Hi,
My apologies for being away for so long. I have been working up some new tube applications for the low powered OTL with matching Autoformer. As I previously stated, an autotranformer has many advantages over a push-pull plate transformer. Small OTL's naturally like high impedances around 100-400 ohms. This is still low compared to the RM-10 transformer which is 12,000 ohms. I will keep you posted on the progress.
This posting was partly inspired by the following discussion on diytube.com:
http://www.diytube.com/phpBB2/viewtopic.php?t=3235. Thanks to Lafish for letting me know and responding to the nay-sayers. I will be referring them to our gentle circle so be ready.
Tube applications are many and varied. Although the data books list 2-5 these are just examples and are meant to help engineers get their circuits going without having to find the right load impedance, voltage and bias currents. There are an unlimited number of other applications that can be created and that is what I find interesting. None of the amps I have produced use published applications. Instead I study the tube, have a look at the actual curves on my curve tracer, see what I can do without exceeding the tube specs. In fact I usually get far longer life than most other applications, in the range of 5000-10,000 hours. The RM-9 was the first amp I know of to state 10,000 hour tube life which has been well proven by the users of 930 units some as old at 24 years now. I have a local customer who bought his in 1987, uses it daily and still has the original Siemens tubes. We did have to replace one that got gassy after 12 years. The other 7 are still going strong and still well matched!
Exploring tube applications starts with choosing the right tube(s) for the job. Tubes are designed and classified by the expected usage. For instance Horizontal output tubes for TV are especially suited for that but they also make fine output tubes yet not for all circuits. These tubes are not well applied in triode or ultralinear circuits but are excellent in certain pentode and OTL circuits. My new OTL series will be using a selection of them based on power desired.
The ultralinear/triode/pentode tubes with which we are so familiar were designed with that in mind and will tolerate the high screen voltages that are found in these traditional circuits, though not if the B+ exceeds 500 volts.
Audio triodes like the 45, 300B and Emission labs series we sell were designed for high voltage low power amps. These tubes allow for very simple circuits and their popularity is in those applications. Western Electric had good reason to create the 300B and thousands of movie theaters around the globe had amplifiers using that tube.
Triodes with low mu (typical 2) were designed as pass tubes in regulated power supplies. These are not good choices for audio output tubes, though many may disagree. Those who have experienced amps built with these tubes find exactly what I predict; drifting bias, extreme power consumption for very little output and generally unreliable operation. These tubes were not meant to hold a steady bias current, instead they are made to vary current widely just as needed in a voltage regulator as the load varies. They have high heater demand and in today’s energy crisis they are the dinosaurs of audio tubes.
Any questions?
