Bridging Stereo Amps

[375]

Greetings people.

I have a pair of stereo 6L6 Allen organ amps and would like to know if it's feasable to bridge each one mono.

I understand it can be done by feeding inverted signals to the inputs, feeding the speakers w/ both positive outputs and making sure that the speaker impedance is double the value of the taps being used.

I would use the 8 ohm taps, wire my woofers for 16 ohm and the speaker negs are already earthed.

Do I risk damage to anything , or are there any other reasons I shouldn't attempt this?

Cheers,
375

[the_manta]

Hi !
A short circuit at the output of tube amp is harmless in the majority of cases. Only make sure that there is a load attached to the output.
Otherwise there is nothing you should take care when you experiment with the outputs...

Regards, Simon

[375]

Thank you Simon!

This is the first I've heard of the dead short being OK.

Good to have learned something new today!

Cheers,
375

[Jeb-D.]

You can also feed the amps an in-phase signal and just reverse the output transformer taps of one of the amps. Of course the harmonic content will be worse, but if there low distortion amps it is simple to do it this way.

[ThSpeakerDude88]

The reason tube amps dont have problems with a direct short is because when the secondary is directly shorted, the tube sees the primary impedance ( a load) and does not harm the tube. However if you leave a speaker un connected and do not short out the output ( place a load on it) , there is nothing resisting all that current flowing through the output transformer and it will most likley fry.

This is why most guitar amp heads use self shorting jacks on the outputs.

[anatech]

Hi 375,
In the "old days" , an amplifier was run in mono by putting the two channels in parallel and using a "Y" cord into the inputs. Normally accomplished via a switch. The 16 ohm tap became your 8 ohm and the taps for both channels were wired together in parallel. Bridging a tube amp as if it were a solid state amp would be odd. I don't think I would do it that way.

One more thing. The 6L6 type amplifier can put out up to 50 watts possibly, depending on the circuit and supply voltages. This is enough power to cause damage to the tubes and output transformers if the connection is wrong and you crank it. It's true they need some load on the output transformer, and that a short is better than an open circuit. The thing is that a short can cause a lot of damage also. What is done in tube amps does not apply to home amplifiers. People pretty much fly by the seat of their pants in the music industry. Just because they do it, doesn't mean it's right or harmless.

Your organ amps may already be in mono at the input.

-Chris

[Tom Bavis]

Reversing the phase of the output transformer is probably not possible, since one side is usually connected to ground to use negative feedback.

[375]

Originally posted by anatech
Hi 375,
In the "old days" , an amplifier was run in mono by putting the two channels in parallel and using a "Y" cord into the inputs. Normally accomplished via a switch. The 16 ohm tap became your 8 ohm and the taps for both channels were wired together in parallel. Bridging a tube amp as if it were a solid state amp would be odd. I don't think I would do it that way.

One more thing. The 6L6 type amplifier can put out up to 50 watts possibly, depending on the circuit and supply voltages. This is enough power to cause damage to the tubes and output transformers if the connection is wrong and you crank it. It's true they need some load on the output transformer, and that a short is better than an open circuit. The thing is that a short can cause a lot of damage also. What is done in tube amps does not apply to home amplifiers. People pretty much fly by the seat of their pants in the music industry. Just because they do it, doesn't mean it's right or harmless.

Your organ amps may already be in mono at the input.

-Chris

This option would certainly save me some hassle, as the woofers are already wired for 4 ohms.

Whichever route I decide to take, I'll be sure to only do one amp at a time. For reference and A/B-ing.

Thanks to ALL of you for the great advice, you're a friendly and knowledgeable bunch!

Craig

[Jeb-D.]

Reversing the phase of the output transformer is probably not possible, since one side is usually connected to ground to use negative feedback.

Ah, yes I was forgetting about closed loop amplifiers. It could be done with some re-wiring, but it's probably impractical.

[kevinkr]

Originally posted by ThSpeakerDude88
The reason tube amps dont have problems with a direct short is because when the secondary is directly shorted, the tube sees the primary impedance ( a load) and does not harm the tube. However if you leave a speaker un connected and do not short out the output ( place a load on it) , there is nothing resisting all that current flowing through the output transformer and it will most likley fry.

This is why most guitar amp heads use self shorting jacks on the outputs.

Actually that's not true at all, what it sees is the primary leakage inductance which in a good transformer is going to be a few tens of mH worst case - pretty close to a dead short actually. If you run any tube amp into a dead short for more than a short period of time you may see the output tube plates start to heat up. Guitar amplifiers took this approach because unlike the open secondary scenario where very large output voltages can be developed (destroying insulation) the over dissipation resulting from a short circuit can generally be tolerated for some period of time.

Think of it this way - in an "ideal" (lossless) transformer the reflected load impedance seen by whatever is connected to the primary is just the square of the winding ratio times the actual load impedance - so a dead short on the secondary is going to be reflected as a dead short to the primary.