For detailed sound, the RCA Clear top is a good tube. I preferred the RCA Black plate with shield for the best all around sound. IMO.
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I use gold lions in my Cary SLP 50A. However, the reason I am replying is that although this is an ok preamp it requires some other changes in order to open up. Some changes that I have made:
The coupling capacitors, which reside on either side should be replaced. Use Mundorf silver and oil. If you can afford it Mondorf gold and oil is even better. Hifi collective out of the UK has the best price.
The next easiest change is to update the power supply. This preamp is sensitive to power surges and dips from air conditioners, refrigerators, and washing machines. The 100 uf capacitor on the side and the 100 uf in the muddle should be switched to ruby gold 200 ufs. The 100 uf in the back by the connectors should be replaced with 47 uf film from Solen. This will make the transformer run cooler and give cleaner power. See parts express.
Finally, if you feel really ambitious update the volume control for a big change. In it's current configuration I could only use 25% of the volume control. I used an Elma switch and Takman metal film resistors. The switch needs to be make before you break, two deck, 26 position. The input signals run through a 220k 1/2 watt, then out to the tubes and the volume switch at the same time position 1 goes to ground, 2 to the lowest homage available, 3 to the next ohmage and so on. The other side of each volume control resistor goes to ground. This will allow you to use over 50% of the volume control and will greatly improve the sound. I can help you with this giving you the values and wiring if you wish. My email is email@example.com.
So in summary this is an ok preamp but if you really want it to sound good a solder iron and high quality parts go a long ways and will make more improvement then any tubes you can choose.
I use the RCA clear tops or the Raytheon black plates or the sylvania black plates in all my Mac and Pilot tube amps..Now before you say how can I use them in the Macs,I converted the 12au7 inverter in the macs and the pilots to the 6cg7s and its a much better tube with lower distortion.They all sound fantastic with the three aforementioned tubes.
I was fortunate to find someone who had several NOS types locally. I tried RCA clear tops, GE clear tops and Sylvanias. the GEs were the best - just a tad stronger and fuller sounding than the others - and I wound up buying a second pair just to have on hand.
AJ, you're right about the volume. I can only take this up to about 1/3. I bought this pre-amp because I needed something decent and reasonably priced in a hurry, although I do like the way it sounds. I may look into those mods in the future. We had it hooked up to a buddy's SS amp last weekend and the sound was very impressive - especially considering the price, which was just a few hundred bucks.