Anyone know anything about wiring a Mic Input Transformer?

[Lotus 7]

Whew, A Masco MA-8 is beyond "vintage" and well into "antique". Welcome to 1940!

The Stancor A-4351 is a standard mic to grid transformer. You should be able to figure ot the windings and the input taps with a ohmmeter.

As you surmised, the Blue and white wires are probably the secondary (grid) output and the secondary common (ground).

To identify the primary winding taps, try this.

1. Use a digital ohmmeter and set it to its X10 range. When testing low level audio transformers, It's generally a good idea to try to keep the DC test current as low as possible to avoid causing any magnetic build-up in the core. Ohmmeters use the most current on the X1 ohms range, but the X10 range should have enough resolution for the task, so use it instead. If it's autoranging, don't worry about it.

2. Pick any of the primary wires and measure to each of the other wires. As you do that write down the dc resistance values between each pair.

For instance, if you picked the Black wire as your starting wire, you might have something like this:

Black to Black Red = 10 ohms
Black to Green/White = 50 ohms
Black to Black/Yellow = 30 ohms
Black to Green = 100 ohms
Black to Green/Yellow = 40 ohms

3. Do this again using another starting wire, and continue until you have measured all possible wire to wire combinations, AND you have found the highest resistance. Those two wires will identify the end points of the primary winding (which will be the 500 ohm input impedance) Although the DC resistance will be lower than 500 ohms.

4. Using one of the end point wires, measure to each of the other "tap" wires to determine the tap order.

Given the common wiring color coding of the day, I'd make a very rough guess that the Blue is the secondary grid with White as the common.

For the primary, I'd guess that the solid black is one end and solid green is the other with the taps arranged as:

Black = Low side

Black/Red = lowest Z input
Black/Yellow = next highest Z
Green/Yellow = Next highest or possibly a center tap
Green/White = next highest
Green = Highest rated input Z

Again, that's just a pure guess, but if that were the case, the DC resistance would increase as you went from step to step, with the lowest reading between Black and Black/Red, and the highest reading between Black to Green.

A few minutes with an ohmmeter and a pencil and paper, should give you a clear understanding of the tap order.

I'd expect a much higher reading on the Blue to White secondary, with obviously an open circuit between either of those and any of the "suspected" primary connections.

Hope this is clear and hope that it helps.

[robanimator]

Hi Lotus, THANKS!

Let me study your reply, lots of info, but understandable. I need to see if my Radio Shack Ohm meter has X10 range..

So if I start with black and measure all around on that side, my goal is to find wires with 50, 200, 500ohms right? Or close to it I bet..

Let me try in the morning..

Thanks again,
Rob

[Lotus 7]

Originally Posted by robanimator

Hi Lotus, THANKS!

Let me study your reply, lots of info, but understandable. I need to see if my Radio Shack Ohm meter has X10 range..

So if I start with black and measure all around on that side, my goal is to find wires with 50, 200, 500ohms right? Or close to it I bet..

Let me try in the morning..

Thanks again,
Rob

Your goal is to find the pair of wires which have the highest resistance reading between them. The DC resistance (which your ohmmeter measures) is not the same as the transformer winding impedance. They will be roughly proportional (on the same transformer winding, a higher impedance winding or winding tap will have a higher resistance) but again they are not the same value.

The 500 ohm, full primary winding, will have the highest DC resistance reading, but that resistance reading may actually be only 50 or 100 ohms. The resistance reading is the simple DC resistance of the wire in the winding, the (AC) impedance is a much more complicated factor that includes many parts of the transformer design and also can change with how the transformer is being used. The secondary loading will change the primary impedance, but won't change the DC resistance. The true AC impedance (and inductive reactance and capacitive reactance) can be measured with an impedance bridge, or a "LRC meter" but that's something most people don't have lying around, and is not necessary in this case.

The rated input impedance of a transformer is ONLY achieved when the loading on the secondary windings is a known, design value. The transformer you're using is designed to feed a vacuum tube grid circuit which might possibly have a 100K ohm grid bias resistor. In other words, it's designed for a very high impedance load. If that secondary was connected to a low impedance transistor circuit, the apparent input impedance (the load that a connected mic would "see") would be much lower than 500 ohms.

Again, to start, you're just trying to find the combination of leads that gives the highest resistance reading (it won't be 500 ohms, and will probably be much lower).

[robanimator]

Hi Lotus,
The results are in..

Using A cheap radio shack Ohm meter on '200' (non X10 switchable)

Black- green (Over Load)
Black - Black/Red 10.2
Black - Black/Yellow 3.6
Black - Green/Yellow (Over Load)
Black - White/Yellow (Over Load)

Green - Black/Red (Over Load)
Green - Black (Over Load)
Green - Green/Yellow 7.0
Green - Black/Yellow (Over Load)
Green - Black (Over Load)

Black/Red - Green (Over Load)
Black/Red - Black 10.2
Black/Red - Green/Yellow (Over Load)
Black/Red - Black/Yellow 6.6
Black/Red - White/Yellow (Over Load)

Green/White - Black (Over Load)
Green/White - Black/Red (Over Load)
Green/White - Green/Yellow 4.2
Green/White - Black/Yellow (Over Load)
Green/White - Green 11.2

Black/Yellow - Green (Over Load)
Black/Yellow - Black/Red 6.6
Black/Yellow - Black 3.6
Black/Yellow - Green/Yellow (Over Load)
Black/Yellow - White/Yellow (Over Load)

Green/Yellow - Black/Red (Over Load)
Green/Yellow - Green 7
Green/Yellow - Green/White 4.2
Green/Yellow - Black (Over Load)
Green/Yellow - Black/Yellow (Over Load)

Can you gather any info from this?

Thanks in advance!
Rob

[robanimator]

Check out this image, I wonder if there is any meaning in the way the wires are intertwined..

[Lotus 7]

Originally Posted by robanimator

Check out this image, I wonder if there is any meaning in the way the wires are intertwined..

Don't see any link to an image, but the resistance readings are clear. Your transformer actually has (3) independent windings.

One is Black to Black/Red with a tap at Black/Yellow

One is Green to Green/White with a tap at Green/Yellow

What is the resistance reading between the White and Blue Wires? Now thet we have (3) windings, it's important to be surethat the White to Blue set is the secondary. It's most common to have tapped primaries, but no taps on the secondary, but measuring the resistance will confirm thet this is correct,

Hang in there, we're almost there!

[robanimator]

White to blue is the secondary, the come out from the other end, away from the 6 others. The label says,
Primary:500 C.T./333/200 C.T./125/50?
Secondary: 89,000 ?
Ratio 1:13.3

[Lotus 7]

It's actually not unusual to have multiple input windings on a mic input transformer that can be used individually or can be used together to create different impedance options.

Lot's of old mics came in different impedance models (Some old Neumanns could be internally "strapped for different impedance operation and some old Shure dynamics even had a screwdriver switch to select for (2) or even (3) impedance settings. for

[robanimator]

Re:What is the resistance reading between the White and Blue Wires?

Switching the Ohm meter to '20K' I get a 2.88

[robanimator]

"Primary:500 C.T./333/200 C.T./125/50?"
This didnt come out as I typed it..

Primary:500 C.T./333/200 C.T./125/50-OHM

The question mark was an Ohm symbol

Rob

[robanimator]

The schematic of the amp says the unit ships with a 50/200/500 Ohm input transformer..

Rob

[Lotus 7]

Originally Posted by robanimator

White to blue is the secondary, the come out from the other end, away from the 6 others. The label says,
Primary:500 C.T./333/200 C.T./125/50?
Secondary: 89,000 ?
Ratio 1:13.3

That's actually very helpful.

It appears that the transformer has dual primaries that can be used independantly or in series to match different impedances. That does slightly complicate things a little, because to get the highest input impedance (with the primary windings in series) they have to be connected "in phase" so teh drive signals sum instead of subtract from each other.

One primary is the Green to Green/White tapped at Green Yellow (that's probably the higher impedance primary (500/333/200) and the other primary.

The other primary is the Black to Black/Red tapped at Black Yellow. That appears to bethe lower one (125/50)

Considering the color coding, to get an input impedance of 500 ohms (which may be OK for most new electronically balanced mics, but is low enough that it's not ideal), I'd try the following:

Connect the low signal input to the Green wire.
Connect the Green/White wire to the Black wire
Connect the "high" signal input to the Black/red wire.

No connections to the Green Yellow tap
No connections to the Black/yellow tap

When "mutually coupled " transformer windings are connected in series, the actual impedance you wind up getting is not the sum of the individual winding impedances. You don't have to be concerned with the theory, but that's the way it works with transformers.

To really be sure of the phasing you can try switching one of the winding polarities.
As:
Connect the low signal input to the Green/White wire.
Connect the Green wire to the Black wire
Connect the "high" signal input to the Black/red wire.

The signal level should drop about 5 or 6 dB. If it goes up 5 or 6 dB, then leave it connected that way. If it drops, revert back to the orginal connections.

The secondary impedance is rated at 89k ohms which is appropriate for a grid driven vacuum tube stage with a input bias resistor of 100K or higher.

[Lotus 7]

Originally Posted by robanimator

The schematic of the amp says the unit ships with a 50/200/500 Ohm input transformer..

Rob

You can hook up the other windings individually to get the lower input impedances, but unless you have a old mic with a 50 or 250 ohm output transformer, they will be of no practical use.

[robanimator]

Hi Lotus,
That is a lot for me to wrap my brain around, whoh!

I'm smart but to a certain extent..

I want to wire a 3-pin XLR connector to the amp, for 500 Ohms..
Then have a second jack for the day I get a 50 Ohm ribbon mic like an Altec 639A..

Can you be more specific for wiring? Like on the XLR, "solder BLANK colored wire to pin 3, then solder BLANK colored wire to pin 2" Etc..

Also what type of connector should I use for the 50 Ohm mic like an Altec 639A?

Thanks,
Rob

[robanimator]

What do you mean when you say 'tapped'..

Ie. "One primary is the Green to Green/White tapped at Green Yellow"

Thanks!
Rob

[Lotus 7]

Originally Posted by robanimator

Hi Lotus,
That is a lot for me to wrap my brain around, whoh!

I'm smart but to a certain extent..

I want to wire a 3-pin XLR connector to the amp, for 500 Ohms..
Then have a second jack for the day I get a 50 Ohm ribbon mic like an Altec 639A..

Can you be more specific for wiring? Like on the XLR, "solder BLANK colored wire to pin 3, then solder BLANK colored wire to pin 2" Etc..

Also what type of connector should I use for the 50 Ohm mic like an Altec 639A?

Thanks,
Rob

OK:

To set this input transformer up for use with a 500 ohm dynamic mic:

This configuration places the two primary windings in series to increase the input impedance.

1. Solder the BLACK transformer wire to the female XLR pin 3
2. Solder the GREEN transformer wire to the female XLR pin 2
3. Solder together transformer wire BLACK/RED to transformer wire GREEN/WHITE and insulate the splice.
4. Connect the female XLR pin 1 to your chassis ground.
5. Be absolutely sure that all the remaining primary wire ends are fully insulated and not touching anything in the chassis. Alternately, they could be soldered to an insulated terminal strip.

This configuration uses only the low-Z (50 ohm) input winding.


For use with a Altec or Western Electric 639A (which has a rated output Z of 40 ohms:
1. Remove (cut) the splice connecting BLACK/RED to GREEN/WHITE.
2. Solder the BLACK transformer wire to the female XLR pin 3
3. Solder the BLACK/YELLOW wire to the female XLR pin 2
4. Connect the female XLR pin 1 to your chassis ground.
5. As above, be absolutely sure that all the remaining primary wire ends are fully insulated and not touching anything in the chassis. Alternately, they could be soldered to an insulated terminal strip.


It would be possible to wire up two XLR's for the (2) different input impedances, but is not a good idea. The connectors will be cross coupled and the splice connection between the two windings will have to remain connected all the time. That could lead to potential noise pick up through the unused connector. It's also possible to install a multi-pole switch, or even use a relay to provide tha impedance change switching, but for now you're probably better off just doing a little re-soldering.

For the 500 ohm configuration only: Again, the testing you've done so far does not confirm the polarity of the two input windings, so you should still try it with the connections of the GREEN & GREEN/WHITE reversed to see which works best. The change will be dramatic - one way will be much louder. This does not apply for the 50 ohm configuration since you're only using one of the primary windings.


Re The "tap":

You can think of a transformer winding (primary or secondary - it doesn't matter) as a long coil of wire. For the example lets let the coil have 1000 turns of wire on the coil.

The ends of the wire are the winding ends and an AC signal connected to those ends will "see" a certain impedance. Sometimes this may be written as "The signal will be loaded by a certain impedance. For this example let's say the signal "sees" a load of 100 ohms when connected to the ends (1000 turns) of the coil.

Now lets strip the insulation off a tiny section of one of the turns of wire on the coil, and a third wire is soldered to that point. Let's call that a "Tap". Let's place the "TAP" on the 250th turn from one end of the full 1000 turn coil.

If we now apply the AC signal to the Tap and the winding end closest to the Tap so that the signal is only seeing 250 turns instead of 1000 turns, the impedance of the winding will drop to something like 1/4 of what it was originally. Now the signal "sees" an impedance of 25 ohms.

In fact a single coil as mentioned above (with one tapped winding) can actually function as a step-up or step-down transformer. If an AC signal of 1 volt is applied to the 250 turn section through then"tap", and a voltmeter or oscilloscope is connected across the 1000 turn section of coil, the meter or 'scope will read 4 volts. This kind of transformer doesn't have an isolated primary and secondary and is called an auto transformer.

Your mic transformer has several windings that are insulated from each other, but are magnetically coupled together. (That's what transformers do). the various "taps" allow all or part of each winding to be used and the impedance the source signal "sees" depends on how much of those windings are connected, and how they are connected.

Hope this is understandable.

[Lotus 7]

Re: The 639A mic. Those originally used a thing called a "442" connector. I believe "442" connectors are nearly impossible to find. If you have a cable with one on one end you are very lucky. If so, just put a male XLR on the other end.

I've heard of 639A's being converted to use male "flange mount" XLRs and it looks like an easy job, although I've never seen one in the flesh.

Possibly posting a "WE 639A connector" post might bring some specific suggestions from someone who has done it or has a converted mic.

[robanimator]

Wow Lotus_7, thank you so much! Literally, I am not worthy!!

Let me dig into that...Looks like a slam dunk!!

Rob



Your genius mind is unmatched!!

[robanimator]

Hopefully that little green bouncy character I chose means he is JUMPING FOR JOY!!

[robanimator]

Hi Lotus,
(I'm back from a small weekend trip)

Now that I'm thinking about the possibilities of this input transformer, can I use it to hook a lo-Z mic up to this amp that is designed with a hi-z input like this one?

Vintage 1951 David Bogen Guitar Harp Amp | eBay

That amp appears to have hi-Z mic inputs..

If so, it would open up other possibilities for me..

Thanks,
Rob

[Lotus 7]

That David Bogan amplfier has the (typical for the era) single pin Amphenol connector and probably has an input impedance of 100K or more. It should be a good match for the 100K secondary winding of your Stancor transformer.

You may want to mount the transformer in the chassis so it's grounded properly. and so you don't have to use the Amphenol connector. Those are probably the worst designed mic connectors ever devised by man - nothing but trouble! The amp on eBay seems to include a Amphenol to 1/4 inch phone plug cable. That's good since the single pin Amphenol connectors can be hard to find, but I'd still avoid using it if you can.

Occasionally similar amps might have a 47 k ohm input resistor, but 100K or even higher is more typical. You can check by tracing the connection from the center pin of the mic connector to the nearby tube. there may be a series capacitor to the grid pin and a resistor to a ground connection. If you're not familiar with resistor color codes, there are plenty of on-line information sources. A 100k ohm resistor will have a color code of brown-black-yellow with the fourth band being either silver, gold or blank.

Usually, amps like that one don't have a lot of excess gain and they almost always have simple AC filament circuits, so are prone to having a little hum (depending on the quality of the tubes). Don't expect pristine, noise-free sound from such a beast. The one in your link claims to have had its electrolytic caps replaced (a good thing), but the rest of the caps are all 1950's era, waxed paper caps which are far from the most reliable. A amplifier using those can be a reliability risk, and a leaky (electronically, not physically) coupling cap can shift the tube bias enough to really increase distortion and shorten the tube life. (Just so your warned about what you're getting into.) I've worked on amps using the same components that were only 20 years old in which the wax/paper caps were beginning to fail. At 61 years old you can certainly expect to have some problems.

Also, if you're not going to actually drive a speaker with the amplifier, but want to record or otherwise capture its tone and distortion "character" you must be sure to load the output transformer with an appropriate "dummy load, or you'll risk damaging the output circuits. For a push-pull 6V6 amp like that one use a combination of wire-wound resistors with a combined resistance of 8 or 16 ohms and a power rating of 15 or 20 watts. Do not operate it without a speaker or a "dummy" load attached.

Messing with "antique" gear like this is exactly like trying to drive around in a 60 year-old vintage car. People who do it don't expect the car to always get them from point 'A" to "B" like a new Honda would, They half-expect to be surprised with some kind of a failure every time they take the old vehicle for a ride. That's part of the "fun"!!

I'm certainly not trying to discourage you, but you should be aware of what you're getting into.

Addendum: The "phono" input on the RCA jack will be "compensated" for a phono equalization curve. There were several mono curves in use in 1951 (FFRR, CCIR,NAB, RIAA, Columbia, Victor, and others), so there's no telling which one it uses. Anyway, don't connect your mic transformer there. That input will have a severe loss of highs for a "flat" source signal.

[robanimator]

Thanks Lotus, this thread has become my personal vintage mic/tube/input transformer encyclopedia, you literally can't imagine. And you have made me smarter and more empowered! Now I can do the things I want to do..

Thanks a trillion fold,
Rob

[Lotus 7]

Originally Posted by robanimator

Thanks Lotus, this thread has become my personal vintage mic/tube/input transformer encyclopedia, you literally can't imagine. And you have made me smarter and more empowered! Now I can do the things I want to do..

Thanks a trillion fold,
Rob

Glad to contribute to your "education" and delighted that you want to learn about this stuff. There are many today who just expect to be able to download an "App" to do anything they want to accomplish.

One other little detail that those of us who work with vacuum tube gear regularly take for granted is to be aware of, and to be careful of the high (lethal) voltages involved and the possibility of charge stored in the filter capacitors. If you are planning on probing around in any line operated vacuum tube circuits and aren't very experienced, it's a good idea to wait 5 or 10 minutes for the charge to drain off, or if you're in a hurry, to carefully discharge the filter caps with a 1K ww resistor before sticking your fingers into the chassis. A 50uF cap with a 275 V charge can deliver a nasty shock or a big spark if you accidentally short it.

Take care, be safe, and above all have fun.

[robanimator]

Hi Lotus,
Now that I totally get this input transformer thing, it opens a world of mic transformers and antique tube amps that are equiped with Hi-Z mic inputs (minus the mic input transformer).

Going back to my previous question above I asked:
"can I use it to hook a lo-Z mic up to this amp that is designed with a hi-z input like this one? Vintage 1951 David Bogen Guitar Harp Amp | eBay"

You mentioned:
"Occasionally similar amps might have a 47 k ohm input resistor, but 100K or even higher is more typical. You can check by tracing the connection from the center pin of the mic connector to the nearby tube. there may be a series capacitor to the grid pin and a resistor to a ground connection."

But some amps I have looked at with Hi-Z mic inputs have a resistor from the hot mic pin to the tube that is 1 MEG.. (1,000,000 ohms [I think..])

So my question is, can a mic transformer like mine be a good match for that with a secondary winding of only 89,000 Ohms?

Thanks in advance!
Rob

[robanimator]

Let me rephrase my question:

Can a mic transformer like mine with a secondary winding of only 89,000 Ohms be a good match for that type of amp that has a 1 MEG resistor between the mic and tube?

Thanks,
Rob

[Lotus 7]

Originally Posted by robanimator

Let me rephrase my question:

Can a mic transformer like mine with a secondary winding of only 89,000 Ohms be a good match for that type of amp that has a 1 MEG resistor between the mic and tube?

Thanks,
Rob

Absolutely, If anything, the transformer will be "happier" with the "less" loading that the 1 meg resistor provides. The 89k (or 100k) secondary winding impedance is really at the upper end of what is practical for iron-core input transformers. Internal winding capacitance becomes a serious problem for transformers with higher turns ratios (higher output impedance), and the 89k (or 100k) winding will already provide plenty of signal voltage, so noise is not an issue.

As long as the resistor is 100k or more, it will work fine.

In a typical VT input circuit, the resistor is just there to provide a charge path for the input coupling capacitor. If the circuit is properly biased, there should be no current flowing through that resistor, so it's really the only load the transformer "sees".

Some "PA type" amps were designed to be used with either dynamic or crystal microphones. The "old" crystal mics work best with very high (1 meg) input Zs.

Addendum: The resistor is not "technically" "between the mic and tube". The transformer connects directly (usually through a series capacitor) to the tube grid and the resistor shunts the grid to ground. Saying "between the mic and tube" can imply that the resistor is in series with the grid which would not be the case.

[robanimator]

Hi Lotus, you are soooo the right guy to talk to!

Then comes the smaller transformers like this one..

Bogen tm-200 input transformers
primary: 200 ohms
secondary: 50k ohms

What scenario are these a good fit? Is it dictated by the resistor?


Thanks,
Rob

[robanimator]

Hi Lotus,

Sorry to burden you..

I had these input transformers on Ebay to sell because I didn't know what they were for, but now with my new education, I took them off Ebay..

I got them when I purchased a Collins 12Z mic amplifier.. I removed them to restore the unit to stock before I sold it..

The guy who sold me the Collins unit described the input transformers as this: "these input transformers give balanced inputs for the XLR inputs".. That was about it..no further description.

For close inspection of these input transformers, see the Ebay page I made with the close photos of the terminals and the ohms specs..

MIC INPUT TRANSFORMER MFP Part C-280-A TF1A11YY Audio | eBay

Also, here is a quick retyping of the data printed on the sides..

WATTS-0.006 RESPONSE -400-3000±3DB
TERMS. 1-2-3 = 0-250-4000 ohms
TERMS. 4-5=150 ohms TERMS. 6-7-8=600 ohms C.T.
TERMS. 4-5=50 MA. D.C. MAX

What can I do with these? Can they be place in front of my Stancor input transformer to make a balance XLR for a 50 ohm mic like an Altec 639?

I noticed that the frequency response is 400-3000 and I like the mid-range effect this might have..

Let me know what you think..
Rob

[Lotus 7]

Rob,

As you noticed, the frequency response of the transformer is limited to the voice or "communications" range of 400Hz to 3k Hz. That's done on purpose with communication gear to eliminate background noise and to improve voice intelligibility. It's probably not even very flat over that range. If you pass music through it, chances are it will sound like you're listening to music through a [very nice] telephone. No bass, no real highs and a slightly distorted midrange. If that's what you're looking for, great!

Generally, it's not common practice to directly drive a transformer from another one, because there is no good reason to do it (You would simply use a single transformer with the correct windings for the impedances you were trying to match), BUT there is no reason why it won't work). It is not the most efficient way to go since each transformer does "lose" a bit of the signal energy, but that's not a major issue here.

Since you seem to be "into" wanting to experiment with the different sounds that can be had using old vacuum tube amplifiers and old transformers, you might want to consider getting a few small chassis boxes (like plain old BUD miniboxes) and mounting a transformer or two, in the box with balanced XLR connectors connected to each respective transformer input and output windings. (Two transformers, two female XLRs for the inputs and two female XLRs for the outputs or even three outputs - use separate output connectors for each secondary winding) and a couple of XLRM to XLRF patch cords).

You could then play around trying different combinations of multiple transformers in series. Because transformers can sound "different", depending on how they're driven and terminated (source impedances, load impedance, frequency dependence of those impedances, etc.), connecting two slightly different transformers in series may have a different sound depending on the order they're connected. Putting multiple transformers in a signal path won't hurt anything, and although it's not a recommended practice if you're looking for the lowest distortion, widest bandwidth, and least phase shift, it is something that can be used as a tool if you're into sound design.

With your "new" mic transformer, I'd stick with trying the 150 ohm, 250 ohm,and 600 ohm windings and avoid the 4K terminal.

Just remember, the "sound" you will achieve with those parts is not going to encourage the Chicago Symphony Orchestra or the Berlin Philharmonic to be knocking on your door to ask you to record their violin sections.