6DJ8 / 6922 tube?

[nordhaven]

Why is the 6DJ8 / 6922 tube a good choice for a tube buffer?

http://en.wikipedia.org/wiki/6922

http://www.pacificvalve.us/YaqinTB.html

[Serge66]

Hi
I was going to ask that question.

Well, this is what I have gathered.
Used as a cascode - with bypassed cathode or diode/LED bias - the gain can be quite high. Used as cascode in a differential pair with constant current source in the commoned cathode is quite a good driver for a push-pull amp. A simple simulation with LTSpice gave a gain of 100.
Hope this helps.
Some reckon that this tube has a nice sound.

Serge

[SY]

It's useful as a buffer because as a cathode follower, the source impedance will be 1/gm and, as small triodes go, the gm is pretty high. You can easily get the source impedance down to 120 ohms.

[nordhaven]

Originally posted by Serge66
Hi
I was going to ask that question.

Well, this is what I have gathered.
Used as a cascode - with bypassed cathode or diode/LED bias - the gain can be quite high. Used as cascode in a differential pair with constant current source in the commoned cathode is quite a good driver for a push-pull amp. A simple simulation with LTSpice gave a gain of 100.
Hope this helps.
Some reckon that this tube has a nice sound.

Serge

Funny!! Your Avatar looks like the one I use on other forums also. I'll put on DIY also.

Yes, the 6DJ8 seems to be the ticket. They can be had for a cheap price as well! I like that there are a lot of tubes in that family to play with.

[BudP]

nordhaven

In my experiance the very common Phillips 6922 has a neutral, precise sound that lends itself to headphone amp drivers, in a long tailed cascode circuit. The 6DJ8 nos tubes I have tried in the headphone amp are all more colored and microphonic than the Phillips 6922. This doesn't mean they sound bad.

AudioPrism produced a very highly thought of tube preamp, using the 6922, called the Mantissa, if that matters to you. Again, a very precise and neutral device.

Bud

[nordhaven]

I was told the JJ Tesla JJ6922 is nice as well.

[Geek]

Hi,

Here's a nice line buffer circuit for you:
http://geek.scorpiorising.ca/GeeK_Zo...30521#msg30521

Only mod I'd suggest is join the power to earth ground through a 100 ohm, 2 watt metal oxide resistor, instead of what's shown.

Cheers!

[ray_moth]

Four objections I've heard/read about the 6DJ8:

1 Microphonic

2 Prone to RF oscillation because of its high Gm (should use stoppers)

3 Not much use if you need a lot of output voltage swing, because of its low max. plate voltage

4 Sounds harsh (maybe due to No. 2 above)

[Geek]

Hi,

Originally posted by ray_moth
Four objections I've heard/read about the 6DJ8:

1 Microphonic

Can be, yes. Avoid el cheapo brands and those Amperex dealies that go for a lot of bucks on fleabay - phoolery. Plain old Philips or on the other end, Brimar or Siemens sound great and no pings.

2 Prone to RF oscillation because of its high Gm (should use stoppers)

"If you don't feed them RF, they will make their own" - Allen Wright.

Stopper is a must.

3 Not much use if you need a lot of output voltage swing, because of its low max. plate voltage

They have drive, no doubt about that. But not meant to drive a 300B, 6080 or some other rediculously low-mu tube. That's where a 6922/E88CC comes in handy though

4 Sounds harsh (maybe due to No. 2 above)

There's some duds for sure, but I've never heard one that fit that. Oscillation was probably the cause.


To take a tangent, 6N1P as *the* most transparent sounding of the family if you can spare the heater current, IMO.

Cheers!

[Shoog]

quote:
2 Prone to RF oscillation because of its high Gm (should use stoppers)

"If you don't feed them RF, they will make their own" - Allen Wright.

Stopper is a must.

I agree. Handled correctly they are sweet and pure. The load should be wired onto the anode pin or a small anode stopper added.
These valves were originally designed for input duty on scopes, they are meant to deal with high frequencies.

Shoog