Identifying PT Primary and Secondary wires

[pjd3]

Hello, would sure like your thoughts on this.

Although I have ohmed out OT's primary's in the past for the sake of biasing using ohms law, I haven't spent much of anytime looking at PT coil resistances to see how they might correlate to secondary voltages, and how I would normally expect a linear ratio of V vs R in a transformer output or secondary. (And the primary for that fact).

The Bogner Alchemist PT that I am planning to "try" to use for the plexi project does not have spec sheets available as just about any other transformers would.
So all I can go by are illustrations and schematics that I find are somewhat vague, and not spelled out clear like "normal" schematics. I've done much internet searching and after hours of searching decided I'd better figure this out by my own testing and speculation.

There are 3 primary wires on the PT. Black, Blue and White. I get the largest reading of 1.9 ohms between the black and white wires, cluing me that they are most likely the line and neutral in. (or the outer taps of the primary coil). The Blue wire, which I would now expect to be a center tap or electrical ground of sorts measures quite differing values when ohmed out in respect to either Black or Blue (Black to Blue = 1.6 ohms, White to Blue = 0.5 ohms). Well, my meter is probably a bit skewed, it is an old digital Radio Shack but, 1.6 + .5 is fairly close to 1.9 so, I had to concur that the Blue wire is a tap somewhere between the Black and Blue primary wire.

Its getting late now and I'm getting frazzled but, in your experience is it customary to find primary "center taps" that measure such a large differing resistance to each end of the primary coil?

Thanks. I'm sure this is my lack of knowlege in PT's that caused such a surprise about this. I just wan't to make sure I'm not dealing with some "red-headed stepchild" of a PT.

good night and sleep tight!

Best
PJD3

[pompeiisneaks]

Hello, would sure like your thoughts on this.

Although I have ohmed out OT's primary's in the past for the sake of biasing using ohms law, I haven't spent much of anytime looking at PT coil resistances to see how they might correlate to secondary voltages, and how I would normally expect a linear ratio of V vs R in a transformer output or secondary. (And the primary for that fact).

The Bogner Alchemist PT that I am planning to "try" to use for the plexi project does not have spec sheets available as just about any other transformers would.
So all I can go by are illustrations and schematics that I find are somewhat vague, and not spelled out clear like "normal" schematics. I've done much internet searching and after hours of searching decided I'd better figure this out by my own testing and speculation.

There are 3 primary wires on the PT. Black, Blue and White. I get the largest reading of 1.9 ohms between the black and white wires, cluing me that they are most likely the line and neutral in. (or the outer taps of the primary coil). The Blue wire, which I would now expect to be a center tap or electrical ground of sorts measures quite differing values when ohmed out in respect to either Black or Blue (Black to Blue = 1.6 ohms, White to Blue = 0.5 ohms). Well, my meter is probably a bit skewed, it is an old digital Radio Shack but, 1.6 + .5 is fairly close to 1.9 so, I had to concur that the Blue wire is a tap somewhere between the Black and Blue primary wire.

Its getting late now and I'm getting frazzled but, in your experience is it customary to find primary "center taps" that measure such a large differing resistance to each end of the primary coil?

Thanks. I'm sure this is my lack of knowlege in PT's that caused such a surprise about this. I just wan't to make sure I'm not dealing with some "red-headed stepchild" of a PT.

good night and sleep tight!

Best
PJD3

On the primary side there are often only 2 leads, extras are for different wall voltage selections. in theory one may be for 120V the other for 130V or the like. The best way to know is to get a variac and get all the unused leads set aside and save, and then ensure the variac is as close to 12V as you can get. Then see what the secondary voltage of what you expect to be the B+ leads is, and it should be 10X less than what you wnat for the amp,
i.e. it may be at about 45VAC, that means you'd get 450VAC at 120.

You could then swap the leads and try the blue as well and see what you get there, etc. This is also a good way to test out what pairs may be the bias windings and heaters etc.

12V into heaters would end up with like .63VAC and then the bias side just depends on what the expected bias would be but for 50VAC or whatever, it would end up 5V (always 10X differences, if you're at 120 AND have the 120V winding).

It will give you some weird/off numbers if you choose the wrong windings to power.

The reason to do 12V is it's a hell of a lot safer to have 30to40VAC than 300 to 400

~Phil

[johnnyreece]

Is it possible this PT was for US/Europe? Maybe the amp had a switch for 120/220?

[sluckey]

The service manual says there were two transformers available for this amp. One PT has a 120v/100v primary for use in the USA or Japan. The other PT has a 220v/240v primary for European countries. The chassis has labels on it that identify which version you have.

***IF*** you have the USA/Japan model the primary leads are probably BLACK=120v, BLUE=100V, and WHITE=neutral. A very simple/easy way to verify which primary wires are which is to connect 6.3vac (either from a variac or from another filament transformer, or even from another amp) to a filament winding on your PT. Then assume the white wire is neutral. Connect your meter between the white and black wires. Should be 120VAC. And you should have 100VAC between the white and blue wires. Be careful and keep all wires separated from each other!

Here's a tip when using donor transformers that you have removed from working amps... Gather as much info about the transformer as you can BEFORE you start ripping stuff out. Identify how it was connected into the circuit. Measure voltages. Group windings together. (I just put a cable tie around primary wires, filament wires, etc.) I know your PT has multiple filament windings. Sort those with your ohm meter. Keep notes with the transformer.

[pjd3]

Thanks guys, what you all say makes a ton of sense. So yes, if its step down one way, then its step up the other way and I can use that to isolate, test and determine whats what.

The amp was bought at a Massachusetts Guitar Center and I've only used it in New England so, never had to consider if I even had a 220vac switch or wiring option.

I don't have a variac but I'm sure can come up with small AC voltages to test things.

Thank you!

PJD3

[pjd3]

I'll keep this short since this is not a dead cow I care to milk any longer. You know those days you should have just stayed in bed? Yup. Here we are.

So, rig 120vac up to the transformer in what certainly appears to be standard fashion. Check everything, all is safe, white to white, black to black, forgot to check the location of the blue primary which was "hiding" under the chassis edge. Volt meter on AC clipped to HV centertap (red/yellow) and to one of the red HV.
Plug it in, hear sounds coming from PT and read:

HV centertap to HV red wire "A" = 83vac
HV centertap to HV red wire "B" = 254vac
HV red wire "A" to red wire "B" = 337vac

Huh?

I find the Blue primary leaning on the grounded chassis - are you kiddin me? Nope, no kiddin. I haven't pulled a bonehead move like this in years, never have even caught myself putting a cap, diode or transistor in backward, and then miss this.
So, figured I just fried the primary of the PT? Reasonable deduction, right.
I ohm out all the primary and HT wires, expecting opens, and to my surprise, they are exactly the same DC resistances as I logged last night, down to the tenth of an ohm, on all primary and HV connections. I totally believed that the weird boiling/gurgling sounds from the PT were the coils melting and boiling the wire insulation. Well, if that were the case, wouldn't that have reflected in some change, any change to the ohmic resistance of these coils?
I do some math, and find that the ratio of measured resistance to measured AC voltage is precisely linear, nearly exact ratio of R to V as I would expect.
And on top of all that, I find that one of the AC power connector wires, I think the white neutral, wasn't even connected. It happens to be an odd ball configuration in the orentation of the power connector itself. I am now one baffled little boy.

I think I really need to just give up on this PT and buy a proper one with specs, history, fame!! If this were a reliable transformer wouldn't I have measured symetrical voltages on the HV wires? Like 330-0-330 (red-red/yellow-red)?
The reason I gave up on this amp is that the 2 channels began swithing back and forth like a machine gun, eventually with no break. As an experiment I pulled each tube out one by one and little by little the severity of the "automatic machine gun" channel switching became less aggressive with each tube.
I'm starting to think that this PT was gradually giving up the ghost and the amp was losing V potential, and showing strange symptoms.
Well I failed, didn't keep this short! Sorry guys. Thanks for your patience. Time to bite the bullet on the big "Chuang" PT.

Building my Fender Blackvibe was so clear without any surprises, everything working like expected. Ha. This amp really is a "Red-Headed Stepchild"
Thanks OK though, I have an order of Sozo snake oil caps coming in to finish off the plexi board. I bet those will be reliable.

Thank you,
PJD3

[pjd3]

Classic tone - 40-18016 -- There we go, that will do. Really a PT for Deluxe Reverb which is what Sluckeys Plexi's power section sorta is.

Keeping it simple - for an extra 66 bucks,

Thanks guys.

PJD3

[tubeswell]

I'll keep this short since this is not a dead cow I care to milk any longer. You know those days you should have just stayed in bed? Yup. Here we are.

So, rig 120vac up to the transformer in what certainly appears to be standard fashion. Check everything, all is safe, white to white, black to black, forgot to check the location of the blue primary which was "hiding" under the chassis edge. Volt meter on AC clipped to HV centertap (red/yellow) and to one of the red HV.
Plug it in, hear sounds coming from PT and read:

HV centertap to HV red wire "A" = 83vac
HV centertap to HV red wire "B" = 254vac
HV red wire "A" to red wire "B" = 337vac

What you think is the primary winding, might not be. Do any of the (other) windings measure 6.3VAC?

[pjd3]

Hey Tubesell, I didn't even get as far as measuring the 6.3vac windings. Looking back I should of checked but, if I wasn't seeing anything near reasonable, usable or expected out of the 3 red HV wires on the secondary, then I couldn't see the PT as something usable.

I would have liked to put a few volts on a secondary coil and see how and where it stepped up the voltage on the primary but, that was hard not having any good low voltage AC sources at home. It just made sense that it should have been OK putting 120vac across the highest ohmic value found across the primary cables. Perhaps I'll look into this PT more as time goes on but, I'm going to just pick up a PT that is well documented and makes some sense to me. This Bogner Alchemist as not a true Bogner made amp but a combination of Bogners design and Line 6's R&D, manufacturing and distribution. They used transformers and chokes made by "Chuang" with little or no documentation. I spent hours trying to find drawings, spec sheets, anything period, and came up with nothing on the Xformers. It was really just an attempt to save a few bucks on project #2 since I spent nearly $1000 on project #1. It won't kill to just order an PT that is well established and therefore give me the confidence to install and implement. I do use my amps for work so, I'll save that Chuang PT for something else once I understand more about it. It has bewildered me.

Thanks for stopping by, this PT will continue to bug me but hopefully enough to learn something from it!

Best,
PJD3

[pjd3]

just to quickly mention, per traditional there were 3 wires emanating from one side of the PT (Black, White and Blue) and a bunch of other wires coming off the other side of the transformer (3 Reds, 4 greens, a green/yellow, 3 other colors which are the 27 volts for the FX board).

Traditionally, if you see 3 wires coming out of one side of a PT that are Black, White and Blue, its hard not to imagine them as anything but primary taps, whether that be 120/220, or 120/100, or some well known international options. Even if the wrong pair was chosen, I would still expect to find a symmetrical voltage reading across the 3 red HV wires, as long as I chose the right pairing of primary wires that included a neutral. Perhaps there exist some convention that I"m not aware of. Maybe it was the Black and Blue that I needed to use (but that didn't make sense at that point. I chose the Black and White, connected to the Black and White 120vac wall cable that I stripped open for direct hookup to the "primary" wires of the Chuang PT.

THanks!
PJD3

[tubeswell]

... was hard not having any good low voltage AC sources at home. ...

You can make yourself a low voltage AC source with any old mains PT and a high-wattage (e.e. wire-wound) rheostat. PTs from discarded transistor amps are good for this because they have low secondary VAC to start with, and then you only need a small 25W rheostat (which is about $15 off ebay ).