Super Reverb Replacement OT Wiring

[bobloblaws]

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

[Leo_Gnardo]

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.

[mozz]

Send off a email to the company asking for specs.

[bobloblaws]

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?

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?

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.

[bobloblaws]

Send off a email to the company asking for specs.

Thanks, but that company doesn't seem to be a going concern any longer.

[Leo_Gnardo]

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.

[Enzo]

ATC-Frost became part of Standex Electronics in 2001. Try contacting them.

https://standexelectronics.com/about/company-history/

[bobloblaws]

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.

[Jazz P Bass]

Try this: info@standexelectronics.com

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

[bobloblaws]

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?

[bobloblaws]

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.

[Leo_Gnardo]

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?

[bobloblaws]

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.

[J M Fahey]

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 (I´m 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.

[bobloblaws]

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.

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?