Finally -- Princeton Reverb

Discussion in 'Shock Brother's DIY Amps' started by theprofessor, Apr 9, 2020.

  1. theprofessor

    theprofessor Poster Extraordinaire

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    Yeah, and that's being driven by a 12AU7!

    I also have a Belton/Accutronics 1AB2B1B medium decay reverb pan that I'm going to sub in for my MOD 4AB3C1B to see how it sounds by comparison. They're both 8Ω input, 2,250Ω output tanks. Only the length of the springs is different.
     
    Last edited: May 17, 2020
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  2. tubegeek

    tubegeek Tele-Afflicted

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    Well, driving a reverb transformer is not really about the gain of the tube driving it so much as the ability to deliver some power. (That's why the AT7, doubled up, is the typical Fender reverb driver.) A better metric is the rp of the tube, the lower the better in this application. AU7 has gain of 20, rp of 8K (which becomes half that when you double up the two triodes.) AT7 has gain of 60, rp about twice that of the AU7, so it's kind of a compromise between AX7 and AU7, lower rp and higher gain.

    The transformer circuit is basically a voltage divider between rp and its own impedance. The lower the rp (without a mu so low that you don't get all the voltage gain desired) the better the voltage transfer into the load. Best power transfer is a 1:1 matched impedance. Since impedance is frequency-dependent, these are all moving targets. So sue me.

    A combination of relatively high mu and relatively low rp means high transconductance (symbolized by gm.)

    High-gm tubes are great, and to my mind, it's better to have high gm than high mu when you are taking circuit impedance into account.

    High-gm tubes have their own difficulties though: they are also usually high bandwidth and so are VERY prone to radio-frequency oscillation which requires a lot of attention to layout and RF filtering. For things like phono preamps and V1 in a guitar amp, though, they are The Balls. The same twitchiness that turns them into RF oscillators translates into enormous sensitivity to these small input signals, the high gm means they can transfer a large current drive into whatever follows, preserving bandwidth and allowing the use of lower impedances in the following circuit block.

    I could blather on about this for quite a while, it's a pet subject. I guess I already have.
     
    Last edited: May 17, 2020
  3. King Fan

    King Fan Poster Extraordinaire

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    That sounds amazing, Professor. A perfect example of great PR tone. And... well played, sir!

    Actually this discussion is really super helpful. Thank you! And you made me go read about rp, where I immediately found this article, which basically restates what you said, but reminded we what rp means:

    "Of the vacuum tubes’ three operating characteristics – Dynamic Plate Resistance (rp), Transconductance (gm) and Amplification Factor (mu) - two are explicit values and one is an implicit value. The gm and rp values are explicit because they come directly from the tubes’ actual operating voltages and currents. The mu value, however, is an implicit value that is mathematically-derived from the product of the rp and gm values:

    mu = gm * rp

    Gm and rp are almost (but not exactly) inversely related. That is, as gm increases with increasing plate current, rp proportionately decreases. This inverse relationship causes mu to be virtually a constant that typically varies less than 10-15%."

    He goes on to talk about *effective* mu and rp and about bandwidth -- let's say a wide-ranging discussion. But I really like your version.

    I was especially interested in your note about how high-mu, low-rp tubes (can you say 12AX7?) are great in V1, and why (I guess by contrast?) AT7, AU7-type tubes make good 'drivers.'`
     
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  4. tubegeek

    tubegeek Tele-Afflicted

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    @King Fan actually I think there are much better tubes than 12AX7 for V1: 12AX7 is not an amazingly high gm tube. 6DJ8 6N1P are lower mu but higher gm, 6C45P "the Rooskie firecracker" is one of the higher gm tubes available.

    Trying to remember the guy who used a 6C45P for V1 in his amp - Ken Gilbert seems to be my memory. He liked it a LOT.

    EDIT: It was 6N1P in a bass preamp, but his website has some amazingly over the top experimentation going on. A real independent thinker!

    http://ken-gilbert.com/all-tube-guitar-preamp
     
    Last edited: May 17, 2020
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  5. theprofessor

    theprofessor Poster Extraordinaire

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    Here's another clip with the Les Paul, this time clean. I think it illustrates well the rich tones and the "pop" I get with this setup, without losing clarity.

    Epiphone Les Paul Traditional Pro with two Seymour Duncan Seth Lover humbuckers. Both pickups selected, vol 10, tone 10. Straight into Lenox Reverb amp, vol. 4.5, treble 7.5, bass 6, reverb 3.8. Recorded with SM-57 slightly off-center of Eminence Copperhead 10" speaker, straight to mini SD card in Zoom H2N. No post-production.

     
  6. King Fan

    King Fan Poster Extraordinaire

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    Nice, sir, again a perfect PR demo. Excuse me if I hang out in the door of the teacher's lounge for a minute more -- I know you guys have the good coffee in there.

    Hey, @ tube geek, did you know our learned friend @theprofessor has been on to these tube parameters for a long time? I shoulda suspected -- he is a tube pro. Back in 2016 he found this nice post from the famous Thom / 'Timbre Wolf' on TGP about mu, gm, etc. Thom is particularly talking about how the 12AY7 worked for Leo in V1, but it raised another question in my mind:

    What combo of parameters make the 12AT7 and 12AU7 good 'drivers' -- I think you mentioned power, but we frosh can't quite get from rp, mu, and gm to power. And do those qualities also make them less good as 'pure amplifier' tubes -- eg, in V1?
     
  7. theprofessor

    theprofessor Poster Extraordinaire

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    Since I haven't yet done a demo with a traditional Tele, I thought I should do one. This is "Ashokan Farewell" -- or at least part of it. I am playing it from how I remember it, so it might not all be correct. I heard a lot of this song as played on a fiddle when I watched Ken Burns's Civil War miniseries in junior high or high school.

    AlNiCo 3 Tele neck pickup, vol 10, tone 10, straight into Lenox Reverb amp, vol. 4.5, treble 7.5, bass 6, reverb 3.8. Recorded with SM-57 slightly off-center of Eminence Copperhead 10" speaker, straight to mini SD card in Zoom H2N. No post-production.

     
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  8. theprofessor

    theprofessor Poster Extraordinaire

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    Hey, no sweat. I wish we really did have good coffee in my teacher's lounge... Anything I know about tubes, I learned from Thom. I have loads to learn, but I guess I could say that I have a pretty good idea of the kind of tubes amongst those typical in old Fenders that can do what I need in various spots. I haven't tried a fraction of those that Thom has tried, but my ears hear exactly what he has described to me. He has a remarkable ability at putting words to sonic qualities. Lots of folks poo-poo this stuff and write it off as "merely subjective." Well it is subjective, of course, but "tone words" are only meaningless if you're not very good at using them. Thom is a master. Here at TDPRI, he was @Opaltone . I haven't seen him around here in a while.

    Here's a nicer copy of a chart like the one @King Fan mentioned above. Thom gave this to me, I think, or I lifted it from a post of his on TGP.

    PreampTubeDataTable_Thom_Opal.jpg
     
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  9. tubegeek

    tubegeek Tele-Afflicted

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    NOTE: I have been doing some edits here for clarity, corrections, and expansions. I'm trying to explain this with the absolute minimum of math and a fairly small number of brain cells.

    1) thanks, @theprofessor for allowing the threadjacking to continue unabated

    2) gm is also known as transconductance. The crucial thing to know is it comes in units of milliamps per volt, mA/V. This makes it the inverse of resistance, which is volts per ampere. (The milli- is just a scale factor but the main thing is the upside down fraction.) There are two names for this category of unit: "mho" (which is "ohm" backwards) or "Seimens."

    As an aside, if you want a unit named after you, you have to start early.

    In other words, how much does the plate current change when the grid voltage changes a given amount?

    This is not the same thing as how much does the plate voltage change when the grid voltage changes a given amount? The answer to that question is the voltage gain. The plate voltage changes as a result of plate current change through the plate load resistor.

    The larger the plate load resistor, the larger the plate voltage change for a given current change.

    Here is the connection, now: the other factor in the voltage change is rp, plate resistance, which forms a voltage divider with the plate load resistor. This is the (dynamic) internal resistance of the tube, and it varies with operating conditions.

    (If you are familiar with load lines, rp can be approximated by taking the slope of the characteristic curve at your operating point.)

    Low rp can be thought of as the tube's leverage on the plate resistor, its effectiveness to cause large current change, because of a minimal loss in the tube portion of the voltage divider. Low rp in combination with high gm will produce a relatively large amplification factor, voltage gain, volts per volt, a unitless measurement.

    The high gm allows the tube to maintain a high mu in a relatively small plate load resistor. This is what we would call the ability to drive a "difficult" load. Another way to look at it is, if we want low rp (to control a challenging load, and preserve bandwidth) and high mu (so we can minimize gain stages) we have to have high gm to create that circumstance, because of the mu=gm*rp relationship.

    Every tube has tradeoffs between these measures as the result of its physical design.

    The focus on low rp is somewhat less essential in voltage gain stages, so we use the high-mu 12AX7 there, and we give it an easy load to drive like a 1M volume control or a tube grid with a proper negative bias and s large grid resistor and we get around its poorer ability to "leverage" the load. But that tube doesn't respond to a voltage change with as much current change as a higher gm tube. So suppose we want to push a wildly variable inductance around or overcome a parasitic, current-hogging Miller capacitance to get into very high frequencies. Or suppose we want to push current into a tubes grid that is biased positive temporarily to avoid hard clipping. This is where low rp high current high gm are useful.

    To be clear: we are not always able to decide arbitrarily what our tube is driving. The special requirements of some loads (I've mentioned grid capacitance and transformer loading as two examples) are something we accept because of other requirements and they may not be possible to fix just by using an arbitrarily large plate load resistor, because they appear in parallel with that resistor or replace it entirely in the case of transformer loading.

    The choice of an infinitely large plate resistor might seem like a great idea, and it is very appealing until you consider the power supply requirements that it implies. ;)

    2a) There is a fascinating analysis of the Bassman/Marshall two stage 12AX7 circuit in Merlin Blencowe's preamp book that makes very good points about how that voltage gain->cathode follower pair gets lovely rounded clipping as the result of the limitations of the tube and a fortuitous circuit arrangement. Must read.

    3) Handy chart! Thanks!

    4) Some measurements of some high-gm tubes (pentodes, pentodes wired as triodes, and triodes) are given at this link. (note that 12AX7 is not included because it's not really in the same league, gm-wise.)

    http://www.pmillett.com/pentodes.htm

    tl;dr : there's more to tube behavior - even in "small signal amplification" applications - than just mu.
     
    Last edited: May 18, 2020
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  10. David Barnett

    David Barnett Doctor of Teleocity

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    That's why they call it a "mho".
     
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  11. King Fan

    King Fan Poster Extraordinaire

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    Thanks for letting me lounge near the lounge! By good coffee, I mean the kind you dons doctor with short carbon fragments. :) And thanks, that table is *way* nicer than the original you see around the internet, which I think Thom himself may have created back in the days of dot matrix printers. Plus I suspect the Pp(W) column is useful in our discussion of driver tubes.

    I took the liberty of pruning the list to preamp nonals I'd heard of, and adding a column of theoretic bandwidth under assumed conditions -- as was done in the original dot matrix table.

    upload_2020-5-18_8-38-48.jpeg

    I recall how you often see 12BZ7 or BH7 for sale on eBay with the note "12AX7 sub." The two tubes are kinda different!

    OTOH, I don't recall seeing 12AV7s discussed much -- those are interesting values for gm, rp, Pp, and BW.
     
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  12. David Barnett

    David Barnett Doctor of Teleocity

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    BZ is rather like two sections of an AX in parallel.
     
  13. tubegeek

    tubegeek Tele-Afflicted

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    A lovely beast!

    A gm perspective will lead you in some uncharted directions - we tend to use the same old same old tubes because that's what Fender and Marshall used, and there are many thousands of tubes, each with their own special flavor.

    I can very clearly remember meeting jc morrison for the first time at Dave Slagle's photography loft where a bunch of DIY tube maniacs were gathered. He discussed the behavior of various tube types as if they were people he knew sitting right there next to him that he knew intimately. jc is a bona fide genius and that was my first impression of him.

    A personal plug:

    A long time ago I did a bunch of DIY phono preamp experimentation, using 6C45P's and later 6N1P's. I learned a lot about what we're talking about here, as I followed my goal of using very low impedance phono filter networks. Here is the web page where I wrote these experiments up, which I was very happy to notice is the second hit when I was searching for "6C45P gm":

    http://home.earthlink.net/~jeremyepstein/6c45riaa.html
     
  14. theprofessor

    theprofessor Poster Extraordinaire

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    That mis-cut on the front right-hand corner of the amp continued to bug me, so I decided to see if I could make it less visible. I think I succeeded! I got out my utility knife, my extra Tolex, and some cutting boards and cut very, very thin pieces using the knife and a ruler, until I got the right piece. Then I super-glued it in. Much better!

    IMG_2334.JPG
     
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  15. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    What mis-cut? I don't see no stinkin' mis-cut...

    Good work professor!

    :cool::cool::cool:
     
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  16. tubegeek

    tubegeek Tele-Afflicted

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    Sl.D - Doctor of Sliverology
     
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  17. theprofessor

    theprofessor Poster Extraordinaire

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    I like that one! I'll add it to my C.V.
     
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  18. theprofessor

    theprofessor Poster Extraordinaire

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    The tremolo sounds great on this amp, but I've got to turn the intensity way up to get it where I want it. I often just turn the intensity straight to "10." Do I understand correctly that I can replace the 1M grid leak resistor to V4, pin 2, with a lower value in order to increase the intensity? And if so, how low would you recommend? 100k?

    This guy, I think. Circled in green.

    Hoffman layout tremolo oscillator detail.jpg
     
  19. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    I may be wrong but I think it would be the 1M from pin1. Just clip in resistors in parallel or put a pot in parallel to come up with a winning value. Then replace the resistor with that new value.
     
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  20. theprofessor

    theprofessor Poster Extraordinaire

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    Ah, yes. I went back and re-read the post I was taking my cue from (https://el34world.com/Forum/index.php?topic=18457.0), and it's the _plate_ of V4 (pin 1), not the grid (pin 2). Thanks.
     
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