How to build a reliable negative bias supply for a Tube Amp. 

Thanks! 👍

@@iainbowman8979 Yes mate it should be ok, but you will 'lose' 1.2 to 1.4 volts across them. If your circuit can still operate with up to 1.4 volts less, then it will be fine. By the way, you must always use diodes within their ratings, of course. Usually rated current & voltage are the main parameters to be sure of.

Sure, I don't see any issue using parallel diodes.

Just added complication. Before there were silicon diodes they used a small tube for this. One could also use a bridge rectifier if you need more voltage, but these seem to function fine as a half wave w/ filtering caps.

I've used auto bias on most of my projects. Sometimes "fixed bias" makes sense and this I feel is a good way to implement it.

Yeah there are ways of pulling the bias voltage off the HT.

Grid to ground measured is 66K with the bias pot adjusted. Yeah this is a little over the published "Grid leak resistor max" but I know these have been run with 100K grid leak resistors + bias supply so I'm good with that.

@@SkunkieDesignsElectronics Thanks. M

Many years ago I modified a pair of Dynaco Mkiii amps for a friend. He picked up some power supply upgrades to replace the old quad cap, and I believe to convert to a SS rectifier. There was probably more but I don’t recall. I also changed the grounding and tweaked the bias supplies so that he could use a wider range of output tubes. But one thing I did was I used a small zener diode string to regulate the bias supply. I don’t recall exactly how I implemented it, but it made the amps bass much cleaner and more controlled. I cannot be certain, but it may have been as simple as adding a small valve resistor after the first cap, then adding a zener string with a total valve of the nominal B- followed with another cap before the biasing components. Unfortunately I don’t have access to the Mkiii’s to verify how I built it.

@@SkunkieDesignsElectronics I've been looking forward to this video and discussion about this limit. I have a PP amplifier (Luxman A3600) with a fixed bias circuit. The bias circuit was modified to take KT88s/6550s after the original power tube went out of production and I've since replaced the KT88s with KT120s. I've noted the concern online about the max grid leak resistance of 51Kohm for KT120s. TBH, I've struggled to calculate the resistance of my bias circuit - its complicated and I'm still learning! However, I think each KT120 tube is seeing somewhere around 60Kohm, biased. No one has indicated what would happen if it is too much for a tube?

This is neg 30ish to 65, KT120 tube is gonna need around 52, EL34 is gonna want low 30's. Why fuse the HT when it's not needed if you simply design a bias supply that's not going to get lost?