45 46 300B Convertible Triode Amp
I’ve built or bread boarded quite a few 45 amps, and have remained impressed at how truly wonderful Gordon Rankin’s Bugle 45 sounds. I also wanted to play around with a few other common triodes including 46 and 300B. This amp does a surprisingly good job with all 3 tubes. ( It is also much simpler to build if the 300B option is removed).
I currently use ST 46’s most of the time. Power output is a modest 1.25w. The 46 is inexpensive (even as NOS) and easy to locate.
This is one of the best sounding amps I’ve yet built!
Some construction notes:
The amplifier section is copied directly from Gordon’s article in Sound Practices (Issue 13), and can be downloaded below. I prefer 5751’s as the driver tube over the 6072.
I’ve added a second cathode resistor of 820 ohms that can be substituted when 300B’s are in use.
The SRPP driver is built on a small PC board, makes for tight wiring.
The power supply has been changed significantly. First I used an 83 mercury rectifier. To allow a few moments for the rectifier to warm up before full B+ is applied I use a 1K/10watt resistor before the first choke which is shorted out by a time delay relay after 15 seconds. If you don’t want to deal with the 83 and a turn-on delay consider either an 83V (indirectly heated) or substitute an octal socket for the rectifier and use a 5V4 for 45/46 and a 5U4 for 300B.
There are separate voltage taps for 330V (45) and 285V (46 strapped class “A”). There are separate sockets for 45/300B (4 pin) and 46 (5 pin) which are wired in parallel. To switch from 45 <-> 46 only a single tap on a terminal strip is changed for the B+.
To switch to 300B the filament is changed from 2.5v to 5v (I use a 5vct transformer to handle both), the rectifier is changed to a 5Z3, a small input capacitor (2uf) is switched into the circuit and the cathode resistor is changed to 820 ohms. All of the wiring that needs to be moved is on terminal strips, changeover takes only about 2 minutes. B+ is raised to 385v for the 300B. The driver section does get a bit higher B+ in this mode, but doesn’t seem to mind.
I find the 300B operates very nicely into a 5K load instead of the traditional 3K output transformer.
The 8uf oil cap in the power supply was a large sonic improvement over a film or electrolytic in this position.
My transformer is a vintage Thordarson, chokes are old stock Stancor. Don’t use a cheap choke for the input – it will buzz like crazy. Also use a choke that is rated for much higher current than this circuit draws.
Coupling caps are “orange drops”, cathode caps are Solen, sockets are old stock ceramic and the 100uf electrolytic is a LCR 50/50uf wired with both sections in parallel.
Schematic:
Bugle 45 Article from Sound
Practices
Power Supply:
Bottom View:
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