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
Thank you for publishing the pictures I sent. I build this preamp last year, after some experiences with non DHT tubes. Also very good, but these older devices have a extra gear at least! In one word they sound more lifelike, so there’s not much else to be added… for hard core audiophiles at least.
The first version had a full tube circuitation (you can see it in one of the pics), a mu-follower with the output connected at the DHT’s plate. It was an extreme solution, working well over 500V. Sound was phenomenal. Sometimes. Totally depending on the following power amp. One of the first listening tests was carried out with a friend’s very expensive Viole Forte SS amplifier. Sound quality was stunning, I remember a somehow scary presence with some voices.
But with other amps, included my DIY 2A3s and 845s, it was quite unlistenable. The reason is “simple”, the Forte amp have an extremely high input impedance, a rarely seen 1M ohm value. Furthermore the connecting cables were very good, I’d guess with low capacitance.
Then it developed a problem due to the high upper tubes dissipation and no forced cooling, I’m sure a simple fan would have fixed it. The planar triode used was developed for RF and was liquid cooled as far as I know. Anyway due to the SQ issues I decided to switch to Ale’s hybrid mu-follower solution. This also allowed to lower the working voltage to a much more civilized 260V 🙂
Now the driving problems are gone, sound is always excellent. Maybe, but I’m not 100% sure, it has lost a bit of the previous unbelievable transparence. But now there are two, AC coupled, stages. Anyway that previous “magic” was too hardly achievable and that preamp simply wasn’t a complete design. Dot. In other four of five systems it plaied from mediocre to awful.
So, thank you Ale for this great sounding, and SO simple to implement, solution!
Fabio
Ale,
I ‘ve tried a few different tubes as drivers for my 300b amps including your scheme for the 6SF7. At this point the most direct way to continue is with separate chassis’s I don’t need all that much gain. What would be the right level of b+ for the 801a preamp to drop in as the driver stage for the 300 b’s?
I am making the 801a preamp including rod’s filament bias. Is the heat of the diode array the limiting condition? What’s the max current that can be run, and what current is needed to drive the 300b’s cap coupled? The diodes are well dunked.
Hi David, the VT-25 preamp circuit is different from the driver. On this page you have a good example, otherwise you can look at implementing this: https://www.bartola.co.uk/valves/2020/12/30/300b-845-amp-driver-options/