PT or OT ?

[crashtm1]

I originally thought this was a (Musicman HD130) OT I had on my stash.. I even marked 2 wires as 4/8 ohm...
When doing a search I've found that it may be a PT..

Schumaker... EIA606-814. Above that ..4-130
Any ideas???

[LooseChange]

The wire colors make me think it's an OT.
Blue - red - brown just maybe the primary.

[DummyLoad]

OT - PP

Blu & Brn are plates Red CT.

Yel & Grn secondaries
Blk common for secondaries.

it was made by woodward-schumacher.  maybe 1978 - 14th week?

[crashtm1]

Thanks  DL....

[plexi50]

OT i say!

[sluckey]

Let your ohm meter guide you. Put one probe on any wire. Now touch the other wires, one at a time. Twist together any wires that have continuity. That's one winding.

Now you have some wires left over. Put one probe on any wire. Now touch the other wires, one at a time. Twist together any wires that have continuity. That's another winding.

Repeat until all wires are accounted for. In this case, I'm betting that blue, brown, and red will be twisted together and black, yellow, and green will be twisted together. And the blue, brown, red group readings will probably be several hundred ohms, while the black, yellow, green readings will be less than 10Ω, probably more like a dead short. These are typical readings for a P/P OT.

D/L already nailed this one for you but the ohm meter can be used to learn a lot about any transformer. Checking winding resistance of several known PTs and OTs will give you a good feel for checking unknown transformers.

[RicharD]

Now inject a small AC voltage into the primary and measure Vout of the secondary.  This gives you the turns ratio = Vin/Vout.  Turns ratio squared times the speaker impedance gives you your primary impedance.  Now you know almost all there is to know about the transformer except it's wattage.  All I ever do at this point is guestimate based upon it's size as compared to a known transformer.

[Dave]

That brings up a good point Butters. Let's say you apply said small ac voltage to an unknown transformer (that happens to be a Twin Reverb OT), now, you do the math, and you find that it is either meant for 4 6L6's @ 4 ohms, or 2 6L6's @ 8 ohms.
Is there a better way than just comparing the size to determine the truth?
The reason that I ask is because I have come accross some transformers (especially lately) that are grossly below/above the normal size range.

Example: I have a pair of OT's that I harvested out of a stereo. They were PP 6V6 OT's and they were about the size of Super reverb OT's.
I have a PT that drove a quad of 6L6's and a handful of preamp tubes that is well over twice as big as a Twin Reverb PT.

I have worked on 2 mesa boogies lately that were rated at 45 watts but the OTs were the same size as Deluxe Reverb OT's.

I have played with a lot of Bogen CHB100's that are rated at 100 watts and have OT's that are much smaller than you would think.

When you eliminate everything else and get to the point where wattage is the only stone left unturned, is there no better way that to guess based on size?

Dave

[RicharD]

I'm sure there is but I don't know the answer.  I rarely come across transformers flying solo.  Usually they come from a derelict chassis which either still has the output tubes or is at least marked.  I remember PRR having a "rule of thumb" x ounces = y watts.  If it was a PT, one could measure the unloaded voltages and the DC resistance of the primary and then plug the values into PSU Designer II and come up with a close value.

[PRR]

Size of an OT is strongly related to lowest frequency of full undistorted response.

> OT's ...out of a stereo. ...size of Super reverb OT's.

Good hifi may be designed for 30Hz-20Hz and 2% THD. Guitar amps may be fine with 82Hz at 10% THD. Why pay more for notes you can't play? Too-clean bass may not cut-through small speakers in a loud tavern, colored bass cuts better. Hifis don't move, and were often judged by weight; stage amps have to be practical and portable.

> Bogen CHB100's ...OT's that are much smaller than you would think.

The last large Bogen tube amps were designed with a remarkably sharp pencil. They had better iron than was common in the 1950s. They are exactly good enough to meet their THD spec at 50Hz, no more. I used to carry 350 Watts of tube Bogen with just two arms, could carry 850 Watts on the hand-truck anywhere it had to go. Very right-size.

[foghornleghorn]

It is an OT for a Musicman 130. Look up the 2100-130 schematic on Schematicheaven. Blk is secondary common, grn is 4 ohm, yel is 8 ohm.