Speaker hook-up to PA Amp???

[jjasilli]

How are the speakers correctly hooked-up to these odd secondary taps??? After tune-up the amps I'm about to put my Stromberg Carlson APH-1100's back into service.  Schematic below.




I remain confused about correct speaker hookup.  I know to use a 25V tap, because in this 100W amp that's equivalent to 6.25 Ohms (per Ohm's Law and the Manual!!). 


*  Which 25V tap?
*  Where does the other speaker wire go: to GRD or to CT???
*  Should CT be connected to Ground?


Thanks for any help here!  (The Manual is of no help)

[jjasilli]

So what I'm thinking is this:  the secondary connections are GRD 70V  25V  CT 25V  70V.  So connecting:


70V to 25V on the same side = 45V (maybe useful at about 20 Ohms for a 16 Ohm speaker)
70V to CT = 70V
70V to 25V on the opposite side = 95V (rather useless)
70V to 70V on the opposite side = 150V (rather useless)


25V to CT = 25V = 6.25 Ohms (@ 100W) Bingo!
25V to 25V on the opposite side = 50V (rather useless @ 25 Ohms)


EDIT:  based on PRR's post down below my voltages, above, are all wrong.  This, from the schematic, is correct nominally:  70  25  CT 25  70.   I.e., the schematic is telling us where to put the speaker wire leads, on either side of the CT, for 25V (6.25 Ohm) or 70V operation.  This confused me, because the correct recitation of actual voltages is 35.5  12.5  -0-  12.5  35.5.    Now it makes sense.  Also, per Ohms Law, or more appropriately the Power Formula W = V² / R, for a 100W amp, a connection from the nominal 25V tap to CT yields about 1.6 Ohms.   

***************
So then, 2 lines of speakers can be used: one line off each side of the OT secondary???
Choosing this:  25V to CT, should the CT "float", or attach to GRD???

[PRR]

BLANK-OFF the "115V" outlet. Aside from being dangerous, the cross-connect to the 70V leaves me uncertain. Do not cut the wires to the 115V; they are used for the NFB.

> 25V to CT = 25V

No. It is 25V BALanced to CT. 12.5V on either side of CT.(*)

"25V" to "25V" should be 100W in 6 Ohms, 78W in 8 Ohms.

With the "115" blanked-off, and NO jumper "115V" to "70V", you may ground any one other terminal on the 25/70V winding. For standard speaker work, ground one of the "25V" terminals and call the other "hot".

From one "70V" to "CT" is 35V which would suit a 12 Ohm load.

From one "25V" to "CT" is 12.5V which would suit a 1.5 Ohm load (OT losses may be high).

Under-loading (load impedance too high) will reduce power but will do no harm. 50% overload (load impedance 0.67 of nominal) will be OK in speech/music application. (That suggests 8 Ohms on the "70V" to "CT" connection.)

> 2 lines of speakers can be used: one line off each side of the OT secondary???

No. Connect two 8 Ohm speakers parallel (4 Ohm total) at "25V" to "25V". This is a mild overload (under-impedance), power output will rise insignificantly, tube abuse rises, but it is clearly designed to drive a 115V motor at FULL power steady, so speech/music signals won't cook the beast.

The "High Pass Filter" is 2dB down at 280Hz. This is not useful for anything except driving honky-horns.

(*) Think about a "6V CT" heater winding. It is 6V end to end, 3V end to CT. In unlikely situations (long-long runs with low-level cables), you ground "CT" to balance the speaker-line so it throws less signal to other lines. We never do this on-stage or in small auditoriums.

[PRR]

If you plan to clip it more than slightly, I suggest a 470 Ohm 5 Watt resistor in each screen lead. The screen current rises abruptly when the plate is bottomed. Some resistance here blunts the abuse.

[trobbins]

If you measure the turns ratio of primary half-winding, or total winding, to a secondary then you should be able to reverse engineer the designed impedances.  If the OT secondaries are taken to terminals, and winding sections are separable, then you may even be able to improve on the use of the windings for standard speaker impedances.

This may help:
http://dalmura.com.au/projects/OT calcs.xls

[kagliostro]

Many thanks for sharing that file Tim

Franco

[mresistor]

It says in one of the notes that 115v can be removed/enabled by a jumper from 70v to ground at the output terminals.

My question is: Why would you want 115v out of a Public address amplifier? and why would you drive a motor with it? A motor in what?

[P Batty]

A motor in a siren (burglar alarm)...

[jjasilli]

Thanks PRR, that info is dead-on and hard to come by. 


The amps are for home theater / mid-fi; not for guitar, and clipping.


@trobbins:  thanks for that technical info, but I was hoping to avoid testing procedures.


@mresistor:  "Why would you want 115v out of a Public address amplifier?"  The short answer is that higher voltages need less current to do work - in this case driving many speakers over long lines.  Less current  yields a smaller voltage drop across this extensive speaker load, and the load of the long hookup leads themselves.  It also means that thinner, less expensive hookup wire may be used.


The long answer:  The ancient history of PA amps is a walk through the weeds.  The problem is how to power a long line of many speakers.  Say, a long line of 30 speakers in a school building of classrooms, offices  and large common areas like hallways & lunchrooms. 


Different wattage speakers might be needed to be heard in different-size areas. The various speakers might vary in impedance ratings.  The combined impedance of the speakers is huge. The long length of the hookup lines also adds-up to  significant resistance which needs to added to the total impedance of the load of all the speakers.  This total could be many hundreds of Ohms. 


As shown in trobbins's table "Sometimes a PA amp provides secondary winding impedances (eg. 40, 60, 100, 150, 300, 600Ω)."
That's how the problem was first addressed; and a complex math formula was needed to reconcile the actual speaker + line load to the closest tap on the OT secondary.  Line matching transformers -- with a plethora of input and output impedances, possibly using "reflected impedances" to get both sides to match -- might have to be selected to use with individual speakers, to get the whole contraption to work. 


This evolved to "constant voltage outputs" which alleviates the former complexity. Now the selection of line matching transformers, along with hookup to the amp's output, is obvious.  Just pick one with the right "voltage input", and a secondary impedance that matches its mated speaker.  Constant voltages became standardized at 25V, 70V and 115V.  The reason for 115V has already been stated; it works better with very long lines, due to less current and voltage drop. "Constant voltage" is a misnomer, because the voltage does change with the strength of the amp's output.  The nominal constant voltage is true only when the amp is actually performing at its rated output. 


With the advent of electrical codes, 115V fell into disuse, because it violates the codes.  By law, that level of voltage may not be run through exposed leads, without conduit, etc. 

[jjasilli]

@mresistor: re "motor"  I believe PRR is referring to the electro-mechanical portion of a speaker driver: the voice coil & magnet assembly is often called a motor.  The line of speakers driven by the amp could be considered as one electric motor, driven @ 115V (if that tap is used) and consuming up to 100W (the rating of this amp). 

[trobbins]

You can ball-park the secondary winding impedances by estimating an rms voltage swing across the OT primary - the schematic shows B+ level, and the 5881 datasheet will give saturation voltage, and the nominal preferred PP load impedance.  With a PP rms voltage, and the secondary tap rms voltages, then you can get ballpark secondary impedances.

[mresistor]

Aha,,,   lots of info, thanks for providing. ;-)

[jjasilli]

You can ball-park the secondary winding impedances by estimating an rms voltage swing across the OT primary - the schematic shows B+ level, and the 5881 datasheet will give saturation voltage, and the nominal preferred PP load impedance.  With a PP rms voltage, and the secondary tap rms voltages, then you can get ballpark secondary impedances.

Yes, but without PRR's help it still wasn't clear to me how to realize the correct secondary taps to actually use on this amp, due to the arcane nomenclature used on these OT taps, which is supposed to be helpful, but is factually misleading.  See my corrected Reply #1 above. 

[trobbins]

Philips and AWA PA amps in Australia generally align the idle B+ voltage (eg. 340V) with say a "100V" OT secondary using a 'PP to output' turns ratio of eg. 3.4.  I've seen that on 5 PA amps. 

A 100Vrms output would then require a 340*1.4 = 480Vpk signal across PP, or 240Vpk signal across each half-primary winding, which is a reasonable design aim given B+ sag at full load, and output valve saturation voltage, and OT winding DCR.

For the APH-1100, the B+ is about 635V.  Using the above rule-of-thumb, if the 70V winding had a 635/70=9.1 turns ratio with the PP winding, and a 5kohm PP was used (6550 datasheet for 600V B+ with 100W output), then the 70V winding would nominally be designed for 60 ohm impedance.  So practical speaker impedance matching would be 16 ohm speaker across '70-CT', or 8 ohm across '25-25'.  If you could split the 70 V winding at the CT, then you could rewire those winding segments in parallel rather than series, and hence get a better winding utilisation for the 16 an 8 ohm output.

I wouldn't say the drawing was factually misleading - you had made an incorrect assumption that CT = 0.

Ciao, Tim

[PRR]

> Why would you want 115v out of a Public address amplifier? and why would you drive a motor with it? A motor in what?

When you bring a recordplayer from England to America, it runs fast because of the 50Hz 60Hz power difference. One hack is to drive a power amplifier with a 50Hz sine wave so the UK motor feels at home. (The voltage difference is easily made-up with a simple transformer.)

When you copy films to NTSC video, the frequencies do not work out unless you drive the projector at 59.9Hz. Then you can get frame-sync, and nobody notices the slight change of speed.

There is a big post-production house in NYC and they had racks of 300 Watt Bogen tube amps driving their transfer projectors. One caught fire. They bought new transistor amps. I scooped-up the Bogens, fixed the smoker, and had what was at the time one of the bigger PA systems in the county.

Tape recordings sometimes came in at the wrong speed; weak batteries, defective governor, gummy bearings. For proper playback the dubbing recorder was run variable Hz/speed with a large power amplifier.

To voice-sync film to tape the tape player is run on a variable-Hz power amplifier to follow slight changes in camera speed.

Many early music production special effects were based on variable speed.

Yes, electronic speed controled motors and computers have made much of this only historically interesting. Which is why I feel safe in advising that the "115" outlet be blanked-off.

________________________________________

> "Why would you want 115v...?"  .... driving many speakers over long lines.  .... less expensive hookup wire may be used.

Sigh

115V is NOT a standard speaker-line voltage. It IS standard (nominal) wall-outlet voltage.

In US Code, speaker lines may run 70V in inexpensive cable. Higher voltages must be run according to power-wire code, MUCH more expensive.

In Europe 100V speaker lines are common, not 115V.