This very high quality vintage amp was available from 1961-1964 as a $170 kit, or about $1400 in 2019 dollars. It was also available “pre-wired” for $270. It uses a pair of 5AR4 rectifiers, a 7199 driver/splitter and a pair 6550 outputs for each channel. It is widely regarded as an excellent sounding amp, even today. My rebuilt one bears this out, and it measures well too.
Here’s an Audio Precision THD vs frequency plot at 1 watt RMS. (The vertical scales are 0 to 1% for THD, +/- 3dB for the freq resp.)
IOW, the freq resp is a very flat 20Hz to 20K, +0/-.5dB at less than .2% THD.
Not bad for a 60 yr old amp!
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As of 2019, I’m working on refurbing one of these for my main system. Then life happened, and it got set aside for a while…
Update: 2023.12.17 – Time to return to this project
– Good NOS 5AR4 rectifier tubes continue to skyrocket in price. And this guy uses two of them! I’m going to install a SS rectifier stack with a thermistor for the slow start/surge control.
– The bias meter isn’t working, I’ll take a look at that too.
2023.01.08 – Looking good.
I dropped the B+ a tad post SS rectifier change, that will keep the occasional 122 VAC line voltage I see around here from causing any havoc.
PIC: HH Scott LK-150 refurbed, and with a few added safety and reliability mods: Grounded AC line cord, extra B+ filter capacitance, and a AC line thermistor w SS B+ rectifier circuit replacing the 5AR4 tube pair.
Measurements from my AP System One
Really flat frequency response (especially for a tube amp), and at about 0.2% THD across the audio spectrum. This amp easily meets OEM specs.
(insert graph here)
Ch B is quiet with 1.6 mV of noise, but Ch A has more noise, 2.8 mV. Hmmm. I plan on using these with my Klipsch Heresy’s which are quite sensitive, 99dB/1watt.
Unfortunately 3 mV will probably be audible.
Looking closer with a scope, it appears to be primarily 120Hz power supply “buzz”, yet it does not follow the driver tube. It goes away when the driver tube is pulled, so a little sleuthing will be needed here.
2024.03.19 – While chasing the noise floor issue, I encountered what looks like a possible schematic wiring error, and an added capacitor in a weird place. The fixed bias circuit pot look mis-wired compared to the usual Scott fixed bias circuit. I’ll change it and see what the effect is, and remove the capacitor. (see previous pic) This might explain the difficulty getting sufficient range to set the idle current for different output tubes.
Last Updated on 2024-03-22 by Daev Roehr