Bridging tube amps, resulting impedance

dcmeigs

I have a few tube amps (console pulls) with just 4 ohm transformers taps. They seem to do very well with 6 and even 8 ohm loads but I was thinking of pairing them up and running them bridged.

It seems to me that a pair of output transformers connected to a single 8 ohm speaker would each see a 4 ohm load. Am I thinking correctly about this?

madmax

That is a very scary question. Since the output is a voltage source rather than a current source my guess is if you wired in series the voltage would double which would mean to draw the same current you would need an 8 ohm speaker. If wired in parallel you would have the same voltage but could draw twice the current (xformer 1 plus xformer 2) which means you could use a 4 ohm speaker. I bet it matters whether its a single ended or push pull amp. Be prepared to repair something.

heiney9

Have an extinguisher and medic on stand by.

H9

zingo

I know you can do it with McIntosh amps, but they can also be a different breed.

nooshinjohn

This sounds like a recipe for disaster.

reeltrouble1

It can be done and is in fact encoraged by certain manufacture's, you should contact yours to see. Certainly BAT is another example, in fact I have known an owner who used six of them for his stereo rig.

RT1

dcmeigs

madmax wrote: »

That is a very scary question. Since the output is a voltage source rather than a current source my guess is if you wired in series the voltage would double which would mean to draw the same current you would need an 8 ohm speaker. If wired in parallel you would have the same voltage but could draw twice the current (xformer 1 plus xformer 2) which means you could use a 4 ohm speaker. I bet it matters whether its a single ended or push pull amp. Be prepared to repair something.

Hummm, you make an interesting point, re the voltage source. BTW, the O/Ts would be wired in parallel in a bridged setup. If I understand correctly, it's your point that the voltage would not change, regardless of the number of O/Ts in parallel. The pair would be able to deliver more current during transients but no change in voltage. So perhaps the speaker impedance doesn't need to change at all. Or in cases where 4, 8 and 16 ohm XFO taps are available, one would continue to use the same tap that one used in the non-bridged setup.

I'm thinking of flashlight batteries in parallel as an analogy; voltage sources not current sources. The DCR of the bulb, or impedance in our analogy, doesn't need to change as more batteries are wired in parallel. Why would the speaker impedance change? It wouldn't.

O/Ts are impedance matching devices. They, through the square of the windings ratio formula, multiply the speaker impedance to match the 6,000 ohms the 6V6 needs to see. In a pp application the XFO is center tapped so as to present the required impedance to each tube (like two SETs in one frame). It would not matter whether the application was PP or SE, IMHO.

Thanks man, I think I get it now.

And guys, you can put your fire extinguishers down now and send Nurse Ratched home too:)

heiney9

reeltrouble1 wrote: »

It can be done and is in fact encoraged by certain manufacture's, you should contact yours to see. Certainly BAT is another example, in fact I have known an owner who used six of them for his stereo rig.

RT1

The BAT's and others are designed for this, the OP is talking about taking a couple of old counsole amps and bridging them himself :eek::D.

H9

madmax

dcmeigs wrote: »

And guys, you can put your fire extinguishers down now and send Nurse Ratched home too:)

I'm not ready for that just yet!

thuffman03

I would not recommend doing that.

FTGV

dcmeigs wrote: »

It seems to me that a pair of output transformers connected to a single 8 ohm speaker would each see a 4 ohm load. Am I thinking correctly about this?

I believe thats correct.You will also need to invert the phase of the input signal to one channel.

dcmeigs

FTGV wrote: »

I believe thats correct.You will also need to invert the phase of the input signal to one channel.

Why would that be?

FTGV

To get the increased voltage swing requires that the channels operate differentially or in opposite phase to each other.

dcmeigs

I learned that FTGV is indeed correct. In a bridging situation, one inverts the signal of one channel and then connects the two speaker leads to the positive terminals of each channel. In that way the transformers operate in series, not parallel. The current is doubled this way. And the voltage.

Since the voltage doubles, one must take impedance into consideration. Using an 8 ohm speaker, one would use the 4 ohm taps. Otherwise the current would quadruple with possible bad results. In most applications the negative terminals (or 0 ohm connections) are connected to chassis ground and are therefore already connected, thus completing the circuit.

There are a few vintage tube amps like the Fisher X-101C for instance that ground the 4 ohm terminals and that would call for a different approach.

Obviously, bridging creates a non-common ground situation; a bad idea for SDA owners.

I was confusing bridging with paralleling, which I'm told can sometimes be a better approach for tube amps. It can be a bit problematic though. In this situation, the two channels are operated in phase and the transformers are wired in parallel. Paralleling the channels does leave the voltage unchanged as mentioned before, and it does double the current available but in order to draw twice the available current you must halve the load impedance. This means connecting the 8 ohm speaker to the 16 ohm taps.

Many thanks to the guys at diyaudio for 'splaining it to me. If you want to read the full discussion you can find it there.