Your journey with lower-watt tube amps -- Can a kit be good enough?

I don't want to interpret for someone else, particularly someone with vastly more hands on equipment building and technical knowledge than Alex Berger, but, I don't think "impossible" meant it cannot be done--its more like it cannot be done optimally.  Clearly Sun Valley is doing it; the question is whether this is a good approach.  I think that there are lots of suboptimal designs that are done to lower cost, so it is helpful that Alex Berger is pointing this out.  I too have my doubts about the plethora of low cost 300B SET amps with 12AX7 drivers and no interstage transformer--this might be less than ideal, so the question is whether this is the right cost-quality tradeoff.  I am not saying it is the wrong tradeoff, but, it helps for buyers to be aware of the issue.

845 is impossible to drive without IT! You get more driver tube distortions than 845 output stage distortions. You also can't get a good deep and fast bass.

@alexberger This statement is entirely false (and is what IT manufacturers would like you to believe).

If the driver is a cathode follower and direct-coupled to the grid of the power tube you'll have no problem with drive and less distortion than you can get with an interstage transformer. SET designers don't do it because you have to have a B minus supply for the driver tube and they don't want to spend the money although a good interstage driver transformer will cost more. The other reason SETs designers don't do it is they didn't think of it!

You'll recall I mentioned this technique on another thread on which you were active on this website.

For the record we've been driving power triodes in exactly this manner for decades (47 years to be more exact). The driver has an iron grip on the grid of the power tube and can easily drive it into grid current with good linearity. It also prevents blocking distortion which is a problem for any tube amp which employs coupling capacitors between the driver and grid of the power tube so you get instantaneous overload recovery.

You might try it since you're into DIY. If you have any questions about how to go about it, email me.

Hi @hilde45 

The problem of 300B they need a driver with 150 volt peak to peak voltage swing. For 845 you need much more voltage swing.  

It is possible to drive 845 with RC coupling, but you get much more distortions vs IT drivers with the same tubes. With IT - the output tube can be driven when it is in class A2 and you get more power and better clipping recovery.

I have a DIY 300B amplifier and I used 6f6 in triode with RC coupling, then I changed to IT coupling. It was a huge difference! It sounded like a different amplifier. And I used super expensive V-Cap CuTF capacitors in RC coupling.

Then I changed the driver tube from 6f6 to 6v6 and to 5881 (6L6) and each time a more powerful driver sounded better, bigger soundstage with better instrument separation. 

A bit late to the party here, but it's a topic near and dear.  I've had Altec 604-8G in one of my systems for many years.  One day a friend and I did some trading, and I wound up with an Alan Eaton SE45 stereo amp.  It was quite pleasant and I enjoyed listening.  He had also brought over a 300B amp made by BEZ.  It was not impressive, very veiled, mushy, etc.  I kept the Eaton amp until I decided to make my own SE45 amp from vintage parts.  Output trannies are Triad HSM-79.  Borrowed and merged bits of circuit design from various sources, and the result was an amp that outperformed the Eaton amp, both on the bench (output at clipping and sq waveform) as well as sonically.  If you're wondering, I listen first, then measure.  I don't recommend scratch building unless you enjoy the journey, but the point of this comment is buy a kit with the best output transformers you can afford, as they are the heart of any single ended amplifier.  As for wattage concerns, 1.6 wpc does seem a bit lean, but if you're speakers are 95 db at 1 watt, they'll be 85 db at 1/10 watt.  Distance from speakers matters, room sound absorption matters, but unless you're looking for slam at disco levels, SE is a fun part of the audiophile journey.  Life is short, build the kit, savor the results.  I also have one of Dennis Had's KT88 Inspire amps, and it brings a little more punch, with up to 6x more wattage on tap.  All good fun.

The mention of a directly coupled cathode follower by atmasphere is interesting.  Today, I just happen to be looking at the schematic for an old Wyetech SET amp and noticed it had a directly coupled cathode follower to the grid of the output tube.