Bridging Amplifiers

Although solid state amplifiers are normally bridged into mono, the same does not apply to tube amplifiers. I thought you might want to know why.

I can certainly convert to mono any of my amplifiers here at the shop. Tube amps are not like transistor amps and the term "bridging" only applies to transistor amps. When transistor amps are "bridged" the input of one channel is inverted and the load is connected across the hots. When drawn out on paper one can put the speaker horizontally across the hots as if they were ends of a bridge and the speaker is the platform between them. To my knowledge this is how the term came into being.

I have been thinking about the RM-200 and looking at options to make it into good performing mono amps. The advantage of bridging has a lot to do with your speaker impedance. Specifically, the impedance range one plans to work in and the speaker sensitivity that will be employed.

In general when we MONO a stereo tube amp the power is indeed doubled (you get the total of the two channels) but the tap impedances are cut in half. So, you put the 4-ohm speaker on the 8 ohm taps to get the full power. If you happen to have an 8-ohm speaker you will not get twice the power though you will get twice the current.

One often overlooked problem in connecting stereo tube amps to mono is the need for critical gain matching within a few tenths of a decibel. If not gain matched the two channels will fight each other. Dynaco solved this problem by requiring two power resistors in series with each 8-ohm output. Of course, that shoots the damping factor in the foot. They did put a mono switch on the front, but that only makes the inputs mono. I am not sure why they even bothered as it only does half the job.

The RM-10 is the best stereo/mono amp I know of. I did the mono switching in a new way and a toggle switch on the back takes care of everything. Mono into 8 ohms is about 45 watts but into 4 ohms you get the full 70 watts.

To mono a tube amp properly the best I can think of will require moving a lot of wires to other places. Too many for a single switch to handle.

[Source: circa 2007]