HH Scott LC-21

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 several 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 – let’s you digitize old 78 and LP records and then re-EQ in post, after you’ve de-noised
  • Polarity invert or “phase” – useful for checking speaker phase and listening to old records and mono sources
  • Comprehensive mono/stereo switching – again useful for old records
  • Buffered line outs – the better to drive an amp, my dear!
  • A sub-woofer output (repurposing the center channel here)
  • Aesthetically pleasing – the gold faceplate looks nice with my other Scott eqpt. and helps with “PAF” (Partner Acceptance Factor)

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

As expected, the audio quality is a bit off. The distortion is quite high, and the frequency response is rolled off at both end.
Those ancient and leaky coupling caps are the likely culprits, the first three I measured were awful.

First up:
– Replace the old leaky output couplers!
– Replace the cathode 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 mentioned in the build guide, btw.)

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.

Last Updated on 2022-02-20 by Daev Roehr