845 Tube vs KT90

Bel Canto also makes(or used to) and SET-80 monoblocks which use two 845 in parallel push-pull for 70-80 wpc. I think it sells for $8k a pair new.

The only problem is that there is no free lunch in audio design and the doubled output power comes with some other drawbacks. As to whether they matter or are audible is a debate for those with more technical experience than I.

I believe they still sell the mono blocks, as well, but the price is signifcantly higher. More like $19k a pair! Darkmodebius, what are some of the drawbacks of doubled output power? I'm not going there, but interested never the less.
warren :)

Hi Warrenh,

I was mistaken in my description, the SET-80 isn't SE, parallel Single-Ended (PSE), bridged, push-pull, or parallel push-pull(PPP). It's a kind of a hybrid of all.

What the advantages are of that, I have no idea. Total harmonic distortion is specified at 0.05% at 1W, 3% at 70W clip.

To quote the Stereophile review from 1998:

The close relationship between Bel Canto's SET 40 stereo amplifier and the SET 80 monoblock is evident in components and parts—and the virtually identical chassis. Ingenious use of a high-quality, studio-grade input-matching transformer transforms the stereo chassis into a monoblock with the additional versatility of balanced and unbalanced inputs. This is a function of the center-tapped, balanced winding of the input transformer, while its secondaries allow phase inversion of one amplifier channel relative to the other.

At the loudspeaker outputs of what was the stereo chassis, we now have a balanced signal that—if appropriately connected and ratio-matched via the secondary windings of the output transformers—provides doubled output power in monoblock mode. Conceptually, the result is an oddball—it doesn't fall neatly into the usual class-A, SE, push-pull, or bridge categories, as it contains elements of all of them. (diagram)

Finally, in fig.3, the SET 80 monoblock version of the SET 40 stereo unit is shown. While the output transformers remain separate, the loudspeaker connection is made via a loop containing both secondary windings arranged as a bridge connection. As in the push-pull design, the tubes operate in antiphase, but no phase splitter is required. This task is accomplished by Bel Canto's addition of the balanced/unbalanced input transformer. The amplifier does have a differential topology that will help reject noise on the high-voltage line. This is run in class-A so that each output transformer is magnetically biased; this is said to improve the incremental linearity of the magnetic material. The cross-connected output infers bridge mode, yet the floating transformer secondaries allow the output to be ground-based, avoiding connection difficulties with subwoofers and headphone adapters.

Those four triodes at the input (two 12AX7s) are connected as a differential pair. SRPP mode is achieved by the second pair, the stage said to deliver a low-distortion signal up to 100V rms from a low 1k ohm output impedance, which will fully drive the 400k ohm input grid resistor of the output triode.

*** Still, I'd got for a really good PP amp over a high powered if not using speakers over 94-97dB, maybe even 100dB.

YOU mentioned that they "come with drawbacks" What are the drawbacks you're talking about?

You're not gonna let off the hook for that, eh?

Any SE output transformer, including PSE, should require an air gap and larger size for the same output power as PP. SE also requires greater power supply attention because it does not benefit from the same noise cancellation as PP. I think these two things increase overall cost compared to a PP amp of the same power.

Soundwise, I can't speak from personal experience, but many posts on AA's Tube DIY forum mention a loss of clarity or focus with PSE relative to SE.

Of course, like all things in audio, those opinions may have more to do with poor implementations of the topology than actual flaws of it.