300b lovers

I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations.  I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is.  Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.  

Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence.  They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music.  They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding.  Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a  unique circuitry to mate with his 300b monos via XLR connections.  Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it. 

I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system.  300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm.  Of course, 300b amps favor efficient speakers so carefully component matching is paramount.

Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public.  Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC.  I will be there to hear what I expect to be an outstanding sonic presentation.  

To allay any questions about the cost of Don's 300b mono, I do not have an answer. 




The sonics of power supplies are different for SE and PP amplifiers, so it is impossible to generalize without specifying the amplifier topology.

The modulations of supply current on the main B+ supply in a single-ended amplifier are simply the music itself, with an addition of noise from the rectifier stack. Single-ended amplifiers are entirely Class A in operation, by the way.

This is not true for a balanced or push-pull Class A amplifier. The modulation of music on the supply is reduced by 30 to 35 dB (depending on balance), and what’s there is doubled in frequency, similar to a balanced-detector in a radio. The balanced-detector artifacts are the result of symmetric nonlinearities in the balanced pair ... if they have 100% distortion, you get a balanced detector.

If the balanced-pair distortion is a small fraction of that, say, 1% or less, then you still get balanced-detector distortion but much reduced in level. If the balanced-pair are 100% distortionless, then current draw is constant, with no variation. But distortionless balanced pairs exist only in fantasy, so there is always some variation in current draw with real circuits.

In a Class AB amplifier, it is worse, with three regimes ... Class A at low levels, and clipped-off Class B at higher levels.

This has an impact on the sonics of the supply. A single-ended amp is simple ... improve the musicality of the supply, since music is directly impressed on it. Push-pull is more difficult ... the modulations on the supply are a mix of residual imbalance and balanced-detector artifacts, and significantly worse if Class AB artifacts appear in the output stage.

This is the strongest argument for stage-to-stage isolation, so distortion artifacts from a high-level stage do not modulate a lower level stage, In the Raven and Blackbird, we go the additional mile by having a shunt regulator for the input section. The shunt regulator operates by having a current draw that is the precise inverse of the audio-circuit fluctuations, so the net current draw is constant.

By contrast, in a generic Dynaco or Mullard circuit, we have several topologies, with only simple RC power-supply filtering between stages. The output stage is typically Class AB semi-pentode, the driver is Class A triode, and the input is single-ended triode. All three have different distortion signatures.

And then we get into the deeper waters of the sonics of the regulators themselves. Some are slow and noisy, and intermodulate with the music. Others are fast and silent. Regulators do not all sound the same, and passive CLC supplies can have a signature too, depending on the capacitors chosen. There is no one-size-fits-all solution.

The tricky bit using regulators is bandwidth.

The output of any regulator has a certain impedance. What you are looking for is a linear impedance curve across the entire audio band. The tube regulators often had a problem with this; the output impedance rises when the regulator meets its bandwidth limit. At this frequency the regulator has to be bypassed with a capacitor that keeps the output impedance as linear as possible. It won’t be perfect- usually you wind up with a step in the output impedance at the crossover point. Too much capacitance can cause the regulator to run hotter and with tubes, you can have reliability problems that might occur when the capacitance is charged during warmup.

Solid state regulators often have much wider bandwidth so don’t usually need so much bypassing, but they can have stability problems so are often bypassed with a small capacitance at the output to prevent it oscillating.

Adding additional capacitance to the output of the regulator will do little to improve sound quality and if enough capacitance is added, will increase the heat of the regulator and could threaten its reliability.

If the regulator is properly bypassed and operated well within its limits then they will tend to be neutral.

To help the regulator along, its a good idea to do as much as you can to minimize noise at the input of the regulator. For example a PI network is helpful to reduce the amplitude of the sawtooth waveform at the regulator’s input; this will reduce the work the regulator has to do, which can reduce its operating temperature.

@lynn_olson - what are your thoughts on using a balanced input stage to drive a single ended 300B?  My front end hardware is all balanced and I like using balanced connections. I’m thinking an LTP with a CCS on the cathodes and a push/pull interstage on the anodes. Output of the IT driving a single 300B with fixed bias connected to the other side of the IT secondary. I haven’t decided which tubes to use for the input stage, but was thinking about triode strapped D3As. 

I have a nice pair of Monolith Magnetics SX-11 output transformers that I want to use. 

what are your thoughts on using a balanced input stage to drive a single ended 300B? My front end hardware is all balanced and I like using balanced connections. I’m thinking an LTP with a CCS on the cathodes and a push/pull interstage on the anodes. Output of the IT driving a single 300B with fixed bias connected to the other side of the IT secondary. I haven’t decided which tubes to use for the input stage, but was thinking about triode strapped D3As.

@jaytor You’ll want as much gain out of the voltage amplifier as you can get, since the more gain also yields better Common Mode Rejection Ratio. But you’ll need a good Constant Current Source to optimize the gain stage. The D3A, triode strapped, will give you a good mu value to work with, although you could get that with a single 12AT7 and be in the same ballpark. A 12AT7 will allow for plenty of bandwidth.

If you use a driver transformer as a plate load, the issue you’ll be up against is imbalance of plate current between the tube sections. The better your CCS the less you’ll have this problem, and matching is a very good idea. The more current that isn’t cancelled in the magnetic core you can also expect greater distortion. So you can see getting the plates to have equal plate voltage is important. To this end, using a balance pot in the cathode circuit to balance the plate currents, while effective, has the effect of also reducing the differential effect, increasing distortion and reducing gain and bandwidth. So matched tube sections and a really effective CCS are paramount.

One thing to consider is its really impossible to get a perfect center tap in a transformer. Its always going to be a little off.

Since its often difficult to get really well matched tube sections (they should be matched on a curve tracer for best results), I prefer to use RC coupling to a cathode follower, which is in turn direct coupled to the grid of the power tube. The power tube would thus obtain bias from the driver tube. If the driver tube gets weak, the power tube will conduct less also, preventing damage. I explained this topology earlier in this thread.

If you are considering using a differential amplifier as the input voltage amplifier, you really should consider a bi-polar power supply of equal B+ and B-. This will improve the differential effect and the effectiveness of the CCS. If you have such a power supply then you have a good way of setting up that cathode follower I mentioned. If you go this route, care must be taken to make sure the plate of the driver tube is well bypassed so even at full output there is no noise, no artifact caused by the signal on that plate. This will really help the amp assume greater authority.

@charles1dad - Thank you. 

@atmasphere - One of the reasons I was thinking of using single section tubes was to improve matching. I've noticed this section matching problem in LTPs I've been playing with, particularly with some of the soviet tubes (e.g. 6N6Ps) where the quality control seems to be lacking. 

Are there any tubes that, in a PP IT loaded design, would be reasonably linear and provide enough drive for a single 300B without a cathode follower stage? 

I'm confused why a bipolar supply would help. I'm assuming I'd need to have some kind of negative supply for the fixed bias circuit which I would also use for the LTP CCS connection, but I would expect this to be around negative 100V. 

Alternatively, I could use a balanced input transformer with the secondary center-tap tied to a positive voltage to allow enough voltage across the CCS tied to ground.

Are there any tubes that, in a PP IT loaded design, would be reasonably linear and provide enough drive for a single 300B without a cathode follower stage?

@jaytor Sure! We are assuming that you plan to use a high efficiency speaker so you’re not also looking for a lot of gain. But I have to get some clarification- do you want the ability to have enough drive for the power tube, along with enough voltage gain at the same time? They are not always the same.

Tubes that can swing the voltage needed for the 300b include the 12AT7, 12AU7, 6CG7, 6SN7 and so on. I would stay away from the 6DJ8 and similar frame grid tubes; while they are very linear and can handle the current you need, they aren’t very good with microphonics as they are not meant for audio.

A bipolar supply supply is helpful because there is always a bit of differential performance left on the table by any CCS. The more negative the supply, the higher the impedance of the CCS can be (which is a good thing). You can, with modern semiconductors, easily built a good CCS that will work on -100V, but If you are already building a B+ supply, its really not all that hard to build a B- supply from the same transformer. I like to use a separate power transformer for the driver and voltage amplifier, so as to minimize noise appearing in the power supply of the output section from messing with the rest of the amplifier. This helps reduce IMD.


Here’s the latest: Don is training the folks in Salt Lake City as we near production.

The Raven preamp is the same preamp you heard at the show, while the Blackbird now has KT88 drivers, at twice the power of the 6V6 drivers, with third-generation Cinemag interstage transformers and Monolith output transformers, If you’re wondering who Monolith is, you heard them in the Songer room on the Whammerdyne amplifier.

Merry Christmas, Happy Hannukah, and a Happy New Year, to all!

Hi @lynn_olson ,

I have a couple of technical questions:

1. Is it a good idea to feed indirect heated tubes AC filament with a separate dedicated power transformer? In my integrated SET 300B amp input tubes receive relatively low level signal after the input potentiometer.   

2. I use interstage transformers and separate PS for driver and input tubes. As a result, I have two separate GNDs: 1st - for output circuit and PS  2nd - for input/driver  circuit and PS. Should I connect both these GND together to the amplifier chassis at one point? Or can I leave one of these GND floating?

1. If the heater windings are not shielded from the high-voltage B+ windings, switch noise from the B+ section can be capacitively coupled to the low-voltage heater windings. Although indirectly heated tubes have significant isolation from line noise, it is not 100%, so some of that switch-buzz gets through. If you listen closely, you will probably hear it.

There are several solutions: A) get a power transformer with electrostatic shielding (copper foil) between HV and LV sections B) go to DC heating with a regulator C) use a separate heater transformer.

When none of these are done, yeah, that’s cost-cutting, no matter how fancy the name brand. If swapping the rectifier tubes makes a notable difference in the sound of the amp, there’s something wrong, or not optimal, with the power supply design.

2. Yes, the two grounds must connect together at one point, and only one point, preferably the chassis. There’s an old joke: when two engineers get together, they will come up with three grounding solutions, and all of them will be right.

Grounds (and shields) should never float, ever. That’s bad practice and a possible safety hazard. Determining the best connection for the ground or shield usually takes a bit of thought (which is why it’s called engineering).

There are whole books on grounding. It is not a trivial subject. Part of the problem is language, because one word covers several things:

A) Safety ground 

B) Current return path for the DC currents that power the tubes or transistors

C) Current return path for the AC audio signals going through the circuit

D) RF and noise shielding

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From Don Sachs:

"First pair of production Blackbirds in walnut... running and sounding really good for only 10 minutes on them :)

Tomorrow we start the first pair of production Ravens...."


2. I use interstage transformers and separate PS for driver and input tubes. As a result, I have two separate GNDs: 1st - for output circuit and PS 2nd - for input/driver circuit and PS. Should I connect both these GND together to the amplifier chassis at one point? Or can I leave one of these GND floating?

@alexberger I can confirm everything @lynn_olson mentioned regarding your post in his above. Pay attention to all those issues!

Regarding this question which has to do with grounding.

You’ll want to ground your chassis if its metal (and if you’re running single-ended, ferrous materials will provide audio-frequency shielding). So the ground connection of the power cord should be tied to the chassis.

The audio circuit, if tied directly to chassis, leaves you open to ground loops. To avoid this, lay out your amplifier circuit so all the points that go to ground do so to the power supply (star grounding is nice) without touching the chassis.

Grid and cathode connections for each tube section (the ground side of the grid resistor and ground side of the cathode resistor) should be common and use a single wire going to the star ground. This forces noise imposed in that wire to be common mode and so will reduce noise.

The spot in the power supply to which you tie your grounds can be a buss tying all the filter cap grounds together.

Once you have all the grounds starred together, at that point you need to reference the chassis ground. That can be done several ways: you can use a simple resistor from the audio ground to chassis ground. You can use a pair of rectifiers in parallel, each opposite of the other, with a resistor in parallel with that. This higher impedance is presented to ground currents between chassis and the audio grounds which otherwise set up the ground loop.

Be sure to have the input connectors also isolated from ground. Most RCA connectors are supplied with shoulder washers for this purpose.

In this way, the chassis can be a relatively quiet shield for the audio circuit. You’ll find the audio circuit to be quieter with this practice.

Hi @lynn_olson ​​​​@atmasphere ,

Thank you for useful information.

Does aluminum provides any shielding? I will use an aluminum chassis.

There is a beautiful picture of Blackbirds. But I don't see inter-stage transformers. Are they under the hood?


Does aluminum provides any shielding? I will use an aluminum chassis.

@alexberger Yes, but only at radio frequencies. No shielding at audio frequencies, and for that reason steel is likely a better choice if you're running single-ended.

We use aluminum chassis in most of our stuff, but its balanced and so does not need shielding to be quiet.

Alexberger, the Cinemag input and interstage transformers are under the top plate. These are the third generation of the custom Cinemag transformers we started with. The two Monolith transformers are visible on the rear of the top plate.

The other things under the top plate (right side) are the B+ and filament regulators, and the master soft-start circuit, with an external 12V trigger. The point-to-point and turret-board wiring under the audio side (left) is very simple, with only transformers, tube sockets, cathode resistors and bypass caps. No current sources or other gizmos.

Wow. Those Blackbirds are stunning looking! They're just a wee bit larger than my Statement 300b amps from Don.

[How does one post photos here?]



The Statements are the previous versions. I strongly urged Don to use a spacious layout, with all components on a single layer. Easier to build and signal-trace. Don ran with the suggestion and improved it by putting all the power supply components on the right side of the chassis, all the audio circuits on the left, with a shield between the two sections.

The wiring for the audio section, in particular, is really simple ... transformers, tube sockets, cathode resistors, and bypass caps. The wire lengths for each half of the circuit are symmetric, while cathode resistors, bypass caps and local grounds go to turret boards next to the tube sockets.

The bigger chassis also run cooler, as you might expect, since heat-emitting components are further apart, and the top plate is now 18" wide. The Blackbirds might look big, but they fit just fine on standard racks.

Sonically, the Statements and the Blackbirds are pretty close. They are all descendants of the original Karna amps, which date back to 2003. Unlike the Karna amps, the Statements and Blackbirds are on two chassis instead of four, which vastly simplifies grounding.

Here’s a picture of Gary Pimm (foreground) and Gary Dahl (background), taken in 2003. The Karna’s have the distinctive blue chassis, and the separate power supply chassis are behind the amplifiers. The AMT-1's are Gary Pimm's speakers.

Gary Pimm and Karna amplifiers

Here’s a close-up of the 2003 Karna amplifier, showing one channel of the audio chassis, with the power supply chassis out of sight. These have been my personal amplifiers until I received a pair of Statements about eight months ago.

Note the aviation-grade Amphenol connector on the rear of the chassis. All connectors and cables are rated for 1.5 kilovolts, using transmitter-grade coax cables to carry the two separate B+ voltages to the audio chassis.

In case you are wondering what all these tubes are doing, the input tube is a 5687 (or 7119), the drivers are old-stock 45’s, the outputs are 300B’s, one pair of VR150’s are for the drivers, and a single VR150 is for the input tube. The single EL34 is a current source that feeds the VR tubes. The four blue-painted cylinders emerging from the chassis are General Electric industrial motor-run capacitors.

The Statements and Blackbirds are an update of this over-the-top project. I was doubtful the Karna could ever be manufactured until Don came along, with his lengthy experience making the Valhalla (KT66) and Kootenai (KT88) amplifiers. Not only is it more compact, the new Don Sachs power supplies are an order of magnitude better ... and they didn’t exist in 2003.

And here’s the very first version, the 2-stage Amity amplifier, built on old Tektronix chassis by Matt Kamna in 1997:

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Hi @lynn_olson @atmasphere

I’m going to use a separate power supply (PS) box for my amp project. It is too big and too heavy for one box. I don’t want a monobloc solution, because it is an integrated amplifier.

I have 2 GND. One is a power cord GND (1) and another one a circuit GND (2). GND1 will be connected to PS and main chassis, while GND2 will be connected to GND1 thought resistor.

1. What value of resistor is recommended?

2. In which box GND1 will be connected to GND2 through the resistor? In the PS or the main chassis?

@alexberger The resistor value isn't critical. 50 to 100 Ohms works nicely. As long as both chassis (power supply and amplifier) are tied together and grounded by the power cord, its really not going to matter where that resistor actually is, since it will carry no current under normal circumstances.

Hi @lynn_olson ,

The Hashimoto PT-100F power transformer that I'm going to use for drivers/input B+ and  drivers/input filament. But I'm not sure if it has an electrostatic shielding between HV and LV sections.

I can see some strips between high and low voltage windings. But as far as I can see they are made from a kind of cardboard.

If it doesn’t say "electrostatic shield" there’s no copper foil between windings. Medical-grade is even better, with primary and secondary on opposite sides of the core.

If you want full isolation, in the absence of these features, you need two transformers, one for HV, and another for LV. The name brand doesn’t change the shielding, or lack of it ... that requires copper foil isolation, which is a definite hassle for the transformer builder.

I should mention the heaters for the rectifiers do not need isolation. Only the heaters and filaments for audio tubes need isolation. If DC regulation is chosen, a higher secondary voltage than 6.3 volts is needed, since regulators need to throw away a few volts to operate. I should mention not all regulators are the same; you can do a lot better than generic 3-pin regulators.

... Actually, sonically the statements and the Blackbirds have similar heritage, but the Blackbirds are much better.   The Monolith Magnetics iron is just sublime, plus the power supply is improved in several ways including the addition of the pair of VR tubes.  The result is the Blackbird kicks it up a notch from the Statements, which were sort of a final prototype before we just pulled out all the stops and made the cases larger on the Blackbird.  I just returned from Utah, where I taught the guys at Spatial the builds of both the amps and preamp.   There will be actual units for purchase in January.

I showed the pix of the Amity (1997) and Karna (2003) amplifiers to emphasize they were developed largely in isolation from the SET community. I found out the hard way that SET practice wasn’t always useful with this approach. Similarly, practice in the traditional push-pull pentode world isn’t always helpful.

Don’s build and design experience with the Valhalla and Kootenai has been very useful, showing me what worked, and what didn’t. So there are elements of the Amity, Karna, Valhalla, and Kootenai in the Blackbird.

Do they sound like SETs? No, they do not. Do they sound like traditional push-pull pentode amps with feedback? No, they do not. The signal flow and functionality is different. The closest similarity are to some unusual designs of the 1930’s, with the very latest 21st-Century transformers and power supplies.

The Blackbird is actually the result of a four-way collaboration ... between Don, myself, the transformer designer, and the power supply designer.

Just heard the first production Raven was completed, and the folks at Spatial Audio in Salt Lake City thought it was a big step up from the Raven preamp at the show. It uses a new Monolith power transformer, a newly designed input transformer, and VR-tube shunt regulators, in addition to the regulator used in the show preamp.

Like the show preamp and Don’s previous preamp, it uses a Khozmo volume control with a remote control that adjusts volume and balance, and selects inputs. Tube lineup is a pair of 6SN7’s (one for each channel in balanced mode), a pair of VR150 shunt regulators (one for each channel), and a pair of damper-diode rectifiers.

The Raven and Blackbird use standard RCA and XLR interfaces, so they can be used with other components, but they sound their best when used with each other.

Most likely not. It can drive 600 ohm phones no problem, but definitely not 16 to 32 planar phones, which are pretty popular these days. They require small power amplifiers in the 2-watt range.

@downtheline  The Raven has a headphone jack, yes, but as Lynn noted, it will only drive the typical old style phones.  It will not drive planars.  Sounds great with my old Sennheisers and phones of that type with 300 ohm impedance.  

As an overview, the Raven is a 1-stage amplifier with a 4.5:1 step-down transformer, and is output-limited by what both sections of a 6SN7 can crank out. The Blackbird is a 3-stage amplifier with a 28.7:1 step-down transformer, and is output-limited by what a pair of 300B’s can crank out.

The ideal solution for planar headphones is a 2-stage amplifier. For example, in the xDuoo TA-10R headphone amp sitting right next to me, a 12AU7 followed by a pair of Class A emitter-followers for each channel. Simple and inexpensive. Or, small power amplifiers in the 2 to 5 watt range ... all transistor, hybrid, or all-tube. The all-tube solutions ideally use step-down (output) transformers to match the load to the capabilities of the output tube.

Vacuum tubes can swing lots (hundreds) of volts, but are current-limited by peak cathode emission, typically measured in tens of milliamps, not amps. Bipolar and MOSFET transistors, in contrast, can pass not just milliamperes, but several amps, which is why they can be direct-connected to low-impedance devices like 8-ohm speakers and 20-ohm headphones.

A quick note on transformers: the voltage/current transformation ratio is the same as the turns ratio, but the impedance ratio is the square of the turns ratio. For example, the output transformer of the Blackbird, and many other other push-pull amplifiers, has a primary impedance of 6600 ohms, and a secondary impedance of 8 ohms. 6600/8 = 825, and the square root of 825 is 28.7228, which is close to the physical turns ratio. In a well-designed transformer, total losses are less than 5%, so can be neglected for this calculation.

So the output transformer of the Blackbird multiplies the peak current of the 300B pair by 28.7 times, offering peak currents of several amps to the loudspeaker. Similarly, the output transformer of the Raven multiplies the peak current of the 6SN7 by 4.5 times, which is plenty for driving a cable, but not really enough for planar headphones which mimic loudspeakers in terms of current draw.

Sounds like the next project for your collaboration is a headphone amp for a wide impedance range of headphones!

@downtheline Actually, next up is a two stage amp with another lovely DHT in push pull for a whopping 3-4 watts at very low distortion.  Not a 300b so not for this thread!  You could hang a custom transformer off it for phones.  You would want a REALLY good custom little transformer for headphones.  Not some off the shelf average quality solution.  Anything worth doing is worth doing to excess.  It would work and drive darn near any cans you would want to plug in.  You could even have multiple windings and perhaps a switch or two output jacks.  One for 300-600 ohm cans and one for the planar crowd.   Maybe in a few months..  First we get the Blackbirds and Raven off the ground (pun intended).  We have just built the first ones in Utah and I expect sales in Feb.  Then I can focus on other projects....

Maybe Peregrine. Something fast and powerful, which are the hallmarks of these designs.

Just a note that Spatial will be at the SW Audio Fest in Dallas, March 15-17.  They will have the final production versions of the Raven preamp and Blackbird mono 300b amps driving their speakers.   I will be there all day Friday and Saturday, and a bit on Sunday morning.  So if you are in the general vicinity and want to hear this gear please stop by!  We will have tons of music on a hard drive, and if internet Gods are willing we will have Tidal and Qobuz available to queue up requests.  



No, not that I know of.  Dallas, and then I think they are planning on going to Seattle again in early Sept.  You should write them to see if they are planning on going to the east somewhere.  You can contact them via their site.

Don and the Spatial team will be in Dallas, but I will be home here in Colorado. If Spatial goes to the Seattle show this year, I will be there along with Don.

I learned my lesson about flying last year. This time, I’ll fly First or Business Class, and will wear an N95 mask while I’m at the airport. The show, though, was a lot of fun, and it was really nice staying at the show hotel and taking a nap in my room when I needed a brief rest. Also great meeting you folks in person, and seeing (and hearing) what’s going on in the industry.

@lynn_olson After Seattle last year, I made sure that I got pre-checked from TSA. Something like that can make your life much more pleasant. 

I did most of those things for the Seattle trip. I hadn’t flown in more than a decade, and I knew it would be Hell on Earth if I didn’t tick all the boxes. So, in the months preceding, I applied for TSA Pre-Check, Instant Bag Check using the QR code on the phone, Early Boarding (for a fee), Lounge Access (lounge was closed), and Extra Seating Room (because I’m 6’ 1").

Most of it worked perfectly. TSA Pre-Check was fast and efficient. Instant Bag Check, likewise. Early Boarding, same. Extra Seating Room, yup, and pre-checked into a window seat both ways. Check. All perfect and worth every penny.

But ... the *&$@# concourse in Denver Airport was miles long, and there are several of them, connected only by a standing-room-only subway. And United changed the gate THREE times, sending a text to my phone just after I hiked all the way to the end of a concourse A, only to find no flight to Seattle after getting there. So then the flight is changed to the end of Concourse B (walking all the way back, taking the jammed subway, and walking all the way to the end of ANOTHER concourse), and then it got changed a THIRD time, right back to the end of Concourse A.

It must have been at least two miles, and there is nowhere to sit on this trek. Hey, airports are supposed to be ADA compliant, and I’m a tired old geezer about to fall over. So I ask a porter, can I get a ride in one of those ride-alongs I see whizzing by. Nope, you have to reserve a seat at least a day ahead with the airline you are flying on. No ride for you!

So ... the airport is grossly out of compliance with ADA unless you reserve at least a day ahead. Too bad if you are a single mom with an exhausted toddler, disabled with a crutch, or just plain old, like me, You get to walk, and walk, and walk, and if you fall over, well, tough. That’s your problem.

Oh yes, and I got Covid when I came home, almost certainly in the Denver or Seattle airports, which are both jammed each way. And then Karna got it 36 hours later, from me. Fortunately, Paxlovid dispatched it in a couple of days.

So I learned two things: wear an N95 mask in the airport, jetway, and airplane, until the airplane A/C is turned on. There’s plenty of fresh air on the flight (especially if you’re flying on a nice breezy Boeing 737 Max).

Second, reserve a wheelchair (by Federal law, no extra charge) at each end of the journey. The walking distances in both Denver and Seattle airports are stupendous. I felt truly sorry for the disabled folks and single moms ... they were at the ragged edge of endurance, with nowhere to sit and pause for a moment. The absence of any seating in the concourse is a scandal, and frankly, a health hazard for some of us.

Aside from the concourse horror, things went fine otherwise. Flying is fine if you have early boarding and a roomy window seat. TSA Pre-Check is worth the $80 charge and the FBI background check. I lead a boring life, there’s not anything to find.

And the show itself was great, as I expected. Sure, it was possible I was exposed to Covid at the show, but I doubt it. The airports were the human zoo, with tens of thousands of stressed-out people shoving past each other.

I very much miss the Portland to Denver Amtrak train, which was wonderful. Book a sleeper compartment and watch the countryside roll by. That was a wonderful trip.

I finished my integrated 300B SET project with two external power supplies.

It took me about 40 hours to drill chassis (thank you for my friend who helped me do it) and to assembly all parts together.

The amplifier sounds from scratch batter than my previous DIY amplifiers which was disassembled for parts for the new project. And it should sound much better after break-in.

Thank you guys for helpful advices.

Spatial Audio now has the preamp and power amp in production (scroll down about halfway). The power amps are now 18" wide, the same as the matching preamp.

The tube lineup is 6SN7 input (balanced), triode-connected KT88 drivers, and 300B output in a fully balanced zero-feedback circuit. Monolith custom-design power and output transformers. The VR shunt regulators are on the right side of chassis, along with the power supply circuits. The audio circuits are on the left side, isolated from the power supply by an under-chassis shield running from front to back.

Revelation Series