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Thread: Ampeg Jet Weird Design

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    Ampeg Jet Weird Design

    Hey All,

    I am coming back to this thing again... I've realized that there is basically only one triode gain stage before the paraphase PI, which I guess does not really provide much gain in this circuit. My main complaint is low volume, though the tone is respectable for the most part. I just feel like it should be louder. But my question for today is,

    What the heck is the purpose of the circled part of the attached schematic? I've never seen anything like this in any other amp. I see the tremolo oscillator, pretty standard, but this seems like a total waste of gain stages here? What's going on? Maybe if this is "useless" I could purloin a gain stage...

    Anyway, thanks for any ideas!

    Justin
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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Supporting Member Randall's Avatar
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    It looks like part of the tremolo to me. It's grid is fed by the plate of the first oscillator triode, and it's output is fed back to the input, modulating it I think. But I don't understand why the plates and cathodes are tied together.

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    It's weird, because it WAS working fine.....

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    That's my question - why the opposite elements are tied together. Seems like an intentional waste.

    I do know this design didn't last long; nor did the 6BK11s in it. But geez, an extra gain stage would be nice.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    First, how does that function? Is it modulating a load impedance on the first stage's anode? That's weird.
    Second, since I don't know how it works, I shouldn't be offering any advice... but unless the oscillator really needs the extra current from two plates tied together, you could put it to use elsewhere.

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    You can't free it up because it is forming "diodes" with the plates and cathodes cross-connected like that.
    Haven't read through all of this, but it's discussed here:
    https://www.thegearpage.net/board/in...ocket.2021824/

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    The trem LFO feeds the grids of the two triodes. That causes currents through the two triodes to vary with the trem pulses. Changing current makes them for all the world a varying resistance. Note this back to back pair are connectd right to the amp input. The trem thus varies the resistance across the input...voila, trem affect on audio.

    If you stole one of the triodes for a gain stage, then your trem would only affect signal on half the trem cycle, ie.either the positive or negative pulse.

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    Quote Originally Posted by Enzo View Post
    If you stole one of the triodes for a gain stage, then your trem would only affect signal on half the trem cycle, ie.either the positive or negative pulse.
    I mis-read the schem. I see now that cathodes and plates are tied 69-style.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    Does the tremolo sound any good? I can't believe that circuit would work very well. Why not drive a fender style Neon/LDR unit with the LFO. Connect the LDR from V2-Pin7 to ground. and free up that dual triode?

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    This way probably works well enough, I don;t recall Ampeg making stinkers. This method uses all parts they have on hand. Dual triode tubes and sockets, basic parts. Fender style means they'd have to make and stock photocell roaches and stuff.

    We'd need to drive the opto. The trem signal coming out from the LFO won't drive the neon by itself, so one of those "extra" triodes would be needed.

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    Quote Originally Posted by dmeek View Post
    Why not drive a fender style Neon/LDR unit with the LFO.
    Patent. Ampeg would have to pony up some licensing loot. To Fender (oh no, the competition!) or whomever, doesn't matter. I use that as the explanation for the proliferation of vibrato & reverb designs in those days.

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    "Does the tremolo sound any good? I can't believe that circuit would work very well. "

    My '64 Jet's tremolo sounds fantastic, very swampy.

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    It's weird, because it WAS working fine.....

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    Thanks for the explanations everyone. To answer whether it works or not, yes. It's not unpleasing, but I feel like I can't give it a fair shae because it's so quiet.

    I've been comparing some old Ampeg schematics & found that MANY of the smaller amps only had one gain stage before the paraphase. There are some that use cathode-biased outputs, a paraphase, and a tremolo section that only uses a singe-stage LFO & maybe one more triode in a somewhat more "traditional" tremolo circuit. I think I'm going to try one of the simpler schemes and then that will free up 1 or 2 triodes for the preamp. I should be able to leave the PI & power anp the same, and there are a couple 2-stage Ampeg preamps out there.

    I took a ton of pictures so I guess if I have to get it back to stock, well, pictures! I'm wondering if te amp is doing its job just fine & the low volume is a product of only that single front-end gain stage.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Quote Originally Posted by Randall View Post
    "Does the tremolo sound any good? I can't believe that circuit would work very well. "

    My '64 Jet's tremolo sounds fantastic, very swampy.
    Is yours the ‘d’ version under discussion here? Earlier jets implement trem with a more common ‘power tube bias variation’ method, later ones use an optocoupler.
    https://el34world.com/charts/Schemat...g_j12a_jet.pdf
    https://el34world.com/charts/Schemat...eg_j12_jet.pdf

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    Coming back for more punishment...

    I'm resigned to "it works." But it's very faint, or, I think so. Given the preamp tubes being $100/each, that's not an option! I do think one is stronger than the other because when I switch the 2 6C10s I get weak or none. So I left in the "weak" one.

    So I see the trem oscillator output is shot into the input. I swapped the input jacks for Fender style though, and also increased the 5.6M resistor on the far left of the schem for a 10M, thinking it was part of a voltage divider? No effect. What do the .1 & 470K do, that are connected to the plate output of the preamp stage?

    Other suggestions to get it pulsing better? I'm going to dink around with it in the meantime.
    Thanks!

    Justin

    Edit: switched 470k to 220k. Sorta better, not much. Next!

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    Last edited by Justin Thomas; 09-20-2019 at 01:44 AM.
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Okay. Last night I swapped out the oscillator caps to slow down the speed. I did 2 of 3; the trem didn't work at all at low speed settings, but once it was a bit higher it would kick back in, but still not strong enough.

    Today I went in & put one of the 2 caps I replaced yesterday back to stock so now there is only one .1 in the oscillator. I spent some time looking at the schematic & decided to try messing with (what seems to me) the voltage divider feeding the other 2 sections, the "diodes" that started this post in the first place. I just clipped in various values that ended up with the total being anywhere from 390k down to about 200k.

    The stock resistor is now back, the 470k/10k network feeding the "diode" grid. I get only a few decemt-depth pulses of trem when I click the switch on before it dies out, so now it seems the oscillator does not oscillate enough. I replaced the cathode bypass cap on the oscillator as a possible, no change. So right now, the only changes are:

    1. One .05 has been changed to .1.
    2. One 470k resistor (near the input in the schematic) is now 220k.
    3. The 5.6M near the input is 10M.
    4. I wired in Standard Fender-style inputs, as neither of the originals were shorting-type.

    One other change, I removed the wires for 6BK11s that grounded the internal shieldd of those tubes. The 6C10 says pin 8 is an internal connection, but I can verify that in MY 6C10s, there is absolutely nothing connected to pin 8, even though the data sheet specs "Internal Connection." Not on mine! I don't like having useless wiring lying around in my amp, especially sincexit was connected to ground with cathodes fronm other sections. The amp actually seems less hummy now as a result.

    Please don't suggest new preamp tubes - the 2 6C10s I have are definitely different and if I swap them I get nothing for trem. Last night I had steady-yet-shallow. And no, I am not going to spend hundreds on new 6C10s!

    Thanks for following on my mad adventure while my mucking about continues.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    My explanation of the tremolo circuit:

    The input tube V2 is wired as an inverting amp with voltage feedback from plate to grid. The first V1 is the LFO that drives the grids of second V1, which act(s) as a variable/controlled resistance which in turn modulates the gain/feedback of the input tube V2 by variable voltage dividing.

    The 69 wiring of the 2 triodes of the second V1 makes sense as there is no DC current. When the plate signal of V2 goes positive, only the right side triode of the pair conducts while the left triode is blocked. When signal polarity reverses the left triode of the pair conducts and the right one blocks. So we get a modulated "bipolar" impedance.

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    That makes sense to me, as best as my limited understanding can make. I've been experimenting with what I see as voltahe dividers to try to make the trem deeper; changes to schematic are as listed above. I've mostly been trying to lower the laeger portions of what I see as voltage dividers. Any other suggestions? As it is now I get a few good pulses then it dies...

    Overall the amp siunds good so I want to get it right; I'll keep mucking around as I think & document any further changes here. The only suggestion I'm not open to: new preamp tubes. Just go price me some 6C10s or 6BK11s & you'll see why!

    Thanks Helmholtz, I'll apply your theory to make tweaks.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Is that cathode bias voltage changing with the first few pulses?
    It may be worth trying LED biasing (rather than cathode resistor and bypass cap), but need to find a suitable LED (or stack a series pair if more bias voltage is needed).
    Is the F node HT decoupling ecap definitely good?
    The lowish resistor values in the phase shift network may be loading the stage excessively, thereby reducing its gain. Instead of bigger value caps, try higher value resistors, especially that 1st 120k after the plate.

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    Thanks pdf64, I'll check out what I can later this afternoon. I think I'll put all the resistors back to stock & start again.

    Justin

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Okay, I've put everything back to stock except for the Fender-style input jacks but still using 47k input resistors & no 100k on the one jack.

    I can get regular tremolo now, with the right tubes in the right position. The volume issues have sorted themselves after I threw a new can cap in it, and hum is greatly reduced.

    In answer to pdf64's questions,

    The oscillator is pulsing steadily. Cathode voltage reads between 1.3 & 1.65V to ground regularly. The AC on the filter cap node is .015V, though I don't know what you mean by "F." I just see A, B, C?

    I don't have any LEDs handy but might be able to get some. If not I'll have to order. The trwm is now regular, but weak - really weak. More like "I left the ceiling fan on in the room" weak. But it's there. So if I could build some strength/depth, that'd be cool. I'll futz with the two 120k resistors in the oscillator meanwhile, & report back if anything changes.

    Justin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    The forward voltage of LEDs depends on color, increasing from red to yellow to green to blue. Red ones are between 1.6V and 2.2V. The required 1.5V lies in the voltage range of infrared LEDs.

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    Quote Originally Posted by Justin Thomas View Post
    I don't have any LEDs handy but might be able to get some. If not I'll have to order.
    You could use a couple of ordinary diodes (not LEDs) in series.

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    It seems I'm getting the bias I need as is. The schematic shows 1.5V, and my average between extremes is 1.46 according to my meter. Are you saying I would want the average to go up? As in use 3 diodes for 1.8? More? I've git lots of regular diodes... They're all 1000V/3A, but, hey...

    One other question I do have: with the tremolo switch actually clicked to "off," the voltage on the cathode steadily rises. I click it off & it goes to 10VDC & rises to 20. I removed my meter then. I've got a 25V cap as the bypass cap & it hasn't blown up yet, so that's good. I just wonder how high would it go?

    Ugh, so much left to learn...

    Jusrin

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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
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    One other question I do have: with the tremolo switch actually clicked to "off," the voltage on the cathode steadily rises. I click it off & it goes to 10VDC & rises to 20. I removed my meter then. I've got a 25V cap as the bypass cap & it hasn't blown up yet, so that's good. I just wonder how high would it go?
    I don't think the voltage actually rises if you don't connect your meter. If you do, the tube uses your meter resistance as a large value cathode resistor causing a small cathode current which charges the cap until some equilibrium is reached.
    To test switch off the tremolo, wait some time and then shortly connect your meter.

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    So I tried upping the two 120k resistors in the oscillator. No help there... I did one first, then added the other. I'll try them separately (one at a time) later tonight.

    Justin

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    The first thing I would have tried to increase tremolo intensity was to increase the cathode cap to say 220µ. The intent is to increase very low frequency gain. Ideally the cathode voltage should not vary with oscillation.
    This might cause a little turn-on delay, though.

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    Thanks! Ill tray anything at this point. Not sure I've got a 220uF but I've dwf got bigger than that...

    Justin

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Returned the resistors back to 120K AND put in a 250uf/25V cap I had. No love. I may just start rebuilding the tremolo circuit one component at a time... All the resistors test well within spec & I've replaced nearly every cap in it. So I'm outta guesses.

    Thanks to all, & if I stumble across the answer I'll post it here!

    Justin

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    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    2. One 470k resistor (near the input in the schematic) is now 220k.
    Which resistor is this? If you increased one of the 47k grid stopper to 220k, this will reduce input gain by a factor 4 (and consequently reduce tremolo intensity). With this inverting amp wiring the grid stoppers are part of the feedback network/voltage divider.

    Unfortunately I don't see a possibilty to increase input impedance without decreasing gain with this circuit.

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    Hey Helmholtz,

    Iwas these two I tried lowering. I tried them separately. The one specifically that you are asking about is the circled one on the left; "near the input" is just a vague location because my schematic doesn't have designators. I'm also working from my phone here so am a bit limited & hafta get creative sometimes.

    I haven't done anything to any 47k resistors. And currently everything except for the input jack wiring & trem oscillator bypass cap (now 250uF) is like the schematic. The grid stops are still 47k. Nothing I'm trying really seems to change much of anything.

    Justin
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    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    Lowering the left 470k will increase feedback and reduce input gain.
    Instead of lowering the right 470k I would try to increase the 10k grid resistor to increase modulator drive.

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  32. #32
    Bent Member Chuck H's Avatar
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    Have you considered trying a 6C10 tube in V2? That wouldn't fix the trem issue, but it would increase gain a bit. Slightly more affordable than the 6BK11 tube too.

    As to the trem issue, I don't even have my head wrapped around the circuit sufficiently. As Helmholtz mentioned, raising the resistance of the 10k shunt resistor would seem to make more sense than decreasing the 470k series resistor.

    EDIT: Nevermind, I see you have 6C10's now and don't want to purchase more.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Thanks Chuck,

    Yeah, when the 7591s are the most easily sourced tubes in your amp, you have a problem!

    I can sort of figure out the trem circuit by folks' descriptions above, and it sorta makes sense. But I'm sort of resigned to this thing just being what it is, and that there's a reason Ampeg abandoned it! (cuz it don't werk so good).

    I've upped the 10k as Helmholtz suggested; tried everything from 20-50k. Settled on 22k because it gave marginal improvement but going higher didn't seem to do much more. The amp itself aounds quite nice, better when I get a proper speaker in it. I've learned the hard way, Ampeg OTs do not like a mismtch & being asked to play loud.

    I think that this trem circuit is kinda like that "local negative feedback on the power tubes" in my AB165 Bassman: "well, that was fun, but we won't do it ever again..."

    I'm gonna clean up the wiring a bit, put the back on it, enjoy it as is, & stay open to any more suggestions that don't involve new tubes or silicooties. Diodes okay.

    It's also interesting to me that Ampeg never moved to the LTPI & stuck with cathodynes & paraphases for so long.

    Justin

    0 Not allowed! Not allowed!
    "Wow it's red! That doesn't look like the standard Marshall red. It's more like hooker lipstick/clown nose/poodle pecker red." - Chuck H. -
    "Of course that means playing **LOUD** , best but useless solution to modern sissy snowflake players." - J.M. Fahey -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

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    The input triode is wired as an inverting amp with negative voltage feedback between plate output and grid input. Its gain is (mainly) determined by the ratio of feedback series impedance between plate and grid (essentially 470k+47k at audio frequencies) divided by input impedance (47k+guitar impedance). The modulator tube taps the impedance between plate and grid, shunting part of the feedback signal to ground and thus periodically reduces feedback thereby periodically increasing gain.

    Consequences of this wiring are low input impedance (47k+ guitar impedance) and low gain.

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    FYI the benefit of diode biasing is that they're effectively 'self bypassed', right down to dc (and so full gain down to dc), so no worry that the bypass cap isn't big enough or that its ESR is too high

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