How to build a reliable negative bias supply for a Tube Amp.
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- Опубликовано: 13 сен 2024
- This by no means covers everything about this subject, but this should cover the basics of building a reliable negative voltage bias supply for a fixed bias tube amplifier.
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Great explanation of a well designed grid leak supply. 😀👍
Thanks! 👍
It drives me crazy when manufacturers of tube power transformers fail to add a bias tap to their H.T. winding. I've never been comfortable with alternative bias arrangements which depend on capacitive reactance in conjunction with voltage dividers to derived bias. In my last amplifier build, I used a 120v=>12v power transformer used backwards on a 6.3v heater winding to get 60vac for the bias supply. Wasteful, but resulted in reliable solution.
would it be okay to have 2 diodes oriented the same way so if one fails, no problems.
@@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.
Hey Skunkie, greetings from NYC. So, I know nothing about circuit design, but your explanations help it to sink in, so thank you. I also love how you are always thinking about how to protect the circuit, and frankly, many/most people do not.
My question is this: At Diode 1 doing the half wave rectification, is there any benefit, or is it even possible to use a tube instead of a diode, or is it inadvisable for any reason?
Thanks for all your help and I hope to one day clearly understand a complete circuit from start to finish...
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.
This is the way! I don't get it why designers don't make the bias adjustment fail-safe. C'mon, it's so cheap and simple - why not do it?
Any thoughts on using even better regulation/filtering on -bias supply? Also, using an "auto bias" type system to hold bias? I tried both of these on my now highly modified m125's (tubes4hifi) and it worked out well. Although its more silicon....
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.
If there is no bias voltage on the transformer, could you take it off the high voltage going to pin 6?
Yeah there are ways of pulling the bias voltage off the HT.
So the grid leak value is 1K (grid stopper) + 51K + 10K (pot with 100k in parallel) + 10k ?
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?
i disagree on that bias circuit I've always copied the fender twin bias circuit and tweaked the resistor etc. to put it in a nut what you are using is a bias balance circuit. the topology for neg bias is a sweep from minim neg say neg 30 volts to neg 50 volts. if you are worried about loosing bias fuse the HT witch you should be doing if building a valve amp
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?