equivalent input impedance

[jtcc1015]

Hi there,

I am trying to design a driver stage using 6SN7 to drive 300B.

Could anyone kindly advise me the effective equivalent input impedance to this typical 300B SE amp circuit.

[jtcc1015]

Oops! I forget to mention that the bais current is set to 70mA.

[Miles Prower]

Originally posted by jtcc1015
Hi there,

I am trying to design a driver stage using 6SN7 to drive 300B.

Could anyone kindly advise me the effective equivalent input impedance to this typical 300B SE amp circuit.

If you're not driving it into Class A2, then the input impedance is simply the 200K grid resistor in parallel with the Ci + Cmiller. I'd estimate that to be 100pF, to account for the likely stray capacitance. At 30KHz, that comes to: Z= 13.0E3 - j49.5E3 Ohms.

At 20KHz: Z= 27.2E3 - j68.6E3 Ohms.

A 6SN7 is could be hard-pressed to drive a load like that. This is probably the cause of so many reports concerning the dissappointing sonics of the 300B: inadequate current sourcing in the driver. In this case, you're probably better off using a power MOSFET as a source follower. If you just refuse to use SS, then probably an xfmr coupled driver would be better, using one of the low u TV vertical power triodes, or a trioded 6AQ5, or something like that.

[jtcc1015]

At 20KHz: Z= 27.2E3 - j68.6E3 Ohms.

parallel with 200k.

Hi Miles,

Thank you very much for your advice. That was very helpful information. The reason I use 6SN7 is that I have quite a lot of them lying around, and I am somewhat familiar with its sonic characteristic under different operating condition. I would probably experiment with them first before I give up and switch to different type of tubes. Besides, I can always use multiple stages of 6SN7s' to drive a 300B in Class A1 operation.

Nevertheless, I am kind of interested in using 310a, too.

Thanks again.

[cerrem]

If you want to use an octal tube similair in size to a 6SN7 but mucho better performance as a follower to drive a 300B...then I suggest the 6V6 as a follower wired as a pentode ....
Then use the 6SN7 in cascode for the the front end voltage gain...
You will not be dissapointed....

Chris

[Miles Prower]

Originally posted by jtcc1015
The reason I use 6SN7 is that I have quite a lot of them lying around, and I am somewhat familiar with its sonic characteristic under different operating condition.

In this case, your best option would be to directly couple a 6SN7 cathode follower to the 300B grids as a driver. At 20KHz, the 300B will require a current of ~1.2mAp to charge the input capacitance. Running the 6SN7 driver at an Ipq= 4.0mA ought to get the job done. Of course, this will require a negative DC rail. I used the same idea (6SN7 cathode followers) to drive the grids of an 807 amp when it became obvious that the 6SL7 phase inverter wasn't up to the task.

[jeapel]

hi

I am not a 300b expert but if you
have quite a lot of them lying around
maybe just try before build a SRPP with
6sn7.

bye

[Miles Prower]

Originally posted by jeapel
I am not a 300b expert but if you
have quite a lot of them lying around
maybe just try before build a SRPP with
6sn7.

There are a lot of possible driver topologies available: SRPPs, Mu stages, Kimmels, followers (MOSFET and VT). So far, he hasn't stated a preference other than that he wants to use those 6SN7s he's doing nothing with.

[jeapel]

hi Miles

can you tell me more about :

1-distortion srpp vs mu (one more capacitor with mu )

2-and the new to me "kimmels" ?

[Miles Prower]

Originally posted by jeapel
hi Miles

can you tell me more about :

1-distortion srpp vs mu (one more capacitor with mu )

With this topology, distortion varies quite a bit with the actual load. Minimum distortion typically occurs when the currents through both sections of the totem pole are equal and out of phase. They also work well into a high load impedance. It's those "in-between" loads that can become problematic.

2-and the new to me "kimmels" ?

The Kimmel was originated to more closely simulate a CCS. Since it includes an external load resistor, you don't have to operate with identical currents going through both VTs in the stack. That way, you can use a high g(m) pentode up top for enhanced CCS operation, and for more drive current. This no longer operates as a push-pull, and so gives less distortion. Some folks don't like this because it doesn't add the 2nd harmonic distortion that is responsible for the "tube sound" and/or softening the sound.