Output transformer and new 1/4 inch speaker jack

[Hebert]

Hi,
Got this output trans in an old PA amp. has many options for speakers. The two connections in the middle for the 8 ohm each have two wires. brown and black. would I keep each set of two wires and use them together as one wire when wiring in a 1/4 inch jack?

[jjasilli]

The CT goes to the amp's chassis Ground.  The matching ohm taps go to the speaker terminals. 

[Hebert]

The CT goes to the amp's chassis Ground.  The matching ohm taps go to the speaker terminals.

thanks meant to attach this photo too. shows the double wires on each post I spoke of.

[Hebert]

guess I'm confused now. is the c.t. the ground for all the speakers? the 8 and 16 ohm and so on. if so is the 8 ohm bracket in front referring to the brown and red pairs?

[shooter]

before you change anything take your meter and OHM from the CT screw to the chassis, should be 0 or real close

[Hebert]

set my meter to ohms and it never moved off of 1, set it to continuity and found continuity between the ct and each of the ohm posts and continuity between each pair of ohm posts but not between different ohms. example: 8 to 8 but not 8 to 16

[shooter]

example: 8 to 8

what color wires did that correspond to?

are you trying to bring it up and see if it works?

[Hebert]

8 ohms I think is the double red and the double brown located on both sides of the c.t.  I am hoping to change this into a guitar am and would like to keep and use the two transformers but the output transformer has so many wires I am having trouble understanding it.

[shooter]

so many wires I am having trouble understanding it.

I would "document" it before you do anything.  like do both wires soldered together go to the OT or does one of them wander off.  IF so, zip tie the soldered pairs together.

the secondary's are small ohms, my fluke can measure the differences, but marginally and years of doing it helps.

Take your time photograph EVERYTHING intact, as is now.  as it comes apart etc.

if you can get to the primary side wires you can inject a small AC signal on the speaker side and measure the primary side.  that will get you turns ratio, I gotta hunt up the instructions though.

[sluckey]

Look for a schematic before you do anything.

[shooter]

+1  spend time surfing.

the 25v and 70v tells me this NOT a typical OT, those are line out drivers for long wires n that's all I know 

you able to open windows word docs?  most of my "how two's" are cut paste to word

[jjasilli]

This OT seems typical for PA amps of the era.  See the Bogen Guide annexed, around p. 9  You can hookup a speaker: 

unbalanced: CT to one Ohm tap
balanced:  CT grounded; both equal ohm taps to a speaker


It wouldn't hurt to find a manual for you amp as sluckey says.


EDIT:  It occurs to me that the amp's rear panel may have antiquated pinout jacks as alternative spkr connections.  The additional wires may run to those other jacks.

[PRR]

> an old PA amp

"An" old PA amp? Which one? Is it a secret?

It looks odd to me. And I don't like how the lines do not line-up with screws. The inside colors don't give sufficient clues. You want to RTFM, or schematic. Which means knowing what model it is.

[PRR]

http://www.angelfire.com/vt/audio/TP45schematic.gif

[Hebert]

> an old PA amp

"An" old PA amp? Which one? Is it a secret?

It looks odd to me. And I don't like how the lines do not line-up with screws. The inside colors don't give sufficient clues. You want to RTFM, or schematic. Which means knowing what model it is.

It is a Webster Electric  PT45

[PRR]

Please use an ohm-meter. Remove any connections from the screws. Measure from chassis (scratch through rust) to any screw ("CT" is logical). Is it infinity (no connection) or is there a less than 100 Ohm reading?

If I read the plan right, and there has been no damage or hacking, the whole secondary winding "floats" (or can optionally be center grounded) which may aid system stability in a large school/factory. For a short (<40') line from a stage amp we can connect an 8r load to a metal jack as shown. Do nothing with the CT. Connect both "8" screws to the jack, either way. Assuming the winding floats, you may mount the jack in chassis, for strength and for safe grounding.

[PRR]

All the standard impedances are available. However not with simple switching. 2 and 4 may be wired real simple; 8 and 16 require more change. On paper it could be done with a 2P4T switch rated 5A, but this is an odd part, and my brain is not up to drawing the connections.

Do not use a Shorting jack. This amp is fix-bias and could hurt itself driven into a short. Do NOT put signal through the amp without a speaker (or dummy load) connected!! No-load and high signal can kick huge voltages in the OT. You might add a 100 Ohm 10 Watt resistor across the two "8" terminals to give "some load" when the operator screws-up. But this won't eliminate damage, just maybe delay it.

[Hebert]

All the standard impedances are available. However not with simple switching. 2 and 4 may be wired real simple; 8 and 16 require more change. On paper it could be done with a 2P4T switch rated 5A, but this is an odd part, and my brain is not up to drawing the connections.

Do not use a Shorting jack. This amp is fix-bias and could hurt itself driven into a short. Do NOT put signal through the amp without a speaker (or dummy load) connected!! No-load and high signal can kick huge voltages in the OT. You might add a 100 Ohm 10 Watt resistor across the two "8" terminals to give "some load" when the operator screws-up. But this won't eliminate damage, just maybe delay it.

thank you so much, you did so much work and I got the answer I was hoping for. I had no idea I could also get a 4 ohm out of it too. I am thinking of setting it up to handle three different speakers 4, 8, and 16 ohm

[shooter]

keep it simple, pick one then when you're jamming n smiling MODIT 

here's a quickstart for the switch PRR suggested, somewhat overkill, but I got bored 

https://www.surplussales.com/Switches/SWRoMisc-2.html

[Hebert]

keep it simple, pick one then when you're jamming n smiling MODIT 

here's a quickstart for the switch PRR suggested, somewhat overkill, but I got bored 

https://www.surplussales.com/Switches/SWRoMisc-2.html

thanks

[Hebert]

Please use an ohm-meter. Remove any connections from the screws. Measure from chassis (scratch through rust) to any screw ("CT" is logical). Is it infinity (no connection) or is there a less than 100 Ohm reading?

If I read the plan right, and there has been no damage or hacking, the whole secondary winding "floats" (or can optionally be center grounded) which may aid system stability in a large school/factory. For a short (<40') line from a stage amp we can connect an 8r load to a metal jack as shown. Do nothing with the CT. Connect both "8" screws to the jack, either way. Assuming the winding floats, you may mount the jack in chassis, for strength and for safe grounding.

are you using red as "-" and black as "+" ?

[sluckey]

Doesn't matter.