Can I use a resistor as a load for a tube amplifier?

[Clubman35]

Here's my question. I have 2 tube amp heads (35 watts & 50 watts) and would like to A/B between them. The thing is, I only want to use one 4X12 cabinet. I'd rather not wire the cabinet in stereo. Is it safe to build a box with a load resistor and send the output of the amp not selected to it? Would there be a split second while switching that there would be no load on the amp? Would switching have to be done with a relay or is a stomp switch okay? Hope this makes sense. I did a quick search and couldn't find what I was looking for. (if this is posted in the wrong area, I apologize) Thanks, guys.

~Jim

[therecordingart]

I use one of these as a dummy load:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=019-020


The switching part sounds tricky. Maybe a make-before-break switch? For that split second your head(s) would see the dummy load in parallel with your cab. If you have an 8ohm cab and 8ohm dummy load that is a 4ohm load as seen by the amp.

Edit: Now that I think about it...way more complicated than that.

http://www.tonebone.com/tb-headbone-vt.htm

[panterafanatic]

It will not harm your amp if using the same impedance, this is fine. Just use a hefty switch to select channels, a relay may be a good option here.

[R.G.]

Good answers.

A resistor will be a fine dummy load for the amp  you're not listening to.

The issue you're worried about, amp damage from unloading, is real. It would probably be OK even if the switch left an amp head disconnected for a fraction of a second while switching, although if the amps are marginally stable, it is possible for them to go into oscillation and cause the voltage-transient damage that is the real danger.

A simple way to step around these issues is to put a dummy resistor of maybe 150-220 ohms on each amp's output jack. This is usually enough to keep short-time open loads from hurting the amp. Some commercial amps include resistors like this, for this reason. It's very likely that if the amps are otherwise not already damaged or especially prone to damage that this will be fine.

Most stomp switches are rated for high currents. The old standard ones are rated at 5A or more. In fact, the were designed for controlling AC power, not signal voltages. Let's do a little power calculation. Power = 50W = I²*R, so I = SQRT(50/R). So if R = 8,  I = 2.5A. If R = 4, I = 3.5A. Both of these are reasonable currents for most footswitches. There may be special "low signal" footswitches being made now. I don't know. It should be stamped on the switch in most cases.

[wavley]

A simple way to step around these issues is to put a dummy resistor of maybe 150-220 ohms on each amp's output jack. This is usually enough to keep short-time open loads from hurting the amp. Some commercial amps include resistors like this, for this reason. It's very likely that if the amps are otherwise not already damaged or especially prone to damage that this will be fine.

Traynors use a 180 ohm 15 watt in this position, something I've always liked for us forgetful folks that sometimes get in a hurry at gigs and forget to plug in the speaker, of course I've never done anything like that 

[Clubman35]

Thanks for the replies...

I had actually drawn up this circuit a number of years ago using a 3PDT switch, but for the life of me, I can't remember how I wired it. I'm sitting here racking my brain trying to draw it up again. I remember it had 1/4 jacks for guitar in, cabinet out, to amp 1 in, from amp 1 out, to amp 2 in, from amp 2 out...... any ideas?

[Clubman35]

This kinda look right? I'm no artist, so sorry about the drawing......

[panterafanatic]

To save the effort, you could create a buffer that splits the signal and sends the signal to both amps, then you've only got to switch the outputs. This is also reduce the coupling from the output to the speakers from coupling into the audio, power amps use a lot of current, this makes this a big factor. Another nice thing about the relay idea is that you could remote switch, reducing the length of cable that the amp outputs have to travel.

[Clubman35]

Thanks.... any ideas how to do this? You obviously have way more knowledge of this type of thing than I do.

[panterafanatic]

http://www.geofex.com/FX_images/ltchrly.gif

That would work if using a latching relay, you can put the 10k at the other side of the switching. Another way if using a non-latching relay is to use a constant current sink, which works nicely because relays are dependent on a magnetic field, and the magnetic field is determined by the current. http://dl.dropbox.com/u/18064636/Remote%20Relay%20Driver.pdf It's not the most efficient design, but will do the trick. The footswitch section can be run right off of a 9v wall wart, the relay driver, the bjt, can be run at any voltage, but you want to have a couple volts of more available than needed for the relay, you'll lose ~1.2v from the CCS. Essentially, the non-latching relay is a spdt switch that is normally closed to one side, and normally open to the other, when the current flows through the relay, the switch changes position, when the relay turns back off, the 10n and diode across the relay windings prevent flyback voltage from the inductor from damaging the components.

[Clubman35]

Um, okay... thanks (way over my head). 

[panterafanatic]

What questions do you have?