OTL amps, my pick for the tube used.


This is my OTL tube pick for an OTL the 6C33C tube.
I don't believe much can compete with it for current, and that the big achilles heel of OTL's  

https://www.navisonaudio.com/uploads/images/thumb_medium/otl-150-1.jpg

https://www.navisonaudio.com/uploads/images/thumb_medium/otl-150-3.jpg

Just had to show this, in it's glowing glory.

Cheers George
128x128georgehifi

Showing 6 responses by atmasphere

They actually recommenced replacing the socket with each tube renewal!
The original Russian spec sheet called for 750 hours! In practice I've seen the tube outlast the socket.
Now I see M Mathews is offering them for $142 so this is either new production or NOS. I was told that there were warehouses everywhere with this tube stacked to the ceiling. Don't worry, never will run out, I was told.
I got this same story- but was also told by Mr. Mathews that the only supply there would ever be was NOS. So I'm not buying the story that New Sensor controls the tooling, but if that actually turns out to be the case (we've not seen any 6C33s made past the 1990s) it would be a simple matter to set the production up to make the tube for the socket used by the 7241- then the sockets would last decades instead of 100s of hours.

The thing that has always concerned me is the cathode flaking I see with Pass Tubes.
The 6C33 is a pass regulator tube but does not seem to have this problem. IME, the key to making **any** power tube last is 'pre-conditioning'. This is the practice of lighting up the filament circuit for an extended period (often several days) prior to applying B+. Each power tube type has a certain minimum amount of pre-conditioning time that is optimal for the type. Once properly pre-conditioned, the life of the tube can be doubled, and premature failure due to arcing (caused by cathode coating failure) is dramatically reduced.
In the construction of an OTL, there are several factors that should be considered in the choice of power tube, and they are fairly practical. The cost of the tube vs power output is one factor- closely related is availability (which should be considered in any design FWIW). Reliability is important as well as things like plate resistance (which will relate to how many tubes are needed to do the job) as well as the filament current the final tube bank is going to need to make it run.

Availability is a major concern because power tubes fail. While we can get 10,000 hour service lives out of our tubes, knowing that the tube will still be there down the road has been important, since our amps are pretty reliable. So since the inception of Atma-Sphere we've had an internal design rule called the 20-Year Rule, which simply states that since our gear is built to run about 20 years without service (other than tubes)- after which time the filter capacitors may need replacement, its a Good Idea to design the amp for tubes that are available and in production. Since we've now been in business 43 years, this rule has really worked well for us- we've seen competition that didn't take this idea into account and consequently had to close down. In the twilight of the tube era (which ironically has lasted longer than its heyday) it reasonable to expect that the diversity of tubes will dwindle so we've always been conservative about tube types. Its helpful when you have more than one source for tubes!

The ultimate OTL tube in many regards is the 7241 but it's expensive and rare- not a good choice. The 6C33 is a lot cheaper and with slightly over 1/2 the power output capacity of the 7241, still a good choice, but its prodigious filament requirements (and resulting heat production) causing it to eat sockets makes it less reliable.

Sweep tubes could be an excellent choice, hampered only by availability as they are decades long out of production- except for the PL509 and its variants. They have good filament requirements though and are otherwise pretty reliable, although linearity isn't great- but that can be improved by wiring them in triode. 

The 6AS7G is the tube we chose as most on this thread know. But rather than use the American types which indeed proved to have fragile cathode coatings, we used the Russian variant, the 6N13C (marked '6H13C' or sometimes '6AS7G'). This tube has reasonable filament requirements, is easy on the sockets, is inexpensive and over the 43 years we've been in business, proven pretty reliable- such that we have always had a 1-year warranty on the power tubes. Linearity is also good- it is a triode after all- and it has a fairly low plate resistance. Since we were uninterested in building an amplifier with a lot of feedback, linearity in the output section was/is pretty important to our design, as keeping distortion low without feedback is a bit of a trick.

I prefer my OTLs of the non-cyclotron, low waste heat, non-autoformer variety.
So do I. A cyclotron OTL would be a thing to behold though if it didn't make you blind :)


The 6C33 is a nice sounding tube. The real issue with it is 2-fold:1) its been out of production for about 20 years. The tooling was destroyed with the plant that made it so there won't be any reissues.
2) the tube was designed to use the socket for the Sylvania 3C33, which is a dual power triode with less than half the filament current. As a result, the 6C33 overloads the socket connections which can fail within the lifetime of the tube itself. The sockets that hold up the best are NOS; the Chinese socket which is the most common is not very reliable. As a result special precautions have to be taken to extend the socket life but sooner or later when the socket fails it simply has to be replaced. This may not be a trivial matter depending on how the amplifier is built. Because of the heat involved circuit boards should be avoided in the output section. 

A further issue that is a common problem with OTLs is how the output tubes are biased. Because with any OTL multiple output tubes are used, the grid capacitance tends to be quite high in the output section. This in turn leads to a fair amount of grid current compared to higher voltage power tubes. The result of all this is that controlling the output tubes properly has been a traditional problem in OTL design which has resulted in the reputation that OTLs are unreliable (in a good number of cases this is well justified). This is caused by the fact that to get bass out of the amp, larger coupling capacitors have to be employed in the output section, but the bigger you make them the more coloration they have. This is countered by the fact that the bias network has to employ fairly low values in order to actually control the power tubes (re. the grid current I previously mentioned); because these two factors run counter to each other, designers often fudge the numbers a bit- and often run into issues like blocking distortion (as seen in the Joule Electra amps, which will literally shut down for a few seconds if overloaded) or bias drift, such as seen in the Counterpoint SA-4. When the amp is properly designed, you should be able to easily overdrive it all day long without the amp oscillating (regardless of the speaker load), bias going off or anything like that- and it should recover instantly from overload.


When these factors are right, the amp is simultaneously more transparent and will have greater authority.



What amplifier are you referring to? What I was pointing out is that George’s statement about 100 amps referred to the solid state ME 1400 and ME 1500
I just looked on the manufacturer's website- and didn't realize it shows a different model. Apparently the models you mention above are n.l.a.


^^ Clearly the 100 amp thing has nothing to do with its output power. That is likely the 10mS power supply short circuit current.
Before George points it out, the datasheet shown at the second link in his last post does in fact say "output current: 100 amps (continuous)," for the ME 1400/ME 1500 amplifiers referred to in that post (as opposed to the OTL amp referred to in his OP).
@almarg If you look at their website, they claim 100 watts into 8 ohms and 150 into 4. So I am assuming that '100 amps' is a typo, not uncommon on the web... but just for fun, since Power = R x I squared, and giving the amp the benefit of the doubt by assuming a 1 ohm load, 100 amps would work out to an amplifier that makes 10,000 watts into 1 ohms, and 40,000 watts into 4... Obviously the amp can't do that, so I'm going with Occam's Razor on this one, IOW a typo is the simpler explanation :)
Those 6C33s in the OP are too closely spaced for reliable operation. The 6C33 is a very hot running tube on account of its prodigious filament.

We re-introduced the Novacron by popular demand, but its a simple fact that other tubes are a lot easier to live with. 
I know this is a good figure for an OTL, but I have to laugh when I look at the flagship models of my amps (100-amps continuous)
Just to be clear, that's 100 **watts** not amps, per the manufacturer's website. With 150 watts into 4 ohms, that's about 6.2 amps total. Our MA-2 can do more than that.