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Low B+ Voltages on Power Up

Discussion in 'Shock Brother's DIY Amps' started by Mr Ridesglide, Jan 2, 2018.

  1. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    Hello Shock Brothers,
    Okay a while back I ran a thread here about a new build suggestion. It was to use a bunch of parts from an Evil Twin Reverb amp and make it into a bass amp. Well I did my best to do that. Here's what I've got now...

    The HT AC voltages prior to the diodes are only at 175 V each - I was fully expecting nearly twice that as the PT that the amp came from would lead me to believe that it would be -

    As a result, I only have about 240 VDC on the B+, and pretty much all the way down the line to the preamp tubes as well the voltages are low by a bunch - not quite half, but say 60% of where they want to be.

    I know that this donor amp did have a 1/4 power switch on it, making it so that the PT would put out less, but I am assuming that with no switching circuitry at all, I should have the full power.

    One for sure thing I'm wondering is the Brown Pair from the PT - The Schematic shows one side going to ground, and the other to the bias circuitry - I do have negative volts, and it is adjustable, but only from -19 VDC to - 33 VDC. It doesn't change the overall voltage very much.

    I only have 243 VDC on the first filter cap - Using the AA165 Bassman layout as my model for this, but with 4 6L6 power tubes instead of 2 as the OT I have is a monster and should have been able to handle this.

    FWIW - the amp sounds really nice, just not so powerful, and when turned up it does get a bit mushy.

    Any suggestions on places to look that would make the voltages only at about 55 or 60% of what they should be?
    I keep going to the bias circuitry part, wondering if I'm not comprehending something - usually I see bias circuits that only are using one red/blue wire or a tap from the rectifier. Is this possibly causing me some grief?

    Thank you folks :)

    Mark
     
  2. Fiat_cc

    Fiat_cc Tele-Holic

    Age:
    35
    510
    Oct 8, 2016
    Melbourne, Australia
    Was the amp an export model with different primary windings (are you running 120V into a 250V primary)?
    Something is amiss across the board, as your bias voltage is only about 60% too.
     
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  3. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    The ET transformer had two B+ taps, so there is probably a second unused tap with higher B+. I think the lower B+ powered the preamp in whichever position.
     
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  4. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    Not an export. Was always in the US, Fiat. Dacious- I don’t see another tap, or on the schematic either. Image1514947186.304410.jpg
    I’m now worried it may be a half wave vs. full wave thing possibly?
    Is it safe for me to remove just the two red HT leads, secure them safely, tun on the amp and measure them outside the new Home for the donor PT?
     
  5. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    Funny thing too - the PT seems more like the red knob twin PT than the one in the schematic for the black "evil" one -
    Forgot to mention as well - the filament voltages are 3.2 VAC
     

    Attached Files:

  6. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone

    Ok, well you almost certainly have an export 240 volt transformer. Are there four primary wires, two joined?
     
  7. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    No Sir Dacious - only a black one and a white one on the primary side. Two Brown, Two Red, Two green and a red/yellow one -
    Do you think it's safe for me to pull the two Red taps out - and measure them without my circuit to see if they are up around 330 or so without the load I have made?
    when I say that the filament voltage is 3.2vac - that's to one side - about 6.55 together... sorry for that confusion
     
  8. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    You can measure that way. Be super carefull. Make connections with amp unplugged using clip on leads. Power up, take readings and shut down, all hands free.
     
    Last edited: Jan 2, 2018
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  9. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
  10. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    Oh OK. Yes 6.55 would be ok unloaded. You definitely have something going on. I'd suggest that maybe each red wire will turn out to be an end of a brown wire with half the anticipated voltage. So say 160-170 volts. Connecting then in series, you.might get the full voltage across the browns.

    Measure AC across the browns, and across the reds - if you get zero AC the windings aren't contiguous.

    If so, then measure across one brown, and each red. If you get AC voltage between a red and brown take note or mark them. Then test the other red and brown.
     
  11. aerhed

    aerhed Tele-Afflicted

    Age:
    59
    Dec 24, 2016
    Boulder, WY
    Do you have the red/yellow wire grounded?
     
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  12. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    Yes Sir. Ty
     
  13. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Rides, do you have a drawing of thatpower supply area?
     
  14. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    Wally, thank you for taking a look here :)

    - the transformer I believe is set up like the one from the twin doc above on post #5 - two brown taps, two red, one red/white and the filaments - without a center tap on the green filament wires.

    The Filter caps are like this:
    BF-Twin-Reverb-Filter-Cap-Board.gif
    and the remainder of the Power Supply is like the AA165 - three diodes per side from the HT that juice up the first two big electrolytics, and on to the standby switch. I used a 15k resistor with a 10k pot - and the brown wires again - one to ground and one to the bias circuit. I did use a 470 R 1 watt as in the AA165 schematic as the entry of the brown wire.
    Is this the info you wanted ? part of me wants to think I need a value change on the bias resistor - but that would seem to be too far off on voltages to be true. When I get home later, my plan was to remove the two red taps and measure them without my circuit involved. But I may need to also remove the brown wires to prove that - at least then I'd know if the PT was delivering what i was hoping for; somewhere about 660 VAC (330 per red tap).
     

    Attached Files:

  15. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Mr. Ridesglide, what I was wanting to see was the drawing for those transformer secondaries.....as you built the circuit. Just curious.....I have a feeling that here is a problem where those red and brown secondaries interact???? I most probably am incorrect on that guess....but when chasing one has to follow every scent.
    Fwiw, it threw me for just a second in your drawing above because the two series caps in the first stage are drawn backwards compared to how Fender did it...but the results are the same.
     
  16. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    my biggest fear here is that on the Twin Amp - it appears that they followed it up with a full wave bridge rectifier, and the option i chose is just a full wave.... at least I think that's what they're called...
    and perhaps this transformer needs the bridge portion of the program. I guess I'll know when I measure the two HT taps... lol i think
     
  17. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    When I look at the "The Twin" PT - they don't show the red/yellow CT going directly to ground ---- for some reason I always thought that a bridge rectifier didn't use a CT -
     
  18. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    with a center tap on the ht secondary.. the bridge is not necessary.

    got a pic of your rectification?

    part numbers on the PT?

    have you spent time with a meter to determine whats connected to what inside the PT.

    I am going to look at the pdf from earlier. I haven't seen it yet
     
  19. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

    Dec 11, 2009
    Bloomington, MN
    IMG_0649.JPG
    Pretty tight quarters there. But I think you can see pretty much all of it.
     
  20. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    700
    Apr 8, 2016
    Oakland, CA
    I don't think this PT is going to function correctly with the red/yellow secondary grounded, despite the fact that it looks like a HT center tap. In both High and Low power modes in the twin schematic, this secondary is above DC ground. One leg of the HT (the lower red lead on the schematic) is connected to the DC ground, through diode 384. This wouldn't be the case in a standard center-tapped -330-0-330 (or -350-0-350, whatever) type of power supply.

    You may need to build something that looks more like the original, even if you don't want the high-low switching function, at least until you fully understand what's going on and can take better control of it.

    Also, I think someone suggested measuring the secondaries with no connections or no load, to have a baseline idea of how much voltage you have across the different leads to begin with, help rule out possibilities like a short dragging voltages down, or a rectification problem.

    Edit: converted PDF to PNG for ease of reading
    the_twin_power_supply.png
     
    Last edited: Jan 3, 2018
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