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--- 4amps:1tu:ccs [2022/01/17 16:54]
staudio [Усилитель на 300B]
+++ 4amps:1tu:ccs [2022/01/21 12:37]
staudio [Усилитель на 300B]
@@ Строка 342: / Строка 342: @@
 In Rev 3 the I added a shunt regulator driven by a CCS on the driver stage. This rates as one of the biggest improvements so far. The dynamics are really wild now. The background is blacker than ever and I'm hearing things I've never heard before. Bass has taken on a whole new personality, it feels totally bottomless. Kick drums are great. This is a SET that never looses it's composure no mater how crazy things get.
 ----
+==== Усилитель на 47 пентодах ====
+{{4amps:1tu:47pp_10_a.gif}}
+The new power supply is performing very well. The power transformer and the output CCS's are running cooler and the input pentodes are at a much better operating point. All the concerns about the old power supply have been addressed.
+The only thing that needs to be addressed it turn on hum. The plate to cathode voltage of the output tubes is set by the current flowing through the feedback resistors. This current is determined by the CCS on the input tubes. The input stage is at full operating current rather quickly and this forces the output stage to try to set the plate to cathode voltage to the standard operating point before the voltage is available. This starves the output CCS of voltage. When the current is below the set level on a battery biased CCS the CCS looks like a resistor with the value of (R1+5 ohms), or in this case about 130 ohms. Everything gets quiet as soon as there is voltage for the output CCS.
+The fix for this (I think) is going to be to use the mosfet only version of the Rev 5 CCS on the input stage. It starts in the off state and slowly increases the current up to the operating point, which is the opposite of the battery biased CCS. The battery biased CCS is on hard until the current increases above the set point. By increasing the value of C5 in the Rev 5 CCS the turnon time increased. The right size cap should delay the input stage reaching full operating current and stop the hum during startup. The only thing that is keeping me from trying this out is I have run out of space inside the prototype case. The darn thing is full!
+Sound wise, it sounds just a touch cleaner and more dynamic.
+******
+Here is the autobalance servo circuit. Basically, it senses the current in the output tubes by the DC voltage drop across the primary of the transformer. Because it is using the primary for the sense, if the resistance of the 2 halves of the primary don't match you need to add resistance to the low side to make it match with the high side. This must be done with the transformer warmed up to normal operating temp.
+When the circuit is balanced the differential amplifier (Q3,Q4) sends equal amounts of the ~3ma bias current provided by the LND150's through both transistors. If the amplifier drifts off center the differential amp will increase the current delivered to the screen of the side of the amp that is drawing more current. This turns the input pentode on harder on that side which in turn pulls down on the grid of the output tube on that side, decreasing the current it draws.
+The auto-balance circuit connects to the screens of the input pentodes and works with the DC balance pots. Once you adjust the output balance it does not drift more than .5ma after warm-up.
+Q5 and Q6 provide both level shifting and cascode isolation to the differential amp (Q3, Q4) The transistors used for Q3 and Q4 are now BC560's. They have a much higher gain than the 2N2907's.
+{{4amps:1tu:autobal.gif}}
+{{4amps:1tu:47_top.jpg}}
+{{4amps:1tu:47pp_under.jpg}}

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