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    DC Heaters @ Elevated Heaters

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    jwb474

    Posts : 8
    Join date : 2014-07-13

    DC Heaters @ Elevated Heaters

    Post by jwb474 on Wed Mar 18, 2015 9:27 am

    Old topic I know, but after reading so much info and different opinions I would like your take on dc heaters for a power amp and pre-amps. On a power amp any advantage on the output tubes or just the driver? What about dc elevation on a cascode driver? Looking forward to all the thoughts.

    tubes4hifi
    Admin

    Posts : 1282
    Join date : 2008-11-30

    Re: DC Heaters @ Elevated Heaters

    Post by tubes4hifi on Wed Mar 18, 2015 11:25 am

    elevated heaters aren't generally used or needed on a power amp, due to the low gain involved, and in fact most amplifiers dont' even use DC on any filaments,
    although theoretically using DC on the driver board might help, but our ST70s and ST120s are DEAD QUIET using AC, as long as the filament leads are tightly twisted
    and kept short and non-parallel to any signal wires.
    For a preamp circuit, all filaments should be DC regulated and elevated, no questions!!

    MontanaWay

    Posts : 781
    Join date : 2014-02-06
    Age : 58
    Location : Brookings, Oregon

    Re: DC Heaters @ Elevated Heaters

    Post by MontanaWay on Wed Mar 18, 2015 11:33 am

    Like a lot of subjects, DC versus AC filament supplies has pretty much equal fans, and some will argue strongly in favor of either.
    For power tubes, I do not think that DC is going to make a big difference. AC has been used pretty much since day one, and even the best out there use AC. In order to provide DC for power tubes, which as a general rule draw quite a bit of current, over 1A with most out there, a pretty hefty DC power supply would required, which adds to the complexity and price. So, stick with AC on power tubes.
    Now, preamps are definitely better off using DC, imho!
    You will notice, on a lot of older preamp/amp designs, a 'hum adjusting pot' on the AC filament supply.
    By going DC, that is one part of the circuitry no longer required. The DC supply for filaments should be well designed though, with good filtering. I have seen many different designs, some are pretty straight forward, basically RC or CRC, others use voltage regulators to more precisely, and even cleaner, get the desired 6.3 or 12VDC.
    I prefer to go the voltage regulator circuit, it is more precise and even cleaner than a RC or CRC.

    jwb474

    Posts : 8
    Join date : 2014-07-13

    Re: DC Heaters @ Elevated Heaters

    Post by jwb474 on Wed Mar 18, 2015 12:01 pm

    For a preamp circuit, all filaments should be DC regulated and elevated, no questions!![/quote]

    So for a cascode design, at what level should the elevated voltage be? This may be a bit more tricky since the cathode of top tube is setting near the plate level of the bottom tube.

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: DC Heaters @ Elevated Heaters

    Post by tubes4hifi on Thu Mar 19, 2015 9:57 pm

    in general, for a cascode design the filaments are lifted to 1/4 of the B+ voltage. I usually use a 300K and 100K, or 270K and 82K for a divider.

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