Hi All,
I would appreciate some help from the community on the Beast Jr amps.
With Bruce Rozenblit having now made the assembly manuals and schematics available online, I am intending building a pair of these amps, having used their Beasts for many years, but foolishly selling them some time ago. I do have a pair of circuit boards for the Beasts and may build another pair at some point, but as I now use horns, I don't need the power and the Junior's seem a much better bet.
I do, however, have one problem, hence the request for help.
The amp uses Transcendent's patented OTL technology and most of their designs now use the EL509 in the output stage. Here in the UK, the JJ EL509's are £50 each (around US$65) and I need 16 of them. A couple of years ago, I built the Atma Sphere M60's when Ralph kindly put the schematic for his earlier version on this forum. During the build, I managed to buy a large quantity of the 6AS7G's and still have plenty to spare. So the question is, can I use the 6AS7G's in the output stage of the Beast Jr, in the same way that Bruce used them in one of his earlier designs in Glass Audio, back in 1990
If so, how many could I parallel up, and what sort of idle current would be suitable.
I would appreciate some thoughts and comments and look forward to a few replies.
Thanks and Best Regards,
Steve.
I would appreciate some help from the community on the Beast Jr amps.
With Bruce Rozenblit having now made the assembly manuals and schematics available online, I am intending building a pair of these amps, having used their Beasts for many years, but foolishly selling them some time ago. I do have a pair of circuit boards for the Beasts and may build another pair at some point, but as I now use horns, I don't need the power and the Junior's seem a much better bet.
I do, however, have one problem, hence the request for help.
The amp uses Transcendent's patented OTL technology and most of their designs now use the EL509 in the output stage. Here in the UK, the JJ EL509's are £50 each (around US$65) and I need 16 of them. A couple of years ago, I built the Atma Sphere M60's when Ralph kindly put the schematic for his earlier version on this forum. During the build, I managed to buy a large quantity of the 6AS7G's and still have plenty to spare. So the question is, can I use the 6AS7G's in the output stage of the Beast Jr, in the same way that Bruce used them in one of his earlier designs in Glass Audio, back in 1990
If so, how many could I parallel up, and what sort of idle current would be suitable.
I would appreciate some thoughts and comments and look forward to a few replies.
Thanks and Best Regards,
Steve.
I believe the back-of-the-envelope answer is that since transconductance is the dominant characteristic that determines the performance of the output tube in an OTL, and an EL509 has a Gm of 18,000 and an 6ASG7 has a Gm of 7,000 for each triode, you need 1.3 6AS7G to get the current drive of 1 EL509.
I've built quite a few otl's first with 6 519's then 8 and finally with 10. The latter with an iq of 100ma/tube triode connected. The earlier versions needed a lot of feedback making the sound lifeless and flat. The last one didn't need feedback and sounded best but at 700w idle while you're listening at only a few watts. I also built a 6as7g version (circlotron) with 8 6as7g's per channel and that one sounded really good. Iq was 50ma/triode. As you have a lot of those tubes I would certainly use them. The more you use the more natural the sound (pwr supply was + and - 150v).