Output transformer wire colour coding

MtB

I have a Weber 5e3 kit that I built a few years ago. The output as per their standard schematic has a green wire providing an 8 Ohm connection to the output socket.

I would now like to use a 16 Ohm speaker. There are 4 wires in total exiting the O/T: Green, Yellow, Orange and Black. Is there a convention (that I've not yet found on line), as to which colour is which tap from the O/T. In other words, which is my 16 Ohm connection?

In fact, what makes things more confusing now I have visited the Weber VST website is that some of (their own) transformers have the green being 4 Ohm  and the 8 Ohm being yellow!! 

Thanks

ecc83

Now, first off. Do NOT  tell me you are forking about with amplifiers but do NOT have a digital test meter? You are NOT telling me that? Good, so.

You know the common colour and the "hot" 8R wire. Connect the meter to the common wire and on Ohms short the other test probe to that tag. The meter will read the test leads loop resistance, usually 0.2 to 0.5 Ohms.

Now test each colour in turn making sure to get as good a connection as possible (maybe sharpen the probes?) and note the lowest resistance found.

As you might have surmised by now, common to 4 R will be the lowest reading and 16R the highest.

BTW. If you have a totally unknown traff you can feed heater, 6V into the speaker side and measure the voltages at the primary so finding the ratio(s) . Bit of Ohms law and transformer 'rithmetic gives you the loadings and thus the valves it would suit.

Dave.

ICBM

What Dave said.

What you should find is some very low resistances, so you will need to measure carefully, but you should get something like 0.4 ohms, 0.6 ohms and 0.8 ohms from the three other wires to the black. The lowest is 4 ohms, the highest (which should be twice the resistance, not four times, since impedance ratio is the square of the turns ratio) is 16 ohms. The green should be the middle one if that's the 8-ohm and theoretically should be 1.4 times the 4-ohm.

Sometimes the numbers will be closer because cheaper meters often have a zero error which is in the same range, but the lowest to highest order will still be the same.

MtB

Dave, thanks for the response. I do have a digital test meter, but I'm not an electrical engineer (apprentice trained mechanical eng) hence the reason for my original post. I wasn't sure whether it would be possible to test the different loops with a meter - perhaps I should have asked in the op.

I have also emailed Weber, an whilst they responded yesterday (which is impressive in itself), none of the O/Ts that they describe is the one that they included in my kit!
That said, I'very been using the amp with an 8 Ohm speaker for the past 5 years, so I guess that the green wire is the 8 Ohm tap.

I'll get the meter out when I get home and report back.

ICBM

MtB said:

That said, I'very been using the amp with an 8 Ohm speaker for the past 5 years, so I guess that the green wire is the 8 Ohm tap.

Not necessarily .

Contrary to popular 'wisdom', it usually does no harm at all to run a valve amp mismatched by a factor of two in either direction, so it would have worked fine even if that was either the 4 or 16-ohm tap.

You can't even necessarily assume the black is the 0 connection on all transformers - although it probably is. But what you will find is that the two wires with the highest resistance between them are the 0 and the 16, and the two with the *next* highest are either the 4 and the 16 or the 0 and the 4 (these should both be about half the highest), so the reading from either of those to the remaining wire will conclusively identify the order.

MtB

So I could save myself some soldering and just directly fit my 15 Ohm Celestion Blue?

ICBM

Probably - although it's not a good idea if the green wire is actually the 4-ohm tap...

Also, while it's *safe* to mismatch by a factor of two in either direction, it's not ideal - you always get the most power and fullest tone with a correct match.

MtB

Understood.

Thanks

MtB

How hum. Weber have just confirmed that the O/T is not one of theirs. But are trying to identify it.

ICBM

MtB said:

How hum. Weber have just confirmed that the O/T is not one of theirs. But are trying to identify it.

It doesn't matter much if you work out the order of the tap wires. If there are three it's a certainty that they will be 4, 8 and 16 since no-one makes 100V Line OTs these days, and 2-ohm is only for specialist applications like 5F6-As and Super Reverbs.

exocet

If its not one of their own transformers then it is likely to be one of the following

Heyboer

http://www.mojotone.com/amp-parts/Transformers_1/Tweed-Deluxe-Output-Transformer-w-4-8-16-OHM-Secondary

MtB

Christ, another Senior moment for me!

Fortunately I keep all my email records of equipment that I buy. So early in 2012 I bought a replacement O/T from Tube Amp Doctor in Germany (and they keep the specs on their web site). 

Crisis over...