This was my 71a DHT preamp incarnation. I reused existing circuitry so it can be improved. Firstly the top BSP129 should be an DN2540 with heatsink. P1 can be replaced with a LND150 depletion MOSFET CCS and a resistor divider to improve the frequency response. R5 should be your preferred 100K stepped attenuator instead.
I enjoyed this very much and will definitely encourage everyone to build it. I recall that the bass response wasn’t as good as the 4P1L despite anode resistance being low as well.
Slight filament starvation to reduce microphony and DC regulation is provided by Rod Coleman boards.
Globe 71 are difficult to get hold of, but worth trying them.
Here are my earlier impressions and description of the circuit.
Cheers,
Ale
Hi Ele,
I’m planing to built a 71a preamp. Any possibility to have it transformer output? How about Tamura A-4714?
Any pro and con?
Regards,
Cheah
Hi Cheah,
Of course you can. OT is great, providing you have sufficient primary inductance (which is not a real challenge with the low ra of this valve) and the transforming ratio of the OT matches your requirements. I believe the tamura A4714 is a 1:1 so you will get same results in terms of voltage gain.
Hope this helps
Ale
Thanks Ale,
I would like to filament bias the 71a tube with Colman’s regulator. Where can I find more information about filament bias?
Cheah
You can check the 26 DHT preamp thread in DIyaudio which has a lot of info on filament bias
Also check the page on my site on Why DC on DHT filaments. You can contact Rod Coleman directly and he will be happy to assist you with the design
Cheers
Ale
Thanks, Ale.
Thanks for sharing.