Extension speaker jack tomfoolery

[ampgeek]

I built an Express'ish type amp for a friend a couple of years ago. He informed me that he had just bought an A/B pedal so that he could easily switch his guitar output signal between the Express and an early 70's silver faced Fender Twin combo.

My first question was: What speaker configuration would he be using? He said that he was going to just plug the speaker out jack from the Express into the extension speaker jack on the Twin and use the same 2 X 12" array for both amps.

I said...woah Simba.....that doesn't sound kosher!!!! He said that he had been gigging both amps in that configuration for years (switching the input from one to the other manually) without any problems.

He does have the Express impedance selector set at the appropriate 4 ohm position.

Is it possible that "hanging" two active, un-isolated OTs in parallel off of a single speaker array is OK?? If not OK, what kind of damage might be looming in the future?

Any thoughts would be greatly appreciated!

Thanks much,
Dave O.

[xtian]

I don't know! But I'm on the edge of my seat waiting to find out!

BTW, a few times I've accidentally plugged a speaker cable from an amplifier out, into...another amplifier's out. Even strummed a few chords before figuring the error. No smoke got loose...

[Leo_Gnardo]

Yes I'm surprised nothing has gone bang yet. Besides driving the speaker, the active amp will be sending current thru the inactive amp's OT secondary, and that will reflect as high AC voltage on the inactive amp's output tube plates. Not a good scenario.

It's this sort of thing, your friend's been "walking on water" and may continue to do so successfully until told he can't. Splash!

I've been asked to come up with switching that will select from 2 amps to drive one speaker. Because of the possibility a relay contact will get stuck I've avoided doing that.

If your friend does get his amps/speakers properly set up he may be surprised how much "extra" volume he gets when one amp is no longer driving the other's OT. If he had transistor amps I don't think this setup would have lasted half a second.

[pdf64]

If he had transistor amps I don't think this setup would have lasted half a second

Yes!
The output impedance of tetrode/pentode tube amps tends to be ~3x the nominal load impedance; global NFB will bring that down, with BF Fender NFB levels it may be about the same as the nominal load.
However, thinking about it, the 'passive' impedance of a tetrode/pentode output stage may be a lot higher than that, because a voltage applied to the secondary will appear on the plates whilst the grids are at static levels, and the screen grid acts to 'isolate' the plate current from plate voltage.
So the passive amp looks like a high impedance and may not put much additional load on the active amp.
But it may stress the screen grids of the 'passive' amp.
And cranking a TR into a passive Trainwreck may cause a problem, due to the higher power level, though I'm a bit more 'iffy' on that?

[sunnydaze]

What kind of AB box?

Weber sells this head switching box
specifically for using two different amp head with the same speaker/s.

http://www.tedweber.com/two-head-fs

If he's using something like this should be no problem.

thanks
mike

[ampgeek]

Thanks guys!

Yep...slamming the output tube plates of the "unused" amp with excessively high voltages sure sounds like a risky proposition. I can see that getting to 1000V pretty easily depending on which secondary coils are being used.

My friend bought a simple amp input signal A/B switching box. I think he said it was made by DOD.

I built this one a few years ago and it has performed flawlessly. It is very similar to the Weber offering Mike identified. I do recognize how damaging a failed output relay could be as Leo intimates.

Thanks again!
Dave O.

[JMFahey]

What kind of AB box?

Weber sells this head switching box
specifically for using two different amp head with the same speaker/s.

http://www.tedweber.com/two-head-fs

If he's using something like this should be no problem.

thanks
mike

Not the same thing at all, in the Weber switcher the amp outputs are never in parallel.[/i]

[sunnydaze]

OK, see now the ab switch he's using is only switching the guitar input. The weber box or the Aiken design headswitch or similar are the way to switch heads using same speakers, still some risk as Leo describes.

I was thinking about building something like this for my apartment. I have my amps and cabinets stacked on steel shelves - kind of a pain to reach around to the back and switch cables/cabs, especially while wearing the guitar.

When I go out and play, usually go in the other direction and minimize the gear load.

That Aiken splitter looks interesting.

Thanks
Mije

[Ron Worley]

Check out the Radial switcher unit, it's designed for this exact scenario.

[ampfab]

why not use one of the twin's speakers for the the trainwreck, one for the twin?

[JMFahey]

In fact what's actually needed is a more complex switcher which at the same time:
a) switches inputs, grounding the unused one
b) switches cabinet between heads, connecting a load resistor to the unused one

Some nice improvements:
c) it should momentarily mute the input signal while switching (Peavey does that inside certain heads, to avoid clicks and thumps)
d) for safest tube amp switching, it should use "make before break" contacts, which can be implemented inside an intelligent switcher, with relays and, say, a PIC or similar little brain.

[guitarmike2107]

not much to add but I blew up as solid state hifi system doing this when I was a teenager and had a friend who blew up his solid state bass amp the same way, so I would not recommend it to anybody unless they nkow exactly whats going on inside both amps

[vibratoking]

Aiken's design does just about everything you need and will operate safely IMO. If you want to go tighten up the switch timing, which I don't think is necessary, then a bit of logic or some RC networks or a 555 timer will do the trick. A PIC looks like overkill to me.

[ampgeek]

Thanks again mates!
I see a couple of options that will suffice. Hanging the two OT's on the speakers in parallel without isolation won't be one of them.
My DIY Aiken switcher has worked very, very well over the years.
However, the Radial Headbone can probably be had for less than the DIY cost.
Cheers,
Dave O.