It was a fellow audiophile friend of mine who brought his beloved preamp for me to repair one day, which had a burnt 6X4 rectifier. The preamp was bought from eBay and it's actually a shrink clone of the famous CAT SL-1. He had all the coupling caps upgraded and valve tubes replaced with NOS ones. I had it in my system and listened for a few days after having it repaired. What it amazed me was its unique sound to my ears, that makes me very attempting to build a version of mine.
After some search on Google, I found members over forums at DiyAudio.com had discussed the original circuit intensively and they pointed out that there were actually some mistakes with the circuit design, but not known if these faults were left on purpose or ignorant. Anyway, the computer simulation does show improved distortion figure after the faults being corrected.
The design of my version on power supply part is emphasized using two dedicated power transformers, one for high voltage supply only and the other for the filament supplies. Starting with Sylvania 6X4 rectifier, then followed by the SALAS SSHV2 shunt regulator, the filament and high voltage circuits are able to fit in the same board as intended:
Using the same chassis as CT-11H used, the whole layout looks like this:
A close-up look at the main board:
Finished wiring :
Front view :
Back View :
UPDATE : 2/1/2019
MK1 version is developed due to gain changing. I found 26db gain is a bit too much for some of the power amplifiers. Due to DC coupling from second stage to output stage, and the first stage's bias is directly determined by the feedback resistor, I ended up re-biasing all the stages, changed the cathode capacitor value for the lower tube of the u-follower, added 47P silver mica capacitor across the feedback resistor. Finally, the gain is about 21.5db.
UPDATE: 20/02/2019
Can I call it MK2 now? As a result of continuously tuning of this preamp, I found 12AX7 tube is not the best for the lower section tube of u-follower, because tubes with high u do not work well at low anode voltage. Tubes like 12AU7, 6DJ8 are way better for this task. In order to replace 12AX7 with say 12AU7, both Rk for upper and lower tubes as well as Rl need to be re-calculated. The input stage is also optimised again to achieve better linearity. Lower plate resistor in the first stage improves high frequency response when connected to the second stage. New operating point is Ia=1.76ma, Ua=160V, Ug=-8.5V, Rl=51k. Pictures below are LTspice simulation results. After completion of all the changes , the preamp sounds much richer , clearer and more detailed to my ears.
Distortion at 1.7 Vp-p /4khz output
FFT chart at 4khz
Frequency response
Tube used:
3X JJ 802S New production
1X 6BQ7A Toshiba NOS
1X 6X4 Sylvania black plate NOS
Filament Arrangement :
12.6V regulated DC for 3X JJ802S
6.3V regulated DC for 6BQ7A
Output Noise Level:
0.1 mV when input shorted to ground
Gain : 21.5db
Power Input:
AC: 230V -240V
Power Consumption: 45W
For more info or an arrangement of trial, please contact peter@consoletronics.com
