A/B box + Attenuator - Avoiding OT damage from switching and other issues

[pistolplc]

Hi - I have a lot going on here.

I have two small tube amps (Vox AC4-TV Mini combo and Epi Valve Junior head) that I want to run through a 4x12 cabinet.  I don't want to have to plug and unplug them to switch which one is hooked up, so I built an A/B box to use sort of in reverse.  That is, plugging the output of each amp into one of the A/B jacks, and run the third jack to the input of the cabinet. 

To complicate matters, I also added an L-pad with bypass switch so that I can turn these suckers down when the kid is asleep. 

Here are my issues. 

1) I realized that during testing, I was switching the speaker load to the valve junior on and off.  E.g., turn on amp; set switch to the dead jack; strum to test for sound; set switch to the live jack; strum and confirm sound.  I don't know how many times I did this, but I realize now that I was probably performing the cardinal sin of driving a tube amp with no speaker load. 
    a) Amp still seems to work.  Chances I damaged the OT?
    b) I would like to make this switch idiot proof.  I have heard something about putting resistors (200-300 ohms) in parallel with the speaker load to help prevent damage from no-load operation.  Is this as simple as it sounds?  How should I determine the resistance value?  I would like to do this just to cover my butt in case I make a stupid mistake again, or if my kids go switching my stuff or turning on amps when I'm not around.  I would want to put the resistor(s) in parallel with both amp inputs so that I can prevent damage no matter the switch settings.

2) I wired the A/B box based on this wiring diagram: (https://cdn.instructables.com/FPQ/HXR6/FSSIL58D/FPQHXR6FSSIL58D.LARGE.gif).  This resulted in a strange artifact - when switched to "B," the signal (tip) from "A" gets connected to the common (sleeve).  When testing the function of the AB switch (with only 1 amp plugged in to the B side), there was some sound coming out of the speakers when the switch was in the A position.  I disconnected the "B" sleeve wire from the switch and everything seems to work fine now.  Is this an appropriate way to fix the phantom-sound problem, or am I missing something? 

Thanks in advance for your help.  This is a really fun project, and I'm really enjoying the troubleshooting process.  Once my confidence is up with this sort of thing I'll tackle the Mojotone tweed champ kit.

[PRR]

> strum to test for sound

"Strum" or "STRUM!" ?

If not driven to clipping, no-load is usually not a strain.

Clipping induces kick-back in the OT which "can" spike ten times higher than B+.

Small self-biased amps, the output may be shorted without harm. (You did note that Beavis' guitar-level switch shorts the idle path?)

If you intend to switch AT FULL ROAR, the cut-over time of the switching becomes critical, and there may be no 100% musician-proof solution.

[pistolplc]

I definitely don't intend to switch under loud or even playing conditions.  I just want the added protection for when I decide to play the other amp but forget to switch the A/B box.  Though I do have a fear that my kid will switch the box while I'm playing...

[eleventeen]

A momentary "wrong-way" on your switchbox (either shorting an output or having it connect to nothing) is not going to do any damage. You could go to extraordinary lengths to failsafe your switching rig; I guess you could get current transformers that would throw a relay that switched the speaker. You could devise an external box that switched both power and speaker connection at the same time. Yet almost *any* arrangement (that comes to mind) would be defeated by having both amps on at the same time, right? How could the speaker select know which one you intended to drive your speaker without being telepathic?


The failsafe is when you decide to change amps, you strum your guitar, hear nothing, then go look at your speaker select switch and see that you didn't flip the switch. Duh. Then you flip the switch and go on your way. I really don't think you're going to cause any kind of damage running 5-10 watt tube amps for a minute or two without load or shorted.

[sluckey]

Put a 100Ω to 200Ω 10W resistor across the speaker jack inside each amp, not in your A/B box.

[pistolplc]

Put a 100Ω to 200Ω 10W resistor across the speaker jack inside each amp, not in your A/B box.

Why do you suggest in the amps instead of the A/B box?  I'm much more comfortable tinkering with my AB box than the amps themselves. 

[sluckey]

It's the amp you're trying to protect. So, put the resistor inside the amp to provide protection against ANY open speaker load condition, even something as simple as "I forgot to plug the speaker cable in."

[pistolplc]

So, despite Sluckey's suggestion, I don't want to tinker with the amps right now.  Does the following schematic seem to accomplish what I want?  Protecting the amplifiers in the event of driving an amp when the speaker load is switched off via the DPDT switch?