KR Audio produces a 50 watt SET, but not using a 300b (of course)
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I am using a JAS array 2.1. a pair of sofia 300bs driving a pair of 805s yielding 45w of class A power. It runs hot (a small muffin fan keeps things cool)but it has that 3d holographic midrange and will drive 4 ohm loads. It has a passive preamp built in but I'm using the power amp section only driven by my Tranquility DAC using Audirvana plus attached to itunes in my Mac Mini, tons of gain and dynamics galor driving my Audiokinesis Prizmas.
In order to produce more power in a SET tube, the potential difference between the cathode and anode has to increase. When this voltage goes up to a point, the tube will start to emitt other radiation such cathode ray, ie electron. Higher still, it will emitt x-ray. I am a radiologist so I know.
There is legal restriction to control this type of potential X-ray tube. No one want to have x-ray examination while running his up power SET amp. That's why.
As Brf indicated, there is only so much power a given tube type can produce. Using two or more power tubes in parallel, in a single-ended configuration, would allow more power to be generated, and is sometimes done. That would be referred to as a PSET (parallel SET) configuration. Alternatively, the VAC Renaissance 70/70 MkIII amplifier I have uses four 300B's per channel, configured as two push-pull pairs in parallel with each other. It is rated at 65 watts per channel. But of course it is not a SET (single-ended triode), because it is push-pull rather than single-ended.
I have been told by a designer that doing parallel single ended is not that easy to do and somewhat defeats the benefits of a simple single-ended circuit. It is hard to get twin output tubes to behave correctly in parallel and a very high demand is placed on exact matching of the tubes. A lot of SET purists also think that the operation of more than one output tube detracts from the purity of the sound of a good SET amp. I would also bet that there is more complexity and sources of sonic degradation from the additional demands on the driver tubes/circuit as well.
For something like 50 watts, one would be looking at four 300Bs in parallel--I know of only designs that work with two. There must be a good reason why one does not see four output tube parallel designs even for cost-is-no-object designs.
Jwm, thanks for the compliment but I'm not planning to make any commercially, for a few reasons. First, I have a demanding full-time job that I'd like to do for another 8 years or so. Second, the amps cost ~$12k just in parts, some of which are painstakingly hand made, such as the teflon tube mounts. Third, the plate voltage on the 833C is 2.3kV...I shudder to think of the potential liability lawsuits if a customer's dog, cat or child (or self) did something stupid.
That said, anyone near central NJ who is curious about the sound is welcome to visit for a demo. Just pm me to set up a listening session...but I won't build you one, that's up to you!
Oh my, that is some serious voltage. I would be so darn nervous testing that baby during build! You are the MAN!
I followed your long thread during the build and was and am fascinated by it. I would love to build my own tube amps with the thought and care you did, but with no more than 300-400 volts:)
Good reply as always.Hi S1nn3r, if you want to use the 300b tube and require 50 watts you'll have to go with a push pull design amplifier. If you really want what a good 300b SET has to offer you'll have to use more efficient speakers. I see no other way around this reality. The 300b is what it is.
K, maybe not 50w... but how 'bout 38w?
Wyetech's new Ruby does just that via a "serial - parallel single ended configuration". Unique and innovative. Check this out:
"300B OUTPUT STAGE
All 4 output tubes also operate in pure class "A1" single ended self-biased isolated circuitry and transformer coupled output.
The 300B gain stage circuitry is exclusive, ground-breaking and unique, being only one of its kind ever to be implemented in a tube output stage.
Simply stated it is a serial - parallel single ended configuration that results in increasing the 300B stage gain by a factor of four, unprecedented and unique.
Two legs of two series connected 300B's are used in parallel by tying the top plates of each leg to drive the primary of the output transformer. The two bottom tubes of each leg is referenced to ground while the top tubes have floating biasing and signal drive.
The current in each leg is monitored for proper operation. If the current goes above or below the designated operational range it will turn on it's corresponding RED LED to tell you it's time to change this pair of tubes.
One of the advantages of this circuit is that it decreases the high voltage supply current by one half, thereby reducing the current requirements for the chokes in the power supply.
Of course, in order to accomplish this you must double the high voltage to this stage. This would be a stumbling block for most designers who tend to shy away from such high lethal voltages necessary here, similar to 211 transmitting tube requirements,
The drive for perfection makes this a necessary measure, since we are using only pure capacitors and inductors [ Chokes ] to filter the dual high voltage power supplies.
Even with this added benefit it still took a large chassis with each monoblock measuring 16.5" x 24" to hold and accommodate the parts needed to implement this revolutionary and innovative circuitry.
Because each leg of both tubes have the same current flowing through them they each have equal bias voltage applied which ensures the tubes are working together in harmony to produce the best possible operating conditions and life expectancy for each pair."
See more at Wyetech's site.
I'm ordering a pair.
Keep in mind that the Wyetech is using "4" 300b tubes per mono block to achieve the 38 watts. So roughly 9.5 watts per 300b tube. 8 to 10 watts is the usual power rating for most 300b SET amplifiers utilizing a single output tube per amplifier. This is the tube's sweet spot for quality sound. You can push an individual 300b to 15-17 watts but now you're simply stressing the tube and diminishing its inherent sonic attributes. My SET is 8 watts, probably 80-85% of the time with my listening levels(75-80 db range) I'm using only "fractions" of 1 watt of power.
The Wyetech appears to be a very interesting model, although they will not have too many takers at $67k. I am not that up on technical aspects, but, the description which talks about an upper and lower leg sounds more like pushpull to me, with each leg in opposite phase. I have no idea how this topology compares with what is conventionally viewed as single-ended. As with any amp, regardless of tube type and topology, the proof is in the listening experience. Based on what I've heard of Wyetech gear, this should be a quite nice sounding amp; a friend had their 211 single-ended amp and I thought it sounded very good.
While I believe this thread is more of a theoretical discussion of why a particular tube can or cannot be used in higher power applications, I hope no one gets too caught up in any one particular topology or tube type as the be-all and end-all of tube amps. Even if one generally likes SET amps and generally likes the sound of 300B tubes, it is not necessarily the case that the best amp will tick both boxes. In the end, it really does come down to auditioning the particular amp.
I personally like SET amps myself and own a parallel SET amp (Audionote Kageki). But, I also own a pushpull amp that uses 349 pentode tubes and it is the amp I currently have in my system. There are tradeoffs both ways in terms of the sound of the two amps. For example the pushpull amp has a tighter and punchier bottom end, but the SET amps bass sounds more subtly variable (i.e., less mechanical sounding). But, the best sounding amp I've heard is a custom built 30 watt OTL amp; I did not consider purchasing it because it is a bit frightening (massive, complex startup procedure, no protection circuitry so there is a risk of blowing up one's speakers). Hence, I don't consider any one topology to be the absolute best--it really depends on the particular implementation.
As for triode tube type, the 300B is probably the most popular. I suspect that a lot of that popularity comes from it being the most practical tube in terms of output. The other small triode tubes that don't require extremely high B+ rail voltages just don't put out enough watts compared to the 300B. Yes, there are a lot of people that flat out prefer the sound of the 300B to other triodes, such as the 45 and 2a3, because of its rich upperbass/lower midrange, but, most of my friends with high efficiency speakers like the 45 tube more (tighter bass and more extended top end). I like the 45 too, as well as the 2a3.
To add further to the complications, different tubes under the same type can sound quite different. A Kron 300B will sound quite different from a Western or a TJ 300B.
If higher power is needed, it makes sense to consider all options: higher powered SET amps, such as the 833 mentioned above, or 845, 211, GM70, or 1610 tube; pushpull amps and OTL amps (I particularly like OTLs in the power range we are discussing) and even solid state amps. In short, avoid the trap of only looking at certain tube types and topologies.
You can push an individual 300b to 15-17 watts but now you're simply stressing the tube and diminishing its inherent sonic attributes.
Not necessarily true as regards to diminishing sonic attributes.
My SET is 8 watts, probably 80-85% of the time with my listening levels(75-80 db range) I'm using only "fractions" of 1 watt of power.
It is nice to see someone that understands the correlation between their listening levels and the power output of their amplifier. I wish more people would understand this.
I don't build or design 300b SET amplifiers (I get to do the easy part, just listen and enjoy). I was repeating what I've been told and read from those with this experience. They believe that the tube has a performance/power curve or ratio. They say if this tube is pushed too hard in search of power output there will be a deterioration in the intrinsic sound quality. I heard your Electra Fidelity 300b SET at CES driving the Cessaro speakers and it was truly a treat, pure natural sound(one the very best rooms at the show IMO). How hard does this amplifier(power output) run the 300b? Are you familiar with amplifiers that maximize the 300b power curve and sound good doing so?
Hi Charles & Clio,
I have no idea myself about optimal output of 300B tubes. However, I know a builder who likes the sound of his pushpull amps when the tubes are pushed hard (I believe he operates the tubes at a pretty high plate voltage relative to the rating of the tube). The obvious downside is that the tubes don't last that long under such operating conditions. In his view, his preferred tubes (45s) are "cheap."
I can't disagree with a word you've written and I don't mean to imply that
the 300b SET is the only route to sonic bliss. Of all the various topologies
I've heard over many years, this is the one that did it for my particular
desires of music reproduction. Certainly there are those who have also
achieved equal satisfaction as I have with all manner of amplifier
alternatives. You reach a point when you just know what you like, that's
where I am these days. Building a audio system allows you the freedom
and choice to get it "your" way.
Good discussion by all.
09-03-14: Charles1dadAs you'll most likely recall, Charles, the VAC Renaissance series is one example. Member "Raquel" (Drake), who is quite knowledgeable about these amps, posted as follows in this older thread:
What the above poster states about the Renaissance amps running 300B's hard is true -- 95% of the WeCo spec maximum voltage. I've owned VAC Renaissance amps for eight years and they require carefully vetted output tubes. In fact, Sophia used to market a special set of the Sophia tubes for VAC Renaissance amps, which were the basic Sophia tube, but carefully tested at plate voltages that resemble the voltage that 300B's see in the Renaissance circuit. Generally, as for transconductance, 300B's for the Renaissance amps should test in the 3,000-5,000 range. In addition, the plate-to-cathode voltage for 300B's in the Renaissance amps is approximately 430 volts dc, with idle current approximately 85 to 90 milliamperes in a self-bias (cathode bias) circuit. Again, this is approximately 5% below the maximum rating for the WeCo spec 300B. The milliamp and transconductance testing for purposes of matching must be done at these voltage levels, or you will find out the hard way, when your amp starts imitating a popcorn popper, that your 300B's didn't make the cut.Looking at a 1939 datasheet I have for the original Western Electric 300B, the absolute maximum plate voltage it is rated to be able to handle is 450V, and its absolute maximum rated plate current is 100 ma (70 ma for fixed grid bias, which is not used in the Renaissance amps). And a cautionary note indicates that the 450 volt and 100 ma figures are not simultaneous ratings. Elsewhere in the datasheet it appears that at 450 volts a plate current of 80 ma would be the absolute maximum. Under those extreme worst case conditions (which are explicitly NOT recommended) maximum rated output power is spec'd at between 11.5 and 17.8 watts depending on load resistance.
The highest rated output power under any of the many RECOMMENDED operating conditions (i.e., recommended combinations of plate voltage, plate current, and load resistance) that are indicated in the datasheet is 12.5 watts, btw. The corresponding set of operating conditions for that rating is 400V, 80 ma, and 2500 ohms.
Designer Kevin Hayes has indicated to me and others that a truly WE-spec compliant 300B will have no trouble in these amps. As he has indicated, though, and as "Raquel" has emphasized in a number of past threads, a number of 300B reissues from contemporary sources do not meet those specs, and can turn this particular series of amps into popcorn poppers. However, I have not read of or experienced (after 3 years of moderate use) any indication that 300B's which are truly WE-spec compliant would have their longevity unduly compromised in these amps.
Regarding the discussion of paralleling multiple output tubes, based on my (limited) understanding of their architecture I would think that the Atmasphere OTL's are an example illustrating that under some circumstances far more than two triode output tubes can be successfully paralleled. Although, of course, as differentially balanced amps they are not SETs.
How hard does this amplifier(power output) run the 300b? Are you familiar with amplifiers that maximize the 300b power curve and sound good doing so?
Our circuit puts 500V on the plate. In addition to other circuit design features this allows us to get 12 very clean watts (low IMD and low IMD rise) out of the tube. As the output transformers are specially wound for a Class A3 circuit, we see negligible increase in stress on the tube.
Designer Kevin Hayes has indicated to me and others that a truly WE-spec compliant 300B will have no trouble in these amps. As he has indicated, though, and as "Raquel" has emphasized in a number of past threads, a number of 300B reissues from contemporary sources do not meet those specs, and can turn this particular series of amps into popcorn poppers.
Thank you for bringing this up Al. As a former owner of VAC Auricle Musiblocs I have great respect for Kevin Hayes and his designs. These are the types of tubes that we recommend in our amps as well. The "new production" faint of heart tubes need not apply. If you can't get a nice set of NOS 300B tubes we recommend the current production of the Sovtek 6A3, which is not really a 6A3 at all if you look at the specs. It is reliable and works very well in our circuit.
Raquel, now that's a name from the past, I always found his comments interesting and informative. What I don't know is this, SET vs push pull circuits, is one inherently "tougher" on tubes than the other? Or is there a wide spectrum in terms of stress/demand with either topology that's the overriding factor?
Clio09,was the version of your SET I heard(CES) using NOS W.E.300b or a current production 300b?
09-03-14: Charles1dadAs you realize, Charles, there are obviously a great many design dependent variables involved. But I suppose that there is probably a loose/partial correlation between SET and greater stress on tubes, due to the fact that SET designs (or at least conventional SET designs) of necessity operate with Class A bias (thereby consuming lots of power at all times, relative to the power capability of the amp), while of course many push-pull tube designs are Class AB.