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Thread: Catching/ flyback diodes on amps.

  1. #1
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    Catching/ flyback diodes on amps.

    Is it a good idea to add catching/ flyback diodes to tube amps that don't have them?
    I know most modern fender amps have these like the HRD.
    Any thoughts welcome.

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    The high-voltage diodes that Fender uses seem to be pretty resilient - I rarely get problems with those. Where there are two or three 1N4007 in series sometimes the whole string is shorted out, so if I get an amp where this has happened I replace them with the Fender type - assuming no other fault found.

    One of the most reliable ways to protect the OT is to install a wirewound resistor right across the output socket such that it's always paralleled up with the secondary. I usually use 220R/10W as this really kills oscillation and doesn't affect the sound. You can go as high as 470R, depending on the amp.
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    you do not really need them with modern amps, older amps used layered paper transformers (no nylon bobbin), with the layered paper approach, you do not have the nylon flange in between the hi-voltage primary and the grounded core, so large transient voltage can spike over to the core (especially if large gauge secondary wire has smashed the smaller gauge primary wire out towards the core, thus compromising the margin space as is often the case) and start a chain of events that will lead to bank account drainage,

    layered paper coils were wound on a multiple coil machine, after the winding is done, they are cut apart with a band saw, if the band saw guy is having a bad day, he might cut off center, leaving one coil with a large margin (space left on layer between the edge and the last turn of magnet wire) and the other with little or no margin.

    a well engineered bobbin wound transformer should withstand transients with no ill effects, to test this you could get a layered paper xfmr and a nylon bobbin xfmr on the bench and use and AC/DC hi-pot machine to record breakdown voltages for both.

    i use stereo jacks for speaker jacks ( non shorting) and solder a 600 ohm resistor across the hot to gnd as mentioned above, this prevents xfmr death if a speaker opens up or your spk cable opens up some how, or if you turn the amp on and start cranking leads on 10 with no load,

    why stereo? you can solder the unused tip (shorter one ) to ground and thus get a bit more contact area for the amperage. and it gives a tighter fit that will not break down as fast as a standard spk jack. may as well, only a few pennies more.

    1/4 inch jacks were never designed for speaker loads, they were designed for telephone switchboards, people are finally starting to get that with the advent of the speak-on cables, which are a PITA but work good for 2000 watt Carvin bass amps.

    hmm, those flyback diodes might protect the tube socket from arcing also. ceramic sockets/ no problem. tired plastic jacks? could be a problem.
    Last edited by cjenrick; 07-17-2017 at 10:01 PM.

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    Quote Originally Posted by cjenrick View Post
    you do not really need them with modern amps, older amps used layered paper transformers (no nylon bobbin), with the layered paper approach, you do not have the nylon flange in between the hi-voltage primary and the grounded core, so large transient voltage can spike over to the core (especially if large gauge secondary wire has smashed the smaller gauge primary wire out towards the core, thus compromising the margin space as is often the case) and start a chain of events that will lead to bank account drainage,

    layered paper coils were wound on a multiple coil machine, after the winding is done, they are cut apart with a band saw, if the band saw guy is having a bad day, he might cut off center, leaving one coil with a large margin (space left on layer between the edge and the last turn of magnet wire) and the other with little or no margin.

    a well engineered bobbin wound transformer should withstand transients with no ill effects, to test this you could get a layered paper xfmr and a nylon bobbin xfmr on the bench and use and AC/DC hi-pot machine to record breakdown voltages for both.

    i use stereo jacks for speaker jacks ( non shorting) and solder a 600 ohm resistor across the hot to gnd as mentioned above, this prevents xfmr death if a speaker opens up or your spk cable opens up some how, or if you turn the amp on and start cranking leads on 10 with no load,

    why stereo? you can solder the unused tip (shorter one ) to ground and thus get a bit more contact area for the amperage. and it gives a tighter fit that will not break down as fast as a standard spk jack. may as well, only a few pennies more.

    1/4 inch jacks were never designed for speaker loads, they were designed for telephone switchboards, people are finally starting to get that with the advent of the speak-on cables, which are a PITA but work good for 2000 watt Carvin bass amps.

    hmm, those flyback diodes might protect the tube socket from arcing also. ceramic sockets/ no problem. tired plastic jacks? could be a problem.
    What wattage on the 600 ohm resistor?

    nosaj

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    They do make higher current 1/4" jacks specifically for speaker use. No one uses them, but they are out there. I think I did see them in an old EV speaker once. The males have a really wide insulating gap.

    Point of information: in a TRS jack (stereo) the shorter one is called the ring contact. Calling it tip is confusing. I do not report this only for semantics.

    Peavey and some others have special jacks made in the bodies normally for TRS, but instead of a ring contact, there are two tip contacts. This doubles the tip contact surface. The sleeve contact is plenty for any current a speaker might draw. The limiting factor in a 1/4 jack is the tip contact. Of course ther is an upper limit even for them, and the speakon is a simple, reliable solution.
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    Quote Originally Posted by cjenrick View Post
    you do not really need them with modern amps
    You mean flyback diodes?
    I find it very odd that Fender would add cost by installing a component in their modern amps that they never used in their vintage designs. Surely they must have a reason?
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    yeah flyback diodes. your right, Fender never used those diodes, it might have to due with plate voltage, 460 was max on the Twin Reverb's and similar models, maybe Leo trusted Shumacher to do a good job,

    Ampeg used them on the V4 and similar models, there plate voltage was a bit higher and the OPT was bigger compared to Fender and Marshall, so there is more kickback voltage from the increased primary inductance,

    Leo did use them on the Music Man amps, which use mega plate voltage, maybe Tom Walker came up with that idea. Ken Fisher used them on some of the Trainwreck amps,

    you could put a storage scope on the pwr tube plates and set the trigger threshold high to capture spikes, might yield some interesting data, Owsley did this to Jerry Garcia's Twin Reverb amps and saw that coupling caps from the PI to 6L6 grids had AC voltages higher than their 400 volt rating when really cranking, so he changed all those to 600 volts.

    wattage on 620 ohm resistor s/b about 1 or 2 watts, 1 K will also work, anything is better than infinite ohms,
    Last edited by cjenrick; 07-18-2017 at 06:23 AM.

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    The Hot Rod Deluxe has them and only 430v B+.

    The problem isn;t crappy transformers, the problem is what happens when the speaker load disappears. Fender puts a shorting jack on the speaker connection, but that does not protect against blown open speakers, faulty cords, cords not plugged in, etc.

    In the old days, no one used them because it just wasn't on the radar. Eventually someone figured out open secondary was a potential death to transformers, and they dealt with it. Then it became widespread.
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    Flyback voltage will arc across the surface of a tube base in contact with a ceramic socket and render the socket just as useless as a phenolic one. I've had them with metallic oxides and carbon fused into the glaze.

    The way I arrive at a suitable resistor is to connect my ring tester to the primary and a 5k pot on the secondary. Adjust the pot until the rings just drop. You don't need to kill it dead. Read off the pot and use the nearest fixed resistor value. The rating is just ohms law. I generally use 2x to 3x the nominal power rating just to cover non-sinusoidal output.
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    Quote Originally Posted by Mick Bailey View Post
    Flyback voltage will arc across the surface of a tube base in contact with a ceramic socket and render the socket just as useless as a phenolic one. I've had them with metallic oxides and carbon fused into the glaze.

    The way I arrive at a suitable resistor is to connect my ring tester to the primary and a 5k pot on the secondary. Adjust the pot until the rings just drop. You don't need to kill it dead. Read off the pot and use the nearest fixed resistor value. The rating is just ohms law. I generally use 2x to 3x the nominal power rating just to cover non-sinusoidal output.
    Well, that settles it. I building the ring tester.
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    those flyback diodes were probably meant to protect the pwr tubes first off,

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