How To: Solid State Rectifier to Tube Rectifier Conversion

Discussion in 'Amp Tech Center' started by Dan_Pomykalski, May 25, 2020.

  1. Dan_Pomykalski

    Dan_Pomykalski TDPRI Member

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    I recently undertook converting my Champion 600 from a SS rectifier to a tube rectifier. It took me a considerable amount of time of reading through websites and forum posts to figure out exactly how to do it, and I wasn't ever really able to find a simple "here's how to do it." Maybe I'm just bad at using Google, but I figured I'd type up a how-to to save others some time. I'm not saying this is 100% the way to do it, but so far it's worked for me. I'm sure someone will chime in if something is wrong.

    Before we get started, obligatory warning about tube amp voltages, make sure your amp is unplugged, the filter caps are drained, etc.

    The first thing I had to do was order a filament transformer. The stock power transformer didn't have any 5v taps. I ordered this transformer: https://www.tubesandmore.com/products/transformer-hammond-low-voltage-filament-open-5-vct

    Once the transformer arrived and was installed and the 5Y3 was wired up, there wasn't any DC voltage at the filter caps. After doing some investigating, I came across the "Hybrid Rectifier with No Center Tap" on Rob Robinette's page (https://robrobinette.com/How_Amps_Work.htm#Rectifiers) under the "all about rectifier circuits" section. After implementing this, I was getting DC voltage to the filter caps. Also if you're not sure, one primary wire from the filament transformer is soldered to the fuse, and one, in my case anyway, is soldered to the white wire that isn't coming from the power cord. In another post of mine, someone (I forgot who. I'll try to remember and check) recommended increasing the fuse by 500mA.

    I didn't know what to do with the center tap of the 5v Hammond transformer I had though. Initially, I tried grounding it, which was definitely not the thing to do, so I eventually just didn't attach it to anything. After doing some more searching, I came across this: https://www.thegearpage.net/board/index.php?threads/converting-to-tube-rectifier.2085288/

    About halfway down the page, there's a post that says to connect the center tap to the first filter cap, instead of connecting the first filter cap to the pin of the rectifier tube.

    And, electrically, that's all there was to it.

    Some background information:
    My Champion 600 used to be the stock amp from whenever Fender "reissued" them however many years ago. I then modified it so it was closer to an actual Fender Champ. Eventually I modified it so much, I decided to do away with the PCB completely. The fact my Champion 600 didn't have a tube rectifier was something that was always in the back of my mind, which is why I decided to make the conversion.

    I made a hole for an octal socket between the existing preamp tube and existing power tube sockets. Originally, I planned on drilling a bunch of holes in the shape of the octal socket. After realizing it wasn't working, I ended up just using a jigsaw with a metal cutting blade. The result was a half circle, half square monstrosity, but the octal socket does an almost perfect job of hiding it.

    I then used the power tube socket for the 5Y3 and the new socket for the power tube.

    TL;DR
    Buy a filament transformer.
    Unplug amp, drain caps.
    Figure out how to make a hole for the new octal socket.
    Figure out where to place the filament transformer.
    Use hybrid bridge rectifier.
    Solder center tap to first filter cap, instead of attaching the first filter cap to the rectifier pin with a wire.

    Let me know if you have any questions, or, again, if something I've said isn't correct.
     
  2. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Thanks for posting this.:cool:
     
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  3. zook

    zook Friend of Leo's

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    A less expensive way to install a tube rectifier is to use a 6CA4 9 pin rectifier. Since the amp uses a bridge rectifier there's no center tap. use two 1N4007 rectifier as the artificial center tap in the same way a bridge rectifier works:
    Rectifier tube artificial centert tap.jpg
     
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  4. Dacious

    Dacious Poster Extraordinaire

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    A 6X4/6X5 would be the business too, I think. I used a 6X4 for my first 5E3 build, using a 275-0-275 radio console trans I got about 315v which was marginal for two 6V6s but worked OK. Probably ideal for a Champion 600 or even a 5F1.
     
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  5. tubelectron

    tubelectron Tele-Afflicted

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    How To: Solid State Rectifier to Tube Rectifier Conversion

    Yes @Dan_Pomykalski , I did it also similarly when I rebuilt my Gretsch G5222 Electromatic RI amp, which is the brother of the Fender Champion 600 RI (same original PCB and circuit) :

    [​IMG]

    [​IMG]

    -tbln
     
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  6. jimgchord

    jimgchord Tele-Afflicted

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    What is the goal here?
     
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  7. EddieLocrian

    EddieLocrian Tele-Afflicted

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    This is how I do it:
    1, drop off amp with tech.
    2, Wait 1 week.
    3, Give tech $coins

    Job done!
     
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  8. Dacious

    Dacious Poster Extraordinaire

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    To introduce the sag of a vacuum tube rectifier. 5Y3s have high internal impedance which resists changes in current draw. You could do similar with resistors in the legs of the high voltage AC.
     
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  9. jimgchord

    jimgchord Tele-Afflicted

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    Isnt it cathode biased class a?
     
    Last edited: May 26, 2020
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  10. Dacious

    Dacious Poster Extraordinaire

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    Cathode or grid (fixed) biasing refers to how tube current draw is kept within optimal parameters for safe operation with good performance of the power tubes.

    The discussion here is about rectifiers - which turn high voltage AC off the transformer secondaries into high voltage DC to power the amplifier.

    Solid-state rectifiers tend to have no lag, are very direct and 'stiff' or hifi.

    Classic 40/50s tube rectifiers were laggier and had characteristics of breakup and compression or note extension.
     
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  11. jimgchord

    jimgchord Tele-Afflicted

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    Im just wondering how sag is introduced when it idles around max.
     
  12. Dacious

    Dacious Poster Extraordinaire

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    Because when you hit a note on a single tube class A amp it goes past 100% maximum dissipation.

    Cathode biased on a 5E3, AC30 or similar push-pull amp does not mean class A. They are class AB amps.
     
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  13. jimgchord

    jimgchord Tele-Afflicted

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    Have you measured this?
     
  14. Dacious

    Dacious Poster Extraordinaire

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    Measured the tube idling at 100%? Yes. In fact 130%.

    Here's the best description I've read from another forum member.

    https://www.tdpri.com/threads/vibro-champ-measuring-plate-dissipation.345798/#post-4388406

    tubeswellFriend of Leo's
    Single-ended amps are designed to run in Class A because there is only one output tube driving the speaker through the OT, and therefore it is more 'optimal'* for the output tube to be 'on'/conducting through the entire signal cycle (which is the 'Class A' operating condition). This is achieved by biasing the tube as close to 100% at idle as possible.

    * but by no means 'essential'.

    With a cathode-biased stage, it is possible to bias the output tube at 100% dissipation at idle and it won't hurt the tube at all when its running, because those parts of the signal cycle where there is increased current through the tube, are compensated-for by the 'auto-biasing' characteristic of the cathode resistor, i.e.: the cathode resistor's voltage drop increases if more current flows through the resistor, which increases the bias voltage, keeping the tube cool. (Its a different story with fixed-bias however).
     
  15. jimgchord

    jimgchord Tele-Afflicted

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    have you read the stuff regarding sag on the aiken site?
     
  16. Dacious

    Dacious Poster Extraordinaire

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    Yes, long time ago.
     
  17. zook

    zook Friend of Leo's

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    Yep, same procedure, different pin connections.
     
  18. telemnemonics

    telemnemonics Telefied Ad Free Member

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    My understanding is that rectifier sag alters the response in class AB but not really in single ended class A (one power tube) amps.
    I forget why this is but I’ve read it in many places.
    Techs and designers have explained that the tube rec in a 5f1 does not add sag to the output.

    I’m not familiar with the 6x4 rectifier but given that something like a gz34 in a 12-18w amp hardly sags at all compared to a rectifier with less capacity like a 5y3, I’d presume that the benefits of adding a 5y3 to a champ would be almost impossible to hear or feel.

    That being my experience with choosing a rectifier that delivers a little less current than the amp needs for a punchy transient, vs delivering more current than the amp draws for a punchy transient.

    OTOH I’m all for cool amp details.
    Just doubtful that this mod is of real sonic benefit in this application.
     
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  19. Nickfl

    Nickfl Tele-Afflicted

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    If you read Aken's white papers closely you will notice that he is at pains to emphasise over and over again that class of operation is relevant at 100% undistorted operating power. he mostly says this in the context of pointing out of that just because an amplifier can have both tubes operating in a full 360° cycle at very low output does not mean that it is class A because it will not be operating that way when pushed to full undistorted output, but he also is clear that things change after you exceed that output.

    He doesn't go into detail about what happens in a single-ended amp when pushed past the point of distortion but it's interesting to note that that's what we're doing with the guitar amps most of the time and there may be something distinctly other than class a happening at that point which could explain why many people claim that choice of rectifier tube makes a difference in the compression and sound characteristics of these amps even though the rule of thumb understanding of class A operations implies that it shouldn't.
     
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  20. tubelectron

    tubelectron Tele-Afflicted

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    I made these measurements some many times and I agree with you, @telemnemonics. The old manuals from RCA (USA) and Tungsram (EU) states this in accordance also : in class A, the tube never reaches the cut off and the average plate current remains constant, as is the plate dissipation, no matter the tube is driven or not.

    This reduces of course the influence of sagging with a rectifier, unless we reach the clipping / distortion territory of the power stage.

    If the current is not constant under modulation, then you are producing distortion or overdriving with a single tube. And then, the sag by a rectifier may occur.

    And if this is a push-pull configuration, then you leave the class A for a class B operation at higher power. Here the sag has an increased influence, since plate current variations are greater.

    So indeed, any VOX push-pull, and most of the cathode biased push-pull amps are class AB operation - more or less, depending on the idling conditions.

    And sure, the fixed bias push-pull allow the possibility of a more efficient class B operation than the cathode bias.

    IIRC : efficiency in class A is circa 30%, AB is 40-50%, B is 60-70%.

    But it's me, OK ? :D

    -tbln
     
    Last edited: May 26, 2020
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