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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 Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    Snfoilhat - thank you too for jumping in the water here to save the overboard fisherman -

    Great chance you are right here. One bit of good news is that I have another PT that will probably be more like what I need here. Bad news is that it's all tight spots and is going to be rotten to remove and replace :) - I don't think I want to be adding a bunch of switches and re-do all the rectifier biz -
     

  2. Snfoilhat

    Snfoilhat Tele-Holic

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    Apr 8, 2016
    Oakland, CA
    This PT can make B+ 460VDC and -65VDC bias voltage, apparently, it's just a matter of figuring out how to wire it in. I am not great at those spatial-visual tasks where you rearrange a picture in your mind, but I think if you totally ignore the red/yellow lead for now, and flex the diode leads around, then this
    the_twin_power_supply_2.png

    looks like this http://www.valvewizard.co.uk/bridge.html
     

  3. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    Here's a similar one from a TR 135 - looks a little bit more manageable for me....
    TR 135 PS.png
     

  4. Snfoilhat

    Snfoilhat Tele-Holic

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    Again in this case the RED-YEL tap is going to the intersection of the voltage-balancing resistors that are a part of that first series set of filter caps that form the reservoir/B+1. It'd be cool if someone could explain the reasoning for this feature.

    This schematic does make it clearer that the RED-YEL is not a center-tap in the normal sense, and that a solid state bridge rectifier is doing that job. Usable voltage is RED to RED, and grounding RED-YEL would reduce the available voltage considerably, just like in the OP's bass amp conversion.
     
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  5. danlad

    danlad Tele-Meister

    244
    Mar 24, 2015
    Here and there
    It's a voltage doubler circuit I believe. The centre tap goes to the middle point of the two stacked filter caps if I'm reading it right.
     
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  6. Finck

    Finck Tele-Holic

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    Oct 11, 2017
    São Paulo - Brazil
    My 2 cents: a transformer with center tap combined with a 4 diode bridge is used in symmetric PSU's, those with positive and negative voltages, measured against a "central" reference, which is connected to ground.

    [​IMG]

    If you take the negative rail V- of the above example as the reference (connecting it to ground), the other two rails (ground and V+) will be both positive in relation to it: the V+ rail will show the full intended DC voltage; the "former ground" will show the half.

    Having two different DC voltages, one highr, one lower, you could think about power scale...

    If you don't want this function, just ignore the central tap. Use a 4 diodes bridge and don't connect the center tap to anywhere...
     
    Last edited: Jan 3, 2018
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  7. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    Ding ding - I woke up.thinking this! Very common in Aussie made guitar amps - lots of high amp, but low HT voltage TV transformers so Moody EL34 amps especially used this.

    That's why B+ is half what is expected - and low power is achieved by reverting to the direct bridge rectifier connection. Advantage is smaller transformer.
     
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  8. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY


    That appears how they are switching between 20w and 100w on the first schem provided
     
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  9. Finck

    Finck Tele-Holic

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    I've just edited my post to include a schematic. Help it makes easier to understand all my crazy words...
     
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  10. danlad

    danlad Tele-Meister

    244
    Mar 24, 2015
    Here and there
    Quite clever to use that as the low power option rather than a triode mode. Now how about a low power triode switch too squeezed in there ;) ?

    Think it was an Ozzie valve amp I read about somewhere that clued me into voltage doublers, probably the same one!
     

  11. Finck

    Finck Tele-Holic

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    I remember that Orange Tiny Terror uses the lower voltage trick to scale from 15w to 7W. But in the original circuit, when you flip the switch, all the circuit is supplied with lower voltage, I mean, both preamp and the power stages.

    It works perfectly but, as some people didn't like the tone when in 7W mode because of the "starved" preamp, someone has developed a mod to maintain the full voltage on the preamp stage, lowering the voltage on the power stage only.

    I think that using a transformer with a center tap and the 4 diodes bridge, the implementation of such feature becomes very easy, offering more options. At the flip of a switch, you can have:

    1 - full voltage on preamp and power;
    2 - half voltage on preamp/full on power;
    3 - full on preamp / half on power;
    4 - half voltage on preamp and power.

    Not sure if it's good, but sure it's flexible...
     
    Last edited: Jan 3, 2018
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  12. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    Well it gets more interesting to me -

    1. With the Red Leads removed and soldered to a test bed - I get a total of 350 volts or so - I forget as it didn't surprise me; and it was about 174 something per red lead.

    2. Remove the Red/Yellow lead and it's funny -
    A. One lead has 160 VAC
    B. The other one has 44 VAC

    So - having said that I'm not so sure that this is going to work out so well for me. Please tell me if this is right -

    Question 1. I can build a simple bridge rectifier.
    a. I would have to order up some .002 uf 1kV disc caps I imagine per the bassman 135 schematic.
    b. Can I use 1n4007 diodes ? they should be good for up to 1000 volts.
    Question 2. I will have to use the famous red/yellow lead somewhere I imagine, as I don't think the rectifier circuit will work with an unbalanced HT. ? is this right ?

    Question 3. Where would I install the red/yellow? somewhere like here ?

    Question 4. This will be my bridge rectifier/Voltage Doubler ? Because if it's not a doubler, I'm no better off than I was at the 240 vdc on the plates, B+, etc...
    BF-Twin-Reverb-Filter-Cap-Board.png
     

  13. Finck

    Finck Tele-Holic

    Age:
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    Oct 11, 2017
    São Paulo - Brazil
    How are you measuring the voltages? Let's say that you have measured 350Vac or so between reds.
    Now, measure between the red/yellow and one red, you will have about 175Vac and the same between the other red and the red/yellow.

    If you don't read aproximate the same voltage on both measurements, it's not a center tap tranny, and I'm not sure what you have, nor will be able to help anymore...

    If it's a center tap tranny: you can use exactly the schematic I have posted before, with a full 4 diodes bridge. That way the first big capacitors will operate with lower voltages, what is good and cheaper (with this configuration 350V capacitors are sufficient).

    You will have almost 500VDC after the bridge, (without charge) measured between the points V- and V+ of "my" schematic.

    Not sure if this voltage is what you need (it appears too high to me, but I believe it will be lowered by the choke and the resistors of the filter). The point V- goes on Ground, the V+ is the highest B+ (prior to posterior filtering stages).

    You can use 1N4007. I believe that there are better options (faster diodes, like UF4007). But don't bother with that.

    The small 0.002 (or so...) ceramic capacitors paralleled with the diodes are snubbers that helps to reduce noise. The PSU will work with or without them. You can add them later. I have never used them.

    The resistors paralleled with the big capacitors are for drain the current when the amp is turned off. This is an important safety resource and also avoid strong "pops" on speaker when you turn the amp on.

    The 2uF capacitors of the filter stage will be under 500Vdc or so. I recommend that you use caps rated for 630V at least. May be difficult to find electrolitics with that voltage (here in Brazil it's impossible). If so, you can try mylar caps.

    Hope it helps!
     
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  14. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    I am still working this out. I think that this scheme looks more like the fender schematic for the Bassman 135 power supply. Would anyone chime in please to verify - before I blow the whole thing up? :) IMG_0655.JPG
     

  15. Finck

    Finck Tele-Holic

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    Yes, that's perfect.

    But look that, as I've written before, the point marked as 0V/GND on above schematic will not be connected to anywhere (supposing you'll not implemented a power scaling method).

    And the point marked as -Ve will be connected to Ground of the amp's circuit.

    And both polarized capacitors must be oriented with their positive leads pointed to the top.

    If you don't do that way, very bad things will happen (not a joke).
     
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  16. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    Finck, I really appreciate all the time you've spent helping me on this. I do not want the half voltage I already have, but would rather get about 490 VDC. Here are a couple i've come up with - That do not use the center tap. I like the idea of the "Greinacher doubler", as I already have three diodes all going one direction on each leg, and would have less de-soldering/soldering to do.
     

    Attached Files:


  17. Finck

    Finck Tele-Holic

    Age:
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    Oct 11, 2017
    São Paulo - Brazil
    My pleasure to help anyone!

    I don't know too much about voltage doublers.

    But I see that you already have a transformer that is able to supply the voltage you need (490V after rectification) without use a doubler. You just need to use the right circuit.

    If you just ignore the center tap and engage a doubler on the red leads of the tranny, I believe you will reach almost a 900V, a no-no way to go.

    If you just ignore the center tap and connect the red leads on a 4 diode bridge, you will get the right voltage of about 500VDC, but your filter capacitor will operate under that voltage. A 630V rated capacitor will be required. I have told before, capacitors rated for such voltage can be hard to find and expensive.

    That's why I have suggested the configuration with 4 diodes and center tap. You will have the 500VDC, you'll be able to use 350V capacitors...
     

  18. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    I only get 175 vac out of the PT. With rectification I only got 240 or so. So why won’t I need a doubler? Btw my first two caps are 450v the remaining ones are 500v. Since they are in series, won’t that work ?
     

  19. Finck

    Finck Tele-Holic

    Age:
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    Oct 11, 2017
    São Paulo - Brazil
    Look at this:

    IMG_0655 (1)-1.jpg

    You tranny has 175Vac between the certer tap (red/yellow) and side leads (red), right?

    With the above circuit, you will get almost 500VDC between the points +B and GND I've marked by hand.

    Each capacitor will be under 250VDC, so, the 450V ones you have are OK.

    Got it?
     
    Last edited: Jan 5, 2018

  20. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Silver Supporter

    Dec 11, 2009
    Bloomington, MN
    Hmmm. So at all times (each phase of the cycle) only one cap will be in use. Thus the need for a larger voltage rating? The fender diagram for the 135 line of amps shows 285v caps in the circuit
     

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