And what about your second DIY amp?
When you finish this one, you're going to have all those lovely leftover valves, and all that hard-earned knowledge and experience. You're gonna have to do something with all that.
Not that I would ever push you over the edge and into the amp-building abyss, of course.
-Gnobuddy
The input (pentode) stage looks a lot like the one in the 1960 VOX AC15, the one that VOX lifted straight from the back of a valve catalogue, which was plagued with microphony problems in guitar-amp use.
Maybe I missed something, but the only change I've spotted is the value of the cathode resistor, which seems to have been 2.2k in the original VOX amps.
The far-too-big 25uF pentode cathode bypass cap also shows the circuit's Hi-Fi roots: for guitar, an appropriate full-bandwidth value would be 3.3uF or 4.7 uF, five to eight times smaller.
IIRC, 65 Amps (boutique company most known for Peter Stroud, Sheryl Crow's guitarist, being one of the two owners) also released a number of guitar amplifier models which borrow the same concept, and often, nearly the same circuitry. They had an amp called the "SoHo" which, IIRC, used an EF86 input stage, and a pair of EL84s as outputs, just like your current project. (But the SoHo had 12AX7s in the middle, unlike your Webster.)
I wonder why the Dr. Z Route 66 was discontinued - microphony problems, lack of sales, something else?
In possibly related news, about ten years ago, I remember reading a number of Internet threads in which a few owners of new "boutique" pentode-input guitar amps complained about endless problems with microphony. The typical lament read something like "A new EF86 cured it, but only for two weeks, then the howling and screeching came back."
It does seem as though many guitarists (and guitar amplifier builders) fall in love with the sound of that EF86 input stage, only to bump up against exactly the same microphony problems that plagued VOX sixty years ago.
I haven't had any microphony problems with my 6JW8 pentode preamps, but I design the pentode stage for relatively low voltage gain, I tend to play at relatively low SPL, I don't play with high gain distorted tones, and I put the pentode near the end of the preamp signal chain, where signal levels are bigger and there is less gain after it to amplify the tiny microphony signal and cause problems. Maybe I've just been lucky, as well.
Good luck!If I can finish those changes this weekend we'll find out soon enough
-Gnobuddy
And what about your second DIY amp?
When you finish this one, you're going to have all those lovely leftover valves, and all that hard-earned knowledge and experience. You're gonna have to do something with all that.
Not that I would ever push you over the edge and into the amp-building abyss, of course.
-Gnobuddy
Haha you machiavellian genius you
I'm wise to your strategies, leading me down the path of no return, into the diy amp building abbyss. But I've already beat you there lol 
I have a lead on my next amp project already but I'll keep it a secret for now until I know the components needed come through okay...As for the unused tubes in this amp, I think I'm going to leave the other two mic channels intact with their input tubes. I have a functioning input transformer and could run a microphone into the amp for a wild distorted harmonica mic type input. Not anything that would sound nice but it would be fun to play with. Got the idea when I remembered Jack White doing something similar in the video documentary "It Might Get Loud".
Jack White's Shure 520DX Green Bullet Blues Harmonica Microphone | Equipboard(R)
And I may try to do something else with the third channel as well. But if I don't have any ideas for that then another EF86 preamp build could be in the cards. In the distant future of course, I really am trying to keep to the one project at a time rule for now... except for the next one I'm starting to buy stuff for already... Oh boy
I might be in trouble lolMine is 6L6GC’s for output, but there are definitely similarities in the preamp design
I’m not sure but I’d guess more toward the sales theory? They also made a bigger and smaller version under different model names. KT-46 and Z-28 I think. All with the same preamp. But I think they only came as head units which can help keep the EF86’s away from too much vibration. I don’t recall hearing anyone complain about microphony, but it’s certainly a possibility. They didn’t have a whole lot of clean headroom either so maybe the sales suffered because they where 1 trick ponies with just gain, and more gain to offer?
Well some quick google searching this morning proved my memory wrong on both counts from these statements yesterday...
Looks like the smaller version (Z 28) came as a 1 x 12 combo, 2 x 10 combo, and a head. It's bigger brothers (Route 66, and KT-45) were only head units though.
Also a quick google search for "Route 66 microphonic" provided all kinds of results from people looking for help with EF86 related trouble.
Man am I glad I found my 6AU6 conversion basses for this current project, here's hoping they hold up a lot better than EF86's
It seems to be the same story for every guitar amp that uses the old VOX AC15 EF86 input stage unchanged, whether expensive "boutique" amp, or budget DIY build.
After a while, reading about all these problems with EF86 microphony starts to feel like hearing from the neighbour who complains that his dog has dug up the same rose-bush from the same location ten times already, and he has to keep on re-planting it. Tragic and funny and hopeless, all at the same time.
I have a hunch that physically smaller RF pentodes, with their lighter electrodes, shorter supporting wires and rods, and correspondingly higher structural mechanical resonance frequencies, may be more resistant to microphony. Basically the moving parts inside are lighter and shorter and stiffer, so they should be less prone to resonate and vibrate at guitar frequencies.
Fortunately for us DIY types, many RF pentodes tend to be quite unloved and unwanted, because Leo Fender didn't use them in his guitar amps and D.T.N Williamson didn't use them for Hi-Fi. Which means NOS ones can still be found inexpensively, though probably not for much longer.
-Gnobuddy
Let's hope E-moose's isolation mounts work for him!
I found a clean schematic of the VOX AC15 EF86 input stage at SluckeyAmps.com, with DC voltages on it. That let me work out the operating point, and some other details (in the attached image.)
The stage has a measured voltage gain of 220, and uses a 220k anode load, that tells us the transconductance is 1 mA/V at this operating point. This in turn tells us the effective "internal cathode resistor" is 500 ohms; and that tells us that the external cathode bypass cap should be about 4 uF for full guitar bandwidth. (The original value of 25 uF was obviously intended for Hi-Fi, not guitar, so bandwidth goes down below 20 Hz.)
The original 220k anode load drops most of the available DC voltage across itself; about 205 volts across the resistor, only 95 volts across the pentode! This is more clear evidence that the circuit was never designed for guitar - it's going to clip extremely assymmetrically, with negative swings at the anode clipping long before positive swings. The designer was obviously going for maximum possible voltage gain, and expected input signals would be very small (much smaller than modern guitar pickups can put out.)
Working backwards: the anode is at 95V, and might be able to drop as low as, say, 35V. That's a 60V swing, so maximum unclipped output signal is in the ballpark of 120 volts p-p. Divide by the voltage gain of 220, and that means the maximum input signal (before the output clips) would be less than 500 mV pp. Guitars with humbucker pickups can put out two to four times that much voltage.
It seems the high voltage gain is also part of the reason for the microphony problems. High gain means high input sensitivity, so the electrodes don't have to vibrate much to generate a signal comparable in size to the input signal, and get the amp howling.
So if we can lower the voltage gain, we might win some ground on two fronts: there should be less unwanted clipping of the input stage, and less microphony.
Thanks to the lop-sided bias point, this looks very easy to do with minimal changes. Simply splitting the 220k anode load into two (120k + 100k) and bypassing the junction to ground with a capacitor should drop the voltage gain to around 100x, instead of 220x. It will also reduce the extent of the assymetric clipping, though I can't tell how much without building and testing it.
The second attached image shows the rather slight changes that I think might make a flubby, woofy, howl-prone, EF86 input stage a bit better behaved.
-Gnobuddy
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Sure thing, very true. Too clean may not be good. Permanently dirty with no accessible clean tone isn't good either, though. Most of the EF86-input amp demos I've watched seem to fall into that latter category, turning them into one-trick ponies.
So where does ElusiveMoose22's amp fall? Since it depends on his guitars and his musical preferences, only he can tell us. If the input proves too "dirty" for his taste, or if microphony rears its ugly head, then the ideas I outlined might help.
The thing that interested me about this old circuit is that Vgk is actually well over 2V, so clipping at the input (control grid) shouldn't really be a problem. It will be the output that's clipping, because of the design choice of very high voltage gain and very offset operating point. (Neither of which, we should keep in mind, was optimized for guitar.)
-Gnobuddy
I finally have some answers to these very questions!
Read on for those answers and more: After working several hours Sunday and then finally clearing some space at home to bring the amp back and do some final tinkering on Monday I am pleased to announce that the mod to Fender AB763 style tone-stack is a success!
I'm much happier with the sound I'm getting out of it now, the Midrange and Bass frequencies are much more under control. Here's a low gain test with single coils:YouTube
And here is a higher gain test with a bridge humbucker:
YouTube
Please take into account that these vids were from a spare room at home this morning so I was only able to test at bedroom volume levels. I don't really have the output section of the amp working too hard at these volumes so all the gain and compression you're hearing is coming from the preamp. As a result the higher gain is a little fizzy sounding in the second vid.
Now to answer the dirty gain/versatility question: This amp is incredibly versatile! When I'm able to open the master volume all the way I can get fully clean output plenty loud enough to gig with, or push it a little more and get an edge of breakup sound. But I can also dial the master volume back to 7-8 and push the preamp volume even more and get some medium to high gain sounds also at gigging volume. It's not quite cranked Marshal territory, but I'd say it's pretty close to a classic AC/30 going full tilt. Some real high end sizzle but still a fairly thick and chewy distortion. I'll try to post a vid of this next time I know my neighbors aren't home and I can get away with the volume lol
I was worried that 2 pentode stages in the preamp would put out too much gain not leaving enough clean headroom for a gig-able clean sound, but that's not the case at all. I'd conjecture this is probably for 2 main reasons, the first being that there are pretty sizable voltage divides attenuating the signal down after each pentode stage, controlling the gain a little. The second reason would be that I have the 2 6L6's in the output section which will also overdrive but want to stay cleaner longer than EL84's which I see most often paired with EF86 preamp designs. I am just guessing here, but those reasons make sense to me. Also the negative feedback loop originally designed for clean Hi-Fi application probably helps keep things quite a bit cleaner as well.
At any rate I'm pretty happy with this overall design and the results I'm getting so far
. I will need to tinker with a few things here and there, but the amp is definitely at a usable stage right now. I'll need to play it a lot more, run pedals through it, and get it out for some gigs before I know what else might need tweaking, but I'd say it's probably 95%-98% there 
Tell me what you guys think, I'm curious to know other peoples thoughts
Tried some pedal combinations this morning to see how the amp takes them. Four pedals on in this vid and the amp still has plenty of headroom and dynamics stay in tact pretty well even with multiple effects running:
YouTube
I'd say it takes almost all my pedals really well except for my tube screamer and klon style pedals. Even after scooping and taming the midrage with the new tone-stack all my drive pedals with a pronounced mid hump sound just a little too honky. They would still be usable in certain situations with single coils, but probably not usable at all with humbuckers. I think it's the pentodes in the preamp just wanting to push frequencies around 500hz-800hz in front of everything else. They grab that mid boost from a tube screamer or klone and run with it lol. Still experimenting though so we'll see what other effect do and don't work well and if the amp can be tweaked to be friendlier with all of them.
YouTube
I'd say it takes almost all my pedals really well except for my tube screamer and klon style pedals. Even after scooping and taming the midrage with the new tone-stack all my drive pedals with a pronounced mid hump sound just a little too honky. They would still be usable in certain situations with single coils, but probably not usable at all with humbuckers. I think it's the pentodes in the preamp just wanting to push frequencies around 500hz-800hz in front of everything else. They grab that mid boost from a tube screamer or klone and run with it lol. Still experimenting though so we'll see what other effect do and don't work well and if the amp can be tweaked to be friendlier with all of them.
Firstly, congratulations! It looks like your foray into guitar amp building has paid off handsomely.<snip>
I'll need to play it a lot more, run pedals through it, and get it out for some gigs before I know what else might need tweaking, but I'd say it's probably 95%-98% there
Secondly, you are now teetering on the brink of the "Shall I tinker with it a little more?" precipice. If you are wiser than some of us, you can make a quick trip down the cliff face and into the valley, and climb back out before you get trapped.
But others among us (nobody I know, of course
) have a splash of perfectionism (which is classified as a depressive disorder!) or a touch of OCD, and that makes it much harder to know when to stop and leave well enough alone. Once we climb down the cliff, it's quite hard to get back up!So where do you go from here with your amp? That's entirely up to you, since the logical thing to do is to start by addressing the problem that most bothers *you*. As you say, start by using the amp, and finding it's strengths and weaknesses.
One thing to consider is that a "dirty" preamp can benefit from following it (rather than preceding it) with delay, reverb, or modulation pedals - which requires an FX loop between preamp and power amp. This can tame unwanted harshness from the preamp (but you may not think it's harsh, in which case this is a non-issue for you.)
You have so many extra valves on the chassis that an all-valve FX loop is a possibility, if you want to go that route. (Normally I'd advocate for MOSFETs for this particular purpose. Smaller, lighter, quieter, cheaper, no heater power needed.)
The subtler remaining issues you mentioned are going to be increasingly harder to tackle without more measurement capability. For instance, the issue with the mid-hump overdrive pedals: the first question to ask is, where is the notch in the tone-stack response? If it more or less lines up with the mid-hump in the overdrive pedal, then you get the intended result. But if the notch and the hump are at two different frequencies, then you get a weird Loch-Ness-monster frequency response, and who knows what that'll sound like.
I remember something in Merlin Blencowe's guitar preamp book about using a variable slope resistor in a Fender tone control circuit to adjust the notch frequency up or down. It might be worth the experiment, to see if it makes your overdrive pedals sound better. Temporarily replace the slope resistor with a pot, to see if moving the notch around makes any difference to how the amp sounds with your FX pedals.
-Gnobuddy
Thank you for saying so! It has been quite a learning process (heck it still is a major learning process haha), but I've enjoyed it a lot and the sense of accomplishment in getting this far is my favorite thing about this amp
Also thank you again Gnobuddy for the continued input and direction, you've been a huge help!
Sounds like pretty sage advice to me. I think I naturally fall somewhere in between on this spectrum of never done OCD and knowing when to stop while I'm still ahead lol. I know there will be a point where the cost in time spent tinkering won't be worth the results it yields and I'd like to stop before I get there. But also I'm still learning and could see myself crossing that line while chasing the tone in my head that I think this amp should be able to produce. We shall see how it goes haha...
Brilliant!
I think this is probably the best place to start and easy enough to do Thank you for that tip!I agree, it's a wonderful feeling to create something yourself. Amps are particularly fun, because they "come to life" in a way. (I've also had that experience with a few other things.)It has been quite a learning process (heck it still is a major learning process haha), but I've enjoyed it a lot and the sense of accomplishment in getting this far is my favorite thing about this amp
I'm glad to have helped - that's another nice feeling, to help someone along their journey!
Sorry if I've been sounding like Chicken Little, warning that the sky is about to fall. My wife and I are both creative and we both have hobbies and interests, so we understand each other, and it's part of what we love about each other. But I've also seen couples where one person had an absorbing hobby or interest, and the other hated him / her for it, causing endless friction and misery for both partners.
I'm starting to tackle a new project myself, but not an electronics one per se. This one involves cats...
By the way, sometime in 2018 I simulated the entire working circuitry of a Klon in LTSpice. As the attached image shows, the Klon "mid hump" is centred at 800 Hz...that might prove useful in tweaking your tone stack, perhaps.
If you post the schematic of your tone stack, I can run that through LTSpice too, so we can see where the notch is, and maybe what component values to tweak to move it to 800 Hz.
-Gnobuddy
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There’s some real life wisdom in that and I appreciate you sharing it on a forum like this. I suspect that not enough people are putting the time, effort, and sacrifice necessary to sustain a marriage into their relationships these days and the percentages of marriages ending in divorce might support that theory. I know I learned some lessons like that the hard way when my first marriage ended poorly. Fortunately I’m making a much better go of it the second time around
. As for these passion projects I take on, my wife works some weekends so I try to keep my work on projects to the days she’s working and we keep her free weekends open for family time and couple time. The projects take much longer but I don’t get lost in them and stay tuned into her needs and then our family’s. I’m not a cat person at all (we do have a dog) but now I’m curious what your project is haha.
That’s interesting, I think I remember reading that the Klon treble knob began its sweep around 400hz or so as well so the mid hump can increase as treble is boosted or decreased a bit if treble is cut? Does your model confirm anything like that? Also my klon style pedals are but lowly klones lol so the circuit would probably be similar but not exactly the same. I’m working on finding some schematics for them, but also I’ll work the amp end too. I want to see how close I can figure the effectiveness of the new tone controls in shaping the amp as is before I go tweaking more. Though if I do tweak more I’ll start with that 100k slope resister you pointed out earlier. Just a quick mess around in the tone stack calculator showed that can be quite effective in adjusting where the lowest valley of the mid scoop falls.
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I agree. Time, effort, and sacrifice. Also compromise, and willingness to listen, and to learn, and to forgive. And kindness. Lots and lots of kindness!
We seem to be in an era where many of us care more about our "stuff" than about our relationships. The prenuptial agreement is the ugliest, most extreme proof: a legal document that spells out "I want my stuff more than I want you, so I'm making sure I keep my stuff when I lose you."
I grew up with both cats and dogs, and loved and enjoyed both as a kid. Also squirrels, birds, shrews, garden lizards, preying mantis, dragonflies, birds, grasshoppers, garden toads, and all the other wonderful and beautiful little creatures that used to be all around us before we wiped most of them out and pushed the rest to the brink of extinction.
It's been years since I've seen grasshoppers leaping out of my path as I walked across the grass - a daily occurrence when I was a boy. I see a few dragonflies a year, instead of dozens every summer day. Same thing with butterflies and squirrels. No more garden lizards at all. Five years ago I still saw racoons in this city. In the last three years, I've only seen racoon roadkill.
We had both cats and dogs a few years ago, when we lived in Los Angeles. Back then, we had a back yard for them to play in. Now we're in an overpriced apartment in a city that's rapidly turning into a cement-and-asphalt lake, and I really don't feel it's a good environment for any pet bigger than a mouse. But cats are long-lived, so we brought them with us, and here they are, stuck in a tiny apartment with us.
The new project? One of our two cats has an auto-immune disorder that makes him throw up a lot. We found we could ameliorate his condition by feeding him just a few grains of cat food at a time, at twenty-minute intervals. This means we now spend about five hours a day feeding the little beast, which is wearing down our patience, and keeping us from ever getting away from home for more than a few hours.
So the project is to try and build an automated cat-feeder that can take over the job. I've got a few ideas, and just went out and bought a few supplies, and hope to start building something soon.
This is half mechanical engineering, not my forte. It might turn out to be a complete failure. But I hope not!
I went back and tweaked my model so this time LTSpice twiddles the virtual "Tone" pot rather than the "Gain" pot. The result is attached. As you said, the hump goes as low as 400 Hz, and as high as 2 kHz!
Same here, the only Klonish thing I own is an Electro Harmonix Soul Food.
Truthfully, I believe most of the pedal "cloners" don't have the knowledge to improve a circuit, and are far more likely to just reproduce it blindly. IMO, chances are the cloned circuits in your pedals are, in fact, exactly the same as the Klon.
I know there are all sorts of lurid Klon-fantasies about incredibly rare and expensive magic diodes made by tame unicorns in Tsarist Russia and blessed by Nikolai II himself before being sealed in a hermetic time-capsule. The guy who created the original Klon spread a lot of those stories himself; whether savvy marketing or uninformed self-delusion, I don't know.
But Bill Finnegan didn't know enough electronics to design the pedal he wanted - he had to hire someone else (supposedly two MIT engineers) to do that. So Finnegan wouldn't have the knowledge to know that there are no magic diodes - whatever magic the Klon has, is the result of clever circuit design by those two MIT engineers. Finnegan supplied the good musician's ears, and the hired engineers managed to create the sound he wanted.
-Gnobuddy
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Fortunately, our friend the difficult-to-locate-extremely-large-member-of-the-deer-family has already progressed well past that point.
-Gnobuddy
A worthy endeavor to care for your feline friend. I wish you luck!
That is interesting, quite a bigger sweep than I thought even!
One of the ones I own haha! A pretty decent and cost effective version for sure. I have a couple different klones as well, I used to run 3 pedal boards for different gigs and had 1 on each. I don't need 3 anymore but kept them all because they all sound just a little bit different. I don't buy into the hype or use them as some "always on" magic sound fixer, but they do a thing with that mid hump and treble boost that can be very usable in certain situations haha.
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