Incept date: 2022-01-19
The HH Scott LC-21 is a clean, straightforward 12AX7 based preamp. Fairly rare, it was only sold from about 1961-63. It was intended as the front end for the mighty Scott LK-150 power amp, which pushed about 60 WPC from it’s 6650A/KT-88 outputs.
The LC-21 has multiple unique features that precisely match my desires for a high quality vintage tube preamp:
- Clean & simple design with commonly available 12AX7 tubes – Easy to service!
- Two phono inputs – I have two turntables
- A “flat”(mic) setting for the phono inputs – Lets you digitize old 78 and LP records and then re-EQ in post, after you’ve de-noised. Also enables real time 78 listening
- Comprehensive mono/stereo switching and polarity invert or “phase” – Useful for checking speaker and cartridge phase and listening to old records and mono sources
- Buffered line outs – The better to drive a solid sate amp, my dear!
- A mono mix output – Useful for a sub or a mono monitor speaker
- Aesthetically pleasing – The gold faceplate looks nice with my other Scott eqpt. and helps with the “PAF” (Partner Acceptance Factor) issue
I’ve wanted one of these for years, but clean examples are tough to come by. Now I’ve finally obtained one, and this page will detail the restoration process.
A quick check of the underside shows no obvious issues or tampering, other than a replaced line cord. All the controls spin freely, I’ll clean and treat them first up.
Fuse is the correct value, so time to variac the unit up for a quick audio check.
Red yellow is THD, blue green is frequency response
(Apologies for the screen shots, the export function on my oldel AP one no longer works.)
As expected, the audio quality is a bit off. The distortion is quite high, and the frequency response is rolled off at both ends. Those ancient coupling caps are the likely culprits, the first three I measured were awfully leaky!
First up:
– Replace the old leaky output couplers! Duh!
– Replace the cathode bypass caps in the tone section. (They’re in the big cans, they never last.)
– Rebuild the filament DC supply. (That selenium bridge is smoke about to happen.)
– I’ll also remove the simplistic high pass filter (C9/C109) by replacing the 560 pF cap with a 0.01 uF coupling cap. (This is discussed in the build guide, btw.)
2022.02.19
Replaced the 3 output couplers and the two “can” electrolytics in the tone section cathodes (C10/110, 25 uF, 25 V)
Wow, what a difference already! The THD lowered and flattened out a lot, so I’m on the right track!
The frequency response irregularities are likely the other couplers.
More to come, once I finish replacing the plate resistors. They all making that spitting noise that says they’re cracked, and their values have drifted a lot too.
Last Updated on 2023-09-13 by Daev Roehr