AC VT25 filament?

[alexg]

I have been looking at VT25 schematic, I have noticed that all are DC.

I built a 2a3 SE amp with AC filaments, somehow I like it better with AC heaters.

Would the VT25 on AC filaments feasible? Has anybody tried?

[Rod Coleman]

The first test for a dc heat solution is to compare the sound to ac.

Raw rectified dc is the worst of all worlds, and has no place in an amp pretending to be 'hi fi'. In this case, it would be better to use a KT88 or EL34 in triode connexion than VT25 heated by raw dc.

With the 2A3, the ac problem is at its least, because the voltage swing is small [2,5V]. Still, with carefully implemented dc, even this DHT can be markedly improved. With the 10Y/VT-25, you are near the opposite end of the scale with 7,5V filament. This DHT needs the highest quality filament supply, which IME should be current-fed.

Some builders are amazed at the degree of difference good filament supplies make. But when you consider that the same internal conductors for the filament supply also run the cathode current to the outside of the tube - i.e.: the noise components of a 1,25 Ampere system are directly mixed with your 0,025A cathode current - then it is little surprise.

There are plenty of DIY solutions for this: LT1084 plus one resistor will give a taste of current-heating. If you like it, and want higher sonic performance. Tentlabs supply built & tested modules, and you can get self-assembly kits of a design that was shown at the 'New DHT Heater' thread - from me!

Other boards are available from Pete Millet (raw dc board -use some regulation after this!), and Tomchr is working on SMPS voltage-output PCB solution.

Many good things are said about choke-fed filament heating too, and if you like this idea, look out for Thomas Mayer's advice on these.