I’d always wanted a valve amp for my hi-fi, so I was very pleased when I picked up the remains of a Crown SXA. This is a 30-watt per channel stereo valve amp, made in 1960 by the same Crown company of Elkhart, Indiana who would later give us the giant-slaying Macro-Tech amps. (Yep, I want one of those for my amp collection, too! A MA10,000 will do nicely 🙂 )
That’s the good news, the bad news was that there wasn’t much left of it. I knew there was no chance of getting it back to original condition. All of the components were missing, and the EL37 valves originally used are now very rare and expensive. What’s more, I hated the look of the front panel! I made the following changes:
New front panel in minimalist hi-fi style.
Converted to 240V, 50Hz mains. (original transformers were 110V only, connected them in series and added a 15V bucking transformer)
Power valves changed from EL37s to EL34s, idle current increased to 40mA per tube
Transformer-balanced XLR inputs, to match my ghetto multi-room sound system which I’ll write about some other time.
Binding post speaker terminals.
Bigger smoothing capacitors, in Pi filter configuration with choke.
10-turn bias pots, one per EL34.
Crown’s bizarre bias test jacks and lamps replaced with 1 ohm cathode resistors and 4mm jacks, allowing bias to be read in millivolts on a so-not-1960 digital multimeter.
I recently measured the THD:
0.038% at 1kHz, 1W
0.22% at 1kHz, 10W
1.00% at 1kHz, 30W
and it’s mostly 2nd and 3rd harmonic as expected. At 100Hz and 10kHz it only gets worse!