300b lovers

RM's minced no words in responding that 6SN7 tubes were a very poor choice of tubes -- "better employed in old TV's", because of their inherent high noise levels.  

@whitestix I encountered that with him as well- although by that time he was beefing about the 6SN7 linearity family of curves. Turned out his source was an early Tung Sol tube date manual that didn't reflect 99% of most production tubes. RCA and Sylvania of course figured things out with the 6SN7 early on and in another year or so, so did Tung Sol. RM had the bad luck to have a Tung Sol manual that was outdated. 

There is a group who prefers this approach and there is a group who prefers the more simple way - using half or the 6SN7 in parallel as one triode and interstage coupled with the next stage, or alternatively using a more capable driver, including a DHT such as 300B itself, 4P1L, 10y, 801, EML’s 20 or 30, etc. 

@ffzz I've found that implementation plays an enormous role in zero feedback amplifiers: grounding, component quality, power supply design and of course getting the operating points right in the circuit design. I challenge the idea that using an interstage transformer is actually a simpler approach- it is once you have a suitable transformer, but a good transformer design is the tricky bit; on that account direct-coupling is IMO easier. It has the same advantage of being able to support class A2 operation as well as instantaneous overload recovery, plus wider bandwidth and lower distortion.

The lower distortion may well be why it sounds less 'dynamic'! The use of that word when associated with SETs is always tricky, since most of the 'dynamic' nature of SETs has to do with how they make distortion. IOW its normal for a lower distortion circuit to sound less 'dynamic'. 

These amps do PP a different way and limit the phase split issue and avoid some SE distortion.   Again, there are many fine examples of all sorts of amp topologies, but these don't sound at all like other PP amps you have heard.  They sound like a great SE amp with the drive of a PP amp.  That is the design goal....

@donsachs +1

I've been harping on this very issue for years. Our amps do not have a dedicated 'phase splitter', relying instead on good CMRR in the differential voltage amplifier.

Thanks @donsachs - looking forward to it.

@atmasphere great post.  My new amp does use an interstage transformer and a larger driver tube.  The 6SN7/300B  amp I'm listening to in the mean time is darn good.  Not great, but darn good.  So lets not walk away with the understanding that all 6SN7 implementations in 300b amps are junk.  but I agree a bigger driver is a good thing.

Jerry

@carlsbad2  Yes the Reichert amp used 6SN7 to drive 300b and though I have not heard one, those who have said it had a sonic purity that was hard to beat.  So it was a good starting point and 6SN7 driving a 300b can sound good.  But it can be improved if you are looking for all out performance.

Thanks for the kind thoughts, whitestix. For those who are curious, this amplifier is a more advanced, second-generation Karna amplifier, first designed around 2005, and the result of a years-long collaboration with Don Sachs. More practical, two chassis instead of four, and benefitting from Don’s decades of experience in what works and what doesn’t, especially in a production environment.

My projects are usually proof-of-concept and kind of out there. Don’s experience makes all the difference for this collaboration, getting it off the ground and into the real world. He has a lot of great ideas, taking it even further, and these will be in the pre-production amplifiers at the Seattle show.

Whitestix, don’t feel left out. The amps you are listening to now are essentially identical to the show amps, just a little earlier in the production cycle.

It’s a fully balanced circuit, with 6SN7 input, 6V6 driver, and 300B output. It’s a high current, high speed design. I tested an early 2003 prototype at 500 kHz at full power, with clean-looking sine waves on the scope, with no visible flat-topping or triangle shapes. No, I won’t repeat that test again, and I don't encourage anyone else to do it, either.