Ok so I shorted the AC to ground for both output tubes at pin 5. The noise was present for both shorts.
As far as the meter goes it's a digital one. I am going to borrow a better meter from my friend later so I will see if that makes a difference. I am curious to check resistance across the two leads of my meter with his and see what the impedence is.
As far as the meter goes it's a digital one. I am going to borrow a better meter from my friend later so I will see if that makes a difference. I am curious to check resistance across the two leads of my meter with his and see what the impedence is.
For clarity, are you saying that when you shorted EACH PIN #5 INDIVIDUALLY to ground (AC wise!) the noise was still present in equal intensity? This is important as you indicated that when the master volume is turned to zero, the noise is gone. In that condition, pin #5 of BOTH output tubes is effectively shorted to ground AC wise. If this is all true, then it means that the noise is coming through on both sides of the push-pull signal. That would likely (but not completely) eliminate anything that is on just one side, as an individual PI plate resistor or coupling cap.
At this point, it would be good to start eliminating components. One safe way to eliminate a number of phase inverter components is to:
1. Short out the 1 meg grid resistors. Notice this is not to ground but across the resistors. You can do this one at a time, and then both at the same time. If either of them are noisy, this will eliminate them from the circuit.
2. Short the point where the 1 meg grid resistors and 470 ohm cathode resistor join, to ground. This will completely eliminate all components in the tail circuit.
3. Do both 1 & 2 together.
The results of these tests should indicate the condition of most of the components in the PI stage.
Dave
At this point, it would be good to start eliminating components. One safe way to eliminate a number of phase inverter components is to:
1. Short out the 1 meg grid resistors. Notice this is not to ground but across the resistors. You can do this one at a time, and then both at the same time. If either of them are noisy, this will eliminate them from the circuit.
2. Short the point where the 1 meg grid resistors and 470 ohm cathode resistor join, to ground. This will completely eliminate all components in the tail circuit.
3. Do both 1 & 2 together.
The results of these tests should indicate the condition of most of the components in the PI stage.
Dave
Ok so one at a time I shorted across the 1M "bootstrap resistors" and the second one I shorted stopped the noise. I am going to go ahead and assume that the resistor needs to be replaced. As soon as I find one in my resistor drawer I will replace and post back.
Which one stopped the noise -- the one on the active input side, or the one on the grounded grid side?
Dave
Dave
Shorting the resistor was more an effort to see where the signal was entering as a resistor in the grid return location would rarely be noisy itself -- it has no current flowing through it to make any noise. BTW, these resistors are not really "bootstrap" resistors -- they are simply the grid return resistors within a long tail phase inverter design. A true bootstrap circuit typically involves injecting a signal from a latter stage back into a given stage to enhance the gain or maximum signal swing of that stage. McIntosh (for example) used the technique in multiple places with great success in the design of their classic 275 amplifier. However, no such action is happening within the basic long tail phase inverter used here.
To absolutely confirm that the noise is from within the inverter stage once and for all, disconnect the .022 coupling cap into the inverter from the previous stage at the inverter stage input. Ground the output of the disconnected cap. If the inverter stage is the culprit, the noise will still be there. If it is, concentrate on current carrying resistors. In particular, if you short out the tail as previously instructed and the noise goes silent, then the culprit is one of the resistors making up the tail connection between the cathode/grid resistor tail connection, and ground.
If the noise goes silent with nothing more than the input cap disconnected and grounded, then the noise is being generated earlier, and was still making it into the inverter in spite of your efforts to short it out earlier with a cap.
Dave
Dave
To absolutely confirm that the noise is from within the inverter stage once and for all, disconnect the .022 coupling cap into the inverter from the previous stage at the inverter stage input. Ground the output of the disconnected cap. If the inverter stage is the culprit, the noise will still be there. If it is, concentrate on current carrying resistors. In particular, if you short out the tail as previously instructed and the noise goes silent, then the culprit is one of the resistors making up the tail connection between the cathode/grid resistor tail connection, and ground.
If the noise goes silent with nothing more than the input cap disconnected and grounded, then the noise is being generated earlier, and was still making it into the inverter in spite of your efforts to short it out earlier with a cap.
Dave
Dave
I disconnected the input cap and grounded the output side of the cap and the noise is still present. Next I grounded the junction where the two 1M grid resistors meet the 470 ohm resistor and the noise is still present.
Ok so I put a jumper across the input side 1M grid resistor and the noise is present (which earlier stopped the noise). Then I disconnected that jumper and placed the jumper across the other side 1M resistor and the noise is present. But when I jump both at the same time the noise stops.
OK -- This sucker is really enjoying itself while we look for it, but now we are starting to really zero in on the problem:
1. The tube is not it: It has been changed out and the noise was still present.
2. Stages previous to the phase inverter stage are not it: The .022 mfd input coupling cap was disconnected and its output grounded, and the noise was still present.
3. The tail components are not it: The tail was shorted out and the noise was still present.
4. Turning the master volume down eliminates the noise.
Therefore, the problem must be originating in the phase inverter stage.
If all of this is true, all we really have left is the 470 ohm cathode resistor, two plate load resistors, two primary coupling caps (i.e. to the top of the master volume), and don't rule out the little 47 pf cap between the plates.
It could be any of these components, but the likelyhood is that it will be one of the components in the plate circuits. All you can do is replace each one at a time to see which one is it.
This noise has been a bit of a pistol to find, but at least you are seeing how a logical approach is used to zero in on it. I'm sure the problem will show itself within these last few tests.
Dave
1. The tube is not it: It has been changed out and the noise was still present.
2. Stages previous to the phase inverter stage are not it: The .022 mfd input coupling cap was disconnected and its output grounded, and the noise was still present.
3. The tail components are not it: The tail was shorted out and the noise was still present.
4. Turning the master volume down eliminates the noise.
Therefore, the problem must be originating in the phase inverter stage.
If all of this is true, all we really have left is the 470 ohm cathode resistor, two plate load resistors, two primary coupling caps (i.e. to the top of the master volume), and don't rule out the little 47 pf cap between the plates.
It could be any of these components, but the likelyhood is that it will be one of the components in the plate circuits. All you can do is replace each one at a time to see which one is it.
This noise has been a bit of a pistol to find, but at least you are seeing how a logical approach is used to zero in on it. I'm sure the problem will show itself within these last few tests.
Dave
The reason I called the 1M resistors bootstraps was because earlier I was reading about the long tailed pair section of the Aiken amplification page. I must have misunderstood what he was saying lol. Here is what the article had to say:
The grid resistors
The grid resistors
These resistors (R3 and R4) provide the grid bias reference voltage. They are the equivalent of the normal "grid-to-ground" resistors in a standard preamp stage, except that they don't go to ground, instead, they go to a different "reference" point, the junction of R5 and R6. The value of these resistors is not critical, but they should be a moderately large value, somewhere around 100K - 1Meg. Contrary to popular belief, in this type of phase inverter, the input impedance is not equal to the value of this resistor, rather it is around two to five times higher, depending upon the amount of negative feedback from the "tail resistor" and the amount of global negative feedback (around two times higher for the circuit shown above, with no global negative feedback). This is why it is not a good idea to use too large a value of coupling capacitors going into the phase inverter input.
This increase in effective input impedance is known as "bootstrapping". It is similar to the effect you get when you have a self-biased cathode follower. There is an AC signal present at the junction of the grid resistor (R3) and the "tail" resistor (R6), since there is current feedback due to the unbypassed tail resistance. Since this signal is in phase with the input signal, the effective current through the grid resistor is lowered. The signal at the top and the bottom of the grid resistor is subtracted, and that voltage divided by the grid resistance gives the input current drawn by the stage. If you divide the input voltage by the input current, you get the effective input impedance. For example, if you apply a 1V AC signal and the signal at the tail node is 0.5V and in phase, the input impedance is 2 Megohms, not 1 Megohm, because there is 0.5V across the 1Meg grid resistor instead of 1V, which results in a current of 0.5uA for a 1V input, and Rin = 1V/0.5uA = 2 Megohms. If the tail resistor is large enough to be considered a constant current source, and there is no global negative feedback, the input impedance will be twice the value of the grid resistor.
This increase in effective input impedance is known as "bootstrapping". It is similar to the effect you get when you have a self-biased cathode follower. There is an AC signal present at the junction of the grid resistor (R3) and the "tail" resistor (R6), since there is current feedback due to the unbypassed tail resistance. Since this signal is in phase with the input signal, the effective current through the grid resistor is lowered. The signal at the top and the bottom of the grid resistor is subtracted, and that voltage divided by the grid resistance gives the input current drawn by the stage. If you divide the input voltage by the input current, you get the effective input impedance. For example, if you apply a 1V AC signal and the signal at the tail node is 0.5V and in phase, the input impedance is 2 Megohms, not 1 Megohm, because there is 0.5V across the 1Meg grid resistor instead of 1V, which results in a current of 0.5uA for a 1V input, and Rin = 1V/0.5uA = 2 Megohms. If the tail resistor is large enough to be considered a constant current source, and there is no global negative feedback, the input impedance will be twice the value of the grid resistor.
Yup I never had this hard of a time with any of my amps before but I am no tech. I would like to say thanks again for taking your time to help and educate me. I will probably just replace everything in the darn circuit to be safe.
Jason
Jason
Ah! As discussed in that reference, the basic concept of developing any significant signal across a large cathode resistance and tapping well up on that resistance for a proper bias reference does in fact "bootstrap" the input impedance -- or make it in fact larger than the simple value of the grid resistor would imply. I was assuming (erroneously) that you were referring to the bootstrapping of gain or of peak output capability (which this inverter circuit does not do), since that was more in line with the discussion at hand. Thank you for providing your reference and clarifying the issue! And, it shows what happens when you assume!
Now don't go giving up on this thing and just replace everything. You learn soldering skills that way, but not diagnostic skills. You're doing great -- and better than some who already call themselves techs!
Dave
Now don't go giving up on this thing and just replace everything. You learn soldering skills that way, but not diagnostic skills. You're doing great -- and better than some who already call themselves techs!
Dave
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I just replaced the two plate load resistors and the noise is still present. I think I might try the 470 cathode resistor next. I can't find one so looks like I am going for a ride to radioshack.
Ok so I replaced the cap going into the phase inverter and the noise is gone. Now I plugged my guitar in and there is no sound, lol. I can hear some normal white noise from the speaker but no guitar sound. I am waiting for the tubes to cool down before I try to see what I dun broke.
Everything looked good so I put the tubes back in. Now I get sound from clean channel but not from the gain channel, I think one of the tubes is faulty. Time to fire up my Hickok 539b.
If anything, this has been a great trouble shooting exercise for you! I have no doubt you'll find out why the gain channel is dead -- but at least you found the noise culprit!
Dave
Dave
Yes, I know I have educated myself and for the people that followed the thread hopefully gained some insight as well. I had so many tubes on my work bench that I think I was so excited that I solved the noise I just threw some tubes in and one might have been bad. I will hopefully give a final conclusion post sometime tomorrow because I am determined to get this done. Thanks again
Jason
Jason
The switch works and the led lights up but no sound coming through. I tested the tubes and they all are fine (I swapped them anyway). Here are some voltage reading I took:
V1b - 200v before plate resistor 88v after, .23v at the grid, .42v at the cathode (800 ohm cathode resistor)
V2a - 200v before plate resistor 90v after, 0v at grid, .6 at cathode
V2b - 200v at plate, 90v at grid, 92v, at cathode
The math looks ok except for V2b I thought should be something around 50v at cathode
V1b - 200v before plate resistor 88v after, .23v at the grid, .42v at the cathode (800 ohm cathode resistor)
V2a - 200v before plate resistor 90v after, 0v at grid, .6 at cathode
V2b - 200v at plate, 90v at grid, 92v, at cathode
The math looks ok except for V2b I thought should be something around 50v at cathode
I tried it again today and I guess there is some sound coming through but the boost channel volume has to be maxed in order to hear it.
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