Post by Steve Deckert on 04/13/22 at 03:36:32
Welcome to the SEWE300B Design Log.
This is a continuation of the the Western Electric to Manufacture Tubes in the USA thread found here:
https://www.decware.com/cgi-bin/yabb22/YaBB.pl?num=1648154321
With the war in Ukraine, Russian tubes are hard to get and Western Electric is stepping up to the plate to bring many of these tubes to market. Right now they have the original 300B tubes but are adding more. To support this USA company I am making a DECWARE 300B amplifier specifically for the Western Electric 300B tubes that cost nearly half the price of the amplifier! Obviously the resulting sound of a Decware 300B amp would have to be so over the top to justify such an expensive tube, but we are confident it is not only possible but likely.
So in the world of 300B amplifiers the $1500 price for a pair of tubes would only be small percentage of the amplifier cost... welcome to high end audio of which we completely are but aren't. If you do the math based on my comments we should be able to hand build a 300B amplifier without tubes starting around 3K that can carry the Decware badge with honor.
Below is the beginning of the design log for this new Decware amplifier. I'm sure it will be interesting because they always are. A tumultuous journey for myself as I try to rise to the occasion and make things happen... these real time journals are import so I don't forget any of the stops along the journey and it gets those who care inside my head so they know what to expect before they ever hear it.
DESIGN LOG - DECWARE 300B
So one of the things that has always given me some pause with respect to directly heated triodes like the 300B is the fact that if you run an AC heater, you have to use a hum balance pot on the cathode. That would be a 2 watt or 5 watt wire wound linear 100 ohm pot where the center wiper becomes the cathode path to ground! This means you would have to have an extremely high quality wire wound pot not to tank the sound right here. This is the critical signal path where even the type of wire used will effect the signature of the amp.
So naturally this is why some of the more high end offerings use DC heater supplies for the tube. That also worries me because we're not talking about a battery here, this would be a rectified AC supply with however much noise and sag you decide to allow. Another scary prospect. Even scarier are the solid state regulated supplies that promise perfection but I only see artifact.
Then I find from the WE spec sheet and literature that while they use the best coatings, DC on the heaters can still cause some stripping and reduce the quality and life of the tube. Obviously we're not going to do that. So then besides the worry about the quality of the pot comes the worry about hum. The other reason many hi end offerings go with DC on the heaters.
To do this right, the power transformer is going to have to be really good. The extra trouble to wind it with reduced stray fields, and no primary capacitance coupling. Not to mention since the cathode is the AC heater for the tube, and there are two tubes, there should be two separate heater windings, one for each. And then a 3rd for the input tubes and a 4th for the rectifier.
I figured out that all the pot is doing (other than balancing AC for zero hum) is acting like the center tap on the heater winding passing the signal from ground to either side. That means that if a heater supply is center tapped, you should be able to use that for the cathode resistor instead of the center wiper of the balance pot.
To make the balance pot work, the bypass cap for the cathode resistor can be placed on the wiper since we're only dealing with a low frequency (50/60Hz).
With the cathode current and signal no longer going through the hum balance pot or it's wiper, the amp stands a chance at being a Zen Triode.
Below is a diagram of how I'm doing it.
The right side seems to be the standard method. The left side would be superior on multiple levels because now the only thing the pot does is balance AC to the ZERO point while the DC current and signal do not pass through it.
So this forms the basis for my plan, along with a custom transformer. The output transformers will be the same UFO transformers used in the SE34I.5 now SE34I.6. The driver stage will also be identical but with different tubes. At least at this point in the development that is the plan.
At this stage I am worried about the giant power supply transformer, worried about possible hum, and worried about too much heat. I'm not going to lie, it's stressful like driving around with your muffler dragging the ground. For this reason I am not wasting any time on this project. It's some new territory for me and let's be honest, the pressure is high. Not only would everyone expect it to be better than other 300B amps because it's a Decware but its performance will be measured against the 25th Anniversary Zen Triode amp which has so far been completely unbeatable. This would be a lot of pressure for an amp builder who only dealt with DHT tubes, but for me, of course it's over the top so as I said no pressure here!
Steve
. Now I have to decide if I want to change my order from a pair of UFO25s to a pair of these. Good thing I have plenty of time.
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