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Cascode Amp responds to Large Signal Burst
Here is a different kind of test on the PP 6V6 AB2 amp that I posted a while ago. And an updated schematic. The PS is a Hammond 271X into a FW CT, 1N4007s into a PI filter of 8 microF, 5H & 100 microF for the +ve side. There is also a –ve supply using 1N4007s on the same HV winding into another PI filter of 10 microF, 1.5K & 10 microF. The lid is held on for the –ve side by VR150 gas regulator. The OPT is a Hammond 125E Universal.
The 6V6 screen leads go to the +ve supply. You can connect the dots. From that one might think not to much is possible but they would be wrong. My objective with this exercise as I had previously stated is to try some different things. That is, discover what is possible in a vacuum tube amplifier with low cost parts. It is well known that by throwing unlimited resources (money & time) at a project that most anything is possible. That is not my intention here! There are many tradeoffs in any system. It is the designer’s task to separate what is important from what is not. In the real World market the end product must be competitive. The trace shows what happens to the +ve supply under pulsed conditions. The top marker indicates 10 watts. The red trace is the one KHz test signal, pulsed. The blue trace is the B+ supply at the 6V6 plates. The pulse rep rate is about 14 Hz. How did I do this? I will post that info later. Cheer to all, John |
#2
Posted to rec.audio.tubes
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Cascode Amp responds to Large Signal Burst
"John L Stewart" wrote in message ... Here is a different kind of test on the PP 6V6 AB2 amp that I posted a while ago. And an updated schematic. The PS is a Hammond 271X into a FW CT, 1N4007s into a PI filter of 8 microF, 5H & 100 microF for the +ve side. There is also a -ve supply using 1N4007s on the same HV winding into another PI filter of 10 microF, 1.5K & 10 microF. The lid is held on for the -ve side by VR150 gas regulator. The OPT is a Hammond 125E Universal. The 6V6 screen leads go to the +ve supply. You can connect the dots. From that one might think not to much is possible but they would be wrong. My objective with this exercise as I had previously stated is to try some different things. That is, discover what is possible in a vacuum tube amplifier with low cost parts. It is well known that by throwing unlimited resources (money & time) at a project that most anything is possible. That is not my intention here! There are many tradeoffs in any system. It is the designer's task to separate what is important from what is not. In the real World market the end product must be competitive. The trace shows what happens to the +ve supply under pulsed conditions. The top marker indicates 10 watts. The red trace is the one KHz test signal, pulsed. The blue trace is the B+ supply at the 6V6 plates. The pulse rep rate is about 14 Hz. How did I do this? I will post that info later. Cheer to all, John +-------------------------------------------------------------------+ |Filename: Schema And Burst Example 10W 20C.jpg | |Download: http://www.audiobanter.com/attachment.php?attachmentid=343| +-------------------------------------------------------------------+ Hi John, What is the purpose of the cathode followers in front of 6V6s? At 320Vdc they can deliver as much as they are capable safely in AB1. Overdiriving the amp with Vg2=320V and going into control grid positive zone will either: a) burn the screen grids if nominal Ra-a load is present; b) burn the plates if the load happens to short out. |
#3
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Hi John,
What is the purpose of the cathode followers in front of 6V6s? At 320Vdc they can deliver as much as they are capable safely in AB1. Overdiriving the amp with Vg2=320V and going into control grid positive zone will either: a) burn the screen grids if nominal Ra-a load is present; b) burn the plates if the load happens to short out.[/quote] You are correct Alex, the loadline needs to be adjusted from Class AB1 conditions. Otherwise the screen grids draw excessive current & are in danger of failing. For the example I gave in the previous post I set the loadline at 5.6K. The attached PP 6V6 plate family illustrates this but these are not the actual loadlines. From the trace we see the B+ changes by about 13 volts in 9 ms. The current is coming from a 100 microF cap. If CE = IT, then the current pulse on the PS is 144 mA. So each 6V6 sees a total plate & screen current increase from nominal of 72 mA. The standing current in each is about 38 mA. So peak current during this test was 110 mA. And Ik adds a bit more to account for the G1 current while +ve. Running as a vertical deflection amplifier the 6V6 cathode in peak rated at 115 mA, so looks OK. But in a real audio situation it is unlikely the amplifier would be run at this level. The program material would see to that. I’ve attached here another trace, this time of music by Peter Gabreil, the beginning of his performance of Salsbury Hill. This one drives into a loudspeaker load. All done while my wife was out, being very loud, far more than one needs while inside! For this one the loadline was at 6.8K. This trial in Class AB2 is similar to what has been published in the older tube manuals showing 6F6s running PP Class AB2 in both triode & pentode connexion. My practice is to set the loadline in a PP pentode stage to be somewhat lower impedance than that which results in the maximum output. It is claimed that this helps to lower the 3H. It is also useful where the loudspeaker impedance rises somewhat above nominal at the resonances in some speaker systems. Cheers, John |
#4
Posted to rec.audio.tubes
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Cascode Amp responds to Large Signal Burst
"John L Stewart" wrote in message ... Hi John, What is the purpose of the cathode followers in front of 6V6s? At 320Vdc they can deliver as much as they are capable safely in AB1. Overdiriving the amp with Vg2=320V and going into control grid positive zone will either: a) burn the screen grids if nominal Ra-a load is present; b) burn the plates if the load happens to short out. You are correct Alex, the loadline needs to be adjusted from Class AB1 conditions. Otherwise the screen grids draw excessive current & are in danger of failing. For the example I gave in the previous post I set the loadline at 5.6K. The attached PP 6V6 plate family illustrates this but these are not the actual loadlines. From the trace we see the B+ changes by about 13 volts in 9 ms. The current is coming from a 100 microF cap. If CE = IT, then the current pulse on the PS is 144 mA. So each 6V6 sees a total plate & screen current increase from nominal of 72 mA. The standing current in each is about 38 mA. So peak current during this test was 110 mA. And Ik adds a bit more to account for the G1 current while +ve. Running as a vertical deflection amplifier the 6V6 cathode in peak rated at 115 mA, so looks OK. But in a real audio situation it is unlikely the amplifier would be run at this level. The program material would see to that. With Vg2=250V (as per your load line plot) yeah, AB2 might be justified to suck everything from the poor 6V6. But for Vg2=+320V in pentode (as per your schematic) I guess AB2 is too much for a poor 6V6.. I've attached here another trace, this time of music by Peter Gabreil, the beginning of his performance of Salsbury Hill. This one drives into a loudspeaker load. All done while my wife was out, being very loud, far more than one needs while inside! For this one the loadline was at 6.8K. This trial in Class AB2 is similar to what has been published in the older tube manuals showing 6F6s running PP Class AB2 in both triode & pentode connexion. My practice is to set the loadline in a PP pentode stage to be somewhat lower impedance than that which results in the maximum output. It is claimed that this helps to lower the 3H. It is also useful where the loudspeaker impedance rises somewhat above nominal at the resonances in some speaker systems. This makes sense. Since in PP you do not have 2H anyway (in theory), you have nothing to lose. Then, the control grid characteristic emphasizes the peaks because of the "law 3/2" curvature, but plate-to-screen current distribution tends to compress the peaks. These two effects somewhat cancel each other at a certain load, thus minimising 3H. Cheers, John +-------------------------------------------------------------------+ |Filename: 6V6GTA 14W 4 PushPull C 5W.jpg | |Download: http://www.audiobanter.com/attachment.php?attachmentid=345| |Filename: 2013_03_21_001 Peter Gabriel Salsbury Hill 8W.jpg | |Download: http://www.audiobanter.com/attachment.php?attachmentid=346| +-------------------------------------------------------------------+ -- John L Stewart |
#5
Posted to rec.audio.tubes
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Cascode Amp responds to Large Signal Burst
"John L Stewart" wrote in message ... Here is a different kind of test on the PP 6V6 AB2 amp that I posted a while ago. And an updated schematic. The PS is a Hammond 271X into a FW CT, 1N4007s into a PI filter of 8 microF, 5H & 100 microF for the +ve side. There is also a -ve supply using 1N4007s on the same HV winding into another PI filter of 10 microF, 1.5K & 10 microF. The lid is held on for the -ve side by VR150 gas regulator. The OPT is a Hammond 125E Universal. The 6V6 screen leads go to the +ve supply. You can connect the dots. From that one might think not to much is possible but they would be wrong. My objective with this exercise as I had previously stated is to try some different things. That is, discover what is possible in a vacuum tube amplifier with low cost parts. It is well known that by throwing unlimited resources (money & time) at a project that most anything is possible. That is not my intention here! There are many tradeoffs in any system. It is the designer's task to separate what is important from what is not. In the real World market the end product must be competitive. The trace shows what happens to the +ve supply under pulsed conditions. The top marker indicates 10 watts. The red trace is the one KHz test signal, pulsed. The blue trace is the B+ supply at the 6V6 plates. The pulse rep rate is about 14 Hz. How did I do this? I will post that info later. Cheer to all, John +-------------------------------------------------------------------+ |Filename: Schema And Burst Example 10W 20C.jpg | |Download: http://www.audiobanter.com/attachment.php?attachmentid=343| +-------------------------------------------------------------------+ Apparently it is useless to allow for class AB2 output stage push-pull operation while class AB1 with elevated Vg2=320V is quite capable of working the output tubes 6V6GT to thier limits. It would seem that class AB2 can only ruin the screen grids when the amplifier is overdriven into clipping. However, upon reflection I can see one advantage of class AB2 in these conditions. Usually, in class AB1 control grids are driven via decoupling capacitors. When an amp is overdriven, these caps can charge by the grid current into class C1 and once overdriving burst is gone, get the amp virtually muted for a while, until they discharge to the nominal bias level. To avoid this coupling caps charging effect it is better to use cathode follower drivers in front of the output tubes, but ( IMPORTANT! ) insert rather large resistors (say 47K...100K) between the cathode folowers and the control grids of the output tubes. In this case, if the amp is overdriven, the coupling caps in front of the cathode followers will not charge as the cathode follower will probably never run into grid current. At the same time, large grid resistors will clamp positive excursions of Vg1 on the output tubes. So 6V6GTs will work in sort of benign class AB2, not much harder than AB1. Thus it will be safer fo the output tubes and there will be no recovery problem after a burst. (These series grid resistros shall be bypassed by small capacitors, say 100pF.) |
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