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RE: LD2+ Revisit of Mu follower

Posted by driguy on September 13, 2011 at 14:33:25:
I forgot that I replaced my power supply transformer in order to get more B+ voltage for my EML 300b XLS tube. Of course, a dropping resistor is still used and the original 62k may still be the correct value. My notes show that it should set the voltage at about 390V with the original transformer which is fine. Be aware the the maximum rating on the 10M45 is 450V. Like most silicon products, the CCS does not like to have it's limits tested repeatedly. Watch the voltage after the dropping resistor when you start the unit up. Be sure you have the 300b installed when you test it to get the proper readings. You will most likely see a voltage spike that goes above your final setting by a fair amount. No need to worry too much but a little added resistance to ensure long life is not too bad. Personally I would not set the quiescent voltage below 375.
Second note: If you use a pot to set the CCS I would chose a setting that you like and install a high quality resistor that matches your setting. 2mA requires about a 1500 ohm resistor according to the spec sheet. You will notice a change in sound with different settings as per a previous post on the subject. I believe the poster wrote that more current resulted in a slightly harder sound.
Third note: I would highly recommend cascading the 10M45's. As per my previous post this is a significant improvement for very little money. I just did this after a couple of years with just one so I was very familiar with the sound. A no-brainer IMO. It increases the PSRR so that the output to the tube is a purer DC. It acts closer to having a separate power supply for the driver with less interaction with the output tube power supply. S/N ratio for that stage is better as well. Purchase some extra 10M45's so if they do fail you have them on hand and your amp is not out of service. I was glad I did. I think I had my voltage a little too close to the limit as I only measured the static voltage, not the voltage spike at startup.
Fourth note: If you really want to go to the end of this path you can go to a shunt regulator for the driver tube. I did this last week and the results are very good. I will post soon about the results and the installation with a picture. It is more difficult by far but it really separates the power supplies to a higher degree. Chassis space is the limiting factor on this upgrade as the photos will show.