I don't want to seem like I'm nitpicking, but... Can you tell us at what voltage you'd lift the heater supply to keep the cathode-heater voltage within limits for both the 1st and 2nd stages of this circuit? Thanks.
Not nitpicking at all -- that's a good question.
According to various datasheets, the 6DJ8/6922 has a DC heater-to-cathode rating of about 130 volts (on the low side). I'd lift the heater supply to about +85 VDC. This will give a heater-to-cathode voltage differential of about 60 volts for each cathode, well within the limit.
Also, I recommend a 2 watt resistor for R4 and a 3 watt resistor for R8. These resistors dissipate about 0.5 and 0.7 watts respectively, so the recommended wattage derates these resistors by a factor of about 4:1. I tend to be very conservative when choosing component ratings.
According to various datasheets, the 6DJ8/6922 has a DC heater-to-cathode rating of about 130 volts (on the low side). I'd lift the heater supply to about +85 VDC. This will give a heater-to-cathode voltage differential of about 60 volts for each cathode, well within the limit.
Also, I recommend a 2 watt resistor for R4 and a 3 watt resistor for R8. These resistors dissipate about 0.5 and 0.7 watts respectively, so the recommended wattage derates these resistors by a factor of about 4:1. I tend to be very conservative when choosing component ratings.
Last edited:
I think the max cathode-heater voltage rating is different for each triode in the envelope.
According to the Sylvania 6DJ8 data sheet, the max Vh-k for the section 1 triode (pins 6, 7, 8) is 50V, while the max Vh-k for the section 2 triode (pins 1, 2, 3) is 130V.
So the way out of this dilemma would be to wire triode 1 (pins 6, 7, 8) as the common cathode amplifier (input stage) and triode 2 (pins 1, 2, 3) as the cathode follower. Lift the heater supply to +40V. The section 1 (50V max) Vh-k would be at -10V below the max, while the section 2 (130V max) Vh-k would be -90V below the max.
6DJ8 is a tricky one that way.
According to the Sylvania 6DJ8 data sheet, the max Vh-k for the section 1 triode (pins 6, 7, 8) is 50V, while the max Vh-k for the section 2 triode (pins 1, 2, 3) is 130V.
So the way out of this dilemma would be to wire triode 1 (pins 6, 7, 8) as the common cathode amplifier (input stage) and triode 2 (pins 1, 2, 3) as the cathode follower. Lift the heater supply to +40V. The section 1 (50V max) Vh-k would be at -10V below the max, while the section 2 (130V max) Vh-k would be -90V below the max.
6DJ8 is a tricky one that way.
The 6DJ8 was intended for cascode operation so you are correct that the two triode sections are not identical. For that reason, I personally don't use the 6DJ8 for anything except "totem pole" configurations, and even then you have to pay attention to the pin assignments. It's interesting that the RCA 6DJ8 datasheet (attached) makes no mention of these odd heater-to-cathode specifications. It only specifies that the maximum "negative grid voltage" is 50 volts.
If you intend to use the 6DJ8 in this particular application then your plan is a good one. I personally prefer the 6922 for general purpose applications like this. It doesn't have any particular quirks (that I am aware of).
Thanks for bringing this up.
If you intend to use the 6DJ8 in this particular application then your plan is a good one. I personally prefer the 6922 for general purpose applications like this. It doesn't have any particular quirks (that I am aware of).
Thanks for bringing this up.
Last edited:
Ya know, it's good to chat about this stuff, because you can learn something! I looked up the 6922 (E88CC) data and you're right, it says the max Vh-k is 100V, no difference between the two triodes. It also says the max Va is 220V, and max Pdiss is 1.5W.
Interesting that the 6DJ8 Va max is only 130V, but the max Pdiss is 1.8W.
Those are pretty big differences, but the plate curves look pretty much identical.
Good to know.
I've attached the Philips 6922/E88CC data sheet.
Interesting that the 6DJ8 Va max is only 130V, but the max Pdiss is 1.8W.
Those are pretty big differences, but the plate curves look pretty much identical.
Good to know.
I've attached the Philips 6922/E88CC data sheet.
Aside from their maximum ratings, the 6DJ8 and 6922 do have very similar characteristics, which I guess is why they are often thought of as interchangeable. For SPICE modeling purposes, they pretty much are.
But to me the two tubes have different applications and I consider the 6922 to be the more versatile. It's one of my favorite tubes for line-level circuits. The fact that it is available as both current production and NOS is a plus. Attached is the datasheet for the Electro-Harmonix 6922. Note that it specifies the maximum heater-to-cathode voltage as 200 volts!
But to me the two tubes have different applications and I consider the 6922 to be the more versatile. It's one of my favorite tubes for line-level circuits. The fact that it is available as both current production and NOS is a plus. Attached is the datasheet for the Electro-Harmonix 6922. Note that it specifies the maximum heater-to-cathode voltage as 200 volts!
Last edited:
