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6DJ8 supply voltage
6DJ8 supply voltage
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Old 26th March 2020, 08:07 PM   #1
eduardospz is offline eduardospz  Brazil
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Post 6DJ8 supply voltage

Hello everyone. I am designing a compressor with the 6DJ8, but it's my first time dealing with tubes. Its datasheet shows maximum plate voltage rated at 130V, as shown below.

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I am designing the amplification stages by biasing the tube from the plate characteristics curves below. At first I was considering the maximum plate voltage of 130V for the bias, as shown in the red curve. The operating point of the tube would be 86V in that case.

A fellow person told me that the maximum plate voltage could be higher than the 130V (as it shows in the "cut-off condition"), and that this value is only there to avoid too much power dissipation, considering the maximum current allowed.

I wanted to know if it is possible to bias the tube say, on the green line, and supply it with ~200V, considering the operating point of ~117V, as shown below. What are the implications of it in a compressor? Is there any danger in going above the 130V?

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Every tiny bit of information and knowledge is really important to me. Thanks in advance, people!
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Old 26th March 2020, 08:09 PM   #2
Pano is online now Pano  United States
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6DJ8 supply voltage
Moved to Tubes/Valves
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Old 26th March 2020, 08:12 PM   #3
petertub is offline petertub  Sweden
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6DJ8 supply voltage
E88CC / 6922 is a tighter spec'ed variant that allows higher voltages.
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Old 26th March 2020, 08:16 PM   #4
eduardospz is offline eduardospz  Brazil
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Quote:
Originally Posted by petertub View Post
E88CC / 6922 is a tighter spec'ed variant that allows higher voltages.
I am aware of that, but all I have now are these 6DJ8, so I don't want to give up on them
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Old 26th March 2020, 10:34 PM   #5
audiowize is offline audiowize  United States
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Plate voltage and supply voltage are different. You can have a 300V DC power supply and operate the tube at 130V on the plate and everything will be just fine.
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Old 26th March 2020, 10:37 PM   #6
6A3sUMMER is offline 6A3sUMMER  United States
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OK.

Use your 6DJ8 tubes.

Use a 260V B+.
Use a plate current, plate resistor value, and tube self bias resistor value that will put your 6DJ8 plate at 130V.
Now let the tube plate swing down toward the cathode (plate on its way to near the self bias voltage), and let the plate swing up to near 260V at cut-off.
It is not going to break.
Notice that the set of curves you put in post # 1 go all the way to 300V.

The 6DJ8 was originally designed as a cascode amplifier, with the tubes stacked in series. In fact, one of the triode sections was designed to be the top tube in the cascode (because that particular triode section had a higher cathode to filament voltage rating than the other particular triode section; only some old data sheets even show that fact).

In the curves, the cathode is at ground potential, the grid voltage is varied, and the plate goes all the way to 300V.
Of course, 300V is a signal condition, not intended to be a quiescent condition.
And using self bias only offsets the cathode, so the cathode to plate voltage is according to the difference between the two.

Or, if you need to, pick a plate voltage of 90V, 110V, etc., and pick a B+ of 180V, 200V, 250V, up to 300V.
Just adjust the plate load resistor and self bias resistor to keep the plate DC volts to 130V or under, the cathode current under its max rating, and the plate voltage times the plate current less than the max plate watts dissipation rating.

Does that fix your problem?
Does that help?

Last edited by 6A3sUMMER; 26th March 2020 at 10:49 PM.
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Old 27th March 2020, 03:27 PM   #7
eduardospz is offline eduardospz  Brazil
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Quote:
Originally Posted by audiowize View Post
Plate voltage and supply voltage are different. You can have a 300V DC power supply and operate the tube at 130V on the plate and everything will be just fine.
I understand. Thanks for your help!

Quote:
Originally Posted by 6A3sUMMER View Post
OK.

Use your 6DJ8 tubes.

Use a 260V B+.
Use a plate current, plate resistor value, and tube self bias resistor value that will put your 6DJ8 plate at 130V.
Now let the tube plate swing down toward the cathode (plate on its way to near the self bias voltage), and let the plate swing up to near 260V at cut-off.
It is not going to break.
Notice that the set of curves you put in post # 1 go all the way to 300V.

The 6DJ8 was originally designed as a cascode amplifier, with the tubes stacked in series. In fact, one of the triode sections was designed to be the top tube in the cascode (because that particular triode section had a higher cathode to filament voltage rating than the other particular triode section; only some old data sheets even show that fact).

In the curves, the cathode is at ground potential, the grid voltage is varied, and the plate goes all the way to 300V.
Of course, 300V is a signal condition, not intended to be a quiescent condition.
And using self bias only offsets the cathode, so the cathode to plate voltage is according to the difference between the two.

Or, if you need to, pick a plate voltage of 90V, 110V, etc., and pick a B+ of 180V, 200V, 250V, up to 300V.
Just adjust the plate load resistor and self bias resistor to keep the plate DC volts to 130V or under, the cathode current under its max rating, and the plate voltage times the plate current less than the max plate watts dissipation rating.

Does that fix your problem?
Does that help?
Yes, perfect! That helps me a lot. Thanks!



As a risk-averse person, I was a little afraid to implement the circuit without having a second opinion from more experienced people. Your responses cleared up my doubts about the tube supply. Thanks again!
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