switching between 2 tube heads on one cab

[per111]

Hi there.
I do have 2 tubeheads(Carlsbro 50 top, and fender Bassman) And I would like to switch between them on stage.
Can this be done with a switch of some kind? and what about the missing ohm load on the "not in use" head.
Thanks in advance.
Per
Denmark

[AL30]

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

AL

[Matt T.]

There are switching systems sold that will switch two amps using the same cabinet. I bought this one once. One potential problem I saw with the one I bought was that the load was removed from the inactive amp. That's supposedly not a problem as long as no signal is fed into the amp. But I wanted to have a dedicated echo/delay for each amp which meant the echo pedals went after the head switcher and would be sending signal (the echoes) to the amp after it had been switched (and thus the load removed). They made no mention of this and when I brought it to their attention they gave me a refund. They also seemed unaware of this potential problem...I had to explain it 3x before they seemed to understand what I was saying (and even then I got a "why would you want to do that?" type of response). I noticed that the unit is "temporarily unavailable until design changes can be implemented". Wonder if they blew up a bunch of amps and decided to pull it?

PS - I did one semi-loud gig with it (using separate delays) and both amps (a homebrew smallbox and a stock 2555 'Jubilee' head) survived with no problems. But I then realized that no way was I going to lug all that crap around and went back to just one amp. That gig was literally at the end of a long pier. The rig sounded great though...just too much too haul.

[hasserl]

I realize there are units available to perform this, but I wouldn't use anything that switches the load on a loaded OT, ever. Maybe I'm overly cautious, but it sounds like a bad idea to me. Get two cabs, or run one cab in stereo, and switch the signal going into the front end of the amps, that's the way to do it IMO.

[jaysg]

Maybe I'm overly cautious, but it sounds like a bad idea to me.

The relays would have to be break-before-make so that the two amp outputs wouldn't see each other. If the subject amps have a 250 ohm 5W or 10W safety load tied across their outputs, they'd never be completely open. I wonder what sort of pops might result

[hasserl]

Like I said, maybe I'm overly cautious. I remember reading something from RG Keen on Geofex about switching impedance switches under a load, I had to go back and search until I found it:

if you have an intermittent contact in the path to the speaker, you will introduce transients that are sharper and hence cause higher voltages. In that light, the speaker impedance selector switch could kill OT's if two ways - if it's a break befor make, the transients cause punch through; if it's a make before break, the OT is intermittently shorted and the higher currents cause burns on the switch that eventually make it into a break before make. Turning the speaker impedance selector with an amp running is something I would not chance, not once.

I realize this is a different issue, but you still are making and breaking contacts under a loaded circuit, which seems to me to be likely to "introduce transients that are sharper and hence cause higher voltages". I don't like it, I wouldn't do it. But that's just me, you pays your money and you takes your chances.

[per111]

Thanks for your answers, I`ll consider it once more..afterall, the solution is pretty expensive, and my old Carlsbro head from `73, maybe all I need...thanks again....
Per.
Denmark

[David Schwab]

That's supposedly not a problem as long as no signal is fed into the amp.

I'm not so sure about that. I don't think you can operate a tube amp with no load... even idling. I had an Ampeg B-15 fry its output transformer because someone rigged it up to bypass the XLR speaker plug and disconnected the cab. They just switched it on... nothing plugged into it.

Moral of the story.... don't loan amps to any one!

It's not a problem with SS amps though.

[Brad1]

Axxess..what the...?

Looking at the webpage, I wonder why anyone would even TRY such a device as that Axxess (sic?).

There are more WARNINGS than there are features!

Look at this crazy stuff:

"Powered by a 9VDC Adapter (like those from Boss - not included) with a 5.5mm/2.1mm barrel connector."

And then:

"ALWAYS use the recommended AC Adapter (see label on the back of the Head Switcher or the User Manual - "Power Requirements" section)." Ummmm.why not just INCLUDE the adapter???

"NEVER use the Head Switcher to switch solid state amplifiers, it WILL damage them and the Head Switcher."
Yes, always disconnect the load from a TUBE HEAD, NEVER a solid state one

"NEVER use a delay with a trail-off/slapback (or any other effect that will continue to generate an output signal after its input has been muted) between the "TO INs" jacks on the Head Switcher and the amplifier inputs or in the effect loop of the amplifiers, UNLESS you can GUARANTEE that the amplifier(s) with such an effect will ONLY be deselected when there is no trail-off/slapback being generated. Otherwise, damage to the Head Switcher and the amplifier(s) WILL occur."
Uh-huh? HUH?

Brad1

[David Schwab]

"NEVER use the Head Switcher to switch solid state amplifiers, it WILL damage them and the Head Switcher."
Yes, always disconnect the load from a TUBE HEAD, NEVER a solid state one

It doesn't say disconnect the load from a tube head.. the output transformer must have a load!

Read their PDF, they say:

A proprietary circuit allows only one amp on
at a time, protects the amplifier that is not
selected and protects both amps and the HS2
relays when switching.

So they have the load built in, and that wont work with a SS amp!

[Shea]

Another potential issue is ground loop hum. To avoid a ground loop, the output from at least one of the heads sound be transformer isolated from the chassis of the switching unit. For a 100-watt amp, that would require a pretty big transformer. Ideally, there would be two isolation transformers, one for each amp.

The input to the unused amp should be muted. So there will also be connections from the switching device to the inputs of the heads, and those connections should be transformer isolated also. And there will have to be some circuitry that drives those transformers.

Ideally, the inputs to both amps should be muted from the time the footswitch is depressed until the relay that switches the speaker load has finished traveling. You'll cut out for a split second. It might be annoying, but it's better than blowing up an amp.

A big 50 or 100 ohm resistor should be permanently connected across the terminals of each of the jacks that connect to the amps' outputs, upstream of the isolation transformers, so there is always some load on the amps no mattter what.

So, I think the perfect head switcher would be about as big and heavy as a whole other head, it wouldn't be cheap, and there multiple areas where the tone could be degraded by using this thing. I think it'd be much easier and cheaper to use separate cabs for the two heads and just switch the inputs using something like a VooDoo Labs Amp Selector.

Shea

[Matt T.]

Regarding that Axxess switcher, the warning below was NOT there when I bought mine. They seemed a bit oblivious to the potential problem when I called them and mentioned it.

"NEVER use a delay with a trail-off/slapback (or any other effect that will continue to generate an output signal after its input has been muted) between the "TO INs" jacks on the Head Switcher and the amplifier inputs or in the effect loop of the amplifiers, UNLESS you can GUARANTEE that the amplifier(s) with such an effect will ONLY be deselected when there is no trail-off/slapback being generated. Otherwise, damage to the Head Switcher and the amplifier(s) WILL occur."

The head switcher box is extremely light, not that that means much. I plugged in a cable to the amp speaker out jacks and measured for dc resistance and they both measured 'open' when the 'other' amp was selected. They measured exactly what the cabinet was when that amp was selected. The switcher was totally silent, no pop at all and as far as sound, worked perfectly. I used it at home long enough to 'build' a dual-amp rig, then used that rig at a medium volume New Years gig and never had a problem (still have both amps) but I didn't like it (was just a scary idea to me) and returned it. I decided I wasn't gonna lug all that crap around anyway. KISS!