Stancor A3802 With Unknown Leads To Figure Out

[rp]

Ok, I got a Rubik's Cube for you all. Apologies to those who hate puzzles.

I got a Stancor OT A3802 off eBay to use with with some old coke bottle 6L6Gs and 5U4G I have in a yet undetermined project. Unfortunately the leads are unlabeled and the colors are pretty much faded. I took the bells off to get a look at where they're less faded and I put the colors (what I could make out) in the tables below. Maybe someone knows the Stancor color code?

I need to find the the CT and the common and determine that this OT is good overall. I've mapped out other unknown OTs and figured the turns with a Variac but I always knew which were the CT and Common. Here it's the reverse.

I scraped the leads shiny and then tinned them for good readings. With a Fluke 79 I measured open on all the taps btwn the two sections which is good. Then took resistance measurements btwn all the taps on each section. Readings are in the tables below.

I think got the secs figured out, luckily the common is still black enough. I labeled it A, and from the measurements I think the first column is correct and the sec taps are: B is 4 ohms, C 8 ohms, D 250 ohms, E 500 ohms. Correct? From the other measurements anyone see any problems on the secs?

Now on the primaries I can't suss out the CT. Also, I think the two primaries @ 3300 & 3800 are so close that it might be hard to differentiate them. Once I find the CT I can try with a few volts on a Variac to figure which is the high and the low taps. I also don't understand that very low reading of ~2.5 ohms btwn some of the taps. Is that reading from the 3300 to 3800?

You guys on AG haven't let me down yet. Sorry if this is a headache, I did my best to organized it to not waste peoples' time. Hopefully someone with experience can just look at it and solve it in a minute. I hope the OT is ok too - it sure would be styling in a 5D4 or 5C8 with ST tubes and maybe octal pres in an open hardwood hifi type chassis, and only old parts allowed inside.

[rp]

I think writing this post helped me solve it. That ~2.5 ohm reading was the clue I needed. Is it this?

[Cliff Schecht]

Yup that looks right. I just measure between the leads until I figure out what I'm looking for. This shouldn't take more than 5 minutes on a working piece of iron.

[marcoloco961]

My best guess for the secondary, .....see attached pic.

[rp]

My best guess for the secondary, .....see attached pic.

Hmmm... now I wonder. I went and put a variac on it and got this:

With 5.0Vac on sec A - C I got on the primary btwn A - C 109.1V
so: 109.1 / 5 = 21.82 x 21.82 x 8 = 3808.9

And with 5.0Vac on sec A - B I got on the primary btwn A - C 152.8
so: 152.8 / 5 = 30.56 x 30.56 x 4 = 3735.6

Looks like I got it right, can anyone else confirm?

FYI I got this too on the other set of primary taps:

5.0V on sec A - C got on primary btwn D - E 101.6V
101.6 / 5 = 20.32 x 20.32 x 8 = 3303.2

5.0V on sec A - B got on primary btwn D - E 141.3V
141.3 / 5 = 28.20 x 28.20 x 4 = 3195.5

I hope I'm getting this ok, feels right, but I'm a bit unsure now. Can anyone tell anything from the fact that sec winding B, the one I believe is 4 ohm, is composed of doubled-up solid cores? Does that tell how it's tapped and if it's the 8 or the 4 ohm?

[Phil_S]

I'm having a hard time following what you are doing...that's me, not you. You are keeping records.

My take on this is that you really need to find the outer ends of each winding first. This is done with the Ohm meter. The highest reading tells you the longest length of wire and so then you know the outer ends.

On the primary, your Ohm chart says outer ends are C and A.
On the secondary, your Ohm chart says outer ends are A and E.

For the primary, there are a great many turns, so the outer windings are longer than the inner windings. This leads to asymetrical readings, but the CT will be roughly in the middle and the difference between the two ends of the different primaries will be similar. In other words, if the winding is 200 ohms end to end, the CT will be at about 100 ohms to each end, but it won't be 100, more like 95/105. There is a similar relationship to the inner primary.

For the secondary, try to figure which of the two, A or C looks like the black lead and that is probably the common. From there, using about 10V on the variac into the outer primary, you should be able to get a good idea of the order of the leads. Write down the voltages on the secondary. That tells you the turns ratio x10. The impedance ratio is the square root of the turns ratio.

[lacrebob]

This is likely too late to matter to the orginal post, but I happen to have an A-3802 with the Instruction Sheet.

Here is what is on the Instruction Sheet:

Primary Connections:
PP (3,800 ohms)
Plate - Brown
B plus - Red
Plate - Blue

PP (3,300 ohms)
Plate - Red-Green
B plus - Red
Plate - Red-Yellow

Secondary Connections:
Common - Black
4 ohms - Brown
8 ohms - Red
250 ohms - Orange
500 ohms - Green-Yellow

[rp]

This is likely too late to matter to the orginal post, but I happen to have an A-3802 with the Instruction Sheet.

Never to late for solid data. This is on a shelf for future use, it's a 75W and I'm not sure what I'm going to do w/ it. A 4 EL34 Express when I go mad?!

My last problem was that the Orange and Brown after 50 years looked identical even under the bell. It's 'B' on the secs picture and for now I've concluded it's the brown and the 4 ohm. If you can kindly look at yours and tell me whether that stiff solid core doubled up lead on the sec is orange or brown that would be conclusive. Thanks.

[tictac]

Apply ac voltage to the primary (with a variac) to get one volts on the secondary

(example: if 25V on primary = 1 volt on secondary)

square this ratio 25:1 = 25 x 25 = 625:1

This ratio is used to calculate primary impedance:

With an 8 ohm speaker on the secondary the primary Z would be:

625 x 8 = 5000 ohms

[lacrebob]

Indoors, both the brown and black wires look black
In sunlight, the doubled-up wire looks brown

I have a second A-3802, and for both I would guess that the doubled-up wire is brown based on what I saw in sunlight.

[rp]

[quote="lacrebob"I have a second A-3802...[/quote]

BTW what are your plans for them? I'm sort of stumped, maybe a bass amp w/ a pair of 6550s / KT88s. I don't have a bass

[lacrebob]

or a stereo bass amp?

I don't have much use for that many watts and I don't have a bass amp.

Or a guitar amp where I want a lot of headroom out of the output transformer.