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Thread: Phaez gives away EL84 design

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    Quote Originally Posted by J M Fahey View Post
    Crossover frequency of a .047/5k6 RC filter? Around 600 Hz. Six-hundred-Hertz.
    Anybody for a "subtle" effect?
    They might just use a car subwoofer speaker, it will roll off at about that frequency.
    Actually, I believe the math is a bit more complex because you have to take into account the load that is in parallel with it.

    Still, I am of the belief that if you need a conjunctive filter, you have a problem somewhere else.
    But then, I like a clear and transparent circuit with minimal filtering.

    That said, the only EL84 amp I've built so far (AC30 clone) didn't sound harsh at all, at least not to me. It sounded glorious once cranked, chiming yet smooth as a baby's butt. Of course it does have sort of a conjunctive filter before the power amp (the Cut control), but I was leaving it off all the time.

    But I have yet to hear a high-gainer using EL84 that I like. It is my experience that, the more gain you try to extract from the preamp, the more power you need to keep it clear and solid.

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    Last edited by Hardtailed; 05-05-2010 at 11:55 PM.

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    regarding fizz, fuzz, bumble bees, etc.... i've noticed much of that coming from the pi when hit hard. from there, the pa compresses and muds-up. speaking from limited experience however.

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Not to agree with Bruce , but I started googling a little and found 2 things
    1) In a page called non other that EHN's AX84 Page , where a fine chap who built all AX84 projects called Nyberg posts some tasty licks, many recorded live, on stage (the best kind ), says the following about the Conjunctive Filter (oh my God, who invented that name):

    Ahem !!
    2) And what would that wonderful cure-it-all 7ļ wonder be?
    Nothing more than a d_mn low-pass filter: a capacitor wired in parallel with the load, somewhat damped by a series resistor which dissipates harmlessly all that unnecessary high frequency energy (who needs highs anyway?).
    Crossover frequency of a .047/5k6 RC filter? Around 600 Hz. Six-hundred-Hertz.
    Anybody for a "subtle" effect?
    They might just use a car subwoofer speaker, it will roll off at about that frequency.
    ...
    Well in his defense, he does mention that he prefers the 5k6 .015uF combo now... about 1900Hz.
    I don't, I'll stick to the 2n2 to 4n7 and around 10K resistance. No sense in snuffing the life out of the amp.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by J M Fahey View Post
    And what would that wonderful cure-it-all 7ļ wonder be?
    Nothing more than a d_mn low-pass filter: a capacitor wired in parallel with the load, somewhat damped by a series resistor which dissipates harmlessly all that unnecessary high frequency energy (who needs highs anyway?).
    Crossover frequency of a .047/5k6 RC filter? Around 600 Hz. Six-hundred-Hertz.
    Anybody for a "subtle" effect?
    I avoided trying a (to use the common associated term) conjunctive filter for the same reason. Every clip I heard of amps before and after the conjunctive filter sounded like someone threw a couple of blankets over the speaker cabinet. But then while building my last design (with EL84's) there was a modest overshoot spike that lent a cracking striking unpleasantness that played inconsistently on the top end. I chose my resistor value based on a couple of articles I read on line and I used the smallest cap value that quenched the spike, 1500pf. A far cry from .047uf.

    IMHO this is a viable solution to the problem. Most EL84 amps I've heard that really push the power tubes exhibit some of this behavior to my ears. Not mine, not anymore. There was no notable loss of any useful top end but the refinement of the clipping improved a lot. The cap values Bruce mentioned are also much smaller than the typical values used.

    I didn't take any scope pics but the sketch below is somewhat representivive of my before and after. Who wouldn't want to make this kind of improvement? Higher cap values did round off the tailing edge of the + swing and had the undesireable effect of killing the livelyness and dynamics of the amps top end. I wouldn't suggest using this type of circuit for tone shaping because of it's negative effect on an amps dynamics when too large a cap value is used. But as a corrective circuit to maximize performance it served me well.
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    Old Timer J M Fahey's Avatar
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    Now speaking a little more seriously, when I saw that whatever-filter I instantly recognized it for what it actually is: our old friend the snubber circuit.
    Very acceptable to dampen unwanted (some would call them "parasite") resonance peaks, usually above the actual frequency band where we are working (or in its top octave).
    It can allow a little bit of extra NFB without instability and so, in an indirect way, let us polish our sound a little more; good!!!
    But to rely on them as an equalization trick scratches me the wrong way.
    There are so many other points where I can work !!
    I prefer my snubbers in my SMPSprojects, where they belong, or at most as their cousin, the Zobel Network, which really "absorbs" nothing to speak of but enhances stability.

    PS: I hate any blanket over speakers; if you need them, then something is very wrong somewhere along the path.

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    I recently built a copy of the Marshall 2061 Lead and Bass 20W amp with EL84's in cathode bias. The pre-amp is low gain (60's design) so the only distortion it produces is power amp. There is no buzziness or unpleasantness to this at all. It is a good classic rock sound. There is also no conjunctive filters, diodes or anything like that - just a simple straightforward power amp design. So I don't think EL84 amps have to be buzzy.

    Also, the amp is seriously loud. It has too much headroom to get into any significant distortion at small club or bar gigs. Maybe a tiny bit of grind at most. My brother uses this as his main amp in the band. For distortion he needs to use a pedal in these types of venues. The cabinet is a 2x12 with Celestions

    Greg

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by GregS View Post
    I recently built a copy of the Marshall 2061 Lead and Bass 20W amp with EL84's in cathode bias. The pre-amp is low gain (60's design) so the only distortion it produces is power amp. There is no buzziness or unpleasantness to this at all. It is a good classic rock sound. There is also no conjunctive filters, diodes or anything like that - just a simple straightforward power amp design. So I don't think EL84 amps have to be buzzy.

    Also, the amp is seriously loud. It has too much headroom to get into any significant distortion at small club or bar gigs. Maybe a tiny bit of grind at most. My brother uses this as his main amp in the band. For distortion he needs to use a pedal in these types of venues. The cabinet is a 2x12 with Celestions

    Greg
    Of course they don't have to be buzzy... there are tons and tons of old EL84/6BQ5 Hi-Fi mono and stereo amps out there from many years gone by that sound glorious and still can make 12- 18 watts.
    As there a many fabulous sounding VOX EL84 amps, Matchless amps, all kinds of EL84 amps that don't sound like trashed out buzz bombs.
    That was part of my point....
    What I said was:
    "Ho hum... just what we need, another wanky, over driven El84 amp..."
    I never said there were no good sounding EL84 amps.

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    Noodle of Reality Steve Conner's Avatar
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    So what you're actually saying is, you hate high-gain amps?

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    Of course they don't have to be buzzy... there are tons and tons of old EL84/6BQ5 Hi-Fi mono and stereo amps out there from many years gone by that sound glorious and still can make 12- 18 watts.
    As there a many fabulous sounding VOX EL84 amps, Matchless amps, all kinds of EL84 amps that don't sound like trashed out buzz bombs.
    That was part of my point....
    What I said was:
    "Ho hum... just what we need, another wanky, over driven El84 amp..."
    I never said there were no good sounding EL84 amps.
    Just to clear up any misunderstanding I wasn't arguing with you about what you said. It was the notion presented in some of the other posts that you had to add conjunctive filters, diodes etc in order to make an EL84 power section sound good. That's what I was responding to. My point being that Marshall did it with a simple design without any of that stuff.

    I also hate high gain amps.

    Greg

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by GregS View Post
    It was the notion presented in some of the other posts that you had to add conjunctive filters, diodes etc in order to make an EL84 power section sound good...

    My point being that Marshall did it with a simple design without any of that stuff.
    You do need to add all that stuff to a high gainer to get EL84's to behave. IME you can't push them into a full square clip like you can with the big bottles.

    Often the right OT and speaker can mask the uglies well enough. I didn't want to search for any magic parts so I just designed the flaws out with brute force instead. I would prefere a simpler more eloquent circuit.

    And FWIW the Marshall 18 watter designs are hit or miss. Even from the factory. Some sound OK and some buzz like bees. There are threads on AX84 that discuss using the right OT and speaker to avoid this. Ok, so now everyone has the same PT, OT and speakers. The same tone I just wanted options since I don't think the "18 watt" design is all that.

    It has intrigued me that rare EL84 amps seem to defy this. In many cases building a second identical amp fails to produce the same desireable result. Fine, so it CAN happen. But if I build two amps I want them to perform the same. So I designed mine that way.

    JM2C

    Chuck

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    And FWIW the Marshall 18 watter designs are hit or miss. Even from the factory. Some sound OK and some buzz like bees. There are threads on AX84 that discuss using the right OT and speaker to avoid this. Ok, so now everyone has the same PT, OT and speakers.
    Actually, this is the 20W Marshall which originals from the 60's to early 70's (when they were produced) are rare and very expensive when they turn up used. Clones seem to be rare also. Marshall relatively recently reissued it in their hand-wired series as the 2061x. I'm not familiar with the 18W version but perhaps the design is different enough that it doesn't suffer the problems that the 18W does.

    Also, I did use clones of the original transformers from Mercury Magnetics and used the same Celestions the 2061x uses and built the cab as closely to spec as I could. Perhaps this has something to do with why it was a hit and not a miss.

    One other thing: the brand of tubes made a big difference as well. The new Sovteks I first tried were prone to redplating so I used some new Chinese tubes. They didn't redplate but didn't sound as good as the Sovteks. It now has NOS tubes in it and is a lot better sounding with these.

    I didn't want to search for any magic parts so I just designed the flaws out with brute force instead. I would prefere a simpler more eloquent circuit.
    But if I build two amps I want them to perform the same. So I designed mine that way
    Makes sense. I understand.

    Greg

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by GregS View Post
    Just to clear up any misunderstanding I wasn't arguing with you about what you said. It was the notion presented in some of the other posts that you had to add conjunctive filters, diodes etc in order to make an EL84 power section sound good. That's what I was responding to. My point being that Marshall did it with a simple design without any of that stuff.

    I also hate high gain amps.

    Greg
    Ah... OK.
    I have buddy who owns and uses one of the 60's Marshall, small box 20 watt Lead El84 amps... no pedals, no distortion effects... well he does uses a small digital reverb/delay.
    It sounds absolutely killer with his 4x10 cab using two reissue C10Qs and two Eminence C10N clones.
    Turned down it is still plenty loud enough to cover all the R&B stuff he does too.

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    Bent Member Chuck H's Avatar
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    Yup, those Marshalls, some Traynors and some Besa Moogies run the EL84's over 400Vp and not much lower on the screens. El84's , to my ears anyway, become a whole different tube at that voltage. They get real loud and tight, the bottom end comes up AND they don't seem to fizz and buzz as much. These amps are often biased OVER max too. I had a Subway Blues that was biased at 14 watts per tube with a Vp of 400+. For this extra oomf there are two concessions: Tube life is cut down, a lot IME. And you lose the classic Voxy chime. This last one may not be a concession if your not about that sound and your more interested in getting big and loud. Actually tube life isn't that much an issue either since these tubes are relatively affordable.

    My little Subway through it's 10" speaker was a piss ant on stage. But plugged into a 2x12 Celestion cab it could handle a room with no problems.

    Chuck

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    I fixed a Haze 40w combo a while back and the tone was surprisingly good. The construction sucked, but that's beside the point.

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    Old Timer olddawg's Avatar
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    I guess it depends on your application. All drummers arenít the same. I would prefer to use a 50 watt Marshall... But MOST of the gigs I play it isnít a good choice. I use an El84 single 12Ē combo almost exclusively nowadays. I can turn the master up and it doesnít kill me to transport it. I play with a guy all of the time that HAS to have his 100 watt Marshall half stack. At this point he has an effects processor plugged directly into the main amp input, so really any power amp would work (but I can never convince him of that). So it just takes up a lot of real estate, weighs a ton, and annoys club owners. And itís so directional itís problematic. Itís more coxmanship than reason imho.

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    Quote Originally Posted by Chuck H View Post
    Yup, those Marshalls, some Traynors and some Besa Moogies run the EL84's over 400Vp and not much lower on the screens. El84's , to my ears anyway, become a whole different tube at that voltage. They get real loud and tight, the bottom end comes up AND they don't seem to fizz and buzz as much. These amps are often biased OVER max too. I had a Subway Blues that was biased at 14 watts per tube with a Vp of 400+. For this extra oomf there are two concessions: Tube life is cut down, a lot IME. And you lose the classic Voxy chime. This last one may not be a concession if your not about that sound and your more interested in getting big and loud. Actually tube life isn't that much an issue either since these tubes are relatively affordable.

    My little Subway through it's 10" speaker was a piss ant on stage. But plugged into a 2x12 Celestion cab it could handle a room with no problems.

    Chuck
    The Subway Blues has another thing happening tone wise, check out the deliberate imbalance in the concertina PI. 68K and 120K for the anode and cathode loads.
    I found the sound actually improved dramatically going to 100K and 120K, of-course that was just my preference.


    Cheers
    Ian

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Gingertube View Post
    The Subway Blues has another thing happening tone wise, check out the deliberate imbalance in the concertina PI. 68K and 120K for the anode and cathode loads.
    I found the sound actually improved dramatically going to 100K and 120K, of-course that was just my preference.


    Cheers
    Ian
    Come to think of it I never looked into the balance on the PI for that amp before gifting it to a friend. Now I wish I had. It's not a concertina PI though. More like a LTP with a constant current tail voltage. Then there's an elevation at the grid of the non inverting triode so that it can act as the NFB input. This would cause more local feedback in that stage. In other words... I'm not so sure the design is imbalanced. Especially since Mesa used it on several amps including some that could not benefit from an imbalance in the PI. My old Subway is local and the PI is stock (perhaps the only part of the amp that is ). If I get the chance I'll try to get it and scope it Just the same, if changing to a 100k load for the inverting triode improves the tone I'm all for it.

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    Quote Originally Posted by Chuck H View Post
    I'm not so sure the design is imbalanced. Especially since Mesa used it on several amps including some that could not benefit from an imbalance in the PI. My old Subway is local and the PI is stock (perhaps the only part of the amp that is ). If I get the chance I'll try to get it and scope it
    When you measure it you'll probably find that it's balanced. The negative feedback only goes to the non inverting triode grid (and not also to the tail as the usual Fender LTP circuit) which makes the balance worse. I suspect they had to make the plate load resistors 68k/120k to correct this.

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    You are right of-course - not a concertina but a LTP.
    https://schematicheaven.net/boogieam...ubwayblues.pdf
    The cathodes of V3 will sit at around, or just less than +2V giving a total of say 17V across that 15K tail resistor. Will tend to force V3a and V3b total current to approximately 1.1 mA but otherwise no real current source (hi Z) behaviour.

    Assumming that V3a and V3b share this current equally then each triode has Ia of 0.65ma and that infers rp of 70K.

    Doing a quick and dirty calc of Gain V3a (A1) vs Gain V3b (A2) from
    A1/A2 = 1 + [Ra +rp/Rk(u+1)]
    gives
    A1/A2 = 1 + [68K + 70K/ 15K (100 +1)]
    = 1.09

    That is, the gain of V3a is 9% higher than V3b.

    From that I would assume that for good balance that 120K should be 68K x 1.09 = 74K

    OR if leaving the 120K then the 68K should be 120K/1.09 = 110K.

    That would give best balance before FB is applied which should be your aim.

    Does this make sense to you?

    Cheers,
    Ian

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Gingertube View Post
    That would give best balance before FB is applied which should be your aim.
    Does it make a difference that the NFB is applied at the cathode coupled, non inverting triode rather than at the tail? It seems to me that it would.

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    Quote Originally Posted by Gingertube View Post
    You are right of-course - not a concertina but a LTP.
    https://schematicheaven.net/boogieam...ubwayblues.pdf
    The cathodes of V3 will sit at around, or just less than +2V giving a total of say 17V across that 15K tail resistor. Will tend to force V3a and V3b total current to approximately 1.1 mA but otherwise no real current source (hi Z) behaviour.

    Assumming that V3a and V3b share this current equally then each triode has Ia of 0.65ma and that infers rp of 70K.

    Doing a quick and dirty calc of Gain V3a (A1) vs Gain V3b (A2) from
    A1/A2 = 1 + [Ra +rp/Rk(u+1)]
    gives
    A1/A2 = 1 + [68K + 70K/ 15K (100 +1)]
    = 1.09

    That is, the gain of V3a is 9% higher than V3b.

    From that I would assume that for good balance that 120K should be 68K x 1.09 = 74K

    OR if leaving the 120K then the 68K should be 120K/1.09 = 110K.

    That would give best balance before FB is applied which should be your aim.

    Does this make sense to you?

    Cheers,
    Ian
    I don't quite agree with your calculations. The gain formula used seems to be incomplete. Using the same numbers and equal triode currents of 0.65ma (resulting in a total current of 1.3mA !) with the exact formula from the attachment below, I get a gain ratio of 1.7 (quite close to the ratio of the plate resistors as expected).

    But what makes things complicated is that the triodes must not be viewed independently. Rather they are coupled by the common cathode resistor. When the current of the first triode increases, the increased cathode voltage causes a decrease of the current of the second triode. This leads to mutual gain degeneration. Also the second triode is forced to follow the first one (being driven by the common cathode voltage), which causes the gain of the second one to be lower with equal plate resistors. For low value cathode resistors, the gain of the second triode approaches 50% of the first one for equal plate loads. So the different plate resistors are needed to compensate for the difference.

    As I don't see an easy way to do a complete calculation I hope that someone would offer to simulate the circuit.



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  22. #57
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Helmholtz View Post

    As I don't see an easy way to do a complete calculation I hope that someone would offer to simulate the circuit.



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    Sim results:


    Using equal 68k plate resistors
    Gain Gaa input A to output A = 24.9 (i.e. same triode)
    Gain Gab input A to output B = 21.8 (i.e. other triode)
    i.e. Gaa/Gab = 1.14

    To get Gaa=Gab you need to make the B plate resistor 78.5K.

    I would have thought that you would need it be be balanced for both inputs. If you get it right for the signal input then the NFB will be way off and so fail to operate properly.

    I do like the simplicity that using the negative supply brought. But given they did that it would have been easy to replace the 10K tail resistor with a simple current source.

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    Thanks nickb,

    I thought I was close.

    I used this as a guide.
    http://www.valvewizard.co.uk/acltp.html

    As Helmholtz says, the circuit has to be treated as a differential amplifier where V3a grid is the +ve input and V3b grid is the -ve input.

    What I understand (or possibly miss-understand) is:
    With any differential amplifier (SS or tube or whatever)
    When balance is perfect:
    - 2nd harmonic distortion is cancelled
    - what distortion remains is mostly odd harmonics and is a result of non-linearities in the devices (triodes in this case) themselves.

    Deliberatly unbalacing a differential amp is a way to allow more of the naturally resulting 2nd harmonic distortion (from the diff amp) to remain (not be cancelled).

    The Push Pull output stage is a differential amp too, if it is perfectly balanced then the same 2nd harmonic distortion (from the output stage) cancellation occurs.

    Feeding the output stage unbalanced inputs causes increased 2nd and other even harmonic distortions to occur.

    AND (a point which is often missed)
    The mechanism which produces Harmonic Distortion also produced Intermodulation Distortion, you simply can not have one without the other. It is often the Intermodulation Distortion products which can sound nasty.

    Cheers,
    Ian

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    Thanks!

    Maybe they chose this asymmetric design to introduce some even harmonics in the powerstage spectrum.

    (simul-posting)

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    I thought I was close.
    OK, now I see. The formula from the valvewizard page A1/A2 = 1 + [(Ra+ra)/(Rk(mu+1))] is valid only for equal plate resistors. But you omitted a pair of brackets. So I couldn't verify your results.

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    Quote Originally Posted by Gingertube View Post
    That would give best balance before FB is applied which should be your aim.
    Quote Originally Posted by Chuck H View Post
    Does it make a difference that the NFB is applied at the cathode coupled, non inverting triode rather than at the tail? It seems to me that it would.
    I've tried that on an AC15 type output stage. Without NFB it was well balanced but when NFB was applied just to the non inverting triode's grid the balance was way off. Increasing the NFB made the balance worse. If the NFB is applied to both the non inverting triode's grid and the the tail then the balance is fine.

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    This leads to mutual gain degeneration.
    This assumption of mine was wrong. Actually the mutual interaction of the cathode currents reduces the degeneration effect and increases gain compared to single triode with the same cathode resistor.

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  28. #63
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by Gingertube View Post
    Thanks nickb,

    I thought I was close.

    I used this as a guide.
    http://www.valvewizard.co.uk/acltp.html

    As Helmholtz says, the circuit has to be treated as a differential amplifier where V3a grid is the +ve input and V3b grid is the -ve input.

    What I understand (or possibly miss-understand) is:
    With any differential amplifier (SS or tube or whatever)
    When balance is perfect:
    - 2nd harmonic distortion is cancelled
    - what distortion remains is mostly odd harmonics and is a result of non-linearities in the devices (triodes in this case) themselves.

    Deliberatly unbalacing a differential amp is a way to allow more of the naturally resulting 2nd harmonic distortion (from the diff amp) to remain (not be cancelled).

    The Push Pull output stage is a differential amp too, if it is perfectly balanced then the same 2nd harmonic distortion (from the output stage) cancellation occurs.

    Feeding the output stage unbalanced inputs causes increased 2nd and other even harmonic distortions to occur.

    AND (a point which is often missed)
    The mechanism which produces Harmonic Distortion also produced Intermodulation Distortion, you simply can not have one without the other. It is often the Intermodulation Distortion products which can sound nasty.

    Cheers,
    Ian
    This would have been really close on if it were not some for something going wonky in the calculation. The formula is

    1 + (Ra+rp)/(Rk(mu+1))

    which comes to 1.136 with mu=100, rp=70k and Ra=68k. That would make other Ra = 77.3k, really close to the sim.

    Thinking about the operation of the feedback, if you simplify and call it class B then on the positive going half cycle of the plate where the bigger Ra is located, the NFB gain will be 1.136^2 times that of the other plate. This has the effect of reducing the gain on that side and so explains why they went all the way to 120K to unbalance it again.

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    Last edited by nickb; 02-18-2019 at 12:23 AM.
    Experience is something you get, just after you really needed it.

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