Bridgeable floating output transformer help

[tizman]

As far as I know, Decware does this for a few of their amps, including the very popular SE84. Does Decware do something that makes floating the output transformers safe?

 

[Avo111]

Stupid question here. Is there much difference running two single ended amps this way and using a push pull scheme? Does this arrangement provide audibly different performance? It would seem possible to take a couple of single ended stereo amps and make them mono blocks. Am I correct in thinking I would need a preamp with a balanced output to drive such an arrangement?

 

[jeff5may]

Leakage or, worst case, failure of insulation can put B+ on the secondary winding, putting speaker wiring-to-chassis at high voltage. Everyone, please, please don't do it.

Much thanks,
Chris

The obvious answer here is don't use a defective or otherwise crappy output transformer. Also, don't try to extract huge power from an undersized iron. The risk of leaky or shorted windings exists even in a single OT.

Remember that in a series connection that the current flow is equal through all devices in the string. At any given output level, all secondary windings in the string will have more current flowing through them compared to non-series operation. At high output levels, impedance matching is important to ensure that excessive current doesn't flow through any transformer in the string.

 

[Avo111]

Jeff,

Does this mean that the secondaries of both output transformers need to be able to handle the combined current of both output stages? If so this arrangement sounds problematic of you do not have oversized transformers in the first place? Would not doubling the resistance of the speaker help to solve this? i.e. series up two 4 ohm windings to feed an 8 ohm speaker?

 

[BinaryMike]

Avo111,

You're correct, but it's a moot point because essentially all high-performance audio output transformers are overdesigned, and because overloading usually reduces the output power of tube amps.

 

[jeff5may]

If in fact you are overbuilding a set of amps (se or pp) as the OP stated he is, that will operate reliably into a lower impedance load without stressing the irons, then the amps should have no problems with double the expected current running through each secondary in the series string. OTOH, if you are trying to squeeze more power out of a design that is already giving all it can, something is going to run out of steam when the volume knob gets cranked up. It all depends on whether the output tubes are powerful enough to overload the transformers or not. If the configuration operates like BinaryMike describes, and the output tubes max out before the transformers are smoking hot, the rig can be tortured mercilessly without massive risk. But if the output tubes are stronger, the transformers can be roasted in a short time.