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Author Topic: Transformer wired into 6x15 Speaker cabinet...trying to figure out Impedance..  (Read 3095 times)

gbloomer

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Okay, This is my first post on this site, heres what im dealing with

 so recently bought an old Sears 250xl cab off of eBay. I haven't been able to find out a lot about this other than it was originally sold with the Silvertone 1426 250w Solid State head. I am wanting to pair it up with my Silvertone 1485 tube head, but before i hook them up together i want to make sure the Impedance matches up so i don't blow either the head or the speakers. I checked over the cab and and there is no labeling showing what ohms the cab is. Since i couldn't find any schematics or info on it i decided to check out how the speakers were wired up to figure out the impedance myself. But when  took the speakers out to look at the wiring i saw something ive never seen before there is transformer wired into the speaker circuit as. Ive looked over the transformer and there is no labeling or serial numbers on it. nothing. If anyone could help me figure out the impedance to this cab it would be very much appreciated!   






galaxiex

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Use an ohmmeter to measure the cab total resistance at the input jack or wires.

This won't get you the actual AC impedance but will give you the DC resistance of the cab.
Usually it is "close enough".

Example; I measure an "8 ohm" labelled speaker with my ohmmeter and get anywhere from 6 to 9 ohms depending on the speaker.

.... but you may want to wait for more experienced members to chime in.  ;)
I don't want to be responsible for something bad happening to your vintage amp.
Fear leads to Anger, Anger leads to Hate, Hate leads to Suffering.

phatt

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Remove the transformer > rewire each side in Series (3 x 4 Ohms = 12 Ohms)
Then wire the two sides in Parallel
2 x 12 = 6 Ohms total load.
Run that from an 8 Ohm Tap of Valve Amp.
The transformer is likely a Choke to wipe of excess treble,, part of a crossover network that may have been used at some point,, you don't need it for guitar.

No time today but I'll draw it up later if you need it. :tu:
Phil.

gbloomer

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Remove the transformer > rewire each side in Series (3 x 4 Ohms = 12 Ohms)
Then wire the two sides in Parallel
2 x 12 = 6 Ohms total load.
Run that from an 8 Ohm Tap of Valve Amp.
The transformer is likely a Choke to wipe of excess treble,, part of a crossover network that may have been used at some point,, you don't need it for guitar.

No time today but I'll draw it up later if you need it. :tu:
Phil.


Okay I thought about doing something like that, but the the Silvertone 1485 OTs only have a 4ohm and 2.66ohm taps. so i dont think i could do that with out changing out both OTs

J M Fahey

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1) disconnect all those speakers, you will have to anyway to wire them properly, so do it now.
2) measure each speaker voice coil on its own, connected to nothing else, what do you get?
3) are sure they are 4 ohms each? Only the above test can confirm it.
4) if those speakers are actually 4 ohms each, you are out of luck mating them with your *weird*  Silvertone, which requires 2 separate parallel sets of *8*  ohms per speaker, 6 in total (3+3) ... what is actually shown on the schematic.
« Last Edit: June 27, 2016, 03:30:38 PM by J M Fahey »

Enzo

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It will not harm or stress the amplifier at all to run that 6 ohm load on the 4 ohm tap.

Loudthud

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Three parallel "branches" of 8 Ohms results in a load of 2.66 Ohms. So connect three pairs of 4 Ohm speakers in series, then connect those series pairs in parallel.

Is the Silvertone 1485 head the one with four 6L6s and two output transformers? That won't work well with this cabinet because you need two loads of 4 or 2.66 Ohms each.

Vitrolin

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you could use 4 4ohm speakers in series/parallel getting 4ohm load for one output, and the two remaining speakers in parallel making a 2ohm load, guessing that 2ohm is close enough to 2,66ohm.

or decide that 6 i almost the same as 4, having a parallel pair in series with one speaker.

you could also wire the OTs in parallel, then it should be loaded with 2ohm or 1,33ohm deending on OT tap.

OTs wired in series would demand a load of 8ohm or 5,32ohm depending on tap

dont try to mix and match OT taps

this might help:
http://www.silvertoneworld.net/schematics/silvertone_1485.pdf

Enzo

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J M Fahey

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I STILL doubt speaker impedance, because as shown in the schematic he seems to have measured in-circuit, that´s why I ask to disconnect and measure speakers separate from anything else.

That said, IF speakers are 8 ohms each, they can be used with that Silvertone; if they are actually 4 ohms each, short answer is: forget it, they are not compatible.

Please measure so we know what you have.

phatt

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Hard to read but 4 Ohms is written on the back of the bottom 2 drivers.
Phil.

Vitrolin

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Hard to read but 4 Ohms is written on the back of the bottom 2 drivers.
Phil.

when i zoom, alot i see that all 6 says 4ohms,

so new speakers or fun with paralleling.



in theory you could also take 2 paralleled speakers in series with a speaker and a 4ohm paralleled
remember heatsink and a adequate power rating for that resistor.
not shure how i would work tonewise.



 

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