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Backup Diodes?

Discussion in 'Shock Brother's DIY Amps' started by The Ballzz, Feb 28, 2021.

  1. The Ballzz

    The Ballzz Tele-Afflicted

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    Looking for a bit of info. When installing "backup diodes" to pins #4 & #6 of a rectifier tube, should there be a small B+ voltage drop? After following @robrob Rectifier "Backup" Diodes tips here:

    https://robrobinette.com/5e3_Modifications.htm#Pop_Reduction

    I'm noticing a 6VDC or so drop in my B+. Is this normal and to be expected?

    I'm kind of expecting that when putting these 1N4007 diodes in series with the HT, it changes each of those 355-0-355 VAC HT leads to pulsing DC? On the feed side of the diodes, I'm reading the expected 355 VAC, and on the load side, it's reading about 56 VAC and 235 vdc.

    Thanks,
    Gene
     
    Last edited: Feb 28, 2021
  2. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Good question. Considering it is in series with the rectifier tube diode, I would have only considered the .7 volt drop of the diode. The 6VDC drop in B+ is more than I would expect.
     
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  3. The Ballzz

    The Ballzz Tele-Afflicted

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    Yessir, when I jumper around (bypass) the diodes, that 6 to 7 volts returns to the B+. Rob specified 1N4007 diodes, but those are "obsolete" without alphabet soup following the number. The ones I ended up with from Mouser are 625-1N4007E-E3/73 and try as I might, I could find no description of what the "E" may denote? I kinda didn't really want to drop my B+ at all!
    Thanks,
    Gene
     
  4. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Hmm. I wouldn't have thought it'd drop much at all. And I'm surprised if there's a species of 1N4007 that drops more volts than 'standard.'

    What amp? Got a pic of the installation?
     
  5. Tom Kamphuys

    Tom Kamphuys Tele-Holic

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    I'm surprised too.

    I'm guessing now, at the risk of looking stupid, but might it have something to do with the fact that it is taking ~1V from half of the cycle of the sine wave?

    Edit: Something like this:

    IMG_20210228_231354414.jpg
     
  6. The Ballzz

    The Ballzz Tele-Afflicted

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    Standard type 5E3 clone. The diodes are towards the chassis side and not really "pic-worthy-able" without removing the socket! With the 5Y3 removed, the diodes seem to test fine/as expected. I've checked, re-checked and triple checked my work! I expected a volt or so drop, but 6-7 is surprising!

    Ya see, I opened my my 5E3 to do a full voltage survey in preparation for my new build. I figured that taking such a survey on a fully functioning and regularly used (and pounded) amp might yield some real world perspective. While it was open, I just figured it would be a good time to add the diodes I had neglected (and didn't even really know about) during the initial build a few years back.

    I was also surprised that upon taking said survey, my B+ was already 3-4 volts lower than initial fire up readings. I had determined that all sounded the best at about 372 VDC, B+.

    Thanks Folks,
    Gene
     
    Last edited: Feb 28, 2021
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Suffix “E” part numbers = smaller diameter of the leads.
    E3 = RoHS compliant.
    73 = packaging type.
    Is that with the same wall voltage? Amp warmed to the same degrees? If so, it could be the current draw has changed with wear.
     
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  8. Intubator

    Intubator Tele-Holic

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    Any chance the rectifier is below spec or on it's last leg?...
     
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  9. LightningPhil

    LightningPhil Tele-Meister

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    What’s your B+ current and is there a choke?

    The 1n4007 drops about 1V at 1A. And if your B+ current was 1A, that’s the average current through the rectifiers. However, there’s only enough voltage in the crests of the sine wave to drive current into the B+ caps. Therefore the current through the rectifier has to be quite a bit higher as it’s only on a proportion of the time. Let’s say it’s 3 A. The higher current will increase the voltage drop - not by a factor of 3, it’s more or less a log curve. You could use a larger diode in it’s place. Suggest something like this:
    https://docs.rs-online.com/ca77/0900766b80923f9e.pdf

    If you look at the graph labelled instantaneous forward voltage you’ll see what I’m rambling about.

    I’ve a preference for silicon carbide Schottky diodes. Great for the current and voltage, with the bonus of being quieter - their turn off characteristic doesn’t require a snubber as they don’t produce little sharp spikes upon turning off. A bit more expensive. But if a job is worth doing...
     
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  10. jimgchord

    jimgchord Tele-Afflicted

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    6 volts wont make a bit of difference
     
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  11. lowatter

    lowatter Tele-Meister

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    I'm no expert but why not make the rectification switchable you can run one or the other? I did that on my 1974X(ish) build and works flawlessly. This layout should be self explanatory.

    [​IMG]
     
  12. The Ballzz

    The Ballzz Tele-Afflicted

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    No need for it to be "switchable" as I'm not looking to change the sound even one iota or have multiple choices! I was simply wanting to install some backup/safety diodes, before the rectifier tube, as suggested by many folks who are much more knowledgeable than me! And yes, 6-7 volts certainly CAN and does make a bit of difference. It's more in the feel than in the perceptible sound!
    Thanks,
    Gene
     
  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    OK... now you have me curious. Show us the voltage chart.:D
     
  14. Nickfl

    Nickfl Friend of Leo's

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    To repeat what @Lowerleftcoast said, are you sure your wall voltage is the same? Wall voltage can vary from day to day and even throughout the day and it wouldn't take much change in mains voltage to change your B+ by 7v, a 2 volt difference would be enough to do it.
     
  15. RollingBender

    RollingBender Tele-Afflicted

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    I’d also suspect different wall voltage. I tested my wall voltage for another 8 hours once and there was a high to low difference of almost 7 volts.
     
  16. The Ballzz

    The Ballzz Tele-Afflicted

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    First is from initial fire up with a 12AX7 in V1 and no diodes and the second was today with a 12AY7 in V1 and diodes installed!

    Gene's Voltage Survey-5E3.jpg 5E3 Voltage 02:28:2021.jpg
     
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  17. LightningPhil

    LightningPhil Tele-Meister

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    Lucky you. Measured 203 V as a low to 257 V high in one day. Nice old road with old wires :(
     
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  18. 2L man

    2L man Tele-Meister

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    Where is multimeter black probe connected when you measure +377 and +355VDC on PT CT?
     
  19. The Ballzz

    The Ballzz Tele-Afflicted

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    Black meter probe is connected to Power Transformer Center Tap/chassis ground. Those readings are for the Output Transformer Center tap. Please note the dates on the two voltage surveys. Also please note that I included both VAC and VDC on the HT, just past the diodes as well as the VAC prior to the diodes. The HT leads are going to the unused pins #3 & #5 of the rectifier tube socket and those lugs are being used as the junction point for the diodes going to pins #4 & #6, respectively!

    I've always been led to believe that the most accurate VDC for B+ is measured at the feed to the Output Transformer and not at the power tube plates, where the B+ has already passed through the resistance of the OT primary winding. Please correct me if my perspective here is wrong?

    I did get my voltages back up to "closer to normal" by swapping in/out various NOS 5Y3 rectifiers although now those voltage are just a tad high, for my tastes. 385 VDC at OT/CT and 377-380 VDC at the plates. I do have a new JJ 5Y3 on the way, just to rule out a weakening tube. It never hurts to have an additional, extra 5Y3 on hand!

    Thanks Folks,
    Gene
     
    Last edited: Mar 1, 2021
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  20. The Ballzz

    The Ballzz Tele-Afflicted

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    Well Folks,
    Given how kind you all were to speak up in this thread, I thought it most respectful to provide an update! I just received a new JJ 5Y3 S, plugged it in and all my voltages are right back where they were originally, within a couple/few VDC. This tells me that at least with the JJ 5Y3, rectifiers do indeed seem to deteriorate with time and usage! And yes, the JJ does provide a few volts less than any of the multiple NOS/UOS units I have on hand! I'm a happy camper!
    Thanks 2 All 4 Playin' Along!
    Gene
     
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