300b lovers

The thing about the transformer coupling is that once done correctly, it just works.  It will not fail.  It protects against a tube that may develop a grid short.  Rare in smaller signal tubes like the 6SN7, but I have seen it.  So for a commercial amp I just want something that works and the owner never has to worry about.  If he or she gets a hum or odd noise from the system, a quick tube change will most likely solve the issue.  Direct coupling has the chance for a tube failure to propagate to the next stage.  Very rare, and if it was just my own amp that I could repair, then I probably wouldn't worry about it.  But if you put 100 amps out into the world....sooner or later a rogue tube will appear.  Lynn's point about servo circuits is also valid.  It adds complexity and again, a possible failure point.  If my own amp in my living room, then fine.  I can fix anything.   But if I put 100 amps into the world I don't want to see one fail.  My business philosophy would be to immediately take care of the problem for the customer, but I would rather just avoid the issue entirely.

Every component and coupling method has a sound.  I found the well designed transformer to have less of a sound than any other method, plus it is totally reliable.  I find it more transparent than any other method as well, with subtle detail more audible.  It has less coloration than anything else.  I think transformers get a bad name because there are a lot a mediocre ones out in the world and that is what most people have heard.  If you go all in on good ones they are quite spectacular.   So that is the way we went.

As to Lynn's comments about capacitors, yes, I have heard a very large subset of the best caps available, bypassed and unbypassed, etc...  You can happily live with many of the best ones, but the transformer coupling kills them all to my ear in this circuit.  Once you hear the really good IT you realize that none of the caps can produce the tonality of the transformer.  The instruments all sound just a little wrong with the caps.  The transformer does the "piano is in the room" thing a LOT better in this circuit built this way.  You may have a different experience in a different circuit built with a different power supply.  RC coupling is certainly easy.  LC coupling a little more difficult because you need a good anode choke and you have to physically find room for one.  Direct coupling has advantages, but again, there are failure modes and colorations. The hallmark of this circuit is the absolute transparency and I just found the transformer sounded best, once a really good one was wound, and it will be trouble-free.

If you do direct-coupling correctly, it can be very reliable. I don’t think it wise to direct couple throughout the circuit though!

The advantage of a direct-coupled cathode follower driver tube are several. First, there is more current available to provide linearity if grid current is produced by the output tube(s) (which allows for class A2 or A3 operation since both of those classes produce grid current), and no worries about Miller Effect, so very wide bandwidth (+30MHz!) is very easy (we bandwidth limit our OTL amps in the voltage amplifier section). The stability of the circuit is very high- in our OTLs (where one driver tube is controlling many triode power tube grids), bias adjustment only needs checking once every 6 months or so; often no adjustment needed. Distortion is kept in check in a way not possible if a cathode follower with coupling cap were used. We went to this circuit because its more reliable and prior to its introduction to the marketplace, OTLs generally had a reputation for being unreliable. This approach was key to making OTLs as reliable as any other tube amplifier; one of the reasons we are still in business after more than 45 years.

How this might work in an SET is a different story- yes, you need a B- supply to really pull it off, but if you want to do it with transformers you’ll have to pay that price too, since decent interstage transformers are not cheap- in fact, several times more expensive than a B- supply.

The other advantages of using a direct coupled cathode follower driver are that the driver prevents output tube conduction until the driver has warmed up and stabilized. This is likely not a concern when DHTs are used, but it is if indirectly heated, since cathode stripping can occur if the B+ is applied while the power tube is warming up.

Another advantage, probably the big one, is that small value coupling caps can be used between the voltage amplifier and the driver tube. Since coupling caps have inductance (since they are wound) no matter what materials are used they will always introduce some coloration on this account. By minimizing their value while still allowing for very wide bandwidth, the coloration they might cause is minimized and the inductive influence octaves above the audio band.

If course you could use an interstage transformer for this latter function as well; the advantage being that’s the least expensive place one could be used and most likely to win the most performance from the transformer.

At any rate, by using this approach you wind up with a very simple circuit that offers excellent linearity. One idea I’ve had for a while for an SET is taking advantage of the B- supply and building a differential amplifier for the input voltage amplifier. This would allow a proper balanced input as well as reducing distortion in that gain stage due to harmonic cancellation.

Of course if you already have a fully differential/balanced amplifier design from input to output, the advantages multiply.

We’ve been building a balanced preamp with a direct-coupled output (for which we have several patents) that uses servo control to prevent DC offsets at its output. The inputs of the preamp are direct coupled as well, but coupling caps are used between stages. The DC servo has proven to be one of the most reliable aspects of the circuit, which has been in production since 1989. During that time we simply have not seen any servo failures! The output of the preamp uses a Circlotron, like our OTLs, so there are no excessive voltages produced as the preamp warms up.

The obvious advantage is a lack of coloration at the output of the preamp (most tube preamps use a large coupling cap, from which, as pointed out earlier, colorations are unavoidable) and very wide bandwidth; to the latter point the direct coupling allows for reproduction of extremely low bass that most tube preamps won’t even acknowledge. Since the preamp uses no feedback, controlling phase shift can only be done by having bandwidth down to 1/10th the lowest frequency to be reproduced, so its good to 2Hz. If phase shift is present, the ear perceives it as a lack of impact, even though the preamp might otherwise be perfectly flat at 20Hz. By direct coupling that problem and the often tubby bass that can occur are both avoided.

So while direct coupling (with or without a servo) might seem tricky, it offers great advantages and can be extremely reliable if properly designed.

@stephenr First off, thanks for you polite post on this thread. I bet your 12B4 preamp sounds great. I have tried removing the Raven preamp and going direct to the amps from the Lampi Pacific DAC via xlr. It sounds very good. It doesn’t sound nearly as good as with the Raven active preamp in the chain though. Over the years I have experimented with various passive preamps built with top notch volume controls and none were as satisfying as a really high end active tube preamp in my system to my ear.

As for the direct coupling, no I didn’t experiment with it in this project. I have heard it in other amps. As Lynn has described above, there were reasons for avoiding it in this particular project. I am sure you can make it work well in your amp though and I suspect it sounds very good.

Don and I are moving towards production of the Blackbird 300B amplifier. We’re now using triode-connected KT88 drivers running at twice the current (56~64 mA per KT88) and one-third the plate impedance (Rp = 700 ohms) of the previous drivers. Not only that, owners will be free to use matched pairs of their favorite 6L6, KT66, KT77, 6550, or KT88’s, with no bias adjustments needed.

The tube lineup is: 6SN7 input, KT88 drivers, 300B outputs, with VR105 shunt regulators. As before, fully balanced Class A from input to output. Special-design Cinemag input and interstage transformers, with custom Monolith output and power transformers. 18"/457mm full-width monoblock chassis with two isolated audio-circuit and power-supply sections, a slow-start circuit, surge protection, and a 12V trigger option. (No AC power goes to the front panel, just DC trigger voltage for the slow-start circuit.)

Don and I anticipate Spatial Audio (of Salt Lake City) will start manufacturing in November 2023.

Amazing to watch the arc of the development and completion of these stellar monobock amps.  Just wonderful.