Bridged Tube Amp - is it really so simple ?

[kagliostro]

I've find somewhere this explanation about bridging a Stereo Tube amp to obtain a Powerful Mono Amp



is it really so simple to achieve ?

Thanks

Franco

[PRR]

> really so simple

Yes but-- secondaries are usually grounded for safety.

[pdf64]

When in bridged mode, I think that for the output valves to be working on the intended loadline, the output load impedance would need to be set for half the cab’s nominal impedance.

[tubeswell]

When in bridged mode, I think that for the output valves to be working on the intended loadline, the output load impedance would need to be set for half the cab’s nominal impedance.

Yes, it's doubling output VAC (with the same current though each secondary). Each OT is still magnetising the same amount of current Primary-to-Secondary as if it were a single OT. There will be minimal internal resistance from each secondary winding, and (assuming the signals from each secondary are evenly matched and in perfect phase) the circuit impedance is mostly happening in the speaker. 2 x the voltage for the same amount of current, requires doubling the circuit impedance (V=I x R so 2V = i x 2R). So the speaker has to be rated to handle the doubling of power and needs to provide twice the impedance (than it would for each individual OT).

[kagliostro]

Many Thanks Friend

Only a pair of questions

1) As is write on that explanation there is no necessity to have the input in some way "opposed" like at the exit of a PI ?

2) The resulting power, bridging 2 x 10W amp, will be 20W or it will be more (as I seem to have read many, but many years ago somewhere) ?

Thanks

EDIT: I've find some very old documentation, before long I'll post something about

Franco

[PRR]

> or it will be more (as I seem to have read many, but many years ago

Transistor amplifiers often deliver more power into low load impedance. Two "20W in 8r" transistor amps, bridged, may make 60 or 70 Watts in 8r (because each amp "sees" 4 ohms).

Tube amps usually have transformers with taps so you get the rated power.

[kagliostro]

I'm busy with my father and so at the moment can't read and try to translate for you the old (1994) documentation I've find


They talk about a 4 x increase of power on a bridged amp (also for tube Amps) but using something like a PI before the input


Franco