Matching ohms

[Spivonious]

Here's my dilemma:

I have a 16 ohm 4x12 and an 8 ohm 1x12. I also have the new Vox AC4TV head, which has a built-in attenuator, and is marked for 16 ohm output.

Is it safe for me to use the head with the 1x12? I really don't feel like lugging the 4x12 to the jam session tonight.

[Endrik]

16 ohm amp into 8 ohm cab isn't a good idea

you can buy a 16 ohm speaker for the 1x12 cab

[Spivonious]

Does the built-in attenuator make any difference? I always thought they presented a fake load to the amp.

[Endrik]

Nope, it's for the watt levels

[Spivonious]

Can you explain why it would hurt the amp? I could see the amp running a bit hotter than normal, since it's drawing more current, but I'd be running at a theoretical twice the wattage (8W). At such low wattage levels, is heat even a concern?

I suppose I could always take a speaker out of the 4x12 to put in the 1x12 for tonight.

edit: Am I making any sense? My numbers might not be right, but the lower load would cause the tubes to work harder. Higher load would cause current to backflow into the transformer which can burn up the transformer.

With that said, I find it hard to believe that a modern amplifier can't handle a 1 step mismatch.

[marcus]

Most amps will be able to handle a 1/2 to 2x difference in loads without a problem. And yes, the attenuator does present a load to the transformer. The most likely scenario is that it presents a 16 ohm load to go in parallel with the 16 ohm speaker cabinet, thus resulting in an 8 ohm load. Putting an 8 ohm output on there will be a 5.3 ohm total, which is more than 1/2 of the load using a 16 ohm cabinet. You will probably be OK here. Does the manual say anything about this?

[Spivonious]

No, it's really short manual. The only line about the speaker output is in the section describing the back panel.

1. EXTERNAL SPEAKER Jack -

This is used to connect the AC4TV to any 16ohm cabinet.

Note !

When an external cabinet is connected, the internal speaker is disconnected.

(FYI - the AC4TV sounds awesome through a VOX V412BN 4 x 12" Cabinet!)

WARNING !

Always use a non-screened VOX approved speaker lead with

the AC4TV Amplifier and extension cabinets.

WARNING !

NEVER operate this amplifier without the loudspeaker load
being connected. Serious damage can occur otherwise.

If it helps, here's the amp schematic: http://website.lineone.net/~m0dzo/AC4TVschem.pdf

[marcus]

The schematic isn't much help. There is just a rectangle that says "attenuator network" without giving much of an indication of what it actually is. My wild-assed guess thus stands as is. Only one way to find out if your amp will blow up or not

Also, I am not sure what to make of the ominous warning about not running without a speaker. Is the attenuator always on?

[Spivonious]

The attenuator circuit is in the bottom left. Looks like just a resistor network to me.

No, you can choose 4W (off), 1W, or 1/4W.

[DonP]

Can you explain why it would hurt the amp? I could see the amp running a bit hotter than normal, since it's drawing more current, but I'd be running at a theoretical twice the wattage (8W). At such low wattage levels, is heat even a concern?

I suppose I could always take a speaker out of the 4x12 to put in the 1x12 for tonight.

edit: Am I making any sense? My numbers might not be right, but the lower load would cause the tubes to work harder. Higher load would cause current to backflow into the transformer which can burn up the transformer.

With that said, I find it hard to believe that a modern amplifier can't handle a 1 step mismatch.

Impedance matching is much more critical on high powered amps.

But a low powered amp made with cheap components can be killed.

Use at your own risk, but I don't think it will be a problem.

I am curious as to what the B+ voltage is.

[marcus]

The attenuator circuit is in the bottom left. Looks like just a resistor network to me.

No, you can choose 4W (off), 1W, or 1/4W.

Guess I didn't look that closely.

There is always a load, if the attenuator is on.