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A question and a sick idea...

8/2/1999 7:15 PM
Mark Buckingham
A question and a sick idea...
Can anyone tell me how to "bridge" tube amps? So if I have a stereo amp (imagine an ax84 p1, but with two output sections), and decide I just want louder mono for a while, what do I do?  
Secondly, (and this is the sick part), is it possible to drive a speaker from both a tube output and a solid state one at the same time? so - let's say you derive an tap off of the output transformer, pad it down (a lot), and then run this through some effects, then through a solid state power section connected to the same speaker as your output transformer.  
The other thing I was thinking of is to get the signal to the fx the same way, but routing the output of the solid state power section to a separate speaker.  
And now, a word from our sponsors:

8/3/1999 3:13 AM
Steve A.

    Before the real experts have the opportunity to answer the questions you brought up I thought I'd just post this "rule of thumb" I've always used with audio circuits: I'll use a y-cord to split the output from an amplifier to 2 separate speakers, but never the other way around (2 amplifiers into the same speaker). Although I guess it could be done if you designed everything perfectly, I'd think that the output from one amp would "fight" the output from the other (possibly causing a short circuit which would blow everything up).  
    Along the same line of reasoning, I will try using a y-cord from one guitar to feed two different amps (although you can run into problems with a ground loop). But I would never try plugging 2 keyboards into a y-cord feeding a single amp (although I've made passive mixers that isolate the two signals through resistors and pots, as well as active mixers that buffer the two signals.)  
    Rules of thumb are meant to be broken (the rule, not the thumb!) so I'm sure that there are countless exceptions but in following this rule I've hardly ever blown up any equipment... (knock on wood!)  
Steve Ahola
8/3/1999 5:16 AM
Don Symes

Bridging the two outputs.... possible, great opportunity for smoke and flames, though. Consider this method:  
1) nail the two output grounds together.  
2) add a PI between the preamp and the two outputs  
3) isolate spaker jack from ground  
4) output A connects to speaker +, output B to spkr -  
In all, not worth the trubl and the risk.  
Two outputs, same speaker in non-bridged mode... NEVER!  
unless you're just a pyro at heart.  
I bet Doc will have a more substantial answer.  
8/3/1999 6:35 PM
ken g
"1) nail the two output grounds together.  
2) add a PI between the preamp and the two outputs  
3) isolate spaker jack from ground  
4) output A connects to speaker +, output B to spkr -"
Don, I don't want to seem like a naysayer, but it seems that all of these steps are not necessary...  
Most stereo amps will have speaker -'s attached to ground anyway, absolutely if they have any NFB. If the two -'s are grounded, then they are connected together. So all that is really needed is to invert signal to channel B, and take output from both + speaker outputs. This will provide twice the voltage swing into a given load impedance.  
(Of course, like any other bridge circuit, the load seen by either "half" is now half of the total + to + load.)  
Do you see what I'm getting at?  
8/3/1999 7:52 PM
Don Symes

For the bridge to work, you gotta invert one output or the other. You could tie output A's (-) to gnd and output B's (+) to gnd. That would get you the inversion. Then connect speaker(+) to A(+) and spkr(-) to B(-), so, you're right about it being simpler than I was saying. NFB will re-complicate things by forcing the inverted ouput's feedback to be re-thought in terms of the (+) side being grounded, leaving only the (-) side to pick FB off of.  
Just hook up two speakers and all these issues just disappear.
8/3/1999 12:55 PM

If I understand the "sick" part of your post, you want to derive a signal from the OT (secondary I presume) modify/amplify it and return it to the same point you derived the signal from, i.e. the speaker/OT speaker leads?  
This looks like a feedback loop to me and, assuming that you can hook it up w/o problems (Don's method) you will either have a quite loud oscillator (positive feedback with interesting phase interactions depending on frequency) or a pretty much dead output as the signals cancel out (again, with some interesting frequency dependent effects).  
Buy a second speaker.  
8/3/1999 1:15 PM
Mark Buckingham
Thanks everybody for the help. I didn't think the idea would work, but hey, it was worth a shot...  
Thanks again.

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