Just when I thought I had come to understand OT secondaries,,,

[pjd3]

Then this happens!

I"m sure there is a simple answer but, I sure don't have it tonight. I'd love to hear any of your takes on this little debacle I ran into with my Allen amps Heyboer TO35MT output transformer.

I am upgrading a RobRob Blackvibe 6L6 to a more complete blackface vibrato/reverb channel but without the vibrato (its tremelo, damn it!). Part of that entailed re-routing the OT tap wires to make room for installing a couple more 9-pin tube sockets. I noticed that I had a black wire, and a Yellow wire connected to the speaker output jack so, made the assumption that the yellow wire was for an 8 ohm tap, and that the black wire was the common.

I was unable to find my "build notebook" for this project plus I failed at finding specs on line for the output transformer. Allen amps no longer carries the transformers used in his amps and kits. No problem. I I'll just do some poking around with my Fluke DMM's, and determine the taps from DC ohm readings, as I had previously noted an expected correlational between the stated/rated impedance and a DCR measurement.

What I "thought" I had come to understand, and seen in action time and time again, was that in reference to the secondary common (which usually seems to be a black colored wire), for an increase in the rated/stated impedance, you should be able to measure a slight but notable increase in DCR.
This correlation to me was simple, that the highest impedance tap, whether it be an 8 ohm or 16 ohm tap, constitutes the entire secondary coil from end to end, and I would expect from common (usually a black wire) to the opposite end of the coil (whatever color that happens to be. So, it has always made good sense that this is the longest part of the coil, the whole coil, thus, would measure the greatest DCR. And that would just play out from the highest tap impedance to lowest, with a corresponding decrease in the DCR, less coil length, less DCR.

These are the 3 secondary wires on this TO35MT output transformer.
Yellow
Green
Black

Now, I know damn well as an R&D electronic fabrication tech that electrons don't give any crap about what color the pigment is in the wire's insulation. But, I was getting the highest DCR reading across the Green and Yellow wire, and not from the Black to the others as I would have bet my paycheck on being. I like to think that when I first built this amp, that I carefully identified the correct wires for common and 8 ohm tap (it was powering an 8 ohm speaker).

Is it possible that either the Yellow or Green wire is actually the common? I used two different meters, and even pre-measured a one ohm, and a ten ohm resistor, and added these in series to the taps to get away from any potential precision problems dowm near one ohm. All the results came out the same, with the notably greatest DCR coming from between the Green and Yellow wire.

If you've read this far, you are an angel. Thank you. But, my understanding of OT secondaries is out the window right now, and its pretty frustrating.

Thanks all for taking the time for this ridiculous find. If the black wire isn't the common, I'm sending someone a very compelling email. Naw, won't do that. But just very surprised at this - and that the amp at one time seemed to work fine, if not just a little clean.
Have a good night!
Phil D.

[maxkracht]

I would guess you made an assumption when putting it together (I know I do this way more than I should), or the manufacturer used the wrong color wire. I just looked at a Hammond with orange as the secondary common...

[sluckey]

Using an ohm meter to determine which secondary tap is which is futile because the DCR is so low. If you really want to know then put an AC voltage on the primary and then measure voltages across the secondary wires. Much more accurate. I would use a variac to put about 50VAC on the primary.

[pjd3]

I just did another sanity check, used both my Fluke DMM and then my Fluke benchtop just to up the confidence on what I was seeing and yup, in both cases the Green to Yellow have a DCR about 30% greater than Yel/Blk and Grn/Blk. To be specific:

Yel/Blk = .7 ohm
Grn/Blk - .7 ohm
Yel/Grn = 1.0 ohm

On the bench top meter the readings are a smaller value but retain the same proportion of resistances.

I emailed Allen Amps to see if they can send me the specs on the OT, particularly the wiring color code. We'll see what they come up with.

Thank for checking in here to help confirm that there is the possibility of odd color designations. I really don't know what else it could be based on my findings.

I'll get back to this thread as soon as I know anything else.

Thank you,

phil D.

[pjd3]

Hi Sluckey.

At some point I started to do that to any and all OT's that came this way, did the winding ratio's, impedance ratios, and then divided into primary impedances, 4K and 8K usually, just in case I even decided to change tubes or a different speaker. I realize the the DCR isn't very helpful per se but, if your meters are decent, it should be able to at least reflect consistent weight relations in terms of DCR - at least it always has played out that way in the past. I've always been curious about that when applying AC and calculating the impedance's for the taps. The DCR always followed the tap weights at least in terms of "increase/decrease". But maybe I will just cut some time and do the AC with this OT. I don't think I did on this one.

Thanks and I'll be back with the results.

Best,
Phil dl

[sluckey]

Of course the DCR is an indicator. But my Fluke 87V can't distinguish the difference in DCR between an 8Ω tap and a 4Ω tap. Can your meter? We are talking about fractional ohms. Meter leads distort the readings. Even if I null the leads with the relative button, the amount of pressure I use on the probes is a factor.

[bepone]

Is it possible that either the Yellow or Green wire is actually the common? I used two different meters, and even pre-measured a one ohm, and a ten ohm resistor, and added these in series to the taps to get away from any potential precision problems dowm near one ohm. All the results came out the same, with the notably greatest DCR coming from between the Green and Yellow wire.

it is possible, any wire color can be common,
Marshall JTM OT has Orange for common..with multimeter with 3 decimals so it is possible to find secondaries easy, confirming with some external small AV transformer to check the ratio.. 0.3-0.4 ohm, 0.5 ohm, 0.6 ohm are normal values

[Phil_S]

The assumption that DCR is a reliable indicator is not right. Sure, it gives some information. At best, it's a hint. If you know the DCR and other wire specs, you might be able to reliably estimate the length of the coil. The problem with using that information to determine impedance is that impedance depends on the number of turns in the coil. The inner turns require only a short length of wire. The outer turns require a longer wire. Attaching the primary (or the secondary) to an a/c power source will give you a much more reliable indication of the turns ratio and then you calculate the impedance ratio. While the impedance ratio (for purists) might not be the end of the determination, I suggest, for what we are doing, it is good enough.

Those Fluke meters are nice to have. They are better than most and considered very reliable. It's just that the information you find on a transformer coil, where the differences are very small, won't give you the answer.

Consider one more thing. We commonly see black as the primary. Green might be 4Ω and Yellow might be 8Ω. We expect there will be a midpoint take-off (essentially a shorter coil) for one tap and the other tap uses the full length of the coil. Your particular transformer might not be wound or configured this way.

In my experience, the simplest way to check the turns ratio is to use the filament winding as the low voltage a/c supply. You really do need to perform this test whether or not you obtain a reply from the manufacturer. There is no saying for sure that the insulation colors are reliable.

[martin manning]

Put some AC on the primary as suggested, and measure voltages on the secondary leads.
You have three leads, Y, G, Bk, so measure all possible pairings:
Y-G
Y-Bk
G-Bk
Which pair has the highest voltage? Say it's Y-G. One of those is the common and the other is the end of the coil, the highest impedance tap, and Bk is the lower impedance tap.
Recall the voltage ratio changes by the square root of the impedance ratio, and assume the two taps have impedance ratio two, 8 and 16, e.g.
Assume Y is common. What is the ratio of Y-G voltage to Y-Bk voltage?
- If it's 1.4, then Y is common, Bk is 8, and G is 16.
- If it's 3.5, then 'G is common, Bk is 8, and Y is 16.

[pjd3]

Thank you all for such informative and insightful responses.

I will pull out a 12volt transformer I have somewhere here and do the voltage/winding ratio/impedance measurements and calculations. I've done that on a few OT's and its always landed the results nearly dead on the usual tap impedance's. And I'll follow Martins suggestion of doing all the tap possibilities and look at the rations of voltage as he's laid out.

Finding the mid tap was the one that was eluding me. The outer taps are pretty obvious but it was the inner tap that I didn't know how to determine. Unless I was able to determine that , I couldn't be certain of which was the common and which was the 8 ohm tap. I did find some info that the OT is spec'd as having an 8 and 4 ohm tap with a 4K primary impedance.

I will follow up with this test.

Thank you again everyone for the great guidance and info.
Best,
Phil D.

[pjd3]

OK, my post of shame,

If you'd like to see:

VAC in: 13.39vac to primary (brown and blue wire)

Yellow/Black = .601 vac
Green/Black = .43 vac
Green/Yellow = .175 vac

So,
.601/.43 = 1.4
.601/.175 = 3.43

According to Martins values, my black wire is common, yellow is 8 ohms and the green is 4 ohms.
I did out the winding ratio- impedance ration and divided those by 4k and yup, it played out just like that.

As it turns out, I did have it wired up correctly when it was first built. It was hard to imagine screwing something like that up as I had all of the specs at that time.
So, my instincts were wrong based on DCR measurements, (I think). But, I had to ponder everything I was seeing, and correlations I had thought to witness in the past.
Thanks again all for seeing this through with me. I can confidently go forward with applying the right taps. And that sure is a good thing.
Good night!
Best,
Phil D.

[martin manning]

Nice!

[wpaulvogel]

Thank you all for such informative and insightful responses.

I will pull out a 12volt transformer I have somewhere here and do the voltage/winding ratio/impedance measurements and calculations. I've done that on a few OT's and its always landed the results nearly dead on the usual tap impedance's. And I'll follow Martins suggestion of doing all the tap possibilities and look at the rations of voltage as he's laid out.

Finding the mid tap was the one that was eluding me. The outer taps are pretty obvious but it was the inner tap that I didn't know how to determine. Unless I was able to determine that , I couldn't be certain of which was the common and which was the 8 ohm tap. I did find some info that the OT is spec'd as having an 8 and 4 ohm tap with a 4K primary impedance.

I will follow up with this test.

Thank you again everyone for the great guidance and info.
Best,
Phil D.

It’s a lot more revealing to put 100-120 volts on the primary and measure the secondary than such a low voltage like 12 volts.

[pjd3]

Sounds like a higher voltage would be more revealing.

I did do out the calculations for impedance ratio and then divide those into 4K and the results landed dead on the tap values (8 and 4 ohms) so, at least the identification was satisfied, which was mostly what I was after. Allen Amps did eventually come through with sending me the transformer specs and my findings were in line with the spec sheet.

I also in the past have applied the 12 volts to each separate tap and measured the primary. Seemed like the same thing just from the other side of the transformer but with perhaps applying voltages that are more in line with the secondary taps than the primary. They have both worked well to establish taps and ratios.

Thank you,

Phil D

[martin manning]

It’s a lot more revealing to put 100-120 volts on the primary and measure the secondary than such a low voltage like 12 volts.

Can you elaborate? For Phil's purposes the low voltage worked fine, and was very safe.