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Thread: Super Reverb Replacement OT Wiring

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    Super Reverb Replacement OT Wiring

    Hello folks, I have a 1974 Super Reverb here for which I am attaching a schematic and a few pics pertaining to my issue. So this amp obviously has had the output transformer replaced. It's kind of funny looking, but hey, I ain't prejudiced. The specified impedance (3800) for the primary looks about right. The wiring for the secondary is where I'm having trouble. There are 4 wires, black, green, white, and yellow. The label indicates there is a 4 ohm tap and a 16 ohm tap. The similar OT at this link http://www.hammondmfg.com/pdf/EDB1760K.pdf also has 4 wires, but black is common and the other 3 are for 8, 4, and 2 ohm taps. Unfortunately I couldn't find any helpful information regarding the installed OT, but at 1st glance I assumed the installer meant to have one of the speaker jacks wired for a 4 ohm load and the other for a 16 ohm load (I do realize this is strange since a SR typically has a 2 ohm load). But even if that were the case, I'm not knowledgeable enough to know whether the wiring is in fact correct for that objective (in any case, it doesn't look right to me) or to know how it should be for certain. As shown in the photo, the black and green wires are connected to the the tip lug of the main jack and the white and yellow wires are connected to the sleeve lug of the other jack. Absent proper documentation, is there a way to figure out the respective purposes of each of the four wires using my DMM? Any insight at all is appreciated.

    EDIT: In case anybody was wondering, there are no speakers currently installed in the amp, so no help there in determining what the intention was of the tech who put the OT in.

    - BL

    cbs_45w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

    img_1389.jpgimg_1388.jpgimg_1391.jpg
    Last edited by bobloblaws; 11-12-2017 at 09:35 PM.

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    Old Timer Leo_Gnardo's Avatar
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    Oh, yikes, a toroidal OT! One thing I've seen about toroidals, they have terrible overload characteristics. To be avoided in guitar/bass amps. In any case, if you believe black is common/ground, and you have green already soldered to the output jack's + terminal, then use any handy load resistor, send a sine wave thru the amp, dial up a low output just a volt or so on your green lead. Measure the voltage on your other leads, and that should help you sort out which is which as far as 2 - 4 - 8. If the previous tech had it "wired for 16" and there's no 16 ohm output, then it's likely he was using the 8 ohm output to drive 16, "close enough for rock'n'roll." Once you know which OT lead is which, you can wire the speaker jacks or put in a switch so the amp can be used appropriately.

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    Supporting Member mozz's Avatar
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    Send off a email to the company asking for specs.
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    Quote Originally Posted by Leo_Gnardo View Post
    Oh, yikes, a toroidal OT! One thing I've seen about toroidals, they have terrible overload characteristics. To be avoided in guitar/bass amps.
    Can you pleas explain what do you mean by that? What are the practical implications?

    Quote Originally Posted by Leo_Gnardo View Post
    In any case, if you believe black is common/ground, and you have green already soldered to the output jack's + terminal, then use any handy load resistor, send a sine wave thru the amp, dial up a low output just a volt or so on your green lead.
    I haven't made any assumptions and I actually haven't changed anything. As in the photo, both black and green are currently soldered to the tip lug of the main jack. Do you want me to un-solder all the leads save for the green one and solder the black one to the sleeve lug and then measure the voltage on the tip and then the remaining two leads?

    Quote Originally Posted by Leo_Gnardo View Post
    Measure the voltage on your other leads, and that should help you sort out which is which as far as 2 - 4 - 8. If the previous tech had it "wired for 16" and there's no 16 ohm output, then it's likely he was using the 8 ohm output to drive 16, "close enough for rock'n'roll."
    Again, the label seems to indicate a 4ohm and a 16ohm output, no 8.

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    Quote Originally Posted by mozz View Post
    Send off a email to the company asking for specs.
    Thanks, but that company doesn't seem to be a going concern any longer.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by bobloblaws View Post
    Can you pleas explain what do you mean by that? What are the practical implications?


    I haven't made any assumptions and I actually haven't changed anything. As in the photo, both black and green are currently soldered to the tip lug of the main jack. Do you want me to un-solder all the leads save for the green one and solder the black one to the sleeve lug and then measure the voltage on the tip and then the remaining two leads?


    Again, the label seems to indicate a 4ohm and a 16ohm output, no 8.
    Whups I mis read the lead/color locations.

    Possibly . . . there are two output windings. Wire in parallel for 4 ohms, series for 16. To find out, let's first unsolder the leads in pairs from each other, then check with your ohm meter function to see if you have continuity thru all 4, or not. If not, that points to the series/parallel setup.

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    Senior Member Enzo's Avatar
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    ATC-Frost became part of Standex Electronics in 2001. Try contacting them.

    https://standexelectronics.com/about/company-history/
    Education is what you're left with after you have forgotten what you have learned.

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    Quote Originally Posted by Enzo View Post
    ATC-Frost became part of Standex Electronics in 2001. Try contacting them.

    https://standexelectronics.com/about/company-history/
    Ok, the toll free number on that page is worth a shot. I had tried emailing standex@standexelectronics.com which I got from another page and the email bounced back.

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    Supporting Member Jazz P Bass's Avatar
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    Try this: info@standexelectronics.com

    It appears that they are based in Cincinnati, Ohio
    tel:+1.866.782.6339

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    Quote Originally Posted by Leo_Gnardo View Post
    Whups I mis read the lead/color locations.

    Possibly . . . there are two output windings. Wire in parallel for 4 ohms, series for 16. To find out, let's first unsolder the leads in pairs from each other, then check with your ohm meter function to see if you have continuity thru all 4, or not. If not, that points to the series/parallel setup.
    Oh, OK I think I'm getting what you are saying. If there is continuity between all leads that points to the black as a common lead and the others as taps of different impedances. In the alternative, the idea is that there is two 8 ohm windings which can be wired either series or parallel depending on the load? I'm thinking I could be wrong because I know it works that way with loads but I've never applied that concept to the transformer windings themselves.

    As it happens, there is continuity (act. 0.5 ohm on my meter) between the black and white and the same between yellow and green. Infinite resistance between yellow and black and same between green and white. Given that information, does the way the installer had it wired make any logical sense at all?

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    Quote Originally Posted by Jazz P Bass View Post
    Try this: info@standexelectronics.com

    It appears that they are based in Cincinnati, Ohio
    tel:+1.866.782.6339
    Thanks, I'll try giving them a dingle.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by bobloblaws View Post
    As it happens, there is continuity (act. 0.5 ohm on my meter) between the black and white and the same between yellow and green. Infinite resistance between yellow and black and same between green and white. Given that information, does the way the installer had it wired make any logical sense at all?
    OK that supports the two secondary winding theory. Looks like the installer had it wired for 4 ohms.

    About toroids not liking to go beyond clipping, afaik it has to do with the core material. The hi fi folks have tried it as you might well imagine. I read a report on it sometime in the early 2000's on Audio Asylum, but dam if I can search it out. I just remember thinking "so much for that idea" as far as running toroid OT's in guitar amps. Who knows maybe yours will prove to work better than expected?

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    Quote Originally Posted by Leo_Gnardo View Post
    Looks like the installer had it wired for 4 ohms.
    It might seem like I'm flogging a dead horse here, but can you expound? I'm not understanding the "why" and "how". (I get that we've determined there are two discrete windings, beyond that I'm unclear)

    And how would it be wired differently if 16 ohm was the desired load?

    Does the second (extension jack) still function in the same manner as the original Fender wiring?

    Thanks, Leo.

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    Old Timer J M Fahey's Avatar
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    I am quite certain that we have a tapped primary (that one is easy) and 2 not-connected secondaries, which in parallel will give you 4 ohms and in series, 16 ohms.

    Your picture shows secondaries in parallel, white+yellow being "ground" and black+green being "hot", in this case 4 ohms, since they are in parallel.

    Jack wiring is classic Fender, with the left one being shorting so the "main" speaker out and the right one being the "extension" out.

    Notice both "hot" are joined with a white/pinkish wire, and both "ground" are joined by being screwed straight to chassis, again classic Fender style.

    Secondary ground leads go to the extension jack, no big deal since they are in parallel anyway.

    Colours do not match original Fender but that means nothing, as long as proper phase is maintained ... which it should, or that amp would never have worked.

    at 1st glance I assumed the installer meant to have one of the speaker jacks wired for a 4 ohm load and the other for a 16 ohm load
    the idea is that there is two 8 ohm windings which can be wired either series or parallel depending on the load? I'm thinking I could be wrong because I know it works that way with loads but I've never applied that concept to the transformer windings themselves.
    Sorry, it does not work that way, and in any case they are in parallel, so ........ 4 ohm total it is, period.

    As of:
    Oh, yikes, a toroidal OT! One thing I've seen about toroidals, they have terrible overload characteristics. To be avoided in guitar/bass amps.
    "Advantage" is that having an ungapped core (which is a loooonnngggg strip of silicon steel wound around a mandrel which is then pulled) and a huge window (the hole in the donut) they have improved magnetic properties, larger inductance possible (which means reaching lower frequencies) , maybe (Iīm not sure about that) lower parasite inductance, which means reacing higher (ultrasonic) frequencies , and lower DCR so lower losses (huge window allows using thicker wire).
    NONE of those is an advantage in a Guitar amp (might be in a Bass one) and we have the HUGE disadvantage that ungapped cores are very easy to saturate with even minuscule primary current imbalance.
    Now in a DIY Audio type Hi Fi amp that might not be a problem, some I saw there use a tightly auto balanced DC servo bias, with Op Amps, MosFets, regulated supplies, etc. to take care of such matters, but a toroidal OT has no place in any classic Leo design.

    Yours may work, but you must make certain both power tube idle currents are balanced.

    At least, add 1 ohm 1W cathode current sensing resistors (each from pin 8 to ground) ande set Bias Balance pot so both read the same.

    Even better, measure cathode to cathode and adjust balance pot to read zero.
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    Quote Originally Posted by J M Fahey View Post
    a toroidal OT has no place in any classic Leo design.
    Yours may work, but you must make certain both power tube idle currents are balanced.
    At least, add 1 ohm 1W cathode current sensing resistors (each from pin 8 to ground) ande set Bias Balance pot so both read the same.
    Even better, measure cathode to cathode and adjust balance pot to read zero.
    Cool, thanks. It remains to be seen whether this OT will stay in, it's my buddy's amp and it will be his call ultimately. Good stuff to know in any case.


    Quote Originally Posted by J M Fahey View Post
    Sorry, it does not work that way, and in any case they are in parallel, so ........ 4 ohm total it is, period.
    Fair enough, but how does it work then? That's really the heart of the matter I'm trying to get to, for my own edification. What properties are inherent in the two windings that result in 4 ohm ->parallel and 16 ohm->series?

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    Old Timer J M Fahey's Avatar
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    They are windings, also generators, not speakers, loads.

    They generate power , not dissipate/sink it.

    Loads have resistance/impedance ; transformer taps do not

    To be more precise, they do have a little resistance: wire resistance, which if properly designed should be 1/10th to 1/20th the nominal impedance they are handling.

    Transformers by themselves do not have "impedance" by themselves , they reflect on one side what is present at the other side; they also "should" not dissipate any power, just fully pass tube power into speakers.

    After these different statements which in the long run mean the same:
    What properties are inherent in the two windings that result in 4 ohm ->parallel and 16 ohm->series?
    although we donīt daily use it in the OT area, maybe because of lazyness or tradition, is the concept that transformersī main job is to convert voltage, not impedance.

    That they also do it is just a happy byproduct of voltage conversion.

    Practical example of what is happening on that transformer:

    * each winding "should" be able to provide 12.6V RMS with amp just clipping.
    so
    *both windings in parallel (now itīs wired so): 12.6V available, into a 4 ohm load that means 12.6*12.6/4=160/4=40W RMS (I always round values to nearest significant one)
    and
    * both windings in series: 25.2V available so power is: 25.2*25.2/16=635/16=40W

    so now we understand why here we have only 4 or 16 ohm options, and not 8 (or 2 or any other), only voltages available are either 12.6 or 25.2 V RMS, period.

    While in a multi tapped winding such as the Hammond one we would have (always rounding values) :
    8.9 - 12.6 - 17.8 - 25.2 VAC which mean 40W RMS into 2 -4 - 8 - 16 ohms.
    Just do the Math yourself.

    As a side note, some expensive Hi Fi "universal" transformers have many (as in MANY) windings, both primary (they improve interleaving) and lots of secondary "voltage" windings, they are not even rated in ohms but each provides, say, 5V and 7V taps, and can be wired series parallel in multiple combinations so you can get practically any primary impedance into any secondary load and keeping excellent performance.
    Of course, that is expensive:
    this guy knows all about transformers, reading his entire site is well worth the time.
    No audiophool babble but good, solid, feet on the ground knowledge:
    index

    These old Geloso (Italy) PA amplifiers have each 4 secondary windings (so 8 terminals total), one can provide any impedance between 1.25 ohms and 500 ohms:





    Impedenze dīuscita: output impedances
    Linea ai morsetti: (speaker) line connected to terminals
    Unire tra loro i morsetti: join terminals together

    It also offers constant voltage, 100V lines, and if needed, some of those speaker outs are balanced.
    A truly universal amplifier.

    This one gives you any speaker combination between 1.8 and 16 ohms.
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    Holy crap, Jaun, this is awesome! I'll definitely study the info in your reply. So at first glance it appears to me that you based your calculations on the fact that the label on my torroidal OT indicated 4 or 16 ohm and that you happen to know that the Super Reverb is a 40 watt amp, is that safe to assume?

    Edit: Well I went and tried it out myself using my formula for power (P=(V*V)/R) plugging in 40 for P and 4 for R and indeed I got V = 12.6. So I answered my own question which was essentially, did you start with the premise that the amp is question is supposed to be 40 watts and work backward, so to speak.

    But I'm still unclear about why the two coils wired in the present manner constitute a parallel circuit. The fact that both leads of one coil are connected to a positive lug and both leads of the other coil are connected to a ground lug is throwing me off. And then what configuration would constitute series wiring in this case?
    Last edited by bobloblaws; 11-13-2017 at 09:00 PM.

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by bobloblaws View Post
    So at first glance it appears to me that you based your calculations on the fact that the label on my torroidal OT indicated 4 or 16 ohm and that you happen to know that the Super Reverb is a 40 watt amp, is that safe to assume?
    Edit: Well I went and tried it out myself using my formula for power (P=(V*V)/R) plugging in 40 for P and 4 for R and indeed I got V = 12.6. So I answered my own question which was essentially, did you start with the premise that the amp is question is supposed to be 40 watts and work backward, so to speak.
    Yes, that.
    "Educated guessing" (basing on having *some* of the data): yes.
    "Divination/Godīs Word" ... not in a Million years.
    But I'm still unclear about why the two coils wired in the present manner constitute a parallel circuit. The fact that both leads of one coil are connected to a positive lug and both leads of the other coil are connected to a ground lug is throwing me off. And then what configuration would constitute series wiring in this case?
    To avoid a special drawing, this PT one will do.
    Which is not *that* unrelated: an OT is a sophisticated PT, just that "Mains" comes from tube plates, in fact you have some 250/400VAC there, output voltage is 10/40 Volts and load is a speaker.
    Oh, and frequency is not fixed 50/60Hz but as wide ranging as 20Hz/20kHz.
    But the transformer "mechanism" is exactly the same.

    Parallel connection of 2 separate but equal voltage windings


    Series connection of exact same windings:


    which also clearly shows the optimum impedance change:

    12V and 2 Amperes mean: 12/2=6 ohms.

    24V and 1 A mean : 24/1= 24 ohms.

    Notice the 2:1 voltage change meaning 4:1 impedance change.
    Juan Manuel Fahey

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    Quote Originally Posted by J M Fahey View Post

    Parallel connection of 2 separate but equal voltage windings


    Series connection of exact same windings:
    Yes, this is what I thought, in fact I had drawn a couple of my own diagrams that were in effect equivalent to these. But the parallel diagram doesn't seem to jibe with the OT wiring on this amp, which we have been saying is wired parallel. For the sake of simplicity we can think of the two jacks as one since they are wired together in the traditional Fender configuration, as you have mentioned, with the tip lugs connected and the sleeve lugs connected through the chassis mounting. In your first diagram (parallel wiring), we can think of the two rightmost leads as one being connected to the tip lug and the other to the sleeve lug. In the diagram one end of the top winding is connected to tip and the other end to sleeve. Same for the bottom winding. Yet the photo shows that both ends of one winding (black/green) are connected to tip and both ends of the other winding are connected to sleeve. Can you see why I'm confused?

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    Quote Originally Posted by bobloblaws View Post
    there is continuity (act. 0.5 ohm on my meter) between the black and white and the same between yellow and green. Infinite resistance between yellow and black and same between green and white.
    Quote Originally Posted by bobloblaws View Post
    Yet the photo shows that both ends of one winding (black/green) are connected to tip and both ends of the other winding are connected to sleeve. Can you see why I'm confused?
    You are saying 2 different things here. If you had continuity from black to white, then that is a winding.
    If the windings were shorted out as implied in the second quote, you would get no output.
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    Quote Originally Posted by bobloblaws View Post
    Yes, this is what I thought, in fact I had drawn a couple of my own diagrams that were in effect equivalent to these. But the parallel diagram doesn't seem to jibe with the OT wiring on this amp, which we have been saying is wired parallel. For the sake of simplicity we can think of the two jacks as one since they are wired together in the traditional Fender configuration, as you have mentioned, with the tip lugs connected and the sleeve lugs connected through the chassis mounting. In your first diagram (parallel wiring), we can think of the two rightmost leads as one being connected to the tip lug and the other to the sleeve lug. In the diagram one end of the top winding is connected to tip and the other end to sleeve. Same for the bottom winding. Yet the photo shows that both ends of one winding (black/green) are connected to tip and both ends of the other winding are connected to sleeve. Can you see why I'm confused?
    You "see" it wrong.
    One winding goes from Black to White ; the other goes from Green to Yellow .
    Black (hot end from one winding) goes to tip together with Green, the hot end from the other winding.

    White (cold end from one winding) goes to tip together with Yellow, the cold end from the other winding.

    Which you had already detected:
    Quote Originally Posted by bobloblaws View Post
    there is continuity (act. 0.5 ohm on my meter) between the black and white
    So Black/White are opposite ends of the same winding.
    and the same between yellow and green.
    So Green/Yellow are opposite ends of the same winding.
    Infinite resistance between yellow and black and same between green and white.
    So Black+White and Green+Yellow define different windings.

    EDIT: now that I think of it, *maybe* you expected to measure 4 ohms between Black and White or Green and Yellow?
    It does not work that way.
    Last edited by J M Fahey; 11-14-2017 at 09:15 PM.
    Juan Manuel Fahey

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    JM and G1, I stand corrected, for some reason I was thinking I had earlier identified green and black as one winding and white and yellow as the other. It all adds up now, thanks!

    - BL

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