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Re: Question for Mark Gilmore(6SL7 or 12SSL7)

Posted by Al Sekela (A) on May 13, 2002 at 19:54:53

In Reply to: Question for Mark Gilmore(6SL7 or 12SSL7) posted by Andy Wilks on May 13, 2002 at 16:10:24:

Andy,

Let me field this one for Mark, as I've been one of the instigators of the *SL7-in-the-current-source tweak.

The original current source used 2 Zeners and one triode in a 12AT7. Ralph Karsten's first 6SN7 modification also used one triode. With a separate filament transformer for the current source tube (to avoid exceeding the heater-cathode maximum voltage rating), this can be further modified to use a stack of three 47-volt Zener diodes to provide 141 volts across the tube, and a 45 K (or 47K) cathode resistor to set the current output to about 3 mA with a B- voltage of -300 volts.

Ralph then adapted the other triode in the 6SN7 envelope to create the cascode current source. This uses 4 Zener diodes total, to set each triode to have a plate voltage of 94 volts, and the cathode resistor is reduced to maintain the 3 mA output.

It is possible to drop a 6SL7 in place of the 6SN7 (or 12SL7 for 12SX7) used for the 3-Zener older version and still maintain negative grid bias on the current source tube. However, a *SL7 will not work in the 4-Zener version because the grid bias will become positive. At this time, I am afraid that prolonged operation of a *SL7 with positive grid bias will result in tube failure and expensive damage to your amplifier. However, I have no experience that guarantees that this will happen.

The benefit of this change is a sharper, tighter focus and more stable treble. The reason is explained by Lew in several posts below. The CCS circuit works more like a current source with infinite output impedance if a tube with higher gain is used.

The reason Ralph adopted the cascode, 4-Zener version was to increase immunity to power supply noise. As Lew points out, this version also has higher output impedance because of the cascode connection. Ralph would not have done this if he did not hear a sonic improvement. I don't think anyone here can tell you for sure which version, 3-Zener with *SL7, or 4-Zener with 6SN7/12SX7, sounds better. All we can agree upon is that *SL7 does not have adequate transfer characteristic to avoid positive grid bias if dropped into the 4-Zener cascode. You should know also that Ralph limits his designs to tube types that are still manufactured, and that he is not happy with the quality of presently manufactured 6SL7s.

There are other tweaks to consider if you are going to modify your amps. You can use the search function to uncover these.

-Al

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