I was running my preamp pretty hot to drive my oddwatt, so I finally decided to make use of the two 9 pin sockets I had installed "just in case" and do something about it.
I bought a pair of what are supposedly NOS Tung-Sol 5687 dual triodes (from TD) a few months back. They have black plates and man they get hot. Now I remember one reason I love octal preamp tubes.
I created a SRPP preamp stage from each tube using 1K resistors I had laying around, bypassing the cathode of the lower triode. Using just a few parts on hand I almost literally "slapped together" this preamp section. It's amazing how quickly something can get done once there are sockets in a chassis with power available.
I saw resistor values from 470 to 1.2k for the exact same SRPP using this exact same tube and similar values for other tubes, so I went with 1K to run a little on the low side. I'm using a B+ of 150 volts and I calculate the bias to be about 1.3 mA which seems a little low.
What's a good value to use with this tube for bias current in an SRPP using a 150 volt supply? Is 150 enough to get this tube to sound good or should I go higher? I can go as high as I want to with 475 volts available. I'll be testing a new high voltage CCS used as a voltage regulator and buffered with a high voltage transistor to regulate the supply to the preamp tubes. This voltage regulator would be powerful enough to provide regulated voltage to the KT88 oddblock. I don't know if it's worth the trouble but it should be doable. For the preamp section I want ultra-clean power. I have no issues with power tube hum, just preamp hum.
It sounds pretty good with the new preamp and I can now play any source no matter how weak through the oddwatt with no preamp needed. It's still a little noisy but I'll have that fixed. It has however lost something. It's not quite as clear and transparent as it was. If I can't fix it I'll put it back the way it was. I suspect rebiasing the 5687 would be a good place to start. That and raising the supply voltage.
Any ideas on what would make this sound a little "dull" to my ears. The schematic is the same SRPP that Bruce uses with the 6SL7 and 12SL7 but the resistor from the top grid to the bottom plate is just wire, and the cathode resistor (which is also 1K, same as the top resistor) is not split up to allow application of feedback, with the resistor fully bypassed with 500uF.
It looks really cool now with a total compliment of 8 tubes but my camera is lost (misplaced

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I was also wondering, should I bring the feedback all the way to this new first stage since I could apply feedback to an unbypassed portion of the cathode resistor if i wanted to , or would it be better to let the preamp SRPP remain autonomous and operate simply as a stand alone signal booster? To put it in the loop or not?
Any suggestions other than replacing it with an octal tube are welcome, even replacing the tube with another 9 pin. I actually didn't really know what I was buying but read it was a good preamp tube. I don't want to compromise the sound of the amp in any way, it was sounding awesome, so whatever it takes it what it takes.