The grid needs to stay connected to ground through R5 (or a similar resistor). But you already measured the grid voltage (0.8 V). Do you know what it is supposed to be?
The DC leakage resistance of C40 and C3 in parallel is 170/0.8 x 475 kOhm = 100 MOhm. I do not know whether that is an acceptable or expected value for these capacitors in that location.
The DC leakage resistance of C40 and C3 in parallel is 170/0.8 x 475 kOhm = 100 MOhm. I do not know whether that is an acceptable or expected value for these capacitors in that location.
Paper in oil? C40 says STY, and C3 PPN. I would assume both to be polypropylene, to achieve 100 MOhm leakage resistance, no?
I will disconnect the caps on one end next and see how voltages change. This will be disconnecting C3//C40 from V1 anode.
I have also 'measured' the impedance across these coupling caps from V1>V2 and my meter is not sensitive or appropriate as they measure open circuit on it.
Rich
I have also 'measured' the impedance across these coupling caps from V1>V2 and my meter is not sensitive or appropriate as they measure open circuit on it.
Rich
I see. Measuring 100 MOhm, or 200 MegaOhm per capacitor, is not easy, even if you build a voltage divider.
Actually, thinking a bit more about it, this (wrong leakage resistance pulling the V2 grid bias up) must be what is going wrong. Tube V2, when biased, simply finds the wrong operating point. The anode to cathode bias self-adjusts downward as the current through the tube increases (via the voltage drop across the anode resistor), and the grid bias relative to the cathode becomes likewise more negative (the cathode becomes more positive via the voltage drop across the cathode resistor), until the tube reaches a valid operating point.
So, I am assuming that the grid DC voltage is actually supposed to be zero, or at least very small compared to the cathode voltage of 1.1 V, otherwise the circuit would rely on a fine-tuned value for capacitor leakage resistances. There is nothing in the circuit that can bias the grid negative, so zero is as low as it can go, and the only natural point for the grid voltage in this circuit.
One should probably look at the data sheet (operational curves) for the tube and confirm that -1.1V grid bias, 150 V anode bias and 0.8mA current are a valid operating point.
TL;DR: there is too much leakage through or around the caps. 2000 MOhm (to make the grid bias 0.1 V or smaller) is apparently possible for different dielectrics, not just polypropylene (see e.g. https://electronics.stackexchange.c...leakage-capacitor-for-nano-amp-current-source ). Also interesting: https://audiokarma.org/forums/index.php?threads/paper-in-oil-caps-replace-on-sight-or-keep.778227/
Did you also clean the caps themselves? Surface contamination on them (and apparently also cracks in the outer surface) could drive the leakage current up.
Actually, thinking a bit more about it, this (wrong leakage resistance pulling the V2 grid bias up) must be what is going wrong. Tube V2, when biased, simply finds the wrong operating point. The anode to cathode bias self-adjusts downward as the current through the tube increases (via the voltage drop across the anode resistor), and the grid bias relative to the cathode becomes likewise more negative (the cathode becomes more positive via the voltage drop across the cathode resistor), until the tube reaches a valid operating point.
So, I am assuming that the grid DC voltage is actually supposed to be zero, or at least very small compared to the cathode voltage of 1.1 V, otherwise the circuit would rely on a fine-tuned value for capacitor leakage resistances. There is nothing in the circuit that can bias the grid negative, so zero is as low as it can go, and the only natural point for the grid voltage in this circuit.
One should probably look at the data sheet (operational curves) for the tube and confirm that -1.1V grid bias, 150 V anode bias and 0.8mA current are a valid operating point.
TL;DR: there is too much leakage through or around the caps. 2000 MOhm (to make the grid bias 0.1 V or smaller) is apparently possible for different dielectrics, not just polypropylene (see e.g. https://electronics.stackexchange.c...leakage-capacitor-for-nano-amp-current-source ). Also interesting: https://audiokarma.org/forums/index.php?threads/paper-in-oil-caps-replace-on-sight-or-keep.778227/
Did you also clean the caps themselves? Surface contamination on them (and apparently also cracks in the outer surface) could drive the leakage current up.
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Well I removed the connection to the Capacitor joining V1 to V2 on channel and got obviously nearly 0V on the grid of V2 and then 160V on the anode of V2 and 170V on the cathode of V3.
So I guess this means the capacitor needs a change?
Thoughts?
So I guess this means the capacitor needs a change?
Thoughts?
Sounds like success, except without the caps you are not amplifying anything anymore...
So, yes, you need reasonable caps. The first link I posted talks a bit about leakage resistance for different dielectrics (=types of caps). Unfortunately leakage resistance does not seem to be a selection criterion on Digikey. Maybe that means it usually is not an issue? You need >2000 MOhm for each of the two caps. I would stick with the type that is specified for the locations, or, if there is no spec, with the type that was in there from the manufacturer.
So, yes, you need reasonable caps. The first link I posted talks a bit about leakage resistance for different dielectrics (=types of caps). Unfortunately leakage resistance does not seem to be a selection criterion on Digikey. Maybe that means it usually is not an issue? You need >2000 MOhm for each of the two caps. I would stick with the type that is specified for the locations, or, if there is no spec, with the type that was in there from the manufacturer.
Well so what I am thinking of doing is using the same ones that now work in the line stage in this location V4>V5, and in this case using the same value 0.47uF which was the value used in the original Mk1. They sound great and it's an almost identical circuit where they are 175V rather than 170V on the anode.
Well they are made by a reputable manufacturer (Arizona Caps) and they represent a copy of an old hermetically sealed West Cap military PIO capacitor that was no longer made.....But of course there is a marketing story of sound quality embedded in there.
However the improvement that they made in the line stage to the sound quality was significant over the original (but old) metalized polypropylene REL caps.
I have changed resistors, capacitors and even diodes and heard differences in sound quality (to my ears, in my system, in my equipment) and also found a lot of the boutique sound quality focused products to be worse than regular industrial products BUT they do all make a difference, and these I do rate highly.
However the improvement that they made in the line stage to the sound quality was significant over the original (but old) metalized polypropylene REL caps.
I have changed resistors, capacitors and even diodes and heard differences in sound quality (to my ears, in my system, in my equipment) and also found a lot of the boutique sound quality focused products to be worse than regular industrial products BUT they do all make a difference, and these I do rate highly.
Ok, but they do need to meet the specs for this amp, which implicitly seem to be >2000 MegaOhm DC leakage resistance. Maybe ask Arizona Caps about that.
There is a big question mark how thry sell substandard capacitors , in high impedance circuits , not only with tubes, are useless
And who knows how the leakage current change with temperature and in time ...
And who knows how the leakage current change with temperature and in time ...
Hey clever people, how come the Grid voltages in this article are negative voltage, is that reference to earth 0V or the cathode voltage?
I am going to use this experience to grow my understanding of valve operating points 🤔
I am going to use this experience to grow my understanding of valve operating points 🤔
Using autobias ( cathode resistor ) if the grid is at 0V to ground and cathode at +1,5V then the grid is more negative than cathode with that voltage
ok, makes sense - so how do you deal with these curves (or how do you create a negative grid voltage in a typical installation?) or do these curves refer to the negative offset of the grid to the cathode?
https://robrobinette.com/images/Guitar/Tube_Load_Lines/12AX7_Plate_Characteristics_Graph_blank.jpg
https://robrobinette.com/images/Guitar/Tube_Load_Lines/12AX7_Plate_Characteristics_Graph_blank.jpg
An interesting thing about valve curves is that they always apply, independent of everything else going on around them - external voltages, feedback, with signal or without, etc. You could think about this by imagining a similarly based curve for a resistor - it's just a single straight line, but it's always true, independent of everything else. For the condition of any particular set of voltages (or current) across the terminals, that intersection of voltage and current will define the other value(s).
All good fortune,
Chris
All good fortune,
Chris