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Thread: OT flyback diode removal

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    Supporting Member Randall's Avatar
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    OT flyback diode removal

    I have a Custom Vibrolux Reverb in for a blown PT primary. As I am powering it up and checking things, I notice it has a few mods, what looks like a NF switch, an added bias pot, and the flyback diodes clipped out. What would be the reason to lose these flyback diodes, and should I put them back in?

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    It's weird, because it WAS working fine.....

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Randall View Post
    I have a Custom Vibrolux Reverb in for a blown PT primary. As I am powering it up and checking things, I notice it has a few mods, what looks like a NF switch, an added bias pot, and the flyback diodes clipped out. What would be the reason to lose these flyback diodes, and should I put them back in?
    I dunno any reason to replace your seatbelts in a car if they are cut out?

    i think they are to protect the OT, they may have done their job and someone didn't want to replace them. I use the R3000 diodes.

    nosaj

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    Supporting Member Jazz P Bass's Avatar
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    To me those diodes are a two edged sword.

    Yes they are used to help protect the OT.

    But if they fail short........

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    To me those diodes are a two edged sword.

    Yes they are used to help protect the OT.

    But if they fail short........
    If they fail short......please finish sentence

    nosaj

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    Yep, what saves the OT can kill the PT. Especially if over-fused. Be sure to check the main fuse value, Randall. Remove the tinfoil, if necessary.

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    Supporting Member Jazz P Bass's Avatar
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    Quote Originally Posted by nosaj View Post
    If they fail short......please finish sentence

    nosaj
    Not too sure of the sequence of events but I have had a number of amps come in with shorted diodes and blown OT's.

    I don't trust them.

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  7. #7
    Supporting Member Randall's Avatar
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    Fuse was blown when I got it. I don't know how that can happen, a blown fuse and an open PT primary. It also had a burnt open 1 watt screen resistor, so maybe a tube shorted, taking out the resistor, and then while running on only one power tube it caused some kind of event that took out the fuse AND the primary? I know that doesn't make much sense.

    Oh, and I forgot to mention, they said it got caught in the rain, but not soaked. I see no evidence of water, nor do I see how that had anything to do with this failure.

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    It's weird, because it WAS working fine.....

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    Quote Originally Posted by nosaj View Post
    I dunno any reason to replace your seatbelts in a car if they are cut out?

    i think they are to protect the OT, they may have done their job and someone didn't want to replace them. I use the R3000 diodes.

    nosaj
    I find that flyback diodes tend to short out and cause the PT and OT to pass fault current; hence in practice they're much more of a liability than a benefit.

    Quote Originally Posted by Randall View Post
    ...It also had a burnt open 1 watt screen resistor, so maybe a tube shorted, taking out the resistor, and then while running on only one power tube it caused some kind of event that took out the fuse AND the primary? ...
    Of course it may have been a bad tube, but if the diode shorted it would pull that tube's plate down to 0V, whilst leaving the screen grid permanently up at HT and so drawing heavy current.
    So a bad flyback diode might have been the cause of of the blown screen grid resistor and PT.

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  9. #9
    Supporting Member Randall's Avatar
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    "Of course it may have been a bad tube, but if the diode shorted it would pull that tube's plate down to 0V, whilst leaving the screen grid permanently up at HT and so drawing heavy current.
    So a bad flyback diode might have been the cause of of the blown screen grid resistor and PT."

    No. There were no flyback diodes in it at the time of failure. I just noticed they had been clipped out prior to this failure during repair. I do think they were removed intentionally for some reason, along with the other mods mentioned.

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    It's weird, because it WAS working fine.....

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    Supporting Member mozz's Avatar
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    I have removed them before, but at the same time I upgraded to a larger output transformer for a customer. Whether you can hear a sound difference, I don't know. Maybe the manufacturer saves a few cents going with cheaper winding insulation and uses diodes.

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by Randall View Post
    What would be the reason to lose these flyback diodes, and should I put them back in?
    Quote Originally Posted by mozz View Post
    Whether you can hear a sound difference, I don't know.
    Some people do notice a difference in harmonic content. I seem to recall loudthud posting scope shots of the difference in the waveform with the diodes added.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by g1 View Post
    Some people do notice a difference in harmonic content. I seem to recall loudthud posting scope shots of the difference in the waveform with the diodes added.
    Back in the late 80's, after I had read & absorbed the Trainwreck pages, I started to put the anti flyback diodes in lots of amps I worked on, 3x 1N4007 as suggested by Saint Ken. Sonically I did note the mod removed a little bit of roughness from the tone of hard-overdriven output tubes.

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    Lifetime Member Enzo's Avatar
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    Of course it may have been a bad tube, but if the diode shorted it would pull that tube's plate down to 0V, whilst leaving the screen grid permanently up at HT and so drawing heavy current.
    If the plate node is grounded by a shorted diode, that doesn't leave any full B+ at the screen node.

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    Member HaroldBrooks's Avatar
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    How about eliminating the diodes and using Varistors between the CT and each leg of the push pull to protect the output transformer ?

    It's my understanding that this is "dormant" until you reach the assigned start of the clamping voltage for the particular varistor.

    Here's the ones I started using recently (suggested by Rob Robinette) : https://robrobinette.com/images/Guit...Protection.pdf

    625v TDK.

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    Member HaroldBrooks's Avatar
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    Here's a link to the actual MOV (Varistor) so you can purchase through Mouser if you are so inclined.

    https://www.mouser.com/ProductDetail...jRPStIMZVHlPc=

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    Supporting Member Randall's Avatar
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    Amp is repaired and back to owner. The diodes were not a part of the problem, and therefore did not get dealt with. It seems it is pretty much 50/50 as to whether they should have been replaced. Someone took them out for a reason I assume. This case is closed.

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    It's weird, because it WAS working fine.....

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