Updated design. What do you think?
Please note noise cancellation tecniques
Yes I've studied a lot on Broskie's articles.
And a theory question: when calculating the resistor needed for a given feedback ratio, in the voltage divider formed by the resistor and the cathode resistor of the input tube, do I have to count the cathode resistor alone or I have to consider the total impedance seen at the cathode (formed by Rk || rk)? In that case, R16 should be halved...
And also, is there a better way to inject feedback in a cascode circuit than this?
Please note noise cancellation tecniques
Yes I've studied a lot on Broskie's articles.And a theory question: when calculating the resistor needed for a given feedback ratio, in the voltage divider formed by the resistor and the cathode resistor of the input tube, do I have to count the cathode resistor alone or I have to consider the total impedance seen at the cathode (formed by Rk || rk)? In that case, R16 should be halved...
And also, is there a better way to inject feedback in a cascode circuit than this?
Attachments
In regards to the Rk || rk question it depends on how big one is relative to the other, if the overall feedback node impedance is significantly lower than R3 by itself (and here I believe it is) then the value of R16 should be adjusted. This probably most easily accomplished at the testing phase of things as the internal cathode resistance is probably pretty small, and not precisely known.
Also you don't really need or want C1 in the feedback path, the values of R3 || R16 can be tweaked to give you the bias voltage you want on the cathode of the 12AT7. I suspect that C1 will introduce quite a lot of low frequency phase shift and also result in boosted response in the subsonic region where your transformer can't handle it, this is probably not an issue unless you play records.
Also you don't really need or want C1 in the feedback path, the values of R3 || R16 can be tweaked to give you the bias voltage you want on the cathode of the 12AT7. I suspect that C1 will introduce quite a lot of low frequency phase shift and also result in boosted response in the subsonic region where your transformer can't handle it, this is probably not an issue unless you play records.
Thank you very much Kevin for your reply.
But... rk isn't (Ra+ra)/(mu+1)? This leads to about 100ohm if I am not wrong, pratically halving the impedance seen at the cathode node.
You're right about C1. But how to mantain the bypassing of rk of the cascode (I must do this or the gain will be too low) and accept negative feedback?
Maybe the upper triode's grid could be a place to put error signal? However there the gain will be quite low, but maybe it will be enought.
But... rk isn't (Ra+ra)/(mu+1)? This leads to about 100ohm if I am not wrong, pratically halving the impedance seen at the cathode node.
You're right about C1. But how to mantain the bypassing of rk of the cascode (I must do this or the gain will be too low) and accept negative feedback?
Maybe the upper triode's grid could be a place to put error signal? However there the gain will be quite low, but maybe it will be enought.
Below is a copy of e-mail correspondence between myself (George) and Giaime. Since this is directly related to this thread and may help to answer a few common questions relating to PowerDrive I posted it here. I have a cookbook approach to PowerDrive web page coming, but it is not ready yet. I have added a few additional comments at the bottom.
Hello George!
I'm Giaime, from DIYaudio forums. You surely remember about me.
I would like first to thank you for your wonderful website and all the good ideas that you're spreading over the internet: I particulary liked the PowerDrive concept.
I already applied it, to an almost unusued tube: the 802. See the
results here:
http://www.giaime.altervista.org/802.html
Now, I designed a 211 single ended A2 amplifier. I'm not planning on building it, because of the high costs (and voltages) involved, however I would like an opinion from you: here it is
http://www.giaime.altervista.org/hivolt.html
I simulated the mosfet driver in a computer simulation program, using a diode in series with a 470ohm resistor to simulate the 211's grid going positive: there wasn't any distortion with the full 100Vpk drive to the 211 grid. I was running the mosfet at 21mA quiescent current, and I was puzzled, since I expected that the quiescent current would have been at least more than the maximum expected current in the 211 grid. So I made a little guess, that's explained in the article in that page: on
negative peaks of the source (so decreasing current in the mosfet) the source follower has only to drive the 200k resistor and the Miller capacitance of the 211's grid. On the contrary, on the positive peaks of the grid waveform current in the mosfet will increase and so necessary grid current will be available. Is this right?
Please feel free to make suggestions!
Hi Giaime:
I saw the post that you did on the diyAudio forums, and replied to it last night. I think that your mosfet drive ciucuit will work fine. I think that an ECC83 will have trouble driving the capacitance of the mosfet with enough bandwidth. I would suggest using a tube capable of higher current, as you suggested, a 12AT7. I would also consider a different fet. See the forum post for more details.
Yes the Fet must source all of the grid current on the positive going half cycle. R11 must discharge the miller capacitance on the negative half cycle. R13 (the 200K resistor)is probably not needed.
George
Hello George,
thank you very much for your kind and precise response. Sorry if I sent this mail to you, I did this because I wasn't sure to open a thread on the forum. Now I did it, and I'll be extremely grateful to you if you continue your priceless suggestions on the forum, when you have time. I know time is a big problem...
Ps, I've updated the design as you can see in the thread. Now, E88CC in cascode for the input stage. Please tell me if the idle current of the mosfet isn't enought: I have now 20mA and want to drive about 40-50mA into the 211 grid.
Hi:
I also used 20 mA as the idle current on the fet. This was set based on the total dissipation and the size of the heat sink. The fet can source as much grid current as the grid will draw. It is only limited by the source resistance of the +150 volt supply and the on resistance of the fet. I have used the same PowerDrive board with an 833A where I measured peak grid currents of over 100mA.
George
There are two distinct paths for current to flow in the PowerDrive (or any similar mosfet grid driver). On the positive going cycle the grid will draw current. As the grid is pushed further positive its dynamic impedance drops quickly. This current flows out of the +150 (in this case) power supply through the mosfet and into the grid of the tube. The grid current is limited by the RDSSon of the fet (on resistance, typically a few ohms) and the source impedance of the power supply (hopefully also a few ohms). This allows virtually unlimited grid current. Some tubes like the 845, 211, 833A, 811A and some 300B's remain linear well into this region. As the grid transitions into the negative (with respect to the cathode) region it ceases to draw current. There may be a small charge stored in the Miller capacitance of the tube (worse with high Mu tubes like 811A) if the transition is rapid. There should be a low impedance for this current to flow. In this case it is R11 and the current flows into the negative supply. 4.7k is sufficient to rapidly discharge the capacitance. I used 10 K. The negative supply must be sufficiently negative to ensure that the fet remains conducting as the grid reaches its most negative extreme. -150 volts is sufficient for a 211 or an 833A. It is not sufficient for an 845. I used -300 volts. The quiescent current of the fet is not terribly critical but it is set by the value of R11 (and the bias voltage). This resistor also sets the output impedance of the driver under conditions where no grid current is flowing. I wanted this to be low, which leads to a high idle current in the fet. The upper limit for fet current is determined by the dissipation capabilities of the fet and its heat sink. I have both fets, both resistors, and both driver CCS's on the same (large) heat sink. I settled on 20 mA as a happy medium between fet temperature, and output resistance. I tried a few different values and found little if any sonic differences above about 10 mA.
Hello George!
I'm Giaime, from DIYaudio forums. You surely remember about me.
I would like first to thank you for your wonderful website and all the good ideas that you're spreading over the internet: I particulary liked the PowerDrive concept.
I already applied it, to an almost unusued tube: the 802. See the
results here:
http://www.giaime.altervista.org/802.html
Now, I designed a 211 single ended A2 amplifier. I'm not planning on building it, because of the high costs (and voltages) involved, however I would like an opinion from you: here it is
http://www.giaime.altervista.org/hivolt.html
I simulated the mosfet driver in a computer simulation program, using a diode in series with a 470ohm resistor to simulate the 211's grid going positive: there wasn't any distortion with the full 100Vpk drive to the 211 grid. I was running the mosfet at 21mA quiescent current, and I was puzzled, since I expected that the quiescent current would have been at least more than the maximum expected current in the 211 grid. So I made a little guess, that's explained in the article in that page: on
negative peaks of the source (so decreasing current in the mosfet) the source follower has only to drive the 200k resistor and the Miller capacitance of the 211's grid. On the contrary, on the positive peaks of the grid waveform current in the mosfet will increase and so necessary grid current will be available. Is this right?
Please feel free to make suggestions!
Hi Giaime:
I saw the post that you did on the diyAudio forums, and replied to it last night. I think that your mosfet drive ciucuit will work fine. I think that an ECC83 will have trouble driving the capacitance of the mosfet with enough bandwidth. I would suggest using a tube capable of higher current, as you suggested, a 12AT7. I would also consider a different fet. See the forum post for more details.
Yes the Fet must source all of the grid current on the positive going half cycle. R11 must discharge the miller capacitance on the negative half cycle. R13 (the 200K resistor)is probably not needed.
George
Hello George,
thank you very much for your kind and precise response. Sorry if I sent this mail to you, I did this because I wasn't sure to open a thread on the forum. Now I did it, and I'll be extremely grateful to you if you continue your priceless suggestions on the forum, when you have time. I know time is a big problem...
Ps, I've updated the design as you can see in the thread. Now, E88CC in cascode for the input stage. Please tell me if the idle current of the mosfet isn't enought: I have now 20mA and want to drive about 40-50mA into the 211 grid.
Hi:
I also used 20 mA as the idle current on the fet. This was set based on the total dissipation and the size of the heat sink. The fet can source as much grid current as the grid will draw. It is only limited by the source resistance of the +150 volt supply and the on resistance of the fet. I have used the same PowerDrive board with an 833A where I measured peak grid currents of over 100mA.
George
There are two distinct paths for current to flow in the PowerDrive (or any similar mosfet grid driver). On the positive going cycle the grid will draw current. As the grid is pushed further positive its dynamic impedance drops quickly. This current flows out of the +150 (in this case) power supply through the mosfet and into the grid of the tube. The grid current is limited by the RDSSon of the fet (on resistance, typically a few ohms) and the source impedance of the power supply (hopefully also a few ohms). This allows virtually unlimited grid current. Some tubes like the 845, 211, 833A, 811A and some 300B's remain linear well into this region. As the grid transitions into the negative (with respect to the cathode) region it ceases to draw current. There may be a small charge stored in the Miller capacitance of the tube (worse with high Mu tubes like 811A) if the transition is rapid. There should be a low impedance for this current to flow. In this case it is R11 and the current flows into the negative supply. 4.7k is sufficient to rapidly discharge the capacitance. I used 10 K. The negative supply must be sufficiently negative to ensure that the fet remains conducting as the grid reaches its most negative extreme. -150 volts is sufficient for a 211 or an 833A. It is not sufficient for an 845. I used -300 volts. The quiescent current of the fet is not terribly critical but it is set by the value of R11 (and the bias voltage). This resistor also sets the output impedance of the driver under conditions where no grid current is flowing. I wanted this to be low, which leads to a high idle current in the fet. The upper limit for fet current is determined by the dissipation capabilities of the fet and its heat sink. I have both fets, both resistors, and both driver CCS's on the same (large) heat sink. I settled on 20 mA as a happy medium between fet temperature, and output resistance. I tried a few different values and found little if any sonic differences above about 10 mA.
Thank you George of publishing our conversation, I strongly think that it could help people to desing PowerDrive circuits (however you could ask me the next time: not everyone like to have their own private mails published).
The question now is: negative feedback. Do I need it? I think yes. A2 operation can, if the driver isn't "ideal", deliver some distortion.
I simulated the output stage with SE amp CAD, and I can get up to 40W on 8ohm load, keeping total distortion under 5%. 1% distortion is about 5W.
Distortion here is essentialy produced by the output stage, like every good amplifier should. So I think that -6dB of negative feedback are needed, and I have the gain to allow that.
I know many "audiophile" designers would avoid nfb in a 211 SE amp, in fact if we want low distortion we wouldn't make a single ended amp, right?
Instead of those semplifications, my idea (from an EE wannabe) is that we CAN design a low distortion 211 SE amp, and a little bit of nfb can further improve it. I've seen JUST TOO MANY 845 amps driven by an ECC82 sold at outrageous prices
The problem is: where could I put it? Is there a way to insert the error signal in the cascode input stage?
The question now is: negative feedback. Do I need it? I think yes. A2 operation can, if the driver isn't "ideal", deliver some distortion.
I simulated the output stage with SE amp CAD, and I can get up to 40W on 8ohm load, keeping total distortion under 5%. 1% distortion is about 5W.
Distortion here is essentialy produced by the output stage, like every good amplifier should. So I think that -6dB of negative feedback are needed, and I have the gain to allow that.
I know many "audiophile" designers would avoid nfb in a 211 SE amp, in fact if we want low distortion we wouldn't make a single ended amp, right?
Instead of those semplifications, my idea (from an EE wannabe) is that we CAN design a low distortion 211 SE amp, and a little bit of nfb can further improve it. I've seen JUST TOO MANY 845 amps driven by an ECC82 sold at outrageous prices
The problem is: where could I put it? Is there a way to insert the error signal in the cascode input stage?
Sorry for not asking first. I read this line in your e-mail:
and took it as a request to take this discussion to the forums. I cut out all but the technical stuff and pasted it here. I did this to help illustrate the PowerDrive concept. I get a lot of e-mail about PowerDrive. Yours was one of the clearest, easy to understand that I get, and it had already been brought to the forums.
I usually have small pieces of time during my work day when I look at the forums, I usually can't look at e-mail from work.
I use no feedback in my 845 amp. I also simulated the amp in SE amp cad before building it. The measured results agree closely with the simulations. I used an 845 for my simulations and the initial amp design. I have tried 211's in the amp, but prefer the sound of the 845.
My amplifier used some real low cost output transformers that are no longer available. It sounds good and makes about 40 watts at 5% distortion. I have found that cathode feedback works well on most (but not all) output transformers to reduce output stage distortion without killing the sound. Cathode feedback increases the drive requirements and I don't have enough voltage on the driver to get any more drive, so I haven't tried it on this amp yet.
I don't have much experience with the cascode circuit that you show, so I didn't comment on it earlier. I would assume that you could apply feedback at the cathode of the input stage as shown in your schematic.
Put a small unbypassed resistor in series with a larger, bypassed resistor, then inject the feedback across the small resistor.
and took it as a request to take this discussion to the forums. I cut out all but the technical stuff and pasted it here. I did this to help illustrate the PowerDrive concept. I get a lot of e-mail about PowerDrive. Yours was one of the clearest, easy to understand that I get, and it had already been brought to the forums.
I usually have small pieces of time during my work day when I look at the forums, I usually can't look at e-mail from work.
I use no feedback in my 845 amp. I also simulated the amp in SE amp cad before building it. The measured results agree closely with the simulations. I used an 845 for my simulations and the initial amp design. I have tried 211's in the amp, but prefer the sound of the 845.
My amplifier used some real low cost output transformers that are no longer available. It sounds good and makes about 40 watts at 5% distortion. I have found that cathode feedback works well on most (but not all) output transformers to reduce output stage distortion without killing the sound. Cathode feedback increases the drive requirements and I don't have enough voltage on the driver to get any more drive, so I haven't tried it on this amp yet.
I don't have much experience with the cascode circuit that you show, so I didn't comment on it earlier. I would assume that you could apply feedback at the cathode of the input stage as shown in your schematic.
Put a small unbypassed resistor in series with a larger, bypassed resistor, then inject the feedback across the small resistor.
Hi Giaime,
The suggestion tubelab had about deriving the bias across the a larger bypassed capacitor in series with a smaller feedback resistor is a good one. Something on the order of 47 ohms seems appropriate.
Since the open loop gain is finite, the ratio of the resistors required to set the closed loop gain will be a little higher than the actual gain ratio, depending on the quality of the 12AT7 model and how well it models the nodal impedance at the cathode your simulation and real world might be a little different as well.
You can think of it this way for a non-inverting amplifier.
The open loop gain A is finite.
The feedback factor which is also the reciprocal of the desired closed loop gain is B.
ACL is the actual closed loop gain so,
ACL = Vout/Vin = 1/B x [ 1/(1+1/AB)]
Note that 1/B might be expressed as the ideal gain provided that A is infinite or at least very large.
The term that follows takes into effect the less than infinite available open loop gain (OLG) and is ignored for most op-amp applications except in ATE and in our case where the open loop gain is finite.
Try the equation with progressively lower amounts of OLG and you will see exactly what I am talking about.
As to whether this is useful or not, I thought you might find a little feedback theory interesting.
The suggestion tubelab had about deriving the bias across the a larger bypassed capacitor in series with a smaller feedback resistor is a good one. Something on the order of 47 ohms seems appropriate.
Since the open loop gain is finite, the ratio of the resistors required to set the closed loop gain will be a little higher than the actual gain ratio, depending on the quality of the 12AT7 model and how well it models the nodal impedance at the cathode your simulation and real world might be a little different as well.
You can think of it this way for a non-inverting amplifier.
The open loop gain A is finite.
The feedback factor which is also the reciprocal of the desired closed loop gain is B.
ACL is the actual closed loop gain so,
ACL = Vout/Vin = 1/B x [ 1/(1+1/AB)]
Note that 1/B might be expressed as the ideal gain provided that A is infinite or at least very large.
The term that follows takes into effect the less than infinite available open loop gain (OLG) and is ignored for most op-amp applications except in ATE and in our case where the open loop gain is finite.
Try the equation with progressively lower amounts of OLG and you will see exactly what I am talking about.
As to whether this is useful or not, I thought you might find a little feedback theory interesting.
Thank you very much George and Kevin, your help is much appreciated.
I'm not sure I understood the nfb connection you suggest. Is this correct?
I also have verified the difference between 1/B and ACL, as Kevin said, and I have to underline that this difference is negligible (0.005%) not only when A is very large, but when the feedback factor is low, so A*B is very large, and 1/AB tends to zero. In fact here I'm using -6dB of feedback only, giving an input sensitivity of 1.72Vrms for a power output of 40W on 8ohm (that I consider the extreme maximum).
Note that I have adjusted the cathode resistor of the first stage, because now rk is in parallel with the feedback resistor (and the OPT secondary, which is negligible) and so to mantain the same value it has to be increased (from 120 to 138ohm).
I have still doubts about the decreased open loop gain due to a portion of rk unbypassed. But: if there's a feedback signal there, won't the 47ohm resistor be effectively bypassed (AC ground)?
Also, but I think those things are meant to be tweaked in the real world, not on paper, we have still to consider the total impedance seen at the cathode of the cascode circuit. If I'm right and I remember well what Broskie said about cascode, the impedance seen at the anode of the lower tube is Ra/(mu+1), so about 300ohm. This, divided another time for (mu+1), gives about 8ohm at the lower triode's cathode. Is this effectively in parallel with Rk?
I'm not sure I understood the nfb connection you suggest. Is this correct?
I also have verified the difference between 1/B and ACL, as Kevin said, and I have to underline that this difference is negligible (0.005%) not only when A is very large, but when the feedback factor is low, so A*B is very large, and 1/AB tends to zero. In fact here I'm using -6dB of feedback only, giving an input sensitivity of 1.72Vrms for a power output of 40W on 8ohm (that I consider the extreme maximum).
Note that I have adjusted the cathode resistor of the first stage, because now rk is in parallel with the feedback resistor (and the OPT secondary, which is negligible) and so to mantain the same value it has to be increased (from 120 to 138ohm).
I have still doubts about the decreased open loop gain due to a portion of rk unbypassed. But: if there's a feedback signal there, won't the 47ohm resistor be effectively bypassed (AC ground)?
Also, but I think those things are meant to be tweaked in the real world, not on paper, we have still to consider the total impedance seen at the cathode of the cascode circuit. If I'm right and I remember well what Broskie said about cascode, the impedance seen at the anode of the lower tube is Ra/(mu+1), so about 300ohm. This, divided another time for (mu+1), gives about 8ohm at the lower triode's cathode. Is this effectively in parallel with Rk?
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That is what I had in mind, except that I usually swap the positions of R17 and the R3 - C1 pair. This way the small amount of AC current that flows from the output through R16, R17 and C1 will not go through C1. With the current phobia about putting electrolytics in the cathode circuit of a voltage amplifier, why stress the capacitor any further?
I think tubelab is right on this one, there will be much less signal current flowing through that cap with his suggested orientation, and regardless of where you locate it is still part of the feedback network.
Another reason to do it the other way is it makes altering the operating point of that amplifier stage very easy as you can basically change the cathode resistor with no interaction with the rfb resistor.
Good low voltage blackgates are still available and are very good. I'm not a big fan of EL types either. In some cases though the circuit performance will be a lot better with them than without. Your other option is to use fixed bias on the grid of the lower tube and connect it directly to the feedback network or ground if global nfb is not used.
In theory at least the impedance at the feedback node will be small but not zero due to the limited loop gain, it may be that reducing the value of the 47 ohm resistor and scaling rfb may give you a little additional loop gain although I doubt it will make a significant difference, it might still be worth trying.
Like Tubelab I eschew the use of global negative feedback and I suspect substituting a lower mu driver tube or using an 845 (which many seem to prefer incidentally) would eliminate the need for nfb.
I tried varying amounts of nfb in my early se designs as well as an otl and pp amplifiers, and invariably came to the conclusion that the amplifier in question (even the OTL) sounded best with no feedback.
Should you need a lower source impedance I recommend you trade power output by choosing an operating point optimized for a higher primary Z - in many cases this will help dynamic linearity as well, while giving you a lower source impedance/higher damping factor.
This is just my opinion, YMMV..
Another reason to do it the other way is it makes altering the operating point of that amplifier stage very easy as you can basically change the cathode resistor with no interaction with the rfb resistor.
Good low voltage blackgates are still available and are very good. I'm not a big fan of EL types either. In some cases though the circuit performance will be a lot better with them than without. Your other option is to use fixed bias on the grid of the lower tube and connect it directly to the feedback network or ground if global nfb is not used.
In theory at least the impedance at the feedback node will be small but not zero due to the limited loop gain, it may be that reducing the value of the 47 ohm resistor and scaling rfb may give you a little additional loop gain although I doubt it will make a significant difference, it might still be worth trying.
Like Tubelab I eschew the use of global negative feedback and I suspect substituting a lower mu driver tube or using an 845 (which many seem to prefer incidentally) would eliminate the need for nfb.
I tried varying amounts of nfb in my early se designs as well as an otl and pp amplifiers, and invariably came to the conclusion that the amplifier in question (even the OTL) sounded best with no feedback.
Should you need a lower source impedance I recommend you trade power output by choosing an operating point optimized for a higher primary Z - in many cases this will help dynamic linearity as well, while giving you a lower source impedance/higher damping factor.
This is just my opinion, YMMV..
Thank you very much Kevin. In fact, I designed the input stage to accept 845 output tubes: a little tweak on bias, mosfet power supplies and OPT primary impedance can allow the 845 to be used, of course one must eliminate global NFB.
I'm very sad that this design will remain on paper for looooooong time
Edit: an updated version of the schematic online soon on my site, at the usual address. I'm working on an 845 variant.
I'm very sad that this design will remain on paper for looooooong time
Edit: an updated version of the schematic online soon on my site, at the usual address. I'm working on an 845 variant.
I even built a few amplifiers P-P and SE with a variable global feedback control. I let several people borrow them and asked the users to tell me what setting they prefered on the knob (I didn't tell them what the knob did). All users that already owned tube amplifiers (and tube friendly speakers) preferred the zero feedback settings on all of the amplifiers, even the P-P amp. Several users that were into wall shaking home theater preffered feedback, often lots of feedback, especially on the KT-88 push pull amp. It is possible that some users preferred zero feedback because the amp will play louder, even though there is some distortion.
I have been experimenting with cathode feedback in the output stage only. I find that you can apply a relatively large amount of cathode feedback without destroying the sound. This is not always practical on some designs however, especially a P-P amp.
My latest amp (SE) design uses a CCS loaded 12AT7 driving a 6L6, KT-88, or an EL-34. It is switchable from triode to UL to pentode. There is also a switch to turn the cathode feedback on or off. With some output transformers I can select different amounts of cathode feedback (by using different secondary taps).
I am making up a good looking amp with no tubes visible from the outside of the amp. Some users may not realise that it is a tube amp. There are several switches (including the ones mentioned above) on this amp that make some changes (like tube or SS rectifier) and some that do nothing. The users will be asked to try each switch and rate what it does to the sound, and find the combination that they like best, in their home with their speakers, and their music. There is a spreadsheet for them to rate the sound of each switch and to list the particular music that was used. I have already got several people waiting to audition this amp (mostly engineers who will be critical), some own tube amps, most do not. I will be interested to see the results of this test. It will be a few months before it is complete.
I have found that I like zero feedback with the amp in triode mode, but it wants feedback in UL depending on program material and volume level, and NEEDS feedback in pentode mode.
This is just one of those times where all the engineering, simulations, and performance testing can not relate to what people want to hear, and everyone hears something different, often based on what they are used to hearing.
That's an EXTREMELY interesting test George, please keep us informed.
The deepest listening test I've done on NFB on tube amps... is like your tests George, hooking a guitar to the amp and a speaker combo
I did this for the EL36 PP amp I'm working on. I liked it very much when cleanly driven hard by a guitar preamp, It's extremely loud even now that has no more than 5W. Obviously, I liked it better without feedback, even if I'm using only 4dB or so.
I have yet to audition it with music and hi-fi rig.
Ps. updated schematic of the 211 beast online.
The deepest listening test I've done on NFB on tube amps... is like your tests George, hooking a guitar to the amp and a speaker combo
I did this for the EL36 PP amp I'm working on. I liked it very much when cleanly driven hard by a guitar preamp, It's extremely loud even now that has no more than 5W. Obviously, I liked it better without feedback, even if I'm using only 4dB or so.
I have yet to audition it with music and hi-fi rig.
Ps. updated schematic of the 211 beast online.
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A question for all 845 amp builders:
what is your favourite operating point, at what OPT primary impedance?
I ask because playing with load lines don't give me good results, efficiency is lower than the 211. I'm trying 100mA, 960V, 10k load and with 180Vpk input signal I have 34W, but I'm puzzled about the low Imax of the tube, only 120mA, and note that most of the time this limit is broken with this operating point.
I know, this is the continuous limit, not peak one... but...
what is your favourite operating point, at what OPT primary impedance?
I ask because playing with load lines don't give me good results, efficiency is lower than the 211. I'm trying 100mA, 960V, 10k load and with 180Vpk input signal I have 34W, but I'm puzzled about the low Imax of the tube, only 120mA, and note that most of the time this limit is broken with this operating point.
I know, this is the continuous limit, not peak one... but...
I use 1100 volts B+. The OPT is 10K ohms. I set the current at 70 ma when using vintage tubes to be safe. I have run the new production Chinese (Shuguang) tubes as high at 110 Ma (over the dissipation limit) with out any issues, although I usually run 80 to 90 mA. There is some sag in the power supply as the current is increased, and some voltage drop in the OPT so the tube sees about 1025 to 1050 volts.
I briefly connected this amp up to a BIG power supply that put out 1500 volts (over the line again). I set the current to 70 mA, using Shuguang tubes (I can get these for $30 each, so it was a relatively low cost risk). The amp positively ROCKED! I could play just about any music at any volume level without distortion.
The 845 has a lower Mu than the 211. This requires a lot of drive voltage. The higher the plate voltage, the more negative the grid must be. This just means that you can drive it harder and get more power. I swing the grid positive to +50 volts on peaks.
I played with the design in SE amp cad before I built it, and I remember seeing 40 watts in the simulations with the grid going positive. I can't remember the details since it was over three years ago.
I briefly connected this amp up to a BIG power supply that put out 1500 volts (over the line again). I set the current to 70 mA, using Shuguang tubes (I can get these for $30 each, so it was a relatively low cost risk). The amp positively ROCKED! I could play just about any music at any volume level without distortion.
The 845 has a lower Mu than the 211. This requires a lot of drive voltage. The higher the plate voltage, the more negative the grid must be. This just means that you can drive it harder and get more power. I swing the grid positive to +50 volts on peaks.
I played with the design in SE amp cad before I built it, and I remember seeing 40 watts in the simulations with the grid going positive. I can't remember the details since it was over three years ago.
Thank you George. So my design could accept 845 tubes, because:
- 10k OPT already in place
- B+ is almost the same
- current is similar so PSU can be quite similar
Things to be tweaked:
- mosfet power supplies: +-150V is not enought, it should be +150 -300 or more
- gain of the cascode input stage is enought if you remove nfb but output swing of the cascode must be tweaked, now it can't swing much more than 150Vpk, I need a little more.
However, I don't understand a thing.
Yes, I HAVE to drive it harder, but power is the SAME because of the lower mu! In fact 845 is even a tad less efficient than 211 at that operating point (regardless of the needed input swing), and 211 has better distortion figures.
- 10k OPT already in place
- B+ is almost the same
- current is similar so PSU can be quite similar
Things to be tweaked:
- mosfet power supplies: +-150V is not enought, it should be +150 -300 or more
- gain of the cascode input stage is enought if you remove nfb but output swing of the cascode must be tweaked, now it can't swing much more than 150Vpk, I need a little more.
However, I don't understand a thing.
Yes, I HAVE to drive it harder, but power is the SAME because of the lower mu! In fact 845 is even a tad less efficient than 211 at that operating point (regardless of the needed input swing), and 211 has better distortion figures.
Giaime have you thought about the GM70 as natural 845/211 substitute?
Higher plate dissipation, higher mu and you can find it for cheap on ebay...even, due to lower rp, you can use a 6/7K opt…
http://frank.pocnet.net/sheets/084/g/GM70.pdf
You'll pay the higher performance losing a lot of power in the heaters but, as this is only an evaluation, I strongly recommend to take this tube in account. It is easier to drive and provide more power than 845 or 211
Mark
Higher plate dissipation, higher mu and you can find it for cheap on ebay...even, due to lower rp, you can use a 6/7K opt…
http://frank.pocnet.net/sheets/084/g/GM70.pdf
You'll pay the higher performance losing a lot of power in the heaters but, as this is only an evaluation, I strongly recommend to take this tube in account. It is easier to drive and provide more power than 845 or 211
Mark
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