300b lovers

@lynn_olson

Will you consider ultrapath capacitor as opposed to cathod bypass capacitor in self-bias?

@charles1dad 

My first tube amp was a Viva Solista Lt - a 845 SET with 6SN7 driver. The Viva Solista Lt produces more than 8 watts that many 300B amps advertise for, but only marginally more.

I had the amp for 6-7 years. Out of curiosity, I tried tube push-pull amps from very reputable brands during those years and found that I much prefer Viva each time!

@atmasphere 

The approach you described is used in DIY world for DHT SET, particularly 300B SET. 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. 

@charles1dad

I belong to the camp that prefers simple triode circuit with interstage coupling in DHT SET. It sounds more dynamic and transparent to my ears.

I don’t think it’s just an implementation issue though. DHT output tubes such as 300B are not that easy to drive and will not sound at its full potential if driver circuit is inadequate. The coupling method affects driver tube’s ability to swing voltage/drive the output tube, hence has direct impact to the final sound.

Interstage coupling allows more voltage swing to be squeezed out from the driver tube in theory and practice. In addition, the output tube can recover from overload very quickly with interstage coupling as the grid current can easily go through secondary winding of the interstage transformer to ground - an unique benefit of interstage transformer. These benefits are not insignificant in DHT SET.

I suspect that interstage coupling is less important for IDHT output tubes since IDHT output tubes are often easier to drive compared with DHT output tubes.

Also, it’s easy for me to advocate for interstage coupling in DHT SET as that’s not difficult to do in DIY - where cost, weight, and space are often less important (in fact, interstage transformers are often cheaper than high-end coupling caps). But if a manufacturer needs to fit everything in a single box, then the choice is very limited.

I prefer preamps with output transformers too. Luckily, there are decent line output transformers around that are not too expensive, certainly cheaper than exotic caps. But sadly preamps with output transformers are not often offered by preamp manufacturers.

Many roads lead to Rome...

As to DHT shootout, using the same driver may suit certain output DHT tubes better than the other ones. Different DHTs also need different output transformers, different B+ voltages and filament supplies, etc. So AB test is not that easy as more than one elements have to be changed for each DHT output tube to have a chance to perform at its best potential. That said, there is no bad DHT in my personal view - assuming adequate power output. So, many good choices are around!

@lynn_olson 

How about using SE as opposed to PP in the first stage and a SE to PP interstage transformer between the first and second stage? 

@lynn_olson 

I did check catalog of one European manufacturer for SE to PP interstage transformers. Within the limited offerings, it does seem that a tube with low Rp is needed in the SE stage - if not a DHT, ones like 6EM7 (the half with 750ohms Rp) may work. The gain will be less, but can still work with an active preamp. 

Ultimately, I agree that it all depends on the design goal - I wonder whether having some 2nd order harmonic would end up being a good thing for someone who prefers sound of SETs. But there is a lot factors to balance in such a design that uses SE in the first stage.

@atmasphere 

I have to admit that the conclusion about SE+PP leads to emphasis on 5th harmonic is a bit counter-intuitive to me from pure math perspective, but I haven’t read his paper and perhaps there are certain situations that contribute to that conclusion (?).

Many report excellent result using a tube pre with a solid state or push-pull tube power amp though. In many cases, such combination includes some SE stages with at least one PP stage. So, SE+PP is not necessarily bad empirically :-)

45 is indeed very very nice sounding.

Cost has gone up quite a bit though. $2k will be barely enough these days for a high quality 45SET DIY project, assuming no fancy enclosures and not using big name irons like ISO or Hashimoto.

For DHTs such as 300B, AC at main frequency is hard to be hum free. But some have tried AC at a high frequency well above human hearing limit with decent result. Sadly this is mostly in DIY world, hence very few commercial offerings with high frequency AC DHT filament supply.

I don’t think DC, IF well done, is any inferior to AC in DHT though. But there are many ways to get DC done. So it gets down to implementation at the end.