I have a rebuilt Dynaco ST70. I am asking these questions because I am trying to gain some knowledge as how this can work. I have attached a drawing with the power supply voltages and Pins 1 through 9 voltage readings. I took some voltages on the pentode-triode pins and have the following questions. I do know the triode is a phase splitter and that the pentode provides some type of amplification.
1. How can the pentode plate be 92VDC and the triode grid be 248VDC without destroying the triode grid?
2. How can the triode plate be at 86VDC and the triode cathode be at 111VDC? Where is the rest of the voltage drop from a 344VDC B+ supply?
3. How can the pentode plate have 92 volts and the cathode have 2.71VDC? Where did the rest of the voltage go from the 271VDC B+ supply?
4. Why is Pin 7 of the pentode only 47 VDC?
I am just trying to understand why this circuit has these strange voltage readings and how it really works like this. I have tried my own research and honestly nothing I find explains this well. I find this community speaks plain language for beginners. I did know enough to troubleshoot and rebuild this amplifier and my PAS 3. As I said I am seeking a better operational understanding of this tube and voltages/current flow.
Thanks to anyone that can help me understand how this all works with such strange voltages.
1. How can the pentode plate be 92VDC and the triode grid be 248VDC without destroying the triode grid?
2. How can the triode plate be at 86VDC and the triode cathode be at 111VDC? Where is the rest of the voltage drop from a 344VDC B+ supply?
3. How can the pentode plate have 92 volts and the cathode have 2.71VDC? Where did the rest of the voltage go from the 271VDC B+ supply?
4. Why is Pin 7 of the pentode only 47 VDC?
I am just trying to understand why this circuit has these strange voltage readings and how it really works like this. I have tried my own research and honestly nothing I find explains this well. I find this community speaks plain language for beginners. I did know enough to troubleshoot and rebuild this amplifier and my PAS 3. As I said I am seeking a better operational understanding of this tube and voltages/current flow.
Thanks to anyone that can help me understand how this all works with such strange voltages.
It can't be. Pin 2 and 9 are common with no components in between. There could be a broken solder joint or trace which would seperate the readings but you should be able to use an ohm reading for good continuity between the socket pins with the tube removed.
I have no broken traces or errors. The amp works great. I just cannot figure out the mismatch or voltages readings I noted. Of course the Dynaco manual does not list any voltages on this tube. It does say that "measurements at these points vary from tube to tube and do not indicate whether performance is normal. Also, these tubes have been running almost 6 hours per day for over a year without a hint of sound degradation.
Is this circuit built with the original pcb, a third party pcb, or breadboarded? Are you counting pins CW as seen from the bottom, measuring with a high
impedance DVM? It does sound like you're counting the pins backwards. Remember some DC supply voltage is dropped across the plate resistors.
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First go back and remeasure with the correct pin numbers, counting CW as seen from the bottom.
Normally, the pentode plate (and triode grid, same node) are at around 90VDC to 110VDC referred to ground. The screen voltage will vary from tube to tube.
For the pentode, most of its DC supply voltage is dropped across the 270k plate resistor. For the triode, around 1/3 of its supply voltage is dropped across the 47k plate resistor.
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Rayma, Most interesting. When I measure at the 82PF capacitor connected to the triode grid through an 18k resistor I get exactly 110 volts, which is almost exactly what is coming from the pentode plate. I think that this tells me that the total potential when grounded is 214 vdc, but the triode grid only sees the 110vdc. Maybe I have now answered my own question again with lots of help.
Sounds like the voltages are right, and maybe the interpretation is the problem. Remember all DC voltages are referred to ground (chassis), and that the meter may affect the results, especially when connected to the higher impedance circuit nodes. If the triode cathode is at 110V to 120V, the circuit must be working ok, since it's all DC coupled.
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Ray, yes it has been working perfectly since the rebuild two years ago. I just got interested in understanding more of the circuitry. I do believe the meter could have an effect on the measurement. I may try looking for a VTVM. we used to have these in high school and i found one at a store. i just do not know how to tell if it is accurate. Thanks again.
VTVM is vintage hobbyist overkill. Just get a midpriced DMM at a home supply or auto supply store.
If it's only $5 or something, take along a new 1.5V battery and see what it reads after warmup. At least the VTVM won't load much. Otherwise, for $50 or so you can get a decent new DVM.
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