6SN7 Cascoded CCS

[korneluk]

I think I've had too much eggnog and Captain Morgan's while putting up my Christmas tree today.

Need help deciphering the following circuit description "series connected 6SN7 cascoded constant current stage. One pair of zener diodes sets the grid of the upper half of the cascode-current source 94V DC below ground via a high-voltage bias supply. The cathode of this stage is coupled to the plate of the lower half, whose grid is biased 94V DC above the negative-bias supply rail. Fixed resistors in the cathode of the lower half set the constant current value".

Follow the link for the complete article:

http://www.atma-sphere.com/reviews/ga/index.html


-- josé k.

[SY]

OK, the stage itself is just a normal long-tailed pair. The long tail is a CCS. Now, triodes are pretty mediocre current sinks (low rp), but that can be aided by degeneration or cascoding. In this case, they do both. The cascode is just two 6SN7 sections stacked.

Let's look at the bottom tube of the cascode. Its grid is set 94V above whatever the negative rail is (presumably something like -300V). The cathode is connected to the negative rail via a resistor. We think of the negative rail as the reference point- with the grid at 94V above the reference, the cathode must be at a few volts above that, call it 97V within some small margin of error. If we want the CCS to be 10mA, we calculate the cathode resistor by R = V/I = 97V/0.010A = 9k7. The effective plate resistance is now augmented by mu times the cathode resistance, in this case, nearly 200k. Now we're getting somewhere.

We repeat the process with the upper tube; its plate resistance is now augmented by mu times the effective plate resistance of the bottom tube. That works out to about 4M, a pretty good CCS.

Doing this scheme involves a lot of separate heater supplies and a high parts count, and the result is a reasonably good CCS, but I am unconvinced that it's better than a bipolar or MOSFET circuit.

[HollowState]

After reading that "tube-pology" I can see that this M60 MK. II is really only a slightly improved version of the old MA-1 of 20 years ago. The major changes I see are:

All 6SN7's compaired to 12AU7 input & 6CG7 driver. This benefits the manufacture by only needing to buy one type of tube instead of two. (aside from the outputs)

A current source in the input tail. Perhaps transistors and LEDs would have been as good and cheaper. Something like the pinkmouse/DIY circuit.

Zener regulation for the upper cascode grids.

2 dB of inverse feedback. (A lot of good that does. The MA-1 had no global FB and had over 12% THD at rated output of 50W.)

From the article:

Atma-Sphere uses its patented Balanced Differential Design™ rather than the Futterman OTL design.

Unless Karsten has another patent I don't know of, he's not manufacturing what was patented back then. (1988) According to patent #4,719,431 the output stage is an anode follower. Not a cathode follower as his OTL amps are.

I would gladly post the MA-1 schematic here but it's a little to large for the board to accept. Any member who wants it can email me through my profile and I'll send it to you.

Victor

[korneluk]

Originally posted by HollowState

Unless Karsten has another patent I don't know of, he's not manufacturing what was patented back then. (1988) According to patent #4,719,431 the output stage is an anode follower. Not a cathode follower as his OTL amps are.

He does have a more recent patent, US6242977 Amplifier and Preamplifier Circuit, granted in 2001. I have the pdf - please PM me for a copy.

Originally posted by HollowState
I would gladly post the MA-1 schematic here but it's a little to large for the board to accept. Any member who wants it can email me through my profile and I'll send it to you.

Victor

Please check your inbox...

TIA,

-- josé k.