Blackheart BH112 16 ohm, turn to 8 ohm?!

[Renegadrian]

Right on, I got myself a Blackheart Little Giant Stack, so it's the 5W head and the 12" cab. this is 16 ohm at 30W.
Now it works good with the amp obviously, as it was born to match it! Just connect one of the two 16 ohm speaker jacks to the 16 ohm amp out and it kicks ass. Nice feature that the amp has several outs so you can hook it to any kind of cabs!

I think I am going to build some kind of mini amp soon, think murder one or similar, and I already have the tranny (which is 8 ohm) - but the cab is 16! I read you can put a big resistor in parallel with the speaker to cut its impedance in half, would it work!? Say a 10W 16 ohm paralleled with the speaker would give 8 ohm?! I guess the resistor rating of 10W would be ok with small wattage amps like the above mentioned...Am I right?! I guess some heat could be generated, but maybe just a little as the low power of those amps and the W rating of the resistor...

Would it be also possible to put a switch to engage/disengage the resistor, to go back and forth 8/16 ohm?! That would be easy to wire accordingly I think...

[Renegadrian]

Else I could just have that resistor in an enclosure and connect it to the second input...
As the cab has two parallel inputs, connecting a 16 ohm (speaker/load) will give the same result, yes?!

[anchovie]

You can run an 8-ohm amp into a 16-ohm speaker without risk. It's if the speaker impedance is lower than the output transformer's that you run the risk of blowing up the amp.

[Renegadrian]

Yeah I read that...just  a different performance...but as a 10W resistor is € 0.26, and I think I can wire it externally via the second jack, why not!? That way I'd get close to the 8 ohm...no?!

[anchovie]

It's not a reactive load, so it would come with the same caveats as using a power resistor for attenuation. To some people's ears it makes the tone flat.

[Renegadrian]

'k James, thx for the replies, I guess that I am gonna try anyway as it's cheap and non intrusive...

[CynicalMan]

You can run an 8-ohm amp into a 16-ohm speaker without risk. It's if the speaker impedance is lower than the output transformer's that you run the risk of blowing up the amp.

I thought it was the other way around, because running an amp with a shorted load doesn't cause immediate failure, but an open load can make the OT arc.

Edit: Nvm. 2:1 seems to be fine. Going higher than that is risky. On the other hand, mismatching down seems to be OK to a certain extent as long as you rebias the tubes. And it won't blow anything up, it'll just stress out the power tubes.
http://aga.rru.com/FAQs/technical.html#imp-1
http://www.ax84.com/bbs/dm.php?thread=104395

[petemoore]

 Current increase = heat increase, overcurrent for long enough [might not need to be that long] can burn output.
   Increasing resistance of the transducer coil decreases current through the output amplifier.

[PRR]

> I thought it was the other way around, because running an amp with a shorted load doesn't cause immediate failure, but an open load can make the OT arc.

Depends on the amp.

Most amps will tolerate 2X/0.5X "center" impedance.

Many transistor amps "center" on 8 ohms, so 4 ohms is already using most of the allowance. However they make more power in lower loads, so we work them at 4 ohms.

Older transistor amps died if shorted. (Those days are long gone.)

Most tube amps do not make more power in lower ohms, so they have the taps.

Cathode-biased tube amps will NOT be harmed by a short.

The highest-power fix-biased tube amps can cook if run shorted for some time.

Most tube amps can ZAP their OT if run UN-loaded with large signal. "UN-loaded" means more than 2X or 4X nominal load. A 2X load is ample to absorb the voltage kick-backs.

A shorting jack is a common way to ensure that cathode-bias amps won't ZAP for simple forgetfullness. This does not save you for failing to plug the other end into a speaker, broken wire, busted coil, etc.