I just sorted through my box-o-tubes and found a stash of about a dozen 6CG7/6FQ7's. I did a Google search and found many preferred them over the 12AU7. I myself have struggled with the 12AU7 over the years in my Foreplay III and the driver of my ST-70. I have "compromised" on the RCA Clear Tops.
I dug up an old thread on the AA forums and modified my Foreplay III to use the 6CG7's with a grounded shield and I was blown away! The bass bloomed and there is more depth to the sound (front-to-back soundstage). Female vocals almost have an eerie 3-D sound to them now as well.
For those considering this upgrade, I give it 3 thumbs up (
)! It only took about 30 minutes with a soldering iron:
1. Unhook heater wires from the 12AU7 tubes.
2. Unhook heater ground reference from Pin 9 of the one 12AU7.
3. Re-wire the heaters to Pins 4&5 of the 12AU7's (observe polarity!).
4. Tie the heater ground reference to Pin 4 of the one 12AU7.
5. Tie Pin 9 of both 12AU7's to the closest ground terminal (one used for cathode LED ground).
6. Replace 12AU7's with 6CG7/6FQ7's.
7. Enjoy!
