the help!
     
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  19. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    D’tar, I’ll pull that sheet out and get those numbers for you.
     
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  20. SacDAve

    SacDAve Poster Extraordinaire

    Age:
    67
    Dec 23, 2009
    Rocklin Ca.
    Thanks Wally I'd like to se them also.
     
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