Page 2 of 2 FirstFirst 12
Results 36 to 67 of 67
Like Tree2Likes

Thread: Ultra linear and phase inverter drive

  1. #36
    Senior Member
    Join Date
    Jul 2011
    Location
    Adelaide, South Oz
    Posts
    621
    I've intimated before that my ideas for guitar amps are "infested" with HiFi Amp design principles BUT this is one area where the HiFi ideas can really help.
    Many commercial bass amps have reliability issues directly relating to grid 1 impedances being way too high.
    The way to address this is mosfet source followers to buffer the PI outputs.
    Direct couple the source followers to the output tube grids with just an appropriate grid stop in that feed. That takes firm control of the output tube grids, addresses bias purturbations and thermal run away issues caused by grid current. It also shunts grid current noise to ground.
    Put the AC coupling caps between the PI and the source followers and apply the bias voltage to the gates of the mosfets.
    This makes the PI perform much better too, not only is the PI more lightly loaded but that load is now a Class A load. This will definitely help for the "usual" schmitt (diff amp) splitter BUT it MASSIVELY improves performance of a Cathodyne (split load) PI where balance depends upon equal loads at the anode and cathode. The Class A loading of the PI also eliminates blocking distortion problems.
    Yes you need a negative rail. A minimum of 3 times the bias voltage is what is required (I usually use 4 to 5 times the bias voltage).
    Someone above stated that you are then at the mercy of the negative rail noise (that is, the negative rail noise will appear at the output tube grid).
    That is true to a degree. Looking into the source follower output pin (the source pin) you will see a lowish impedance to 0V (signal ground). The amount of noise which appears at the top of the source follower load can be determined by simple voltage divider theory. The "bottom" arm of the divider is the AC impedance looking into the source pin. The top arm of the divider is the source follower load resistor back to that -ve rail.
    Since you are already adding in a bit of SS (the mosfet source follower) then the answer is to add in another 2 bucks worth of SS and replace the source follower load resistors with a current sources. The VERY high AC impedance of the current source (compared with a resistor) will divide that -ve rail noise down to insignificance. For the current sources I generally use a "Ring of Two" transistor current source. A BC547C as the bottom transistor with an MJE340 on top. There are of course better and more sophisticated current sources but these simple ones are more than adequate for the task.

    Can I give you a schematic - well not a bass amp but I used the same scheme in a HiFi Amp for which the schematic is here:
    EL84 Amp - Baby Huey - Page 61 - diyAudio (See post #602)
    It is "infested" with a few other HiFi'ish ideas (forced balance for example) but the current source loaded source follower buffers between the PI and the output tubes is there for all to see. My current HiFi Amp is this circuit but with 6V6 Outputs and 6SL7 splitter.

    Hope this gives you some ideas, stimulates some discussion or at least a "Pick up your HiFi Marbles (if you can find them) and go home" comment.

    Cheers,
    Ian

  2. #37
    Senior Member
    Join Date
    Jul 2011
    Location
    Adelaide, South Oz
    Posts
    621
    Another +1 to add to Chuck H.
    The lower frequency roll off is set by the output tube internal impedance rp and the primary inductance of the output tranny. Ultralinear connection reduces that rp by a factor of about 3 times shifting the low frequecy roll off 3 times lower. The problem then becomes core saturation at low frequencies and to address that BIG iron is what you need.
    Cheers,
    Ian (Cork Sniffer ??)

  3. #38
    Senior Member
    Join Date
    May 2006
    Location
    Cornelius, Oregon
    Posts
    1,222
    Quote Originally Posted by Gingertube View Post
    Another +1 to add to Chuck H.
    The lower frequency roll off is set by the output tube internal impedance rp and the primary inductance of the output tranny. Ultralinear connection reduces that rp by a factor of about 3 times shifting the low frequecy roll off 3 times lower. The problem then becomes core saturation at low frequencies and to address that BIG iron is what you need.
    Cheers,
    Ian (Cork Sniffer ??)
    Big iron is definitly king for tube bass amps. The early Sunn 2000S (UL with a cathodyne inverter I might add) with the Dynaco transformers (before they switched to Schumacher) has a huge OT and will amplify cleanly down to 10 Hz! But that kind of OT costs dearly these days.

    Greg

  4. #39
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Location
    Glasgow, Scotland
    Posts
    7,251
    I think we all agree that the bigger the OT, the better!

    However, I saw a Wireless World article somewhere arguing that ultralinear allows a smaller OT for the same bass response. The lower Rp allows the transformer to have lower inductance without losing bass. You can then stack the laminations in bigger chunks to get more distributed air gap, and that helps with the saturation.

    If applying the Paul Ruby mod to a fixed bias amp, the relevant voltage is twice the bias voltage. If you have your power tubes biased at -35V, the signal flaps them between 0 and -70, so your zeners have to be 70V or greater.

    Fender's 300PS and 400PS were AB2 all the way. The power tube grids were driven by a transformer the size of a Deluxe OT, which in turn was driven by a triode connected 6V6 or 6L6. That's what I call an output stage! The same topology was used on a miniature scale in the Musicmaster Bass practice amp.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  5. #40
    Old Timer
    Join Date
    Apr 2008
    Location
    Oceanside, NY
    Posts
    1,649
    I'd like to point out that the old Marshall Major design accomplished all of this nicely with a UL OT and a pretty routine LTP 12AU7 PI (chosen for it's lower plate impedance) driving 4- KT88's. You don't REALLY need cathode followers unless you are worrying about blocking distortion, which is not happening unless the amps' is operation at constant overload. Lookee here: http://www.webphix.com/schematic hea..._bass_200w.pdf

    It's also worthwhile to note that while the coupling caps to the power tube grids are at a fairly-high .47uF, the grid load resistors have been reduced to 68K to reduce the time constant.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

  6. #41
    Senior Member
    Join Date
    Sep 2010
    Location
    Edmonton AB Canada
    Posts
    257
    Yeah the Major has been a grood referance for me, but I was puzzled by it's PI design, as the second grid seems to be connected back to a gain stage before the one that drives the other half of the 12AU7, is that a drawing error?

    I also have been using the amp book calculators and was wonder what Kinda time constant I should be shooting for, as far as what is an acceptable bias excursion recovvery time, or is bias excurssion a different thing? I was gonna use 57K grid load resistors and .1uf caps as they should allow enough low end, maybe a .22uf

  7. #42
    Lifetime Member
    Join Date
    May 2006
    Posts
    4,088
    Quote Originally Posted by Gingertube View Post
    The way to address this is mosfet source followers to buffer the PI outputs.
    Direct couple the source followers to the output tube grids with just an appropriate grid stop in that feed.
    I like the idea, Ian. In fact, it's pretty much what I've proposed here MOSFET Follies in 2000, and flogged on this forum a number of times.

    You're right about worrying about hifi heresies - the current source is the obvious choice, but I could never bring myself to go ahead and say it. I caught enough flak here and other places over actually saying to introduce silicon into an tube amp.

    People who have tried it have reported back that it works well.

  8. #43
    Old Timer
    Join Date
    Apr 2008
    Location
    Oceanside, NY
    Posts
    1,649
    Quote Originally Posted by Tage View Post
    Yeah the Major has been a grood referance for me, but I was puzzled by it's PI design, as the second grid seems to be connected back to a gain stage before the one that drives the other half of the 12AU7, is that a drawing error?
    In fact, it is. The junction of the 1M grid-load resistor after the volume control and .047 coupling cap should be joined to the junction of the 2.7K and 100K cathode resistors. The stage just after the volume control is actually configured as a cathodyne PI, which then drives a long-tail pair PI. Cool and simplistic design.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

  9. #44
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    11,379
    Quote Originally Posted by R.G. View Post
    I like the idea, Ian. In fact, it's pretty much what I've proposed here MOSFET Follies in 2000, and flogged on this forum a number of times.

    You're right about worrying about hifi heresies - the current source is the obvious choice, but I could never bring myself to go ahead and say it. I caught enough flak here and other places over actually saying to introduce silicon into an tube amp.

    People who have tried it have reported back that it works well.
    I remember it being proposed here... By you. I thought it was well recieved and valid. The only rub is that it doesn't fit into the ideal of an "all tube" circuit (for marketing) and it has become passe in light of new digital sampling amps (main signal OR effects) and the uber gain preamp into a 100W power amp modern designs that all the wankers use. Better power amp distortion is sort of a tube purist crank kind of thing that MOSFETs don't really fit into. And the metal kids don't crank their amps or they couldn't get that V notch, mid scoop wanker tone. For builders like us it's great. But I don't know if it has a place in any non technically inclined demographic.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  10. #45
    Senior Member
    Join Date
    Jan 2011
    Location
    Australia
    Posts
    285
    That's probably true. Blackstar use a lot of solid state components in their amps, and have copped a lot of flack from prospective buyers, because their not 'all tube'. Though for the more technically inclined home builder, I think on some level we just like to bring our haphazard creations on stage and gloat. Sometimes I tell people my amp is solid state just for fun (it's not far from the truth either!)

  11. #46
    Senior Member
    Join Date
    Sep 2010
    Location
    Edmonton AB Canada
    Posts
    257
    Yeah shortly after I posted I found a better drawn schematic and realized that that was going on there, I kinda like that design, and I may implement it should just using a 12at7 not work out the way I hope. as far as the UL taps I think I'll just wire them away and try it with screen resistors and then with the taps and see which the customer prefers.

  12. #47
    Senior Member
    Join Date
    Jan 2008
    Location
    Doylestown, PA
    Posts
    1,075
    For me the advantage is at the blurred edge between power amp grind and clean preamp distortion. I would argue that both power amp distortion fans and preamp distortion fans would be able to hear and appreciate the difference. You get the ability to avoid grid blocking and bias each output valve independently of the others. That's worth the parts count to me!

    jamie

  13. #48
    Old Timer defaced's Avatar
    Join Date
    Dec 2006
    Location
    York Pa
    Posts
    1,394
    Quote Originally Posted by Chuck H View Post
    I remember it being proposed here... By you. I thought it was well recieved and valid. The only rub is that it doesn't fit into the ideal of an "all tube" circuit (for marketing) and it has become passe in light of new digital sampling amps (main signal OR effects) and the uber gain preamp into a 100W power amp modern designs that all the wankers use. Better power amp distortion is sort of a tube purist crank kind of thing that MOSFETs don't really fit into. And the metal kids don't crank their amps or they couldn't get that V notch, mid scoop wanker tone. For builders like us it's great. But I don't know if it has a place in any non technically inclined demographic.
    You know, I used to read this forum on a daily basis because people had worthwhile stuff to say. To say the least, I learned alot here, but not so much anymore because of the attitudes. You old farts need to get off your high horse. There is life outside of a cranked plexi and strat/tele/LP. Your attitudes suck and it doesn't shine well.

    Thanks,
    Some wanker

    -----

    Back on topic. If you searched for MOSFET follies, you probably stumbled upon my schematic of RG's DC coupled source followers. It does work and isn't a difficult build. I encourage trying it. I designed it to be modular so you can build it as is and try it without hacking into the circuit too much.
    -Mike

  14. #49
    Senior Member
    Join Date
    Sep 2010
    Location
    Edmonton AB Canada
    Posts
    257
    I don't build my amps for myself, my customer is an AC30 guy who like to run his amp all out. He asked for a bass amp that had "good power tube distortion." As soon as he told me this I knew he was the type of guy who listened to a lot of marketing, but I told him I would do my best to create a bass amp with good crunch. I Have now decided that some preamp distortion and sheer power from the power amp will be more than reasonable crunch, without deterring from the fundamental bass tone.

    And FWIW if a customer asked me for an amp that had a V notch and scooped mid tone, that's what I would give them. Most of what I have built and been ask to build has really been the opposite, the Hardcore guys I build and modify for want more and more mids, because they know live they need that to cut through. I'm happy to oblige that pursuit to a point too. I don't think wanting a notched mid range makes some one a wanker though, but then again I'm Canadian so Wanker is just something I hear them say on Top Gear.

    Let's stay on topic now...

  15. #50
    Senior Member
    Join Date
    Jan 2011
    Location
    Australia
    Posts
    285
    I don't know if it's been explicitly mentioned in this thread, but using the direct coupled source follower method with low-medium value grid stoppers I find allows a 'rounder' sounding power amp distortion (I mean that both figuratively and literally - on the scope), even if extra power output isn't your goal. If you've ever scoped the output of an amp into a resistive load driven into clipping, the output is usually pretty damn square shaped, with a good amount of crossover distortion. Though in the end, it does sound slightly different; they/you may not like it, although you can return it to it's usual behaviour by sticking larger grid stoppers on the tube (not a good idea with triodes, however, due to the miller capacitance). This bias shift effect will obviously still be missing, although most people tend to try to eliminate that anyway.

    If it were me, I'd stick a thimble on top of the fet's and tell people they're nuvistors, haha.

  16. #51
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    11,379
    Quote Originally Posted by Tage View Post
    I don't think wanting a notched mid range makes some one a wanker though, but then again I'm Canadian so Wanker is just something I hear them say on Top Gear.
    Didn't mean any offense. When I say wanker I'm implying any of the following:

    A person who's always yanking his locking trem around (which I have been known to do).

    A metal player that likes his tone to sound like a booming buzz saw. (which I have also been know to do)

    A kid that doesn't know better than to turn the bass and treble up and the midrange to zero trying to get a dynamic sound while trying to play as fast as possible (which I used to do)

    Sorry to stay off topic. Done now.
    Tage likes this.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  17. #52
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Location
    Glasgow, Scotland
    Posts
    7,251
    I'll stand up and be counted, I do those things too! Except for the locking trem, I hate them.

    And I now know better than to try and get the scooped mid tone at gig volume with a sensibly sized amp. I admire Ken Gilbert's solution, which was to use an insanely sized amp, but I can't be bothered carting a rig that size around.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  18. #53
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    11,379
    Quote Originally Posted by Steve Conner View Post
    I admire Ken Gilbert's solution, which was to use an insanely sized amp, but I can't be bothered carting a rig that size around.
    +1

    Sort of an icon here. For those who haven't seen it, it's still up for viewing at his site:

    The Big Ass Guitar Amplifier | ken-gilbert.com
    Tage likes this.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  19. #54
    Lifetime Member
    Join Date
    May 2006
    Posts
    4,088
    Quote Originally Posted by exclamationmark View Post
    I don't know if it's been explicitly mentioned in this thread, but using the direct coupled source follower method with low-medium value grid stoppers I find allows a 'rounder' sounding power amp distortion (I mean that both figuratively and literally - on the scope), even if extra power output isn't your goal. If you've ever scoped the output of an amp into a resistive load driven into clipping, the output is usually pretty damn square shaped, with a good amount of crossover distortion. Though in the end, it does sound slightly different; they/you may not like it, although you can return it to it's usual behaviour by sticking larger grid stoppers on the tube (not a good idea with triodes, however, due to the miller capacitance). This bias shift effect will obviously still be missing, although most people tend to try to eliminate that anyway.
    Good point.

    The MOSFET follower idea first occurred to me to replace cathode followers for driving tubes into class AB2, with signal excursions into positive grid voltage. It's not widely appreciated, but practically every tube will go a bit more into conduction if it's grid can be driven positive. It's just difficult to do this with a high impedance drive, like the plate of a preceding tube. The grid impedance goes from near-infinite to a few K over a couple of tenths of a volt, and a high impedance drive will flatline right there. It's easy to see on a scope, as you note. A low-impedance follower drive will push the grid further above the cathode (because it can!) and will not only eke out some more watts on a power stage, but the hard clipping is converted into softer, mushier current/emission clipping; again, rounder and easy to see on a scope.

    For the purposes of this thread, a DC coupled follower driving a power tube grid is in capable of doing the grid-current rectification into a coupling cap that causes bias shift and blocking distortion because there is no capacitor to store grid conduction charge into.


    If it were me, I'd stick a thimble on top of the fet's and tell people they're nuvistors, haha.
    I like this very much. I think what I ought to do is to make up some wacky-looking metal cylinders with wanky wire leads coming out and tell people they're Elbonian power pentodes with cold field-emission cathodes, surplus from the Elbonian space program.

    Inside would be an IR832 (or similar) with a resistor from one wire to the "cathode" lead and a diode to the "plate" wire to fake a useless but convincing screen current, a "grid" lead with protection zeners and maybe some small caps to fake a suitable lower frequency response, maybe some other tomfoolery. The positive "grid-cathode" could be explained as a side effect of the cold-cathode construction.

    I bet I could come up with some suitably over-the-top hifi tweako language ( aged in flours-of-suphur in used wine-barrel casks then cryogenically tempered to 53 Rockwell hardness... ) and sell these as very high-end replacements.

  20. #55
    Lifetime Member
    Join Date
    May 2006
    Posts
    4,088
    Quote Originally Posted by Chuck H View Post
    +1
    Sort of an icon here. For those who haven't seen it, it's still up for viewing at his site:
    The Big Ass Guitar Amplifier | ken-gilbert.com
    It has many appealing characteristics, including completely solving the equipment theft problem.

  21. #56
    Senior Member
    Join Date
    Sep 2010
    Location
    Edmonton AB Canada
    Posts
    257
    MMM, the BAGA... a real feat of tube power.

  22. #57
    Supporting Member Chuck H's Avatar
    Join Date
    May 2007
    Location
    pacific north west
    Posts
    11,379
    Quote Originally Posted by R.G. View Post
    I like this very much. I think what I ought to do is to make up some wacky-looking metal cylinders with wanky wire leads coming out and tell people they're Elbonian power pentodes with cold field-emission cathodes, surplus from the Elbonian space program.

    Inside would be an IR832 (or similar) with a resistor from one wire to the "cathode" lead and a diode to the "plate" wire to fake a useless but convincing screen current, a "grid" lead with protection zeners and maybe some small caps to fake a suitable lower frequency response, maybe some other tomfoolery. The positive "grid-cathode" could be explained as a side effect of the cold-cathode construction.

    I bet I could come up with some suitably over-the-top hifi tweako language ( aged in flours-of-suphur in used wine-barrel casks then cryogenically tempered to 53 Rockwell hardness... ) and sell these as very high-end replacements.
    I met a guy at winter NAMM 2009 who did just this. He built transistor circuits that could retrofit in a tube preamp environment and stuffed them in preamp tube sized cylinders with a male 9 pin on the bottom. He had a couple of amps in his booth using his preamp tube replacements. I couldn't get to his booth once things were under way and we weren't allowed to make any noise prior to opening. So I can't offer a personal evaluation. Nor can I remember the name of his operation.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

    "...less ear-friendly but handy for jazz." Leo_Gnardo

  23. #58
    Noodle of Reality Steve Conner's Avatar
    Join Date
    May 2006
    Location
    Glasgow, Scotland
    Posts
    7,251
    I've seen quite a few like that. I also designed my own version, based on a LND150 with a resistor and diode network to make the curves more triode-like. I don't think it works properly in every application, for instance I wouldn't trust it in the Marshall cathode follower or the SLO cold clipping stage. But I heard from someone who got good results in less critical places like the FX loop.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

  24. #59
    Old Timer
    Join Date
    Apr 2008
    Location
    Oceanside, NY
    Posts
    1,649
    Quote Originally Posted by Chuck H View Post
    I met a guy at winter NAMM 2009 who did just this. He built transistor circuits that could retrofit in a tube preamp environment and stuffed them in preamp tube sized cylinders with a male 9 pin on the bottom. He had a couple of amps in his booth using his preamp tube replacements. I couldn't get to his booth once things were under way and we weren't allowed to make any noise prior to opening. So I can't offer a personal evaluation. Nor can I remember the name of his operation.
    Jet City sells these. They actually sound pretty good.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

  25. #60
    kg
    kg is offline
    Lifetime Member kg's Avatar
    Join Date
    Aug 2006
    Posts
    618
    Quote Originally Posted by Tage View Post
    MMM, the BAGA... a real feat of tube power.
    and remarkably still running on the original, and basically no-longer-available, kt90s.

  26. #61
    Senior Member
    Join Date
    Sep 2010
    Location
    Edmonton AB Canada
    Posts
    257
    Ok so I have now finalized my design and am in the process of making up my PC boards and prepping the chassis. I decided to go with the Mosfet drivers, just because the more I've learned the more benefits I've seen from them. really there seems to be no drawbacks. I'm running them after a pretty standard 12ax7 split load PI, with a big grid stopper, running ~350V on it's plate with .1 coupling caps. The signals then go to a potential divider of 100k/470k. the 470k going to ground and the bias voltage being fed in @ their junction. there shouldn't be much if any voltage dropped across the 100k, were it then enters the gate

    I just have a couple questions still... My phase inverter will be driving with an AC load line of 100K//570K? I suppose this is the advantage of using the mosfet drivers, the PI is not heavily loaded and the Power tube can have nice low grid leaks and high grid stoppers, I'm using 33K grid leaks/source resistors and 10k grid stoppers. Are the grid stoppers still as necessary after the Mosfet buffers?

    I'd love to upload my schematic but I don't have any good software for drawing out the schematic on my Macbook...

  27. #62
    kg
    kg is offline
    Lifetime Member kg's Avatar
    Join Date
    Aug 2006
    Posts
    618
    Quote Originally Posted by Tage View Post
    Are the grid stoppers still as necessary after the Mosfet buffers?
    yes. always use grid stoppers, pretty much 100% of the time. the values can be tweaked of course, and the lower the output tube stoppers in this case the more the drivers can pull the tube into grid current, but you absolutely should use SOMETHING.

  28. #63
    Lifetime Member
    Join Date
    May 2006
    Posts
    4,088
    kg's right. Grid stoppers 100% of the time.

    The reason is that grid stoppers are there to prevent local oscillation of the tube they're "stopping". The wires leading to the tube grid have inductance and capacitance, and also capacitance to the output/plate. This can and does make the tube go into RF oscillation if the stars line up just right/wrong.

    It's the same reason to always use gate stoppers on MOSFETs used as source followers. These things easily oscillate at frequencies you can't even see on oscilloscopes with only 20MHz bandwidths.

  29. #64
    Senior Member
    Join Date
    Sep 2010
    Location
    Edmonton AB Canada
    Posts
    257
    I've got 10k stoppers drawn up, more than sufficient, but I figure I can go bigger because of the follower set up being able to sink the grid current.

  30. #65
    Lifetime Member
    Join Date
    May 2006
    Posts
    4,088
    Generally stoppers don't usually need to be big. The idea is not to choke off current flow, it's to damp the resonances that set the thing up to oscillate. Beyond that, you're making a lowpass filter from the capacitance from grid to cathode and the Miller-amplified capacitance to the plate. For instance, usually 100R to 1K is OK for MOSFET followers.

  31. #66
    Senior Member
    Join Date
    Apr 2010
    Location
    London
    Posts
    202
    We built a 50W head with DC coupled grid using a 12AU7 cathode follower and initially running the output stage UL.

    This amp sounded good clean and had plenty of headroom, but for guitar sounded better in pentode mode when overdriven, although it did sound nice "clean" in UL.

    Initially we were using KT77s which had just been reissued, and are designed to be run UL, but then ended up using EL34s which aren't (although several hi-fi amps have done this).

    We were trying to build an amp that would not go into blocking distortion, so the idea was to use a CF to DC couple the pre-amp to the fixed bias phase splitter (we've used this before with good results), DC coupled CFs to the output valve grids, and UL output stage without global negative feedback.

    The amp ended up being quite complex, and ultimately we didn't pursue the design further, so I can't really comment on the suitability of UL, and at some stage I would like to revisit this configuration.

    As discussed above the advantages of the DC coupling where elimination of blocking distortion, reduced grid resistance, possible AB2 operation, and reduced loading on the output of the phase splitter, so no "nipple".

    It also allowed us to easily add a post phase splitter master volume.

    One issue we were aware of was the heater cathode insulation of the CF driver stage. MOSFETs seem a very good engineering solution all round.

    We were also quite wary of oscillation. The relatively high output impedance of standard long-tail pair forms a low pass filter with the Miller capacitance of the power valves. You now would have low impedance driving the power valves so more potential for instability as you have raised the HF roll off.

  32. #67
    Senior Member
    Join Date
    Apr 2010
    Location
    London
    Posts
    202
    Quote Originally Posted by exclamationmark View Post
    That's probably true. Blackstar use a lot of solid state components in their amps, and have copped a lot of flack from prospective buyers, because their not 'all tube'. Though for the more technically inclined home builder, I think on some level we just like to bring our haphazard creations on stage and gloat. Sometimes I tell people my amp is solid state just for fun (it's not far from the truth either!)
    Most of the flak Blackstar received was not so much for using solid state components, but more for their marketing which implied that certain products were all"valve", when they weren't by most people's definition of all "valve".

Page 2 of 2 FirstFirst 12

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. unbypassed cathode resistor and ultra-linear operation?
    By CopperWings in forum Theory & Design
    Replies: 6
    Last Post: 07-06-2011, 11:24 PM
  2. Bad PS hum on Ultra Linear Twin
    By wizard333 in forum Maintenance, Troubleshooting & Repair
    Replies: 5
    Last Post: 08-17-2010, 03:24 PM
  3. Help with 6L6 ultra linear with ef86
    By walkman in forum Theory & Design
    Replies: 5
    Last Post: 04-22-2010, 11:59 AM
  4. Help with SF Fender Twin tremolo (ultra linear model)
    By Wes in forum Maintenance, Troubleshooting & Repair
    Replies: 1
    Last Post: 08-03-2009, 10:09 AM
  5. Ultra Linear Hi-Fi ?
    By Amp Kat in forum Other Amps
    Replies: 2
    Last Post: 06-14-2007, 05:01 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •