Hi,
I just thought I would pop in and answer a question about the use of carbon composition resistors.
Carbon composition resistors are not inductive, and therefore find use in high frequency circuits. The series resistor for the grid connection is to stop (grid stopper) high frequency oscillation. Therefore you should use carbon composition resistors. I have found inductive metal film resistors before that shows up even at 100 KHz. Prime trouble territory.
Noise. Carbon composition resistors are apparently only noisy when you pass a DC current through them. Since the grid circuit ideally is a high impedance circuit, these resistors should not add any additional noise. Now when you have grid current flowing, the signal will be high and the output will not show the noise. Besides, you normally want to stay completely away from the area where you do get grid current. In class AB2 amplifiers (where grid current does flow), they use a low impedance driver so that the distortion is minimized. Personally, I stay in the AB1 classification as I don't try and squeeze every watt possible from a pair of tubes.
Just a note about resistor power ratings. In the bad old days, 1/2 watt composition resistors had about a 600 volt breakdown rating. Modern metal film and carbon film resistors may only have a breakdown rating of 50 volts for a 1/4 watt resistor. Often you will need to go to a 1 watt resistor just to withstand the applied voltage. Now, you can calculate the normal operating voltages, but do not forget to look at conditions before the tubes have warmed up. Your B+ voltages will be very high for that period of time, so make sure your capacitors and resistors can withstand those voltages.
Another thing that should be apparent is that you need to look at the data sheet for the parts you use. That means you must buy your parts from some place like Digikey, Mouser or one of the other proper distributors. Assume all other parts have worst case ratings, like 50 VDC for a resistor. Mystery parts have no place in your projects or repairs. I can't stress this enough.
For those of you who have already used 100 V resistors (say) and they haven't failed yet, that is only because there is a safety buffer between actual failure and the rated voltage. However, flashover can begin with creep which tends to generate noise. But rest assured that some of these will fail at some point. They may take more expensive parts with them, so get out your tools and replace them with proper rated parts.
I have talked about a resistor breaking down due to voltage. The other failure mode is excessive dissipation, so watch that as well. The voltage rating takes over when you have a high value resistor, say 1 meg, that fails unexpectedly. These failures come out of the blue and surprise everyone. Especially those who don't believe me.
Just for practice (and to prove my point), have a look at the data sheets for some resistors. Digikey might be easier for that since the data sheets are a little more clearly located. Look up the metal film and carbon film types.
-Chris
I just thought I would pop in and answer a question about the use of carbon composition resistors.
Carbon composition resistors are not inductive, and therefore find use in high frequency circuits. The series resistor for the grid connection is to stop (grid stopper) high frequency oscillation. Therefore you should use carbon composition resistors. I have found inductive metal film resistors before that shows up even at 100 KHz. Prime trouble territory.
Noise. Carbon composition resistors are apparently only noisy when you pass a DC current through them. Since the grid circuit ideally is a high impedance circuit, these resistors should not add any additional noise. Now when you have grid current flowing, the signal will be high and the output will not show the noise. Besides, you normally want to stay completely away from the area where you do get grid current. In class AB2 amplifiers (where grid current does flow), they use a low impedance driver so that the distortion is minimized. Personally, I stay in the AB1 classification as I don't try and squeeze every watt possible from a pair of tubes.
Just a note about resistor power ratings. In the bad old days, 1/2 watt composition resistors had about a 600 volt breakdown rating. Modern metal film and carbon film resistors may only have a breakdown rating of 50 volts for a 1/4 watt resistor. Often you will need to go to a 1 watt resistor just to withstand the applied voltage. Now, you can calculate the normal operating voltages, but do not forget to look at conditions before the tubes have warmed up. Your B+ voltages will be very high for that period of time, so make sure your capacitors and resistors can withstand those voltages.
Another thing that should be apparent is that you need to look at the data sheet for the parts you use. That means you must buy your parts from some place like Digikey, Mouser or one of the other proper distributors. Assume all other parts have worst case ratings, like 50 VDC for a resistor. Mystery parts have no place in your projects or repairs. I can't stress this enough.
For those of you who have already used 100 V resistors (say) and they haven't failed yet, that is only because there is a safety buffer between actual failure and the rated voltage. However, flashover can begin with creep which tends to generate noise. But rest assured that some of these will fail at some point. They may take more expensive parts with them, so get out your tools and replace them with proper rated parts.
I have talked about a resistor breaking down due to voltage. The other failure mode is excessive dissipation, so watch that as well. The voltage rating takes over when you have a high value resistor, say 1 meg, that fails unexpectedly. These failures come out of the blue and surprise everyone. Especially those who don't believe me.
Just for practice (and to prove my point), have a look at the data sheets for some resistors. Digikey might be easier for that since the data sheets are a little more clearly located. Look up the metal film and carbon film types.
-Chris
Is there only one primary winding on the power transformer? If there is, then I see a few options:
a)Return to seller if possible and buy the 120volt model
b)Buy a 120 to 220 step-up transformer and plug the amp into that.
Hammond Mfg. - Isolating Line (Step-Up) Transformers - (298T Series)
c)Buy a new (appropriate secondaries) 120 v power transformer and install it into the amp.
a)Return to seller if possible and buy the 120volt model
b)Buy a 120 to 220 step-up transformer and plug the amp into that.
Hammond Mfg. - Isolating Line (Step-Up) Transformers - (298T Series)
c)Buy a new (appropriate secondaries) 120 v power transformer and install it into the amp.
I used the "second lower capacitors" because I'm using the diagram proposed in post#225 (https://www.diyaudio.com/forums/att...el34-a9-tube-amp-a9-schematic-mods-rev1-2-pdf), but I be glad to remove them if they don't bring any benefits to the circuit.
I've installed the two 4K cathode resistors and the two 1K grid stop resistors and it definitely improved the bias balance within the 6SL7 tube. They are now within 10%. see table below.
BTW, I'm currently using 470uF / 63V capacitors in the 6SL7 preamp tube but they are too big and look ugly. Since Vk is only 2.6V, can I use a smaller size capacitor 470uF / 25V?
Thanks,
Frank
Attachments
Yes, a lower voltage cap will be fine IMO.
Also, 470uF seems 'big' in that spot - perhaps 6A3 or another person can explain the considerations for choosing a particular size (uF value) cap for cathode bypass.
Rod Elliott agrees with 6A3 on the idea of paralleling electrolytics with small-value film caps.
This is a useful article, IMO:
Capacitor Characteristics
Read down and you will find in Section 3.1:
Fdlima,
A. In regards to ways of testing how effective using a smaller uF bypass cap across a larger uF bypass cap is, I only know of a few test examples:
1. Use a Very capable LCR meter, and test the capacitors' ESRs (Series Resistance), and Dissipation Factors at both low and high frequencies. Then calculate the amount of improvement in the particular circuit you will use them in. i.e. given a 1000uF cap with an ESR of 1 Ohm (rather large for that), and the smaller 0.1 uF cap with an ESR of 0.05 Ohms . . . what is the result of using a 1000uF cap, with or without the 0.1uF cap in parallel (i.e. to bypass a 1000 Ohm self bias resistor and effective cathode impedance of 500 Ohms)? Very few will have access to that kind of an LCR meter; and some of them may not know how to calculate the results in the circuit under question.
2. Very accurately check the frequency response (especially at the high frequencies), with and without the 0.1uF cap in parallel with the 1000uF cap.
3. Measure the Harmonic distortion of the amp. Use a super low distortion sine wave generator, and a spectrum analyzer that has 85dB dynamic range (and either very low distortion, or use a 1kHz notch filter, and a 5kHz notch filter to reject the fundamental, and increase the measurement dynamic range). Check at 1kHz and 5 kHz.
4. Use your ears to hear the difference, with and without the 0.1uF cap in parallel with the 1000uF cap. Be sure to use the Double Blindfold testing method.
Most will pick # 4 above, but without the Double Blindfold method. The other tests are just a little out of range for many of us.
B. You can use a 10V, or 16V rated 470uF capacitor. Some electrolytic capacitors do not work as well if their rating is many times the voltage they will be used at, i.e. 63V rating working at 3V.
C. I am glad that using the individual self bias resistors got the more balanced voltage result you were looking for (and individual grid stopper resistors is just good to do). I was not surprised at the improvement.
A. In regards to ways of testing how effective using a smaller uF bypass cap across a larger uF bypass cap is, I only know of a few test examples:
1. Use a Very capable LCR meter, and test the capacitors' ESRs (Series Resistance), and Dissipation Factors at both low and high frequencies. Then calculate the amount of improvement in the particular circuit you will use them in. i.e. given a 1000uF cap with an ESR of 1 Ohm (rather large for that), and the smaller 0.1 uF cap with an ESR of 0.05 Ohms . . . what is the result of using a 1000uF cap, with or without the 0.1uF cap in parallel (i.e. to bypass a 1000 Ohm self bias resistor and effective cathode impedance of 500 Ohms)? Very few will have access to that kind of an LCR meter; and some of them may not know how to calculate the results in the circuit under question.
2. Very accurately check the frequency response (especially at the high frequencies), with and without the 0.1uF cap in parallel with the 1000uF cap.
3. Measure the Harmonic distortion of the amp. Use a super low distortion sine wave generator, and a spectrum analyzer that has 85dB dynamic range (and either very low distortion, or use a 1kHz notch filter, and a 5kHz notch filter to reject the fundamental, and increase the measurement dynamic range). Check at 1kHz and 5 kHz.
4. Use your ears to hear the difference, with and without the 0.1uF cap in parallel with the 1000uF cap. Be sure to use the Double Blindfold testing method.
Most will pick # 4 above, but without the Double Blindfold method. The other tests are just a little out of range for many of us.
B. You can use a 10V, or 16V rated 470uF capacitor. Some electrolytic capacitors do not work as well if their rating is many times the voltage they will be used at, i.e. 63V rating working at 3V.
C. I am glad that using the individual self bias resistors got the more balanced voltage result you were looking for (and individual grid stopper resistors is just good to do). I was not surprised at the improvement.
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Fdlima,
I have been a proponent of using a large value of capacitance for bypass capacitors.
This will give a way to calculate the bypass capacitance (it will have some approximations and simplifications, but will get you close enough). We are not doing a moon shot that does not have a midcourse correction, even NASA did not attempt that.
Lets look at your 6SL7. Transconductance is 1600 micro mhos Mu (u) is 70
The individual self bias resistor in your circuit is 4000 Ohms.
The 6SL7 transconductance is 1600uMhos. If the plate was tied to a low impedance (but it is not), then looking into the cathode we would see 625 Ohms impedance, due to 1 / transconductance.
But because the plate sees a high impedance, we have to take that into account (it affects the impedance of the cathode). The 6SL7’s 2 plates are sharing 75k Ohms (each plate effectively sees 150k Ohms).
In addition to the 625 Ohm cathode impedance, we have to add the impedance the plate sees, divided by the tube u.
150k Ohms / u = 150k / 70 = 2143 Ohms.
Now, looking into the cathode, we see 625 Ohms in Series with 2143 Ohms.
625 + 2143 = 2768 Ohms effective cathode impedance.
The 4000 Ohm self bypass resistor is in parallel with 2768 Ohms cathode resistance.
4000 Ohms in parallel with 2768 Ohms is 1636 Ohms.
We have to bypass 1636 Ohms.
If we are willing for that circuit to be -1 dB at 20 Hz, we need the capacitive reactance to be 1636 Ohms / 11. 1 dB is where the voltage drops to 89% (an 11% voltage loss).
1636 / 11 = 149 Ohms
Capacitive Reactance,
Xc = 1/(2 x pi x F x C) 1 / Xc x (2 x pi x F) = C 1 / (149 Ohms x 2 x 3.1416 x 20Hz) = 54uF
A 50uF bypass cap across the 4k resistor (working with the rest of the 6SL7 circuit) will have a frequency response that is -1 dB at 20 Hz, versus the response at 1 kHz.
Want that circuit to be -1 dB at 10 Hz, use a 100 uF cap.
Remember, if you have 3 circuits that are -1 dB at 20 Hz, the total response will be -1dB x 3 = -3dB at 20 Hz. So, 3 circuits that are -1 dB at 10 Hz, will be -3dB at 10 Hz.
I have been a proponent of using a large value of capacitance for bypass capacitors.
This will give a way to calculate the bypass capacitance (it will have some approximations and simplifications, but will get you close enough). We are not doing a moon shot that does not have a midcourse correction, even NASA did not attempt that.
Lets look at your 6SL7. Transconductance is 1600 micro mhos Mu (u) is 70
The individual self bias resistor in your circuit is 4000 Ohms.
The 6SL7 transconductance is 1600uMhos. If the plate was tied to a low impedance (but it is not), then looking into the cathode we would see 625 Ohms impedance, due to 1 / transconductance.
But because the plate sees a high impedance, we have to take that into account (it affects the impedance of the cathode). The 6SL7’s 2 plates are sharing 75k Ohms (each plate effectively sees 150k Ohms).
In addition to the 625 Ohm cathode impedance, we have to add the impedance the plate sees, divided by the tube u.
150k Ohms / u = 150k / 70 = 2143 Ohms.
Now, looking into the cathode, we see 625 Ohms in Series with 2143 Ohms.
625 + 2143 = 2768 Ohms effective cathode impedance.
The 4000 Ohm self bypass resistor is in parallel with 2768 Ohms cathode resistance.
4000 Ohms in parallel with 2768 Ohms is 1636 Ohms.
We have to bypass 1636 Ohms.
If we are willing for that circuit to be -1 dB at 20 Hz, we need the capacitive reactance to be 1636 Ohms / 11. 1 dB is where the voltage drops to 89% (an 11% voltage loss).
1636 / 11 = 149 Ohms
Capacitive Reactance,
Xc = 1/(2 x pi x F x C) 1 / Xc x (2 x pi x F) = C 1 / (149 Ohms x 2 x 3.1416 x 20Hz) = 54uF
A 50uF bypass cap across the 4k resistor (working with the rest of the 6SL7 circuit) will have a frequency response that is -1 dB at 20 Hz, versus the response at 1 kHz.
Want that circuit to be -1 dB at 10 Hz, use a 100 uF cap.
Remember, if you have 3 circuits that are -1 dB at 20 Hz, the total response will be -1dB x 3 = -3dB at 20 Hz. So, 3 circuits that are -1 dB at 10 Hz, will be -3dB at 10 Hz.
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I looked at Section 3.7 of my (new
) copy of Merlin Blencowe's 'Designing High-Fidelity Preamps' where he discusses cathode bypass caps. The simplified formula for finding the bypass cap value gives a result of 40uF for a 4k cathode bias resistor. So 220 uF or even 100uF would probably be 'plenty' for your 6SL7 cathode bypass around a 4k resistor.BTW, the Blencowe book is excellent - much better IMO than Morgan Jones, which I own but where I usually 'can't find the answer to my question'.
EDIT: See, I knew 6A3 would be here with a good answer! Thanks - we were typing at the same time, 6A3.
Regarding small capacitors in parallel with larger electrolytic (or other) capacitors.
In audio circuits, the main issue with large capacitors is dielectric absorption (dissipation factor). Putting a small capacitor in parallel is a complete waste of a component. The smaller capacitor has almost no effect on the total dissipation factor and the issue here is energy lost to the dielectric (= distortion). The smaller capacitor does exactly zero to return that energy. So there is no free ride here. You can't economise by buying a cheap electrolytic and make it all better with a film capacitor. BTW, ESR means nothing in this context. It only matters when you have a lot of energy flowing through the capacitor. That would then be a power supply capacitor, a power coupling capacitor or a crossover capacitor (don't use film here, you need foil types to handle the current).
In power supplies the effect is different. here a smaller capacitor works to reduce the overall impedance of the power supply at higher frequencies. So in this case, putting a smaller capacitor or capacitors in parallel with a larger electrolytic type capacitor does help and is a sign of good engineering. But, in this case you should also mount the smaller film type capacitors across the power supply close to the load.
I never use ESR measurements. When deciding if a filter capacitor is good or not I merely turn the equipment on and look at the waveform across the capacitor. This tells you the state the capacitor is really in no matter what the ESR meter says.
I use an HP 4263A LCR meter with its proper test leads ($$$), usually the Kelvin leads. I have a fixture to measure parts up to 100 KHz. The Kelvin leads are only good to about 10 KHz, but most capacitors have gone south by that frequency.
Best, Chris
In audio circuits, the main issue with large capacitors is dielectric absorption (dissipation factor). Putting a small capacitor in parallel is a complete waste of a component. The smaller capacitor has almost no effect on the total dissipation factor and the issue here is energy lost to the dielectric (= distortion). The smaller capacitor does exactly zero to return that energy. So there is no free ride here. You can't economise by buying a cheap electrolytic and make it all better with a film capacitor. BTW, ESR means nothing in this context. It only matters when you have a lot of energy flowing through the capacitor. That would then be a power supply capacitor, a power coupling capacitor or a crossover capacitor (don't use film here, you need foil types to handle the current).
In power supplies the effect is different. here a smaller capacitor works to reduce the overall impedance of the power supply at higher frequencies. So in this case, putting a smaller capacitor or capacitors in parallel with a larger electrolytic type capacitor does help and is a sign of good engineering. But, in this case you should also mount the smaller film type capacitors across the power supply close to the load.
I never use ESR measurements. When deciding if a filter capacitor is good or not I merely turn the equipment on and look at the waveform across the capacitor. This tells you the state the capacitor is really in no matter what the ESR meter says.
I use an HP 4263A LCR meter with its proper test leads ($$$), usually the Kelvin leads. I have a fixture to measure parts up to 100 KHz. The Kelvin leads are only good to about 10 KHz, but most capacitors have gone south by that frequency.
Best, Chris
anatech,
Good point about dielectric absorption versus ESR.
It would be good to test the sound differences: I am still waiting for somebody in any of the local clubs here to do a double blindfold test with bypass caps:
Electrolytic
Electrolytic in parallel with a small foil cap.
High capacitance foil cap. I am not holding my breath until they run that test here (held it for 2 or 4 minutes when I was young, obviously it did not affect my memory).
It also would be good to measure the waveform across the bypass capacitor, and measure the harmonic distortion of that waveform. Perhaps it would allow us to "bypass" the double blind listening tests.
Good point about dielectric absorption versus ESR.
It would be good to test the sound differences: I am still waiting for somebody in any of the local clubs here to do a double blindfold test with bypass caps:
Electrolytic
Electrolytic in parallel with a small foil cap.
High capacitance foil cap. I am not holding my breath until they run that test here (held it for 2 or 4 minutes when I was young, obviously it did not affect my memory).
It also would be good to measure the waveform across the bypass capacitor, and measure the harmonic distortion of that waveform. Perhaps it would allow us to "bypass" the double blind listening tests.
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Hi VictoriaGuy,
I have both books (actually both from Morgan Jones) and find them all to be excellent texts. For anyone trying to learn, I would say that all these would be a good place to start.
Perhaps you just don't "get" Morgan Jones. He is extremely knowledgeable as well, and his book covers a lot of the nuts and bolts stuff you need before reading another text. He also points out several issues with various tubes you could sidestep using the information in his book. That and the outright examples where he points things out clearly. He has been very helpful to me personally in a one on one basis.
-Chris
I have both books (actually both from Morgan Jones) and find them all to be excellent texts. For anyone trying to learn, I would say that all these would be a good place to start.
Perhaps you just don't "get" Morgan Jones. He is extremely knowledgeable as well, and his book covers a lot of the nuts and bolts stuff you need before reading another text. He also points out several issues with various tubes you could sidestep using the information in his book. That and the outright examples where he points things out clearly. He has been very helpful to me personally in a one on one basis.
-Chris
Problems with Volume Potentiometer
Hallo Gents,
My name is Marcus and I have finished the A9 Amp because of this Forum.
I had some struggles with the Original schematics, but nether the less I have the amp running.
I am facing problems with the Volume control. It seems that whenever I full turn it on, that the amp delivers way better treble compared to when I turn the volume to room volume.Another problem is, that I have a very high volume difference between the record player and CD player. After implementing the below mod, the CD Player input is much too high, so listenening to that is not possible anymore.
Has anyone any idea what is the reason for that?
I added the following mods to the original schematics:
R101 added,C103 exchanged against 470yf+10nf paralleled ( same as circuit A in the schematics)
Hallo Gents,
My name is Marcus and I have finished the A9 Amp because of this Forum.
I had some struggles with the Original schematics, but nether the less I have the amp running.
I am facing problems with the Volume control. It seems that whenever I full turn it on, that the amp delivers way better treble compared to when I turn the volume to room volume.Another problem is, that I have a very high volume difference between the record player and CD player. After implementing the below mod, the CD Player input is much too high, so listenening to that is not possible anymore.
Has anyone any idea what is the reason for that?
I added the following mods to the original schematics:
R101 added,C103 exchanged against 470yf+10nf paralleled ( same as circuit A in the schematics)
Marcus;
Welcome to diyaudio and the Boyuu 'experience'
I cannot open the image of your 'mods' .
Using the 'Manage Attachments' button below the reply window will let you attach an image file that you have in your computer to your message.
Are you attaching the output from a phono cartridge (turntable) directly to the input of the Boyuu?
If you look back a few pages in this thread, LongRoad posted some details of a simple voltage divider added to the input which reduce the signal level for one input.
You might find that useful.
Welcome to diyaudio and the Boyuu 'experience'
I cannot open the image of your 'mods' .
Using the 'Manage Attachments' button below the reply window will let you attach an image file that you have in your computer to your message.
Are you attaching the output from a phono cartridge (turntable) directly to the input of the Boyuu?
If you look back a few pages in this thread, LongRoad posted some details of a simple voltage divider added to the input which reduce the signal level for one input.
You might find that useful.
Hello All,
I've traveling so I was disconnected for a few days.
From what I'm reading, there is no need for the "second lower capacitors" in parallel with the electrolytic caps so I have removed them from my circuit. So with this mod, the final diagram of my amp is this one shown in the picture. (Note: I only draw the left channel. Right channel is exactly the same)
I have question: Almost all transistors amps circuits I've seen have a small value decoupling capacitor on the input to block possible DC from external sources. is this cap also recommend for tube amp? (BTW, it's showing in the diagram but I have not installed it yet)
Thanks
I've traveling so I was disconnected for a few days.
From what I'm reading, there is no need for the "second lower capacitors" in parallel with the electrolytic caps so I have removed them from my circuit. So with this mod, the final diagram of my amp is this one shown in the picture. (Note: I only draw the left channel. Right channel is exactly the same)
I have question: Almost all transistors amps circuits I've seen have a small value decoupling capacitor on the input to block possible DC from external sources. is this cap also recommend for tube amp? (BTW, it's showing in the diagram but I have not installed it yet)
Thanks
Attachments
Most of my tube amp builds have that type of input cap - I just followed the published schematics for projects. I think it's a good idea. 4.7uF seems like 'a bit much' though, I think 0.47uF or even 0.1uF would probably be OK and a film cap in those values wouldn't take up too much space.