Diode rectifier: How to Lower the B+?

Discussion in 'Amp Central Station' started by Jewellworks, Mar 26, 2020.

  1. Jewellworks

    Jewellworks TDPRI Member

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    im a newbie when it comes to tube amps. i repaired my Vibrochamp some years ago, but that was the first and only time ive been inside an amp. now im building a 5F1 / 5F2-A. i got the chassis and most of the parts fairly cheap at a guitar show, and although the amp "worked" it sounded terrible. i never intended to keep the amp the way it was, and wanted it to be a project for me to experiment with. so i gutted it, and took it all the way down to basic components.

    the original amp had a 5Y3 tube rectifier, but somewhere along the line, the power transformer was replaced, and the TX doesnt have a 5v tap to run a tube rectifier, so im forced to use a diode bridge rectifier, -which is what it had in it when i bought it.

    that said, the diodes were way out of spec and leaking a ton of AC through, so i replaced them with new 1N4007 diodes.

    when i hooked it all up and was ready to test it, i put tubes in it, and a speaker and didnt hear anything. but i did hear a POP through the speaker. when i turned it off and checked it, i got NO ohm reading through the primary of the output transformer. (the B+ comes straight off the diodes/1st filter cap, and passes through the primary of the output coil to the plate of the 6V6.) Somehow, the coil opened on the output transformer? i checked my schematic and my wiring several times to make sure i didnt do something wrong, and it looks good.

    i got a new output transformer (rated for a 5watt tube amp) and the ohm reading across the primary is just over 500 ohms. so that verifies my old output TX is bad.

    so my concern now is... did I break the old output transformer by feeding it too much voltage? my DC voltage reading off the diodes is 424VDC. (and all my Caps are rated at 450V, so im awfully close) -i have the Tubes in it, as well as all the rest of the "guts", but until i pass the B+ through the new output transformer, the 6V6 wont draw anything...

    im worried im running really Hot and maybe i popped the original output transformer? so im REALLY nervous about putting the new one in, and then checking voltages again afterwords, in case theyre too high, and i fry the new one. according to my schematic, i should be getting 360+VDC at the B+. so that means i have to loose 60 VDC? dont i?

    next question... how?

    somewhere i read an old trick the radio guys used to do back in the day, was to run some resistors in parallel with the diodes to bleed off some voltage from going through the diodes. -which again, thats what was originally in this circuit. ((but i just took it all out, and i didnt take any voltage measurements)). -plus im REALLY not sure how to do it...?

    plus it raises questions, such as:
    what keeps the 2 AC leads from frying when you put them together? -even w resistors? what value resistors?
    wouldnt that allow power AC to pass into the 1st filter cap? when all we want is DC? what happens to the AC?
    if i have AC bleeding through my DC, then send the AC through the output transformer, wouldnt you hear that through the speaker? the only "ac" we want is whats coming off the plate...

    see? i dont understand how this works. IF it works...

    for what its worth, my power transformer has a center Tap, tied to power ground. -as well as the filter caps negative side, tied to power ground.
    (TX is 336VAC from each leg of the 2ndary to the center tap. so about 650VAC across)

    i hope all this makes sense...

    what should i do?
     
  2. Dacious

    Dacious Poster Extraordinaire

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    You can use a 6x4 or 6X5 tube rectifier to drop B+. What is your power trans from?

    High rectified voltage won't pop an output transformer. Your caps wouldn't give out either. Did you measure plate voltage before installing tubes?
     
  3. AndyPanda

    AndyPanda Tele-Holic

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    Please post a schematic of how you have the diodes and PT wired. You said early on in your post that it was a "Bridge Rectifier" (did you mean full wave instead of bridge?)... then you said you are reading 424VDC ... but then you say the PT puts out 650VAC.
    DiodeTransformer.png
     
    Last edited: Mar 26, 2020
  4. Jewellworks

    Jewellworks TDPRI Member

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    full wave. like the first example.
    heres a quick sketch...

    file.jpeg


    the 424VDC is before i have the output transformer in the circuit. but of course, the filter caps are there
     
  5. Jewellworks

    Jewellworks TDPRI Member

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    no... and now im very hesitant to put in the new transformer, in case its too much for it.
    how much can the 6V6 plate handle? is it bad to have 60VDC too much than what the schematic says??

    as for the other available options for a tube rectifier... this is my first amp build, more or less from scratch. it is what it is at this stage. when i build my masterpiece, ill definitely go w a tube rectifier. but for now... i just want to hear this thing work, and give me a platform to experiment with. thats all im hoping for with this
     
  6. JuneauMike

    JuneauMike Friend of Leo's

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    Have you spent much time on this website?
    https://robrobinette.com/How_Amps_Work.htm

    That sorta looks like a full wave w cap load. Right? The cap is bled to ground. But I don't know much about rectifier theory.
     
    Last edited: Mar 26, 2020
  7. Dacious

    Dacious Poster Extraordinaire

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    I'm confused. Is that B+power tubes out? Voltage will be way high with no tubes in. Adding as diodes to an amp wont push rectified voltage up by 60v in operation.

    Something else is wrong.
     
  8. Jewellworks

    Jewellworks TDPRI Member

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    everywhere ive read says that using diodes will have higher voltage than using a tube rectifier, because there is significantly less forward voltage drop. what im wondering is.. is it TOO HIGH? ill put the output transformer in the circuit, and hope it doesnt pop like the other one did, and measure voltages again. both before and after tubes.
    if the voltage is "too high", is there a way to drop it to where it should be? and im guessing it "should be" around 360VDC, with tubes and all.
     
  9. Snfoilhat

    Snfoilhat Tele-Afflicted

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    For a single-ended power amp that is going to draw constant-ish current, why not add an additional filtering stage before the output transformer. This was done on the 5F2 with a choke.

    A choke has a big advantage over a resistor -- it doesn't drop much voltage in your power supply. Because you want to drop voltage, a resistor is cheap, small, effective. The only challenge is the added cost and space of one large filter capacitor.



    Single-ended-reverb-amp_II.png
    Single-ended_Reverb_Amp_III.png
     
  10. Jewellworks

    Jewellworks TDPRI Member

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    im digging the resistor to another cap to drop the voltage...
    dont want to go through the trouble of mounting a choke to this -already small- chassis.

    im about to make the changes... back soon...
     
  11. Jewellworks

    Jewellworks TDPRI Member

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    ok so... i DONT have any 1.5k, 5 watt resistors, or anything close enough... so tomorrow ill have to swing by the local surplus store and see if i can find something in the range i need. im very lucky to have an electronics surplus store, and that its still open.
    meanwhile, i added the Cap, (but didnt hook it up, just giving it a place to live) , changed out the output transformer and plugged it in...

    my VDC readings were:

    NO LOAD/NO TUBES:
    B+ @ Cap : 450
    @ 6V6 pin 3 : 450
    @12ax7, pin 1 : 445

    W/Load/ W/Tubes and 4ohm spkr:
    B+ @ Cap : 407
    @ 6V6 pin 3 : 385
    @ 12AX7 pin1 : 180

    seems oddly low on pin 1...

    there is a very slight hum (120hz) that varies w the gain, and is much more noticeable when cranked.
    i played a guitar through it, and i need to swap the gain pot for an audio taper, BUT, it gets unbelievable overdrive and really nasty, past the point of horrible.
    doesnt seem right to me...
    i swapped the 12AX7 for a 12AU7, and its much more manageable. i couldnt get past 2 o:clock w the 12AX7. i can turn it up all the way w the 12AU7 and its doesnt distort into oblivion like it does w the 12AX7.
    im sure the bias will need adjusted w the higher plate voltage.

    any thoughts?
     
  12. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Zener diode on center tap.
     
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  13. Snfoilhat

    Snfoilhat Tele-Afflicted

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    That schematic I posted above was only an example and I should have added some info. That amp's power tube is a 6DL5, which you could think of as 1/2 of a 6V6GT. So when it's biased and running well it draws a lot less current than a 6V6GT.

    For your amp, you'd want to estimate about how much current it will draw, and using that plus how much voltage you aim to drop (was it about 60V?), you do a quick calculation of how much resistance and power rating your resistor should have, So a resistor I'd estimate is a little smaller than 1.5k and a power rating a little higher than 5W.
     
    Last edited: Mar 26, 2020
  14. Jewellworks

    Jewellworks TDPRI Member

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    what does that do? i have another schematic that shows a Zener Diode on the center tap.
    what rating? im headed to the surplus store, so let me know!
     
  15. Jewellworks

    Jewellworks TDPRI Member

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    im wondering now... do i NEED to drop the voltage? cant i adjust the bias to run w the higher plate voltage? whats the pros and cons of having a higher plate voltage?
     
  16. Luthier Vandros

    Luthier Vandros Tele-Holic

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    I haven’t read through this entire thread. Just taking a quick shot at it before my day gets too hectic.

    If your overall voltage supply is too high (say 50v from preamp plates to power tube plates etc), you can buy a 50v zener and globally reduce your HV by placing it between the center tap of your pt and ground.

    Installation is easy. You locate a nice spot near the PT, then drill a hole and install. The chassis will act as a heat sink. You then unsolder the ct (red/yello) wire and solder it to the lug on the zener. You can select a range of voltages to shave off when choosing which one you need.

    You can’t bias your way out of every situation, especially when an over-spec’d pt is used. Best to look at the tube chart and see what the max specs are for your particular tube.
     
    Last edited: Mar 27, 2020
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  17. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Let me know what size resistor you need if you go that route and I’ll see if I have one on hand. I’ll snail mail it over to ya. Try to stay indoors until the virus abates.
     
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  18. Luthier Vandros

    Luthier Vandros Tele-Holic

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    Another thought is you could run a JJ 6v6 at those higher voltages. I like my preamp plates around 140-160v, but that’s my preference.
     
  19. Jewellworks

    Jewellworks TDPRI Member

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    HA! thats funny! i just ordered a JJ 6V6!

    i got a 22V (as well as a variety of others) Zener Diode, stud mount. the others are in-line, and im not sure theyre big enough to handle the voltage... 22 V down is about where i want to be, after the amp settles.
    if that doesnt work for some reason, i also got a handful of 5watt resistors, from 600 to 1.5k. -for the dedicated cap/power to the OT idea...

    i thought the preamp plates should be up over 200? 210ish? even 250??
     
  20. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Here's the example I should have used from the start, but had forgotten that I built. This thing is very close to the 5F2-A, with a power tube that uses 12.6VAC heaters instead of 6.3VAC. Caveat: I did not follow the "double the power rating for safety margin" rule here -- so where it says 3W it should be 6W. Idle voltages shown to give an idea of the amount of voltage drop.
    Single-ended_12AB5_amp_as-built_22DEC2019.png
    There isn't a right or wrong answer. All these design decisions are about trade-offs.
    One thing that led me to this choice was that single-ended amps don't have the built-in noise cancellation that a push-pull power amp gives you, so a little extra filtering of the power supply is a bonus. Another was that since I was using this particular 4-section capacitor anyway, why not make use of the extra section?
    IMG_3639.JPG
    IMG_3640.JPG
     
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