I would like to know if B+ can be too high (within reason) so long as the plate voltage of a given tube is okay. Say, if I have a B+ of 400 volts, would that be a problem for tubes like a 6DJ8 if I'm sure to keep the plate voltage and current within spec? If not, why? I ask because so many designs use lower B+, and the curves charts for the 6DJ8 among many others often don't even go up as high as 400v. I have been trying to do a bit of reading on the subject, but I haven't seen anyone discussing that. I'm open to an article or two if someone knows of any.
I appreciate all the help I have gotten here.
I appreciate all the help I have gotten here.
If in operation the plate-cathode voltage is around 100V , the Va max for a cold tube is 550V ... so no worries
The B+ is 400V but what it the voltage measured at the plate after the voltage drop? 130V max plate voltage is based on a grounded heater and one of the two triodes being "on top of the other" as a cascode. You can actually far exceed this before you draw an arc when using the tubes as a grounded cathode amp..
As Depanatoru said, you're within the specs for a cold tube.
As Depanatoru said, you're within the specs for a cold tube.
I haven't actually built it yet. Just trying to wrap my head around what can and can't be done before I start. I had intended to use a large enough plate load resistor (and bias) to keep the plate voltage within spec. The 6DJ8 may not have been the best example though as the datasheet does list a maximum supply voltage. Many that I have seen don't. They just list maximum plate voltage. That's what had me somewhat confused.
I've run 700V B+ to a circuit using 6N8S (330V max) without fail... The same 700V B+ to a 6F12P, also no issue.
Tie B+ to grid (oops) and you get lightning though 🙂
Tie B+ to grid (oops) and you get lightning though 🙂
Many tubes have to Operate at 2X B+.
By Operate, I mean with the tube warmed up, not cold.
A tube that is used as a class C amplifier, will go to Almost 2X the B+, when the plate current goes to zero, and as the resonator swings through the rest of the cycle.
The same holds true for many audio amplifiers. They will go to Almost 2X B+.
example:
A push pull output stage has 350V on the plates.
As one tube plate swings down to 50V (350v - 300v down swing = 50V), the other plate swings up to 350+ 300V = 650V.
Take a look at tube data sheet curves. The plate voltages of those curves far exceed the 'rated' maximum plate voltage.
Maximum plate voltage most often means the maximum quiescent plate voltage (max voltage before signal is applied).
By Operate, I mean with the tube warmed up, not cold.
A tube that is used as a class C amplifier, will go to Almost 2X the B+, when the plate current goes to zero, and as the resonator swings through the rest of the cycle.
The same holds true for many audio amplifiers. They will go to Almost 2X B+.
example:
A push pull output stage has 350V on the plates.
As one tube plate swings down to 50V (350v - 300v down swing = 50V), the other plate swings up to 350+ 300V = 650V.
Take a look at tube data sheet curves. The plate voltages of those curves far exceed the 'rated' maximum plate voltage.
Maximum plate voltage most often means the maximum quiescent plate voltage (max voltage before signal is applied).
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Personally, I would never put 500V on a 6DJ8 plate, not even during amplifier warm up time.
I am too much of a 'worry wart', I can visualize 500V arcing from the plate to the grid.
By the way, the original 6DJ8 data sheet had two specifications for maximum filament to cathode voltage. One section had a much higher rating.
That is because one intended application was as a series stacked Cascode amplifier.
The upper cathode had the higher maximum filament to cathode voltage rating.
But you had to pay attention to which triode section was which.
I think all the manufacturers got tired of circuit designers not paying attention to which section was intended to be the top triode, so they started using the same material and maximum filament to cathode voltage ratings for both sections.
History is always interesting.
Just my opinions.
I am too much of a 'worry wart', I can visualize 500V arcing from the plate to the grid.
By the way, the original 6DJ8 data sheet had two specifications for maximum filament to cathode voltage. One section had a much higher rating.
That is because one intended application was as a series stacked Cascode amplifier.
The upper cathode had the higher maximum filament to cathode voltage rating.
But you had to pay attention to which triode section was which.
I think all the manufacturers got tired of circuit designers not paying attention to which section was intended to be the top triode, so they started using the same material and maximum filament to cathode voltage ratings for both sections.
History is always interesting.
Just my opinions.
Thanks for the replies, guys. I've got some 160v zeners and high voltage transistors on order to attempt my first regulated power supply. I've also salvaged a couple of transistors and mosfets from induction cooktop drivers and microwave oven inverters. I'd like to try those too. May not work, but it'll be fun nonetheless. Would a regulated power supply not hold the voltage down and keep it from spiking up while the tubes are cold? I've built a linear power supply with a 40,000uF smoothing cap for the filaments and am going to be regulating it as well to keep the voltage stable if tubes are removed or if a filament opens. It's alarming how much that voltage can pop up in the absence of the proper load.