AX84 BBS Threads with new messages as of Thu, Oct 30th, 2014 @ 13:42:04 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com Adventures in Digital Control(by Chris Hurley) As some of you know, I've been thinking about making an amp or preamp with some amount of digital controls for a while. I certainly enjoy the traditional feel of an amp with regular knobs but I also enjoy tinkering outside the box a little bit. Some things are really easy to bring under the micro-control. Muting part of one channel while unmuting another is easy with LDRs or other switching devices. Cathode bypass caps should be easy to switch in and out. The beauty of the microcontroller is that you can pretty much make it bring high or low any low-current on/off device that can run on 5v in any order that you command. I immediately start to think about all the things I might switch in or out of a circuit- bypass caps, shunts to ground to reduce gain throughout the circuit- maybe even whole tone stacks. The sky is the limit. Yes... I know... Some of you just prefer to turn a knob and "who would ever want a computer in the amp?" I get that. This thread isn't for you- its for people who are interested in doing more than that for whatever crazy reason. I can imagine collecting up various groups of switch settings into channels. "Turn off all the bypass caps, attenuate the gain and activate tone stack A" could be a clean channel. "Turn on all bypass caps and activate stack B" could be a lead channel. With MIDI, all of these things could be controllable as continuous controllers- easily. One of the harder things for me to handle so far has been digitally-controllable interstage attenuation (i.e. pots). Here's what I've either tried or researched: * Controlling the current to a MOSFET for the purpose of draining more or less preamp audio signal to ground or as the bottom part of a voltage divider: It works but it distorts the signal. Presumably because the MOSFET acts as a diode and only conducts one half of the signal. Evidently, you can put a pair of MOSFETS back to back to pass AC signals but it appears the behavior between all the way off and all the way on varies wildly between units. That would mean you'd have to match pairs of them. I assume this does something bad in terms of load to the preceding stage too. * Using photoresistors (LDRs) in a pair as a potentiometer. These also seem to vary wildly so that X milliamps of current on one unit might produce 220k of resistance while the same current on another produces 470k of resistance. More matching. They could be used as switches- the best ON resistance I can get is about 300ohms. * Digital potentiometers: Where to begin? They're generally limited to the IC supply voltage, so you have to attenuate the signal to 5v or whatever it is before the pot. Thats bad enough but then you also can't have a signal that goes below 0v (which our AC-coupled audio does) which means you'd have to reference the signal to a positive DC around 2.5 volts. Of course, you have to amplify it back up again. * Resistor ladder with relays or mosfet SSRs- this will probably work but it gets big and expensive in a hurry- the more steps of attenuation you want, the more resistors and relays you need(*). * turning on bypass caps and connecting hard-wired voltage dividers in groups. This would work but it isn't very granular. Am I missing some obvious way to control gain between stages via a digital device? Perhaps there are other ways I should be considering. (*) I've been working on a board design for such an animal- I made a 10 position "digital" potentiometer with a switchable bright cap. The microcontroller on the board would be sent simple up/down commands to change which relay (wiper) was active. If anyone wants a link to the oshpark shared PCB project, let me know. Thu, 30 Oct 2014 13:23:37 -0400 http://ax84.com/bbs/dm.php?id=486908 tonight's random? picks(by Kursad K) <a href="http://www.youtube.com/watch?v=6xxr3-CBams " target="newwin1414690925">[link]</a> <a href="http://www.youtube.com/watch?v=PptHWHfnCnY " target="newwin1414690925">[link]</a> <a href="http://www.youtube.com/watch?v=xHNVK9NY4JE " target="newwin1414690925">[link]</a> <a href="http://www.youtube.com/watch?v=0sML3fbgiA0 " target="newwin1414690925">[link]</a> <a href="http://www.youtube.com/watch?v=vxuhNmjnK9M" target="newwin1414690925">[link]</a> Thu, 30 Oct 2014 12:37:30 -0400 http://ax84.com/bbs/dm.php?id=487091 advice on building and designing amps for a living(by CChurchill) I fell into this rabbit hole of amp, guitar, and effects building about five or six years ago. To make a long story short, building tube amps quickly became as much of an passion as playing guitar. I've been reading everything I can get my hands on regarding the subject. I want so badly to build and design amps for a living, and the thought of doing anything else blows(quite frankly). It can be frustrating learning on my own, but there aren't many job postings for Tube amp apprentices. I'm looking for any advice I can get. Thu, 30 Oct 2014 11:35:18 -0400 http://ax84.com/bbs/dm.php?id=486788 Firezog and headphones(by DutchMarco) I was wondering about the Firezog mod. Would I be to be able to use it with a headphone? Or would it blow up a headphone? (I only have 9mm earbuds, not the shell models like used in studios), and the same question in amps other than the Firefly, does tthe same schematic work OK? Also: is the 1M LineOut Level pot audio taper, or linear? I'd guess it is Audio taper BTW, I was referring to <a href="http://web.archive.org/web/20050627183340/home.cfl.rr.com/dbhammond/Firezog.gif" target="newwin1414690925">[link]</a> Thu, 30 Oct 2014 10:41:43 -0400 http://ax84.com/bbs/dm.php?id=487104 Inkscape for Panel Layout(by Tim Mc) I'd like to use Inkscape to design some front and rear panels. I'm struggling with how to draw a circular legend for knobs like a volume or tone control. I imagine there's a tutorial somewhere but I don't know what terms to use when I search. So, if I wanted to draw the legend for a knob with say, the numbers 0 to 10 around the outside, or perhaps just 10 little "tic" marks spaced evenly over a 270 degree arc what do I call that feature so I can search for a tutorial? It won't hurt my feelings if someone has a direct link or instructions either!! Thanks, TimThu, 30 Oct 2014 09:22:41 -0400 http://ax84.com/bbs/dm.php?id=487086 Wood working shop(by tgray) I'm interested in making a few P1 type amps but I don't want to get some of the wood working tools to do a good job. There are several reasons for this, the main reason is that I don't want to risk cutting my fingers off with a saw or router. I'm wondering if there are any wood working shops that would cut pieces of wood for me and round the edges etc... I think I could assemble the cabinet but I don't want to buy a big table saw and router. I'd like to find a place that could cut wood to my drawing. Anyone ever done this or know where I might try to get this done? tgrayThu, 30 Oct 2014 06:35:57 -0400 http://ax84.com/bbs/dm.php?id=487109 OT primary impedance(by tgray) Does anyone know the consequences of running a tube amp with a mis-matched output transformer impedance? For instance, if you don't hook a speaker up to the output and leave it open circuit. With infinite impedance on the secondary will that make the primary impedance go very high, and if so does that cause harm to the OT or the tubes? On the other side of the coin, if the output is shorted what harm will that cause. Also, on a multi-tapped OT (4,8,16 ohm) if you use the 16 ohm tap but hook up a 4 ohm speaker, will that cause harm to tubes or OT? tgrayThu, 30 Oct 2014 04:37:59 -0400 http://ax84.com/bbs/dm.php?id=487058 Bias and Hum(by JB Angelo) I get a hum in my power amp that goes away when i adjust the bias. Let's say i slowly adjust the bias and all is good. I then turn off the amp and turn it back on and sometimes i'll get a hum that goes for about 3 seconds then quiets down. Sometimes it just comes back and i need to readjust bias. my power tubes are biasing about 70% on tube one and maybe 50% on tube 2. I'm suspecting a tube imbalance. If i were to install separate bias pots where i can make the bias the same on both tubes, would that solve this? thank youThu, 30 Oct 2014 04:28:50 -0400 http://ax84.com/bbs/dm.php?id=487028 Mic preamp phantom power(by DanGu) Hi. I'm designing a tube mic preamp (2 channels), and for the power supply I decided to use an hammond 370AX. So with this transformer I decided to use the 50V bias tap to generate the 48VDC phantom power for the mics. Obviously it as to be regulated, so my choice fall to those LR8 regulators (12-450V regulation range), using 2 of them (one for each channel). My only doubt is: Can this regulator supply enough current to power on the mics? Some say it has 10mA max output, others 20mA of max output. Or should I consider to use something else like a tl783(1.25-125V 700mA out)? The max current draw of each channel (if shorted to ground) would be 14mA with 48VDCTue, 28 Oct 2014 21:42:08 -0400 http://ax84.com/bbs/dm.php?id=486907 Build finished, red plating with a 6L6GC(by JayWalker) Hello there! I just finished my P1-eXtreme combo build and I'm having some issues/questions. I'll start by mentioning that my build is slightly modded, but I don't think it's too far from the original. In any case, differences from the design on site is: Using Edcor transformers. OT is essentially equivalent (5k->8ohm, 15 watt) and the PT outputs just a hair over 390 volts after rectification, unloaded. In the power supply section I'm using a 33k resistor to the preamp tube filter cap instead of 47k for higher voltage (don't think this would affect the output section). I'm using a Big-Muff style tone control. Values have been increased appropriately for playing nicely with tube impedances, I used the Duncan tone stack calculator to figure values that would give it a response similar to the Fender style stack that is stock, but with less loss. Besides that, the amp is pretty much the same as what's on the schematic. So here's my problem: after having the amp on for a little bit, particularly with the master volume cranked, the output tube starts to glow red. I've tried it with two different 6L6GC tubes, and the situation remains. This only starts happening after the master volume is turned up a bit. So I did some measuring around the power tube, and came up with some ideas. Let me know if I'm doing something wrong or should check something else. Note that I took all these measurements without my guitar even plugged in, so the amp is not amplifying anything but intrinsic noise. The cathode resistor I'm using measures 331 ohms. With master volume ALL THE WAY DOWN, the anode voltage of the power tube is 353 volts. Screen grid measures 351, so that's safe. Voltage across the cathode resistor is 25.1. So when we do the math to find dissipation: Rk = 331 Vk = 25.1 Va = 353 Vk / Rk = 0.075 amps flowing 0.075 * Va = 26.475 watts dissipated. This is under the 30 watt rating for a 6L6GC, so all is well. Now, if I turn the master volume ALL THE WAY UP, the situation changes. Measurements become this: Rk = 331 Vk = 53.8 Va = 315 Vk / Rk = 0.162 amps flowing. 0.162 * Va = 51.03 watts dissipated. WAY high! There are a few weird things about this. Firstly 162 milliamps should've smoked my PT, but it didn't. Also, if the tube is dissipating 51 watts it should be glowing WAY brighter than it is. It's a rather dull red glow (only visible in a dark room at first) that gets brighter over tens of minutes. The amp SOUNDS great, I can play through it cranked for a while and it seems to behave just fine save for the slow reddening of the power tube. Also it was my understanding that the current through a cathode biased power tube shouldn't change regardless of signal applied, much less master volume and NO signal. So am I measuring something incorrectly or is something funky going on with my output section? I certainly wouldn't expect the master volume to affect the bias of the output tube. Lastly with a simpler question, what's an appropriate cathode resistor for a 6L6GC in this amp? I know it's designed with an EL34 in mind but says the output tube can be swapped. I would think to decrease the current, I want to increase the cathode resistor right? Tue, 28 Oct 2014 20:56:50 -0400 http://ax84.com/bbs/dm.php?id=487094 AX84 1x12 Tools Needed(by ledzepp007) Hi all, I apologize if this has been discussed (I have not found it through the search). I am just getting started with this and I thought I'd start with the woodworking side of things (speaker cab, head cabs) before I dive in to the electronic side. I have a cheapo Seismic 1x12 with a WGS speaker inside (the speaker is fantastic). It sounds ok, but it is an eyesore. I'd love to build the AX84 cab, but I have several limitations: 1) I don't have a lot of cash right now (saving for a wedding, just moved), 2) skill, 3) lack of space (live in an apartment in which there isn't a ton of space), and 4) no real woodworking skill (that I know of). The things I have going for me are that I'm pretty smart and I've been reading about wood joinery, different woodworking tools (saws, routers, etc). My question is this: in order to make the AX84 1x12, what is the bare minimum in terms of woodworking told that I would need, keeping in mind that they would need to be in a small work environment? I really appreciate your help in advance.Tue, 28 Oct 2014 16:45:08 -0400 http://ax84.com/bbs/dm.php?id=486947 maximum speaker voltage level(by Gruszie) Ok so I feel this might be a stupid question and for how long I have been building amps I should know this already but obviously I don't. Ok so I have been digging more into the output stage design and was wondering what the maximum output level should be for a particular speaker say a vintage 30. Obviously a speakers power rating is volts x current and I was wondering what the maximum voltage level speakers are designed for because its current draw is proportional to the voltage level needed to push the speaker to that voltage level so obviously increasing the voltage level will increase the current consummation. So my question is what is the maximum voltage level speakers can take so I can match that level less 10 percent to maximize output. Thanks in advance.Tue, 28 Oct 2014 13:19:22 -0400 http://ax84.com/bbs/dm.php?id=487026 8-inch 3.2 Ohm Speakers(by GeoMan) This is tangential to the thread I've made about having to rebuild my buddies amplifier. I need to find a 3.2 Ohm 8-inch speaker. I know I can get them from Weber, but I was wondering if there were any other suppliers out there, and if anyone has had any experience with any of them. Also, what might the repercussions be of putting in a 4 Ohm speaker?Tue, 28 Oct 2014 08:45:11 -0400 http://ax84.com/bbs/dm.php?id=487025 Speaker Circuit Question(by Tim Mc) I'm getting ready to start my P1-eX build using the Revision 10-10-04 documents. I would like to put in a 3-position switch to select a 4/8/16 Ohm external speaker vs. using three separate output jacks as shown on the schematic. I believe that R5, a 220 Ohm 5W resistor, provides a small load for the OT if no speaker is connected. Am I correct that if I use a selector switch I should continue to wire R5 so that it is always connected from the white (16 Ohm) OT transformer lead to ground regardless of my switch setting? TimMon, 27 Oct 2014 14:52:39 -0400 http://ax84.com/bbs/dm.php?id=487050 SEL, a bit more bass response?(by infinitebaffle) Hi, I made a SEL a year ago and each time I play it I think that it could use a little more bass. This morning I swapped out several speakers and while there were a few with more bass response than others they were all sort of lacking in what I am hoping for. One thing I have noticed is that the Bass knob in the tone stack seems to have very little effect on the tone, so I am wondering if I should consider swapping some components in the tone stack or perhaps the cathode bias area. I built the project as stock from the documents and I am using a JJ KT88 power tube. Does anyone have any suggestions of where I might start? I'd like to get to a point where sweeping the bass knob provided a sense that something was changing dramatically. Thank You!Mon, 27 Oct 2014 14:25:53 -0400 http://ax84.com/bbs/dm.php?id=487009 Finished my Firefly (and cab) finally(by Stack) It took me a lot longer than I thought to finish this project, but hey it was summer after all! As you can see I built the speaker cab as a "tall boy" combo style. I have a few ideas for using up that space but I'll save that for later. <a href="http://s1377.photobucket.com/user/StacklerAmplification/media/2014-09-20171521_zps45338150.jpg.html?sort=3&o=0" target="newwin1414690925">[link]</a> Mon, 27 Oct 2014 13:56:17 -0400 http://ax84.com/bbs/dm.php?id=486480 Plate load on cathode follower stage(by Martin Bergheim) What would happen if a plate load resistor (820R to 10k) was added to a cathode follower stage? Would it somehow bias the grid like the cathode resistor in a normal gain stage? Yes, I know it is a far out question. And yes, I really should go to bed.Mon, 27 Oct 2014 10:34:31 -0400 http://ax84.com/bbs/dm.php?id=487018 HO SEL Advice(by Denny Gracey) I'm ready to tackle my 2nd amp build; first was an 18watt baby will pcb which was great for learning the basics (the 18watt guys put up with all my 101 questions) and now i'd like to do a turret build, learn more about gain stages, and use a big bottle tube with adjustable bias which led me to the HO SEL. If kits are still available, would love to buy one including punched chassis. If not: * I'd like to use a KT88 and could use PT recommendations * I'd prefer to use a pre-drilled turret board, better yet with turrets installed; ditto for a chassis - any suggestions appreciated Tons of great info on this site; i'll do my darndest to search for answers before hitting you guys up. The theory document is excellent! Like being back in school. Looking forward to the adventure.Mon, 27 Oct 2014 07:23:45 -0400 http://ax84.com/bbs/dm.php?id=486500 Getting Ready to Build(by Tim Mc) I'm stoked. Yesterday I received a message that parts for my P1-eX have shipped. This will be my first tube amplifier. Years back I earned a living as an electronics tech and even though that was all pretty much solid-state gear I'm not concerned about the electronic build itself. How are people finishing and enclosing their amplifiers? I've considered just painting or powder coating the chassis and mounting it on a simple wood base for the world to see but I also like the idea of a nice, professional looking enclosure. Is there a gallery of finished user projects to look at anywhere to get some ideas? If you build in a cabinet, how are you labeling things or making faceplates? Thanks in advance! TimSun, 26 Oct 2014 18:46:03 -0400 http://ax84.com/bbs/dm.php?id=486979 5E3 project questions(by Audiosalvage) Hi all, I'm starting a conversion from a 1955 capehart push pull 6v6 amp to a 5e3 deluxe. It has a layout unlike your usual guitar amp. It's more square shaped with the PI between the 6V6s. Since it has all the pots already mounded and little or no AC hum when cranked, should I just gut the preamp and point to point the new components or start fresh with a more conventional fender layout? It is pretty tight to add a turret board. Another thing I'm concerned about is that it is not that loud even at full volume. Is this just an impedance issue because I'm feeding a guitar through it? ThanksSun, 26 Oct 2014 14:59:21 -0400 http://ax84.com/bbs/dm.php?id=486749 design around ignorance(by Kursad K) OK, this is off topic: <a href="https://answers.yahoo.com/question/index?qid=20080917070550AAOR81j " target="newwin1414690925">[link]</a> It says you cannot use %100 pure antifreeze because it is a poor heat conductor and the maximum amount of antifreeze in an engine coolant should be %70 and the most common compromise is %50-%50. Makes sense. But I have often seen that this advice is completely ignored. If a mechanic has even remembers to change the engine coolant, they just replace the radiator contents with %100 pure antifreeze. Assuming that some of the coolant is in the engine, the result is not immediately %100 pure antifreeze, of course, but it would only need a few replacements to exceed the safety limit of %70. Obviously the switch to the long life antifreeze has a reason: if they'll do it wrong in every time, it's better to reduce the number of wrong replacements. I don't know if this interests anyone here. Just wanted to share the info. KursadSun, 26 Oct 2014 08:51:02 -0400 http://ax84.com/bbs/dm.php?id=486972 VVR in october club(by arjepsen) Hey. I'm still working on a layout for this project, and I'm debating with myself whether I should add a VVR. I got the board from Dana Hall, that also adjusts the bias. My question is about were the best place for the board is - front or back. I presume it comes down to which place is most troublesome, conserning noise. Here's a link to the layout I'm working on - it shows two possible locations. <a href="https://www.dropbox.com/s/dkm65krag5cjyg2/OctClub%20layut%20in%20progress.pdf?dl=0 " target="newwin1414690925">[link]</a> What do you guys suggest? Regards AndersSun, 26 Oct 2014 06:41:32 -0400 http://ax84.com/bbs/dm.php?id=486556 Weber-esque 5F4 using '60 Conn Organ Chassis(by Jonathan Haynes) First post here. I come from ham radio kit and homebrew land, and this is my first project. I have several donor electric organs on hand and dove in with a Conn 430 chassis. Looking it over, I saw a 5U4GB rectifier, a pair of 6V6GCs and a trio of 12AX7s, along with a very husky PT, OT and perhaps other salvageable parts. After stripping it down to the bare metal, I decided to use a Weber 5F4 layout and schematic as a guide, and built the amp freehand from there. I've learned a lot so far--especially about observing layout conventions to GREATLY ease troubleshooting and servicing the finished amp. After finding a few goofy mistakes the amp started coughing and sputtering to life--but I have problems. First of all, I cannot get any clean tones whatsoever. My measurements and calculations based on the Uncle Doug video to determine plate dissipation. It appears I have avg 14.8Watts of output, but I may have put the cart before the horse. So I will back up--What considerations should I have in mind when determining the usability of this PT with the Weber design? I don't have the OT voltages on hand, but supposing they are 20% or so too high, what is the typical method of dropping these voltages, and where do I find out what is recommended? I have a current limiter and variac, as well as various DMMs and an oscilloscope, although I don't see one typically used. Please point me toward the right publication, site--or if you don't mind, let's chat on the phone or textually online. I have a feeling my questions will be simple enough for any relatively patient amp-a-holic to answer. This amp's right on the edge of being worthy of building a cabinet--but not quite yet. Thank you, JonathanFri, 24 Oct 2014 11:07:37 -0400 http://ax84.com/bbs/dm.php?id=486903 Epiphone Valve Special plate voltage(by chee16) So I have done a lot of mods to my Epiphone Valve Special, essentially converting the preamp to as close to the Matchless Lightning as possible, eliminating the DSP and now trying to bias the EL84 properly. I used this tutorial for biasing (though it is for the Valve Junior) <a href="http://web.archive.org/web/20071213061717/http://www.diycustomamps.com/valvejunior.htm " target="newwin1414690925">[link]</a> So using the method I have my dissipation at about 14watts, that is with a 270ohm/2w resistor from the cathode to ground, where the stock resistor is 220ohm. My cathode voltage is 11.75V, and my anode voltage is 339V which seems way to high. Using those numbers for the dissipation gives me just over 14watts. Too high. I measured my wall voltage and it was 122V. In the article the guy talks about upping R10 from 220ohm to 1kohm. That sound fine except the Valve special doesn't have an equivalent resistor. Here are the VJ schems <a href="http://www.sewatt.com/files/sewatt/StockVJ_1-3.pdf " target="newwin1414690925">[link]</a> Here is the schem of the Valve Special, I put in the values as they were really blurry <a href="https://lh5.googleusercontent.com/-NqQklh3ewMA/VEGkz-jhFHI/AAAAAAAAC_Q/_--sf54jIok/w388-h221-no/evspowersection.png " target="newwin1414690925">[link]</a> Version 1 of the VJ doesn't have the R10 resistor either, so I am thinking I could safely add one into my circuit. Does anyone have any insight into this? I doubt specifically to this model amp as the VJ craze is pretty much gone, but hopefully someone can tell me if I am thinking correctly. It should bring the B1,2,3,4 voltages down correct? Looking the the VJ schems and comparing Version 1 and Version 2 it seems that when they added R10 (220ohm) it dropped the B1,2 and 3 voltages down. Thank you, I hope I was clear in my description. Fri, 24 Oct 2014 11:00:47 -0400 http://ax84.com/bbs/dm.php?id=486812 loose farty bass(by bullpeters) I got loose loose farty bass in my build. It seems I am not the only one.I have read Lots of threads lots of proposed solutions but no posts that I could see with a solution. I have listened to the clips and didn't hear a tight lower end.but that could be me. It could all be me :) so is it my build, my cab, or inherent to the schematic/system. I love everything about this amp, execpt the bass, which sounds like a fuzz pedal. I just want to know if its fixable or just part of the "flavour" of the amplifier. opened back cab 12 inch celestion, and I have regulated all the power. Ht lt bias etc. The power supply seems to be fine. The layout is as per the document. Fri, 24 Oct 2014 00:40:15 -0400 http://ax84.com/bbs/dm.php?id=486926