This is the first installment of the tube of the month series in which I will present tubes I like and use a lot or which I find remarkable. I will mostly introduce some tubes which are less known for their usability in audio circuits, but you will also read about some of the tubes which are quite common in audio circuits.
Although many of you might know that I'm ususally very fond of directly heated triodes, especially those with thoriated tungsten filaments, I will open this series with a tube type which you probably would not expect to be raved about from me: An indirectly heated beam power tube, meant to be used for horizontal deflection stages in TV sets, the 6CB5A.
Why such a tube you might ask. What brings a die hard user of directly heated triodes to the odd TV tube? Here is the story behind this: About two and a half years ago there was a discussion in a german tube audio forum called Röhren und Hören (translated: Tubes and Listening) about a DIY tube power amp. Details about that amp will be covered in an upcoming series of posts about a SE amplifier platform. One of the requirements for the amp was moderate parts cost. If you are like me and don't feel comfortable to start an amplifier project unless you have at least a dozen spares of the tubes, tube price becomes an important factor in the overall cost. Even the cheapest chinese 300Bs are $100 now. Considering just a single spare set that is $400 for output tubes alone. A dozen NOS 6CB5A will set you back just $60 or even less. And you don't even have to search a lot to find them at these prices. Almost every reputable tube dealer lists them for $5.
The 6CB5A curves are shown in the left screen shot and the right one shows a Western Electric 300B for comparison (original manufacture not a reissue from the 90ies). The right most curve is a bit closer due to a glitch in the tracer, so ignore that one. Admittedly the 300B is more linear, but the 6CB5A looks quite similar. Definitely worth trying triode connected in a single ended amplifier and a low cost alternative to directly heated triodes.
The maximum plate dissipation of the 6CB5A is 26W. So it should be good for 6-7W in SE mode. Due to it's low cost there is no need to be nervous about running it at it's limits or even a bit beyond, so maybe up to 8W can be squeezed out. But more about that in upcoming posts.