2A3 SET SRPP Loftin-White (Fi Primer 2A3)

2A3 SET SRPP Loftin-White (Fi Primer 2A3)

image This handsome amplifier uses the 2A3 Tubes for its output.  Based on the Loftin White Amplifier, Brendon does not give much detail about the final design that he settled on.  However the specs are here and the pictures tell a story themselves.

The biggest lesson to be learned from this project is just how important a wiring scheme and also wiring types make the overall success of a Valve (or for that matter any audio project.

This article was origionally published on Brendon’s Geocities hosted site.  In the interests of preserving the article Brendon has kindly given permission to re-home it at Justblairs Audio and Electronics Pages.  Thanks again Brendon.

Specifications

Tubes:
  • 2A3 Sovtek (new series)
  • 6SL7 Sovtek pre-amp/driver
  • 5V4G Sovtek rectifier
Transformers:
  • 374BX Hammond (universal primary) power supply
  • 375-0-375 RMS - 175ma DC secondary
  • 5V ct @ 2A ; 6.3V ct @ A4 filaments
  • 50 VAC Bias Tap
  • 30VA 2.5V ct x 2 (dual secondaries) for 2A3 filaments custom made by Harbuch  Electronics in Australia
  • 1627SE Hammond output transformer
    2.5Kohm 25 watt (max DC bias 160ma)
Capacitors:
  • All are 630v and 400v Solen polypropylenes.
Modifications:
  • No hum pots used as the donut is custom made and balanced for minimal hum.
  • HT lead is taken from 5 volt CT rather one leg to rectifier
Measured Voltages:
  • B+ right after rectifier tube = 431 VDC
  • 2A3 plate voltage = 420 VDC
  • 2A3 filament voltage = 2.55 VAC
  • 2A3 plate to filament = 260 VDC
  • 2A3 grid / 6SL7 plate voltage = 111 VDC
  • 6SL7 plate voltage = 220 VDC
  • 6SL7 filament voltage = 6.6 VAC
  • 6SL7 plate to filament = 165 VDC

March 21, 2006: After spending a week rewiring the whole amp inside, I COULD NOT believe the improvement in sound quality. I removed all the silver teflon hookup wires and used conventional PVC multistrand 1.5mm copper wires. Somehow silver to this amp was like poison to the sound.

WHAT A DIFFERENCE MAKES with neat wiring layouts! Between my collection of 2A3 tubes, I get between 0.5 -> 3.5 mv AC! The TJ Full Music tubes are the most quiet.

image Before Shot - MESSY!!!!

image Need I Say More?

image Sweet and Tidy!!

image Star Grounding.

image TJ Full Music 2A3