Converting Stereo SE to Parallel SE Monoblock

[sirdelah]

Hello all, can't seem to find an actual answer to this, so I thought based on some schematics I've seen, would it be possible to simply take a stereo SE amp and make it mono PSE by putting the inputs, output tube cathodes, anodes and output transformers in parallel, without changing any of the other circuit components?

The amp in question uses a 6N9P and an EL34 per channel, and eventually I would upgrade to a single larger output transformer, just wanted to know if this is a reasonable approach to "converting" an existing amp to single ended parallel?

Also, what kind of changes would be needed to use KT120s in place of the EL34s, assuming the amp is self-biasing? Besides upgrading the power supply, of course.

 

[stevenrotterdam]

Just connect the input and output. But it will only give you a better damping factor. Or more power if you connect a 4 ohm speaker to the 8 ohm output.

 

[Eli Duttman]

Schematics definitely help. If you are are lucky enough to be dealing with units whose O/P trafo secondaries "float", simply connect the channel I/Ps in parallel and wire the secondaries in series. Use 8 Ω taps for 16 Ω speakers and 4 Ω taps for 8 Ω speakers. The method does not work, when the speakers are 4 Ω.

Forget KT120s! You need "iron" and circuitry suited to the type. Success with KT120s requires a from the ground up approach. We will not discuss the fact that the KT120 has serious linearity problems.

 

[sirdelah]

By I/Ps do you mean the primary winding of the output transformers? And when you mention transformers that "float", is that referring to a lack of a ground connection?

 

[Eli Duttman]

By I/Ps do you mean the primary winding of the output transformers? And when you mention transformers that "float", is that referring to a lack of a ground connection?

I/Ps are the input RCA females.

Yes, "floating" secondaries means the absence of a ground connection.

 

[sirdelah]

And as I understand from another post above, this will not increase power output, but only improve damping factor? The goal was to increase output, hence the thought of putting the output tubes in parallel (before the transformer, based on schematics).

 

[Eli Duttman]

Each of the 2 O/P tubes will produce the same amount of power as it currently does. The "strapped" composite delivers the power of both tubes to the single speaker.

 

[pof@]

What happens if the amps are not equal in output? Are some current running from one output to the other?

 

[stevenrotterdam]

Disagree. The amps don't produce power but voltage. When parallelling voltage stays equal, so does power unless you als use 2 speakers.

 

[Eli Duttman]

Disagree. The amps don't produce power but voltage. When parallelling voltage stays equal, so does power unless you als use 2 speakers.

The method of paralleling I/Ps and wiring "floating" O/P trafos in series sums the voltage swings of the 2 sections in the load. As each amp section "sees" only 1/2 of the load, 16 Ω speakers work with 8 Ω taps, etc.

 

[stevenrotterdam]

Agree. Good luck finding floating secundaries...with regard to feedback and grounding situacions.

 

[Eli Duttman]

I've got a pair of DECWARE SE84s whose O/P trafo secondaries "float". They do quite well, "strapped".

In the case of units containing a common ground, "bridging" can be employed, but that requires driving the I/Ps with out of phase signals. Getting the out of phase signals is an expense.