For quite some time now I wanted to share one of the variations of the famous VT-25/10Y pre-amplifier. As usual due to work, business travel and family duties, I’ve struggled to find the time to update the blog.
Anyhow, here is the variation of this great preamp with a slight modification of the filament bias arrangement. I discovered time ago that introducing the SiC diodes instead of the wirewound resistors provided an improvement on the overall sound and detail level. You can read around this point on my ETF.18 lecture or over here.
The circuit looks like this:
The only change I’ve made is replacing the filament resistor for an array of 6 SiC diodes. Happily used my SiC PCB which holds also six diodes. You can bolt them to the chassis or can leave them without heatsink. They will withstand individually the power dissipation as each will get about 1.2W. You will need good ventilation though.
Fabio’s build
Fabio Valente from Italy built this preamp recently and sent me his feedback and pictures. This encouraged me to dig the circuit out from my archive and post his impressions and comments which may encourage others to build this great preamp:
Ciao Ale,
Fabio, Valente. Italy.
I want to send you a couple of pictures of my preamp. Last week I optimized a few details and installed SIC diodes for the bias (in place of a 3R 12W resistor). With four of them I get 5.1V (1.27V on each diode), I can’t install five as I have 16VAC only transformers. I’d only have 0.5VDC headroom.
The B+, and also the 4VAC feeding the AZ1 mesh rectifier’s filament, is regenerated. There’s a 50Hz DDS based generator, preamplified and suitably delaied, feeding two powerful class AB amp followed by step-up transformers (EI with separated windings everywhere). Filtering is a LCLCLC with the last one in the preamp’s box. Only russian KBG paper and aluminium foil are used. 18+18+12uF only.
The volume is a relay based resistive logarithmic attenuator connected to the DHT’s grid. On the signal only USA made Jupiter PIO and copper foil capacitors are used.
I’m very happy with the sound of this preamp. I wouldn’t know how to try to improve it. But I know that it’d be possible somehow
Fabio
