Compact ~20WPC Tube Amp

@atmasphere

Based on your comments your speakers are nowhere near efficient enough to work with your SET.

Which leads me to suspect that the 96 db sensitivity rating is exaggerated or (More possibly) the speaker’s impedance curve and load just isn’t suitable for an SET 300b.

My speakers are 94 db but a 14 ohm impedance with an appropriately designed/damped woofer to not require much amplifier DF (Damping Factor). They literally sing with my 300b SET mono blocks and replaced the previous 100 watt amplifier.

So, much depends upon speaker design and intended use.Push pull tube amplifiers will usually have lower output impedance and thus higher DF. Also generally they utilize some degree of NFB which increases DF.

Most SET amplifiers are zero NFB (Or very low) and will require different/compatible speaker loads. OP’s speakers just need a good quality push pull amplifier and they’ll do well.

Charles

Which leads me to suspect that the 96 db sensitivity rating is exaggerated or (More possibly) the speaker’s impedance curve and load just isn’t suitable for an SET 300b.

@charles1dad That wouldn't surprise me at all. The impedance thing is why I wrote this article since so many speakers that are rated 8 Ohms actually have 4 Ohm woofer arrays and simply aren't meant to be driven by zero feedback tube amps! That was part of why I convinced Israel to try making speakers with higher impedances. It doesn't help that speaker manufacturers seem to often overstate their numbers as well.

The Sun Audio 300B amp, if I’m not mistaken, uses two 6SN7 stages to drive the 300Bs and is the main issue with the amplifier distorting prematurely. The 300B and many other DHTs will draw grid current long before their power rating is reached and will "slew" causing distortion (the coupling cap is discharged faster than the driver can charge it leading to rolled off highs and sloppy bass). This is really common with all of the 2 stage 6SN7/300B amps out there. The 300B really needs a stout drive capable of kicking it in the @$$ to really allow it to open up. While a push pull tube amplifier will certainly give you the merits of more power, if you’re looking to stick with the overall sonic character of a single-ended amplifier, there are other options out there. My single-ended KT120 for example, with a solid 6SN7 SRPP driver will sound more like a 35 WPC amplifier, but can still produce 18 watts per channel with excellent cone control/damping. However it does use NFB as non DHT tubes really do not sound good without it unless they are "triode-strapped", which is an inherent form of local feedback. With most SETs the driver is the amplifier’s downfall- not the power rating of the amp, or the 300B. 

Best regards, Aric

@aricaudio 

With most SETs the driver is the amplifier’s downfall- not the power rating of the amp, or the 300B.

Aric this is precisely what the builder of my Coincident SET (And speakers) explained years ago. The main limitation with some SET amplifiers is the choice of weak/inadequate driver tube. Just as you say, this driver needs to be robust. The DHT output tube demands this. He chose the 6EM7 rather than the 6SN7 for that reason.

@atmasphere 

Good advice you gave to Israel Blume (Coincident). I’m happy that he took note of that wise counsel,😊

As you’ve pointed out before, higher speaker impedance means less amplifier stress/work and less distortion.

Charles