Grid bias vs cathode bias
Using grid bias allows higher amplication which is useful for crystal mikes, but gain is not such a great priority in your application. The downside is that the tube does not amplify in a linear fashion, so frequency response is reduced - again, not a problem with xtal mikes which have a limited frequency response, or AM tube radios where upper frequency response is severely limited. Cathode bias allows a tube to amplify in a more linear fashion.
Hifi amps which use grid bias (like the Mullard 3-3) use large amounts of NFB to overcome the non-linear amplification of the first stage.
The lower value grid resistor allows your MM cartridge which has relatively low impedence to feed into a low input.
With your amp, I doubt that the values of the cathode resistors are too critical, especially as gain is not a problem and your source is not going to overload the input - you could find that any value between 1.5k and 3.3k would work reasonably well.
Using grid bias allows higher amplication which is useful for crystal mikes, but gain is not such a great priority in your application. The downside is that the tube does not amplify in a linear fashion, so frequency response is reduced - again, not a problem with xtal mikes which have a limited frequency response, or AM tube radios where upper frequency response is severely limited. Cathode bias allows a tube to amplify in a more linear fashion.
Hifi amps which use grid bias (like the Mullard 3-3) use large amounts of NFB to overcome the non-linear amplification of the first stage.
The lower value grid resistor allows your MM cartridge which has relatively low impedence to feed into a low input.
With your amp, I doubt that the values of the cathode resistors are too critical, especially as gain is not a problem and your source is not going to overload the input - you could find that any value between 1.5k and 3.3k would work reasonably well.
I've been busy destroying rectifiers
As previously mentioned, a MM catridge expects to see a 47k load. There is no possibility of using the existing circuits to achieve RIAA correction and gain. However, there is no reason why the existing valve bases, and possibly, valves, should not be used.
There's lots of confusion about RIAA. Records are cut very nearly constant amplitude, but with a step in response between 500Hz and 2.1kHz. Crystal cartridges (which were amplitude sensitive) could thus play RIAA records, but suffered from the step in response. Since crystal cartridges, and their associated equipment were so poor, the step was the least of their problems. MM and MC cartridges are velocity transducers, which means that their output voltage doubles for a doubling of frequency. RIAA correction (as it is understood today) primarily corrects for this characteristic, and adds an inverse step to correct for the recorded characteristic. The final characteristic required 19.1dB boost at 50Hz, and 19.6dB cut at 20kHz. The dynamic range of this equalisation makes it quite hard to achieve satisfactorily.
Older designs placed the RIAA network in the feedback loop of a valve (usually an EF86). Better designs applied the feedback over more stages (allowing more feedback). None worked very well because there wasn't enough gain for the feedback approximation to work properly at LF. (Can be done with one valve.)
Some designs used a passive network between two stages to achieve equalisation. This works, but the necessary loss of >19.1dB simultaneously causes noise and overload problems. It's really hard to design an RIAA stage like this that works well, but it can be done. (Needs two valves.)
The more modern approach is to split the equalisation into two stages, and apply the 75us HF cut first, then the 3180, 318 pairing after the second stage. Its is much easier to get around the noise/overload problems with this technique. (Needs three valves.)
It's a loaded question, I know, but which strategy do you want to adopt?
As previously mentioned, a MM catridge expects to see a 47k load. There is no possibility of using the existing circuits to achieve RIAA correction and gain. However, there is no reason why the existing valve bases, and possibly, valves, should not be used.
There's lots of confusion about RIAA. Records are cut very nearly constant amplitude, but with a step in response between 500Hz and 2.1kHz. Crystal cartridges (which were amplitude sensitive) could thus play RIAA records, but suffered from the step in response. Since crystal cartridges, and their associated equipment were so poor, the step was the least of their problems. MM and MC cartridges are velocity transducers, which means that their output voltage doubles for a doubling of frequency. RIAA correction (as it is understood today) primarily corrects for this characteristic, and adds an inverse step to correct for the recorded characteristic. The final characteristic required 19.1dB boost at 50Hz, and 19.6dB cut at 20kHz. The dynamic range of this equalisation makes it quite hard to achieve satisfactorily.
Older designs placed the RIAA network in the feedback loop of a valve (usually an EF86). Better designs applied the feedback over more stages (allowing more feedback). None worked very well because there wasn't enough gain for the feedback approximation to work properly at LF. (Can be done with one valve.)
Some designs used a passive network between two stages to achieve equalisation. This works, but the necessary loss of >19.1dB simultaneously causes noise and overload problems. It's really hard to design an RIAA stage like this that works well, but it can be done. (Needs two valves.)
The more modern approach is to split the equalisation into two stages, and apply the 75us HF cut first, then the 3180, 318 pairing after the second stage. Its is much easier to get around the noise/overload problems with this technique. (Needs three valves.)
It's a loaded question, I know, but which strategy do you want to adopt?
Attachments
COMBINE.
Hi,
A combination of active an passive RIAA correction is not often used but it has the advantage of:
Keeping things simple and short as far as the signal path goes.
An odd example is doing some correction between the anodes of say, one 12AX7s and some passive correction inbetween the output/input of both triodes.
It's not the easiest circuit to calculate but it has some elegance going that I find quite attractive.
RIAA isn't easy to implement correctly but when done right it beats any CD based system hands down.
Cheers,
Hi,
A combination of active an passive RIAA correction is not often used but it has the advantage of:
Keeping things simple and short as far as the signal path goes.
An odd example is doing some correction between the anodes of say, one 12AX7s and some passive correction inbetween the output/input of both triodes.
It's not the easiest circuit to calculate but it has some elegance going that I find quite attractive.
RIAA isn't easy to implement correctly but when done right it beats any CD based system hands down.
Cheers,
Since records are my only material for mono recordings, I would be willing to rebuild the amplifier without a line level input; this means RIAA equalization could be done all throughout the signal chain. I am also willing to not have bass and treble adjustments. This should leave plenty of room for active and/or passive adjustements, and plenty of ability for gain; the 6SC7, 6SJ7, and 6SL7 are all twin triodes, so this is the equivalent of 6 tubes I have to work with to get from the input to the drivers.
The only tube changes I would be willing to do would be to swap some of the tubes for more 6SL7 tubes, as I have several of these from another salvaged device. I hope this helps in deciding how to implement the RIAA compensation.
The only tube changes I would be willing to do would be to swap some of the tubes for more 6SL7 tubes, as I have several of these from another salvaged device. I hope this helps in deciding how to implement the RIAA compensation.
EC8010:
The 6SL7 and 6SC7 (twin triodes) both have a mu of about 70; with feedback/attenuation, couldn't these be adjusted to have a gain of 10-20? If the third option is the "best," then I would like to pursue it, so long as it is possible with the tubes I have.
Fred:
Can you get me a schematic with values for your suggested acative/passive RIAA compensation? Also, to verify, you are suggesting that I use a ~100pF capacitor in parallel with 47K for the input? Is this just to filter off RF interference?
Thank you.
The 6SL7 and 6SC7 (twin triodes) both have a mu of about 70; with feedback/attenuation, couldn't these be adjusted to have a gain of 10-20? If the third option is the "best," then I would like to pursue it, so long as it is possible with the tubes I have.
Fred:
Can you get me a schematic with values for your suggested acative/passive RIAA compensation? Also, to verify, you are suggesting that I use a ~100pF capacitor in parallel with 47K for the input? Is this just to filter off RF interference?
Thank you.
Cost
Unless you have a huge stock of resistors and precision capacitors, you are going to have to spend some money to achieve the equalisation - even if you salvage parts for everything else. Why not spend a little more money to buy another 6SJ7. or a 7N7 plus Loctal base (cheaper than a 6SN7).
The input capacitance of a 6SL7 will push the electrical HF resonance of your MM cartridge down into the audio band - which is why we only want a mu of 10-20 for the first stage (when using Octal valves).
If we limit the mu to 10-20 on the second stage, we can use passive 75us between the stages, and passive 3180, 318 after the second stage.
The final stage can be half of the 6SL7 as a cathode follower. Normally, I would not want to use this valve as a cathode follower (insufficient gm), but you can get away with it here.
Unless you have a huge stock of resistors and precision capacitors, you are going to have to spend some money to achieve the equalisation - even if you salvage parts for everything else. Why not spend a little more money to buy another 6SJ7. or a 7N7 plus Loctal base (cheaper than a 6SN7).
The input capacitance of a 6SL7 will push the electrical HF resonance of your MM cartridge down into the audio band - which is why we only want a mu of 10-20 for the first stage (when using Octal valves).
If we limit the mu to 10-20 on the second stage, we can use passive 75us between the stages, and passive 3180, 318 after the second stage.
The final stage can be half of the 6SL7 as a cathode follower. Normally, I would not want to use this valve as a cathode follower (insufficient gm), but you can get away with it here.
One of the main reasons that I am reluctant to buy replacement tubes is the large increase in overall shipping cost of buying items from several different vendors. I usually buy resistors/capacitors from digikey (I have already put in an order for components to replace everything in the current configuration); they do not sell tubes. If you can direct me to a vendor where I can get both the passive components and the tubes all in one order to consolidate shipping costs, I would consider buying different tubes. Note: this would have to be a vendor selling low-priced, commercial grade components; not $10 resistors, $50 capacitors, and $300 tubes! There is no possibility of replacing the sockets at this time; I am working out of my dorm room, and do not have equipment for mechanical alterations to the chassis.
If I do not change tubes, then with what I have, how would something like this work:
Use the 6SJ7 that I have for the input, since you say that the 6SL7 has too high capacitance. Use one or both sides of the 6SC7 as following gain stages, with feedback to limit gain to a reasonable value, followed by the 6SL7 driving the output tubes as it currently does.
By the way, although I initially guessed that the amp was running push-pull class AB or class B, I just checked and it is running class A (symmetrical output even with one of the 6L6 removed). Since this much output power is already more than enough, what do I do to convert the output to triode operation?
If I do not change tubes, then with what I have, how would something like this work:
Use the 6SJ7 that I have for the input, since you say that the 6SL7 has too high capacitance. Use one or both sides of the 6SC7 as following gain stages, with feedback to limit gain to a reasonable value, followed by the 6SL7 driving the output tubes as it currently does.
By the way, although I initially guessed that the amp was running push-pull class AB or class B, I just checked and it is running class A (symmetrical output even with one of the 6L6 removed). Since this much output power is already more than enough, what do I do to convert the output to triode operation?
Triodes and all that
The easiest way to achieve triode operation would be to connect the screen grids (g2) of the 6L6s to the anode (directly or via a small resistor - 100 ohms or so) instead of directly to the HT. The cathode resistors would also probably need to be raised in value also, but without knowing the present values it is difficult to say (you might have to replace the cathode bypass caps also, if the voltage rating is insufficient for the increased voltage drop across the resistors). Remember also that the optimum matching to the output tranny will change also - 6L6s in triode operation have a lower output impedence than in beam tetrode mode. It will still work of course, but not at maximum efficiency. You mentioned in an earlier post that 10w output is enough for your needs, but in triode mode output would be nearer 6w. One issue to bear in mind that in triode mode the 6L6s will consume less current, and as a result overall HT voltage will rise - this will place a greater burden on the old electrolytic caps still in your amp.
In following the discussion on the changes needed for RIAA equalisation, how much change do you want to make to the amp? If you are committted to using as much of the original components as possible, remember that the amp was designed for PA work, so it was deliberately made to restrict frequency response, high treble and bass being undesirable in PA work. This is why the output tranny is small and I suspect that your interstage caps are of low value, possible 0.01 or 0.02uf in order to reduce bass response, and NFB will be limited.
If you redesign the front end for good RIAA equilisation would it not also make sense to fundamentally rebuild the entire amp, including replacing the output tranny? Otherwise you might still be disappointed with the output, as the rest of the amp will not match the better quality of the input stages.
The easiest way to achieve triode operation would be to connect the screen grids (g2) of the 6L6s to the anode (directly or via a small resistor - 100 ohms or so) instead of directly to the HT. The cathode resistors would also probably need to be raised in value also, but without knowing the present values it is difficult to say (you might have to replace the cathode bypass caps also, if the voltage rating is insufficient for the increased voltage drop across the resistors). Remember also that the optimum matching to the output tranny will change also - 6L6s in triode operation have a lower output impedence than in beam tetrode mode. It will still work of course, but not at maximum efficiency. You mentioned in an earlier post that 10w output is enough for your needs, but in triode mode output would be nearer 6w. One issue to bear in mind that in triode mode the 6L6s will consume less current, and as a result overall HT voltage will rise - this will place a greater burden on the old electrolytic caps still in your amp.
In following the discussion on the changes needed for RIAA equalisation, how much change do you want to make to the amp? If you are committted to using as much of the original components as possible, remember that the amp was designed for PA work, so it was deliberately made to restrict frequency response, high treble and bass being undesirable in PA work. This is why the output tranny is small and I suspect that your interstage caps are of low value, possible 0.01 or 0.02uf in order to reduce bass response, and NFB will be limited.
If you redesign the front end for good RIAA equilisation would it not also make sense to fundamentally rebuild the entire amp, including replacing the output tranny? Otherwise you might still be disappointed with the output, as the rest of the amp will not match the better quality of the input stages.
The only things I want to keep are the tubes, the iron, and the chassis. I am perfectly willing to replace all the othe components, and completely redesign the amp.
The frequency response of the amp as is is not that bad. Using sinewave test tones, the amp goes up to 20KHz without much attenuation. The amplifier has sufficient low frequency response for my smallish speakers; the RC constant for the interstage coupling is typically around .029 s. The amplifier was not designed for voice PA alone: according to the "License Notice" on the inside of the bottom panel, the device is licensed for use in "public address systems, phonograph distribution systems, systems for distribution from radio broadcast receiving sets or musical instruments, and in speech input systems and monitoring systems."
I realize that I could greatly improve the amplifier by replacing the output transformer, the input transformer, the tubes, the sockets, the connectors, even the steel chassis (which adds significant microphonics to the output) -- by this point I would be building an entirely new amp, and bringing the cost to several hundred dollars. At this point, it is simply not worthwhile. I have so many other components of my system that need improvement -- the first thing I need to do is replace my speakers; I could do with a better turntable, also. If this were a stereo amplifier (or if I had two identical ones), I might invest more in it; as is, it is only useful for listening to the dozen or so monaural records I have, which I could also listen to on a stereo amplifier. What I am trying to do here is learn about tube amp building, without spending lots of money, so that some day when the rest of my system is worthy and I have money available, I will be able to build a good tube amplifier. I am rebuilding this one more for fun and challenge than for excellent results.
The cathode resistor on the output is 125 ohm. What is the difference/advantages/disadvantages of connecting g2 with as opposed to without a small resistor?
The frequency response of the amp as is is not that bad. Using sinewave test tones, the amp goes up to 20KHz without much attenuation. The amplifier has sufficient low frequency response for my smallish speakers; the RC constant for the interstage coupling is typically around .029 s. The amplifier was not designed for voice PA alone: according to the "License Notice" on the inside of the bottom panel, the device is licensed for use in "public address systems, phonograph distribution systems, systems for distribution from radio broadcast receiving sets or musical instruments, and in speech input systems and monitoring systems."
I realize that I could greatly improve the amplifier by replacing the output transformer, the input transformer, the tubes, the sockets, the connectors, even the steel chassis (which adds significant microphonics to the output) -- by this point I would be building an entirely new amp, and bringing the cost to several hundred dollars. At this point, it is simply not worthwhile. I have so many other components of my system that need improvement -- the first thing I need to do is replace my speakers; I could do with a better turntable, also. If this were a stereo amplifier (or if I had two identical ones), I might invest more in it; as is, it is only useful for listening to the dozen or so monaural records I have, which I could also listen to on a stereo amplifier. What I am trying to do here is learn about tube amp building, without spending lots of money, so that some day when the rest of my system is worthy and I have money available, I will be able to build a good tube amplifier. I am rebuilding this one more for fun and challenge than for excellent results.
The cathode resistor on the output is 125 ohm. What is the difference/advantages/disadvantages of connecting g2 with as opposed to without a small resistor?
Now we know where you're coming from.
What model of cartridge are you intending to use, and what load capacitance does it require? The significance of this is that the older Shures actually needed a large capacitance (400-500pF), so a 6SL7 could be used as the input valve.
If you want to use this as a learning project, can we assume that you have access to an electronics lab, and that calculators/maths don't frighten you? It would be best to rebuild the power amplifier section first, make that work, then progress to the RIAA stage. RIAA stages are harder to do than power amplifiers.
The g2 resistor serves two purposes. It decreases the Q of the potentially resonant circuit around g2, thus preventing oscillation, and it limits g2 current in the event of a fault. Don't use a wirewound type. Carbon composition is ideal, but the (more common) metal films are fine.
What model of cartridge are you intending to use, and what load capacitance does it require? The significance of this is that the older Shures actually needed a large capacitance (400-500pF), so a 6SL7 could be used as the input valve.
If you want to use this as a learning project, can we assume that you have access to an electronics lab, and that calculators/maths don't frighten you? It would be best to rebuild the power amplifier section first, make that work, then progress to the RIAA stage. RIAA stages are harder to do than power amplifiers.
The g2 resistor serves two purposes. It decreases the Q of the potentially resonant circuit around g2, thus preventing oscillation, and it limits g2 current in the event of a fault. Don't use a wirewound type. Carbon composition is ideal, but the (more common) metal films are fine.
Cathode resistor for 6L6
What is the HT voltage at the centre tap of the output transformer? Is it around 270 volts? This would be normal for 6L6s in class A1 operation, but the voltage would be a bit low for optimum triode operation, and the output less than the 6w I quoted. In these conditions the resistor should be about 330 ohms and your output tranny would actually be too low an impedence - I am guessing about 5k anode to anode, whereas for triode operation nearer 8k is preferable. You would suffer loss of bass as a consequence.
What is the HT voltage at the centre tap of the output transformer? Is it around 270 volts? This would be normal for 6L6s in class A1 operation, but the voltage would be a bit low for optimum triode operation, and the output less than the 6w I quoted. In these conditions the resistor should be about 330 ohms and your output tranny would actually be too low an impedence - I am guessing about 5k anode to anode, whereas for triode operation nearer 8k is preferable. You would suffer loss of bass as a consequence.
Sorry about not making my intentions clear enough earlier. Don't worry... math doesn't scare me (I'm double majoring math/physics).
My "electronics lab" is in my room; I have an oscilloscope, a multimeter, and a solder station; I think this should be enough.
I am using an Ortofon FF 15 XE Mk II; I don't know what capacitance it expects.
I just connected g2 to the cathode (without resistor for now); the amps now appears to be running class B with horrible crossover distortion
. Is this due to the wrong cathode resistor?
My "electronics lab" is in my room; I have an oscilloscope, a multimeter, and a solder station; I think this should be enough.
I am using an Ortofon FF 15 XE Mk II; I don't know what capacitance it expects.
I just connected g2 to the cathode (without resistor for now); the amps now appears to be running class B with horrible crossover distortion
. Is this due to the wrong cathode resistor?
You need to find the ratio of your output transformer. If you can obtain an oscillator, connect it across the 15 ohm secondary, carefully use the oscilloscope to measure the voltage across the primary (which will be quite large), then divide output voltage by input voltage to find the turns ratio. Impedances change by the square of the turns ratio, so you can then calculate the primary impedance.
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