Question about bias setting on a Hot Rod Deluxe.

Discussion in 'Glowing Bottle Tube Amp Forum' started by Strato50, Oct 27, 2020.

  1. Strato50

    Strato50 Tele-Afflicted

    Mar 30, 2017
    Port Arthur TX
    I had no idea first off this has the Fromel upgrade kit installed. Good caps all that Jazz. I’m setting the bias. It was running right at 60 MAs. So I bumped it up to 68. What is the consensus on a number for these? I like the most dynamics out of the amp.

    Attached Files:

  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    One needs to know the plate voltage at whatever current draw setting one has measured in order to ascertain the plate dissipation. That dissipation number is the defining number for Biasing. The schematic calls for 431vdc at the B+. Plate voltage will be a bit lower...depending on the Biasing.
    Your amp is at 68ma of current draw for both tubes.....34ma per tube. For discussion purposes, let’s say you have 425vdc on the plates of the 6L6s, which have a max plate dissipation number of 30 watts. At 425Pvdc and 34ma of current draw....
    425 x .034 = 14.45 watts. 14.45/30 = 48.2% of max plate dissipation. This is to the cool side. I have encountered only one Blues/Hot Rod Deluxe or Deville that would bias out at more than 52%....and it hit 60% before it ran out of adjustment. Imho, anything above 50% sounds better than anything under 50%. One wats to take care how hot one runs an amp. Most use 70% as a maximum plate dissipation limit, but one can go beyond that IF one likes what happens sonically. There are diminishing returns above 70% for the increased wear and tear on the tubes, but some people pay that price. ime, you could take that bias adjustment to the maximum current draw it will achieve with no worries, but I do not know if the Fromel mods affect the bias circuit or not. If you are not confident/experienced in taking plate voltage measurements on a live amp, that might give you pause in ascertaining that measurement.
    What is the largest current draw number you can achieve there?
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