Safe Output Transformer Speaker Connections

moody_gaz · Post by **moody_gaz** » 15 Sep 2009, 11:30

What's the best way of wiring the speaker jack sockets (4-8-16 ohm) so that the OT will not fry if the speaker is left disconnected?. Would it be better to install a dummy load over a short circuit.
This is my first foray into analogue electronics since the early to mid seventies when I was an apprentice!
I have a little 5W tube amp that I'm going to modify (been reading Modmans topic).
Cheers for any help!

KindaFuzzy · Post by **KindaFuzzy** » 15 Sep 2009, 12:55

I usually use a shorting mono jack for the speaker, and connect a dummy resistor to the shorting bar. For a 5W practice amp, a 5W resistor should be fine. If you've got 4-8-16 ohm taps you'll have to use a 2p3t switch that switches the dummy resistors at the same time the impedance is changed, so the right dummy resistor is connected.

jt · Post by jt » 15 Sep 2009, 19:35

Another way to protect your OT is to solder one 270k 5 watt resistor from 16 ohm tap of OT to ground.This way your OT is protected even if your speaker cord fails or your Impedance switch goes wrong.

Fuzzer · Post by **Fuzzer** » 16 Sep 2009, 17:16

¿Could you please back up your former affirmation? Copared to 16 Ohms, 270k Is an open circuit. That is what we want to avoid.

lolbou · Post by **lolbou** » 16 Sep 2009, 17:26

Fuzzer wrote:Compared to 16 Ohms, 270k Is an open circuit.

No. Current does not flow in an open circuit. But it will in a resistor, even being low...

What are to avoid: shorts and open circuit. Anything inbetween will work provided you respect the OT power max ratings...

See the schem in my first post here viewtopic.php?f=2&t=4151&hilit=dissected+laney(laney lc15), it has this type of resistor...

JiM · Post by **JiM** » 16 Sep 2009, 19:13

Hmmm, it seems to me that the truth is somewhere in between ...

The protection resistor on the LC-15 is 220 Ohm, not 220 kOhm.
So it would let flow enough current to protect the OT in case of speaker failure, while being significantly bigger than a regular load.

lolbou · Post by **lolbou** » 16 Sep 2009, 19:23

Sold to the frenchman with the electrons around the head!

My mistake about the 220 ohm (I should have known this, I played my LC15 remake 5 minutes ago)...

jt · Post by jt » 16 Sep 2009, 19:29

Sorry,the 270k value was a silly mistake.270R is what i should have write in the first place.220R is appropriate value too.

dai h. · Post by **dai h.** » 17 Sep 2009, 12:56

I've seen a number of cases where apparent newbies would stick in an extra "k" when referring to a resistor (i.e. "820k" when referring to an 820 ohm R). Probably where that comes from. Good thing someone pointed this out and there was a correction. Could make a significant difference in some cases such as with this.