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--- 4amps:1tu:dn2540 [2022/01/14 09:49]
staudio создано
+++ 4amps:1tu:dn2540 [2022/01/14 15:51] (текущий)
staudio
@@ Строка 59: / Строка 59: @@
 http://hiend-audio.com\\
 [email protected]\\
+----
+https://www.bartola.co.uk/valves/2017/02/12/pentode-driver-with-gyrator-load/
+**Pentode driver with gyrator load**
+{{4amps:1tu:img_0094.jpg}}
+If you need gain and good drive, our friend the pentode is there. However, with the high anode resistance, it’s hard to implement as a driver. With a resistor load you get good results, but not optimal. The gyrator load (as a hybrid mu-follower stage) brings a good option to the pentode driver. The workaround to the high gain of the stage has been cleverly addressed by Gary Pimm. Here is just a brief summary of how to implement it:
+The circuit can be explained easily. The pentode (U1) is loaded with the gyrator (g1). The pentode screen has a stable voltage (vs) which is provided by the voltage regulator (U2) and the CCS formed by M1+R2. You can implement the screen voltage source that best suits you. Anyhow, the input is provided to the grid (g1) and the grid resistor (Rg) provides ground reference. The cathode resistor (Rk) is un-bypassed. Quite unusual for a pentode. The thing is, we have gain to spare, but thanks to the gyrator, the output impedance of the stage isn’t mu times the Rk. Hence we can afford adding this resistor which also linearise the stage thanks to the negative feedback introduced. Ra is required to provide a stable output and limit the gain. The gain is therefore Gm times the Ra, Gm is degenerated due to Rk (unless you bypass it). Ra could be also be placed in parallel with G1, but as Gary Pimm well explains, it’s better to have it referenced to ground to improve the power supply noise rejection (PSRR).
+The output is take from the mu output of the gyrator. The load is connected here. If you need all the gain from this stage you can bypass Rk or better replace Rk with a series of diodes (SiC) or LEDs. Whatever you please.
+This stage can be a great driver for a SE stage. Like a 300B. A 4P1L will work brilliantly here. As most of the Russian pentodes.
+Also if you want to go further, you can implement a pentode output stage and provide plate to plate feedback (a la Schade) and create a fantastic amp. Michael Koster and Anatoliy have covered this topology at length in DIYaudio, check it out. If you elevate the cathode of the output stage you can DC-couple it. Great stuff and sounds amazing, I did implement this with my 814 SE Amp.
+As you can see, a very flexible stage, thanks to the gyrator. Once again, a very handy topology to use.
+----
+**Improving the driver with a gyrator load**
+After the early experiments with the 4P1L driver in pentode mode, I decided to look at improving it somehow given advice given. The gyrator load is not a good match for a pentode unless the reflected impedance is low enough to control the gain of the stage. Gary Pimm recommends:
+“In the driver experiments the plate resistor was increased to a value larger than in traditional Pentode driver stages to get more gain.A CCS was placed in parallel with the plate resistor to add plate current to compensate for the high value plate resistor. This allows you to have independent controls of the gain and operating current. The resistor is chosen to set the gain and the CCS is used to set the Pentode operating current.
+To maximize the circuit performance the resistance in the screen circuit is adjusted for minimum distortion. There are draw backs to this- The circuit has to be tweaked for each tube. As adjusting the screen voltage and resistance also effects the gain of the stage you have to compromise some to have the gain match between 2 channels. This is not a circuit where you can swap tubes around without “calibrating” the stage on the test bench.
+Another interesting way of applying the circuit is to place the plate resistor in parallel with the Pentode and have the CCS supply all the current needed by the stage. This allows the Pentode driver stage to have PSRR similar to CCS loaded triode stages. It also makes the signal current loop very small including only the Pentode, cathode, and plate resistors. The noise and capacitor colorations of the power supply are quite effectively removed.”
+So I opted for adding a resistor in parallel (RL) to adjust gain, minimise distortion and improve PSRR:
+{{4amps:1tu:4p1l-pentode-driver-test1.png}}
+The load resistor is 68K. I optimised the operating point to reduce distortion at maximum swing (i.e. 200V peak to peak). The input impedance of the soundcard interface which is 100K didn’t produce a significant impact on the distortion when measuring from the anode output or in the mu output:
+P1L pentode filament bias RL=68K
+{{4amps:1tu:4p1l-pentode-filament-bias-rl68k.png}}
+Interesting to see that distortion is now nearly half of previous operating point and 0.27% for 200Vpp is very good.
+The screen current is approximately 1.8mA at 81V bias.
+----
+http://www.pimmlabs.com/web/drivers.htm
+Here are the results of some Pentode driver stage experiments done awhile back. The idea was to see what could be done combining Pentodes and CCS's. Generally, you can't load a Pentode with a CCS as you are effectively trying to series couple 2 CCS's. This issue is resolved by having a plate resistor in circuit so there is a defined resistance that the Pentode and CCS can work into.
+In the driver experiments the plate resistor was increased to a value larger than in traditional Pentode driver stages to get more gain.A CCS was placed in parallel with the plate resistor to add plate current to compensate for the high value plate resistor. This allows you to have independent controls of the gain and operating current. The resistor is chosen to set the gain and the CCS is used to set the Pentode operating current.
+To maximize the circuit performance the resistance in the screen circuit is adjusted for minimum distortion. There are draw backs to this- The circuit has to be tweaked for each tube. As adjusting the screen voltage and resistance also effects the gain of the stage you have to compromise some to have the gain match between 2 channels. This is not a circuit where you can swap tubes around without "calibrating" the stage on the test bench.
+Another interesting way of applying the circuit is to place the plate resistor in parallel with the Pentode and have the CCS supply all the current needed by the stage. This allows the Pentode driver stage to have PSRR similar to CCS loaded triode stages. It also makes the signal current loop very small including only the Pentode, cathode, and plate resistors. The noise and capacitor colorations of the power supply are quite effectively removed.
+Having the CCS in circuit allows you to use the MU output to drive the following stage. On the stages tested the MU output impedance was in the 500 ohm range.
+Here is the schematic of a driver stage using a 6BQ5 Pentode and CCS combo. Highlights of the circuit performance are gain of 125, output impedance of 500 ohms and distortion of .25% at 90 volts RMS. Not many driver stages can do .25% at 90V RMS! The operating parameters and measurements are on the schematic.
+{{4amps:1tu:6bq5_driver.gif}}
+Some more variants using different tubes-
+The 6W6GT has good performance and operates at a very low screen voltage. The gain is slightly lower than the 6BQ5 version.
+{{4amps:1tu:6w6gt.gif}}
+The 6BN11 also performs nicely but the gain is too high at 192 for practical uses.
+{{4amps:1tu:6bn11.gif}}
+The 6CL6 does not perform up the the standards of the other Pentodes tried.
+{{4amps:1tu:6cl6.gif}}

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