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--- 4amps:1tu:ccs [2022/01/17 16:54]
staudio [Усилитель на 300B]
+++ 4amps:1tu:ccs [2022/01/21 12:48]
staudio [Усилитель на 47 пентодах]
@@ Строка 343: / Строка 343: @@
 ----
+==== 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}}
+==== Усилитель на  6BQ5 ====
+The David project was an entertaining exercise in how much stuff can be crammed into a small box... The donor chassis' were a pair of Motorola SE 6BQ5 stereo amps. One of the amps came from a Motorola mini console, the other amp came from an AMC mini console.
+In each amp the power supply was changed to a full wave bridge with choke input to get the supply voltage up for the active loading. One of the 6BQ5's was retained as the output stage CCS. One of the original SE output transformers was used as the power supply choke. Managed to cram 3 CCS boards into the chassis... The output transformer is a 70 volt line matching transformer, the cheap one that Radio Shack used to sell.
+The David uses a 6AN8 triode/pentode as the input tube. The triode section is used as the input stage while the pentode section is used as the CCS for the input stage. The input stage is RC coupled to the EL84 CCS loaded 12B4 triode output stage.  The driver stage of the David has a CCS fed shunt regulated power supply.
+The pair of David monoblocks are used as my lab amps and see almost daily use. They put out 1 clean watt that is very transparent and has great bass. At full power of 1 watt the frequency response is 30 to 22K, at low power levels the response is better going from ~20 to 30K.
+One of my friends uses a pair of David's to power his Klipsh LaScala's. More power than he needs in a small room ;-}
+The schematic...
+{{4amps:1tu:david.gif}}
+{{4amps:1tu:david_front.jpg}}
+{{4amps:1tu:david_rear.jpg}}
+{{4amps:1tu:david_under_with_reg.jpg}}
+{{4amps:1tu:david_ccs.jpg}}
+{{4amps:1tu:david_shuntreg.jpg}}
+==== "Каменный" вариант ====
+http://www.pimmlabs.com/web/solid_state.htm\\
+Quite awhile back I got an email from John Swenson asking if the active circuitry used in the self bias could be used as an amplifier. I mentioned that I thought it would work and in fact had thought of the ideas several times in the past but had never pursued the idea. John went on to do experiments and came up with several interesting tranconductance amplifier ideas.
+After hearing of his successes I decided to try some ideas of my own. One of the early experiments was to compare a single IRF820b MOSFET to a modified self bias CCS amplifier. I started calling the basic building block the Solid State Pentode based on the way the circuit works.
+While the characteristic curves of single MOSFETs looks very Pentode like they do not have the low input capacitance of a Pentode. The cascode circuit below acts very much like an ideal Pentode as the gate to source capacitance of the upper MOSFET is shunted to the source of the lower MOSFET via C5.
+Here is the schematic of the basic SSP building block.
+{{4amps:1tu:ss_pentode.gif}}
+The next experiment was a CCS loaded differential stage. This stage is a tranconductance amplifier, meaning that an input voltage is converted to an output current. The goal was building a solid state version of the Tabor amplifier with an interstage transformer between the input and output stage. I was using 6.6K as the load for the input stage as this is what the estimated input impedance of the Tabor output stage. Later I substituted a 6.6K output transformer and found that by increasing the operating current and B+ voltage the single stage could deliver 10W into 8 ohms at quite low distortion. Not bad for a single stage amplifier. The output impedance is very high. With the amplifier delivering 10V rms into 8 ohms I clipped a second 8 ohm load resistor in parallel dropping the load impedance to 4 ohms. The output voltage dropped to 5V rms.
+Here is the schematic of the input tranconductance stage with values for both input stage and tranconductance output stage amplifier uses.
+{{4amps:1tu:differential_tranconductance.gif}}
+After the success of the input stage I wanted to mate it to the output stage and see the results. Here is the schematic of the first prototype of the "Solid State Tabor".
+{{4amps:1tu:ss_dif_poweramp.gif}}
+{{4amps:1tu:ss_amp.jpg}}

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