Joe Roberts' piece, "I Never Met a 2A3 Amp That I Didn't Like," from Sound Practices Issue 15, is very persuasive. I built jc morrison’s micro 2A3 amp soon after I received the issue via snail mail fresh from the printers in the mid '90s. After rereading the article in ‘07, I got inspired to make a Radiotron SE2A3 variant.
The voltages found in the following circuits can be lethal! Build at your own risk!
|jc morrison's micro|
|Fi "X" by Don Garber|
from a Sound Practices ad
During the March 2020 lockdown, I was motivated to review my audio DIY to-do list which led to a long-overdue parts bin inventory. I turned up a 700 VCT @ 150 mA power transformer pulled from a junked Scott 99D mono integrated. This iron is not a stranger since I've used a pair in my PP2A3 mono-blocks in the olden days. I dug up a couple of 6C8G grid capped/octal based twin triodes as well.
Despite the equal transconductance, the 6C8G isn't a drop-in equivalent of the 6SL7. Aside from the different pinout, the mu and Rp are lower. When the triode sections are paralleled, the Rp is closer to a medium mu/gm triode like a 76.
|morrison micro, reimagined by JE Labs|
This 2-stage direct coupled circuit is a reimagined morrison micro using a 6C8G front end. The paralleled triode sections draw about 3.4 mA and the 510 ohm cathode bias resistor is bypassed with a 100uf @ 16V cap to maximize input sensitivity.
Each 2A3 is AC heated by a Hammond 166M2 (2.5V @ 3A) filament transformer. The operating points are well within the modest specs of the Scott 99D power transformer.
The fullest potential of an amplifier circuit can't be realized if it's inhibited by a mediocre output transformer.
|Nature Sound NS70SE|
|Tamura F7002 Permalloy|
|Budget Output Transformers |
Noguchi PMF-10WS, Edcor GXSE15-16-3.5K, Noguchi PMF-15WS
|Noguchi (now G-trans.co.jp) Finemet|
|Kasuga KA6625ST Square Waves|
L > R: 100Hz, 1kHz, 10kHz
|4.34V rms squared/8 ohms = 2.35W|