Two amp heads -> 1 cab. Easy and not risky solution?

[JaviCAP]

Hi mates,

I've been lucky to get a Valvestate 8100 head dirty cheap and nearly mint for being a 20 years old amp. Yeah, I love this amp, was my first serious head  many many years ago, and I always regreted  the day I sold it.

Well, I just have a cab (2x12) and don't wish to get another  one, so, would like to share this cab with my two heads.

Not together, just one at a time. The problem is that the Cub Head is a full tubes one, and so, you have to be carefull with these heads, cabs and loads.

I think that a single a-b like box would be enough: wire from head 1 to "A" jack, wire from head 2 to "B", a switch to choose which jack is gettin' the signal and a single output wire to the cab.

Is this enough taking in mind that I don't wish to use both heads at the same time?

I know that when using the Cub Head, I should choose its position on the "a-b" box before I switch on the amp, and that should switch off the amp and wait aboput 2 minutes before I set the "a-b" to the valvestate position.

Do you know of a better solution?

[induction]

You can rig a DPDT so that when you switch to the solid state head, the tube output goes through a power resistor to ground. The power resistor should have the same resistance as the impedance of the speaker cabinet the head expects to see, and it should have twice the power rating of the amplifier.  Eg, if the head is 50 watts and expects to see an 8 ohm speaker load, the power resistor should be 8 ohms and rated for 100 watts. The DPDT also has to be rated appropriately.

Let me know if you need help figuring out the connections on the DPDT.

[IvIark]

Headbone TS?

http://www.tonebone.com/headbone.php

[JaviCAP]

Yeah Mark, the Headbone is a great solution, but I'd wish something cheaper and diy if possible :P

Thanks Induction, if you don't mind showin' the wiring, I'd be very grateful, I think that I know how to do it, but when dealing with valve amps, I prefer to  be 100% sure.

Thank you all!

[JaviCAP]

By the way, Induction, the CUB Head is 15w and the cab 8 ohm, so I think this power resistor would be enough:

http://www.ebay.es/itm/8-Ohm-8R-25W-Watt-Power-Metal-Shell-Case-Wirewound-Resistor-/390650169057?pt=LH_DefaultDomain_0&hash=item5af49086e1&_uhb=1

ot it would be too tight and prefereable to choose a 100w rated one?

BR

[induction]

This is one way.



As for the power rating of the resistor, 99% of the time, 25W will be fine. If you don't leave the amp turned on when you're not using it, then this is just a failsafe for that one time you forget to turn it off. If you never crank your amp up to 10, then 25W should be ok. I'm a big believer in over-engineering stuff that's not for mass production, so for the cost of one extra resistor, I'd go ahead and use two 15 ohm 25W resistors in parallel. Then you're covered for 50W and you don't have to worry about that 1% of the time when you're distracted and forget to turn off the amp. You can even leave it on if you plan on switching back and forth, which will save some thermal stress on the tubes.

In principle, you should heat sink the resistors. Just attaching them to a metal enclosure should do the trick. If you use the parallel resistors, then the heat sink probably isn't necessary.

[JaviCAP]

Thanks you very much for all the info. I have just ordered  some 15r 25w power resistors, so I'll build the box as soon as they get here.

Thanks again, mate!

[JaviCAP]

Well, just to soy a big THANKS! again. I received today the power resistors. I used as you recommended two  15ohms / 25W ones in parallel, reading finally 8,4 ohms and the selection box runs increible!.

Used too a 5 amps dpdt and 2mm wire, the result is great and fully  functional, now I can even get both amps switched on if I want to.

Here's the box, surely this weekend I'll drill the enclosure and screw the power resistors to the enclosure itself, hope you like it:

:

[induction]

Looks great! Glad I could help.

[GuitarGas]

Sorry to bump this old thread, but would these be ok to use for this project?
edit. I swapped the link to 15ohm, the previous link was to a pair of 8ohm resistors...
http://www.ebay.com.au/itm/Green-15-Ohm-5-50-Watt-Aluminum-Shell-Wire-Wound-Resistor-2pcs-/301423452923?pt=LH_DefaultDomain_15&hash=item462e3ce2fb

Cheers