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cathode bias vibroverb style amp. need help for choke and cathode math

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  • #16
    'i could use a rectifier, like the gz34 or the 5u4g, to lower B something below 400v. 2x6L6gc cathode bias, and 2x12ax7 as preamp+cathodyne. and i could add a 6bm8 i have, as reverb driver & recovery stage. i think i could make room for these tubes, and the total heather current is 3,2A which is about 65% of the total.
    the 300mA could cover the 50mA of the preamp and the output relatively easy, i think (but i'm not sure. can it?)

    in this scenario i could use a 400v rated choke, which is light and cheap. the Hammond 156R is rated 200mA. is that enough?
    ..can this solution work?'

    That seems ok, however the overall gain may be slightly lower due to the cathodyne rather than an LTP and the overdrive character may be a little different.

    200mA seems a lot more than necessary for the choke rating, unless you intend putting the whole B+ through it as per the 5F4 Super http://www.webphix.com/schematic%20h..._5f4_schem.pdf

    With the normal arrangement, between plate and screen nodes, the choke rating can be much lower, eg 50mA.

    Consider having independent cathode bias resistors for each power tube; by that, the static cathode currents tend to balance out, even with poorly matched tubes.
    Pete
    My band:- http://www.youtube.com/user/RedwingBand

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    • #17
      i need to use the cathodyne because i have very little space under the chassis, so if i use a tube rectifier, i need to cut a preamp tube and the cathodyne comes in handy. basically the preamp structure would be the same as the blackface princeton, with a 6bm8 reverb. 3 tubes total.

      i think i will insert the 6bm8 after the choke, so the draw may be around 100mA
      the 200mA choke may be overrated but it's 5cents less than the 100mA one, and the specifics say they weights the same.
      is there a backlash using a bigger choke? it is rated 1,5H 56Ω. is this good?
      the 100mA is 3H 86Ω. i don't understand chokes, so i don't know which one is better.

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