12AT7 Post-PI Driver Tube

[mobisimo]

Hi friends,

I had picked up an inexpensive Fender 75 and thought it might make a decent SSS-style platform. Its PT has a separate bias winding, so I was going to use it to supply the post-PI driver with martin manning’s voltage multiplier: http://ampgarage.com/forum/viewtopic.php?f=4&t=29882

I also came by some nice NOS 12AT7/6201 tubes and thought I remembered reading that some folks liked them as a post-PI driver tube. My question is would anyone recommend changes to the circuit to optimize for a 12AT7 instead of a 12AX7 in that spot?

Thanks for your help!

[martin manning]

You can use a 12AT7 and reduce the cathode resistor by half (use 56k), thereby doubling the driver tube current (wrt a 12AX7) at the same power tube bias point. Be aware that the PT bias winding will have to supply a lot more AC current, but I think you have 100 mA available from that dedicated winding, which should be sufficient.

[mobisimo]

Thanks very much for your reply, Martin. I will try that and let you know how it works out!

[martin manning]

Output impedance of a cathode follower is ~1/gm, so 12AT7 should have ~182 ohms Zo as opposed to 12AX7's ~625 ohms.

[mobisimo]

Thanks again, Martin. I think the lower impedance makes a better driver- correct? If so, I wonder why Dumble used a 12AX7 there in the SSS. Maybe a 12AX7 was good enough and/or the bias current was limited, as you mention above.

[martin manning]

The 12AT7 will provide a little more current and go a little deeper into Class AB2, if you are playing at full volume. I Think the 12AX7 is sufficient to eliminate the grid bias excursion and blocking distortion that the typical capacitively coupled PI arrangement can suffer from.

[bepone]

12AU7 is even better..also 6CG7, ECC99, 5687 are nice choice.. 12AX7 is completelly wrong from technical aspect, tonewise i dont know, didnt test it

[mobisimo]

Thanks for your replies, Martin and bepone. I actually have a couple GE 12AU7/5814As, so I will try one of them and throw the 12AT7 in the reverb driver spot. It looks like the cathode resistor value should decrease even further, to 18k or so based on Merlin’s article here: http://www.valvewizard.co.uk/accf.html

Based on another of Merlin’s articles, on the long tail pair, I was thinking of using a 12AY7 (got some of them, too) and his circuit here for the PI: http://www.valvewizard.co.uk/acltp.html