AX84 BBS New Threads as of Wed, Jul 29th, 2015 @ 15:40:34 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits DC Heater assist(by Denny Gracey) I've added the DC heater circuit from Merlin's preamp book to my UHG build - used 4001's and an 8700mf 16V cap. Getting proper DC voltage but it's still carrying 150mVpp AC which is getting coupled into the anode causing noise and nasty oscillations. The wave shape is 50mV clipped flat on top with a near sine shaped lower lobe 100mV. I've got a 500R humdinger pot installed up front in the chain, and heater elevated 28V off the cathode (KT88) - A few other bits: * DC circuit is mounted on the board, fed by KT88 heater. I did this for convenience and to minimize length of AC wire to the DC circuit. The preamp tubes/circuits are mounted along front of amp. * If heater elevation disconnected, the lower lobe disappears but there is still ugly AC riding the DC with hash from recto switching transients * PT is a 270FX (115V AC and my wall is 120V+) so I dropped the heater voltage by 1.2V (now 5.8V) using a pair of .22R resistors * I rewired the amp using star ground, which all but eliminated 60hz hum * B+ to input stage has no ripple What might I be doing wrong here? Any suggestions much appreciated! Mon, 27 Jul 2015 08:03:09 -0400 Discovered a little gem in my closet tonight. (by Alva Goldbook) Thought you guys might enjoy this. A few years ago my wife rescued this little combo amp for me. I never looked at it too closely. I figured it was solid state at the time and I just put it in a closet and forgot about it. So tonight I was looking around the house, gathering a few things for a yard sale, and by chance I turned this amp upside down and saw tubes! It's a little Made in Japan Lafayette amp. I opened it up and there was about 50 years of dust and cobwebs in there, but once I got it cleaned up a bit, I plugged it in, and surprise, surprise it works. I'm pretty sure it was purchased new and never played. The speaker looks like it's a 6 inch alnico, and the amp is pretty dark sounding with a humbucker equipped guitar, but it sounds pretty nice with my tele. Sort of like a Blackface Fender Champ. The tube compliment is interesting. Looks like it has 2 6AV6 preamp tubes and a single 6AR5 power tube. I suspect it would sound quite a lot better with a better speaker. A few pics. <a href=" " target="newwin1438198834">[link]</a> <a href=" " target="newwin1438198834">[link]</a> <a href=" " target="newwin1438198834">[link]</a> <a href=" " target="newwin1438198834">[link]</a> <a href=" " target="newwin1438198834">[link]</a> <a href=" " target="newwin1438198834">[link]</a> <a href="" target="newwin1438198834">[link]</a> Wed, 29 Jul 2015 06:30:52 -0400 Thank you to this group for helping me through a successful buil(by Dave Tainer) Yes, I'm a lurker. I never wanted to use the crutch of having to ask a question of this vaunted group because I thought after so many years you probably didn't want another newbie asking the same questions! :) So I followed all the instructions (but not buying a kit, i specced it out myself because I wanted to mod specifically for my purposes), and I used search to find the answers to all the questions everyone had already asked. Yes, I did have a couple that weren't answered by search, but by using the theory document, the schematic and the build document, I was able to figure it out for myself. This didn't come with a light it up on first try success story, but between the docs and all the links, I was able to troubleshoot on my own, knowing that if I really needed it, you guys would be there. So thank you to all of you for all the posts over the years, from the newbies that had the same questions as me, to the "elders" who had all the answers: I thank you, because I now have a kicking amplifier that I built by and for myself! If you want to take a look, see the links below. I also posted one on the facebook page of AX84. I am very happy with the sound with only a little bit of playing and plan on exploring all of the tones I can get out of this on the weekend! <a href=" " target="newwin1438198834">[link]</a> <a href="" target="newwin1438198834">[link]</a> Wed, 22 Jul 2015 19:27:34 -0400 Question about deprecated projects(by ScrubeTeamer) Hello i am new to the forums. Massive investigation and web search brought me here and i am very pleased to see such an active community focused on valve instrument amps. There is a lot of knowledge preserved here, so i decided to build a guitar amp out of these resources. My ultimate goal is an amp in the neighborhood of 50 watts output power with a fair amount of gain and two (footswitchable) channels. The first thing i found was the Renegade project that perfectly matched what i was looking for <a href=" " target="newwin1438198834">[link]</a> . But as it is labeled as deprecated i am not sure i should build that amp. Especially because i got a few questions related to the schematics and i expect no further support on this one. Could someone point me a safe way to go with "Core Series" components? PS: I worked as electronic technician for several years and got the chance to focus on audio related projects. So no worries, i am not gonna die with the tongue on the tube sockets ;) PPS: Excuse my sloppy english. Got a bit rusty over the years.Thu, 23 Jul 2015 18:08:20 -0400 The raffler project (by steve25) Years ago when I printed off the schematic from here for the raffler amp ( 2003 I think ) I did not notice till now that in the clean channel of the amp after one of the pots there is a coupling cap with no value marked for it. Can anyone point me to a updated schemstic, or pass along that cap value? Thanks in advance!Wed, 22 Jul 2015 16:09:58 -0400 NON-Amp related power supply advice needed(by RickTown) I'm in the middle of adding laser engraving to my CNC. It's a light duty 445nm diode laser for mostly wood burning and such. I'm driving the laser with a smoothed PWM signal to simulate analog voltage input from 0-5V. I'll have 8 bit (256 shades) control. At maximum output, I'll have the driver tuned to output about 2.5A to the laser. Here is my power supply for the laser driver. <a href=" " target="newwin1438198834">[link]</a> I only show the rectified output voltage rather than the transformer, rectifier, and filtering so that spice will show me actual dissipation numbers for my source follower and also run much faster. At the moment, here are my tranny thoughts: 100VA 16VCT FWR @ roughly 11.7V 100VA 10V Bridge @ roughly 13.3V 100VA 12.6VCT FWR @ roughly 9.2V ??? My concern is keeping the output regulated at 7V while minimizing dissipation in the NPN follower. Dissipation in the NPN with the minimum 9V needed for regulation on the collector, still sits around 5W and goes up the expected 2.5W/Volt. I'm in Phoenix and this machine is in my garage where it's already hotter than hell in the summer anyway. Adding more heat is not good. Let's say I go with the first tranny listed. Assuming my output voltage is close to my guess, would a couple series diodes on the NPN collector be the best option for sharing the diss. while maximizing the regulator's input voltage? Would I be better off with parallel TO-3's? Or is there a better choice? And I am also using liquid cold plate cooling for the laser heatsink along with transistor heatsinks and multiple fans. I might still be limited on using the laser during cooler months. Hopefully, this post will get me a few good bits of advice.Tue, 21 Jul 2015 16:27:46 -0400 How would you arrange this chassis?(by Darren Porter) I am going a different direction with my next build. I have all the parts (I think) on-hand but I don't know how I should arrange the turret cards in the chassis. What I am going to do is build the 50W push-pull (October stage power amp really) and mate it with two preamps... the Simple Preamp and the Crunch Preamp with 60s/70s switch. I have a Hammond 17x10x3 steel chassis that I want to fit it all in. How should I arrange the turret boards so I won't have any layout-related noise issues? I haven't decided yet if I want to do relay switching or just do a two input amp and use an ABY footswitch either. Thu, 23 Jul 2015 16:32:38 -0400 Audio Cyclopedia,2nd Edition on its way(by CChurchill) Just snagged a printing of Audio Cyclopedia; 2nd Edition by Howard Tremain! Should be arriving in a couple of days. If it is no longer under an active copyright, 'm thinking of scanning it, running it through an ocr, and uploading it.Tue, 28 Jul 2015 10:17:00 -0400 Doberman kit: switches too close together?(by snaptastic) I bought a P1 Extreme kit including punched aluminium chassis in 2012, which I am only now in the process of building (delay due to moving countries). I installed the two switches (On/Off and Run/Standby) into the pre-drilled holes and I see that they are very close together, so close that they need to be insulated from each other (probably - I think nothing could short out in the current configuration, but I don't want to carelessly reinsert the switches in years to come and cook the amp or myself). What's the easiest/neatest solution to this? Put some heatshrink around the terminals and stop worrying? Slice the terminals off with a dremel? Buy smaller switches? I will post a photo if needed. Thanks in advance.Thu, 23 Jul 2015 09:16:53 -0400 Possible OT problems.(by Stevo) I have a Musicman 2475 on the bench right now, and I'm suspecting the OT is problematic. First of all, the owner has complained he no longer can hear himself in the mix with his band, since changing tubes a couple of weeks ago - this coming after a failure of some sort and noticing the amp smoking. I wasn't the one who replaced the tubes, so I'm not sure what else was checked if anything. Without a working scope, the tests I've done are a little open to interpretation. However, this is what I've found. I fed a 1khz signal into the input of the amp and hooked up to an 8 ohm dummy load, I read a maximum voltage of 12VAC across the resistor. From plate to plate of the power tubes, my meter maxes out at 600VAC (presumably it gets much higher than this since the tubes seem to be in good working order). This gives me a turns ratio of at least 50. So, to double check, I disconnected all OT winding connections and hooked up a 6.3VAC from the heater winding of the PT, and read a resulting 180VAC when connected to the 8 ohm tap. With these symptoms am I correct in concluding the OT is likely dead?Tue, 21 Jul 2015 03:15:38 -0400 Shelving or Notch filter Help. (by T Kaye) Hi,,,I've built a P1 amp and need to incorporate a Tone Shelving Circuit to help control the bass frequency of the amp,,,I would like it to be Variable as well,,,Has anyone done this and if so,,do you have a schematic you could share. Thank you.. Terence. Tue, 21 Jul 2015 13:11:27 -0400 Clean round holes(by GeorgeG) How do you guys get clean round holes in the aluminum chassis? I use a punch to prevent wandering at the start of the hole but its hard to stabilize the floppy chassis in the drill press for the front and back holes and I get wobble at the end. Do you guys use a hand drill or a drill press? What type of drill bit or hole saw? Thanks, George Sun, 26 Jul 2015 14:54:24 -0400 October Club tonestack(by Norsetone) I built a Club back in 08. Its been a good amp but, I've never been able to hear much variance in the tonestack when adjusting TMB. I've see but not confirmed with my ver. that the pot values changed in newer ver. schematics ...was this a problem others had that was remedied with the changes or do I need to troubleshoot for something else?Thu, 23 Jul 2015 07:30:08 -0400 A vacuum tube theremin(by John Polstra) This isn't exactly a guitar amp, but I thought some of you might be interested in a project I've been working on for the past year. For various reasons, I got excited about the theremin, and I decided to design and build one -- using vacuum tubes, of course. I just finished it, and it came out working very well. I wrote an article about it, with photos, schematics, and a sound clip. If you're interested, check it out: <a href=" " target="newwin1438198834">[link]</a> And, by the way: if you think the theremin is good only for making cheesy sci-fi sound effects, search on YouTube for videos featuring excellent players such as Pamelia Kurstin and Carolina Eyck. A good theremin player can make truly beautiful music with the instrument. JohnThu, 23 Jul 2015 16:13:16 -0400 5.5V AC heater voltage, any danger?(by sajyho) Hi guys, I recently have built a PP class AB amp using two 6AK6 power tubes,a 12ax7, a 12au7 as PI, and a EZ80 rectifier heated using the same heater tap with the other tubes. The power transformer is winded by a friend and the B+ voltage is good, the problem is the heater supply that is 5.5V when loaded. I found tubes tend to bias cooler with this filament voltage and that isn't my problem. My problem is "Cathode Stripping", that some folks mentiond it as a result of excesive low filament voltage and it will shorten the tube life. So is 5.5V too low? Or I'm just too sensitive about this... ThanksSun, 19 Jul 2015 05:34:58 -0400 years passing too fast(by Kursad K) <a href=" " target="newwin1438198834">[link]</a> Not very long ago they used to look young and had better compositions. they still play well though. Kursad Wed, 22 Jul 2015 15:45:50 -0400 Harmony H400 Rebuild help(by ModelBuilds) I am rebuilding a H400 for a friend <a href=" " target="newwin1438198834">[link]</a> I put an isolation transformer in it via this schematic <a href=" " target="newwin1438198834">[link]</a> They called for a 133R 5w resistor. I had a 120R 5w. I used the Triad N-68X transformer I replaced all of the caps and resistors. I used modern caps, except one and NOS AB resistors except 2. I turn it on and get nothing. The 12AU6 barely glows. There is the physical transformer hum and a faint hum from the speaker. When I was researching this amp I found many of the people who posted about putting in an isolation transformer in it stopped asking questions after they made the iso tranny install. But, I also found information about half wave rectifier issues. Hence why I am here. Now, I am not going to lie, I work with low voltage (effects pedals) and the proposition of using my cat2 meter on this thing while live scares the heck out of me. So I come to you for suggestions and or ideas on how to fix this little amp and not get zapped too badly. Thanks again MBTue, 14 Jul 2015 17:41:29 -0400 Layout Questions(by turtle441) Hi, Working on finalizing the layout on a 2 channel amp that's been on the back burner for a while. Schematic is here: <a href=" " target="newwin1438198834">[link]</a> Layout is Here: <a href=" " target="newwin1438198834">[link]</a> Layout's a bit different than I might have chosen as both trannies are laydown style and I'm using a recycled Deluxe Reverb chassis. I'm trying out the 5V regulated power board and optoswitching board from Granger amps to switch between the 2 tone stacks. . Questions for the group: 1. When I was initially laying out the switching wiring, I'd thought "I'm running a power supply right under the board... I'm going to need to shield that cable". (The 5V from the supply board, which pulls power from the heater circuit to give 5V DC regulated power.) Now that I'm thinking about it, I realized that there's DC supply of several magnitudes greater strength running all over that board, so maybe I'm safe, unless I get some weird induced currents that mess with the switching. (Seems like a long shot, but I had a similar issue with some thermostat wiring last month.) 2. See anything else glaringly wrong with the layout? (Yes, I know, the heater wiring isn't there. I couldn't figure out a time-efficient way to show heater wiring using DIY layout creator, so I opted to skip that step.) Thanks!Fri, 10 Jul 2015 12:43:53 -0400 Power Transformer/Supply design??(by CChurchill) I find myself in a vicious cycle... Here is where I'm at; I know what my needs are for (ideal) voltages, and have the figures for the quiescent current, and can figure the load current under max conditions(I have load lines for each stage). That's nice, right? Until I have to create a power transformer with real primary/secondary source resistances, high rectified DC voltages under light load, fluctuations in mains voltages, etc. So, half the data I need for making accurate power supply calculations exists in a power transformer.. which doesn't exist?! Many of you cats have been at this a long time. I need the transformer. What's a good next step?Wed, 29 Jul 2015 15:06:57 -0400 5f6a style amp - feedback oscillation? Help!(by juliusnick) I am in the process of rebuilding my 5f6a bassman with some extras. My build was working fine, but now after having reassembled it, I am running into some problems: There is low frequency oscillation when raising the MASTER above a certain level. It's like a tremolo sound, whoop whoop whoop, very deep. Once started it will not stop unless I turn off the amp and power it back on. It doesnt sound like puttering motorboating noise, more like a very deep wou wou wou wou wou Volume and Tone controls, presence, have no effect on the oscillations. Same goes for the reverb circuit that I have encorporated (Spring reverb driven with a solid state mosfet circuit). I am not using the other half of v1 yet, I built only one channel. Will use the other for tremolo once i sorted this out. I have added a PPIMV with a dual gang pot. This was already there when i first built it, so I am thinking this should also not be the problem. I have solid state rectification with a voltage doubler circuit. I have tried switching OT transformer leads, this also did not solve the problem. I have tried disconnecting the NFB, the problem still persists. The problem persists in both fixed bias and cathode biased mode ( I added a switch to have both ). I just hope its not the positioning of components (Tubes / Transformers), I am working with a small enclosure, and there is no room to change anything. I am not having any hum problems, so I am thinking this shouldnt be the problem. I oriented PT/OT in 90 angle. OT is fairly close to the power tubes, but I have seen amps where it is similar. I am wondering whether it could be a grounding problem. I am using a star ground scheme, where there are 2 star points. Preamp, PI, Pots, etc all go to a ground point to the chassis at the input jack. Power amp, wall outlet ground lug, center tap, power tube cathodes, bias circuit are grounded to a point at the other end of the chassis. Speaker Jack is grounded to the chassis. I have shielded the grid wires for the power tubes, and put 3k3 grid stoppers on the tube socket pins. Voltages are: V1: 1: 232 2: 3: 1,92 4: 5: 6: 7: 8: 9: V2: 1: 208 2: 3: 1,32 4: 5: 6: 362 7: 208 8: 210 9: V3: 1: 282 2: 30 3: 50 4: 5: 6: 290 7: 32 8: 50 9: V4: 1: 50 2: 3: 491 4: 493 5: -51 6: 495 7: 8: 9: V5: 1: 50 2: 3: 491 4: 493 5: -51 6: 495 7: 8: 9: If anyone can help me out - it would be greatly appreciated!Thu, 09 Jul 2015 08:53:51 -0400 Wire colors on layout(by Jordan) Looking at the layout diagram, I'm wondering what the wires in red represent. I'm pretty sure the ones in blue are the shielded wires, but I can't figure out what red is. Anyone know?Wed, 29 Jul 2015 08:05:55 -0400 Jcm 800 2204 voltage confusion(by Audiosalvage) Hi all, I'm having a little trouble deciphering these voltage readings. Can anyone read these and tell me what's going wrong here? V1-190-0-3-h-h-104-0-1-h V2-167-0-1-h-h-283-166-168-h V3-242-25-40-h-h-239-0-40-h V4/5-0-h-494-490--44-495-h-0Sun, 05 Jul 2015 19:25:44 -0400 Tube Amp Breadboard and Prototyping(by CChurchill) I have some time right now to start planning and constructing a proper, comprehensive workstation for prototyping tube amplifier and effects designs. (I'm currently waiting on having a power and output transformer made to finish up a build I'm working on, so I want to use the time productively) I've started doing some preliminary searching for inspiration, ideas, and products. There are some nice octal and noval tube sockets connected to screw terminals available, which should save me the time and work of building those. Also, initially, I like the idea of screw terminals as a secure and quick method of component changing without having to expose terminals and components to heat(ie- from soldering and desoldering). I know Wiha sells screwdriver sets with an working AC voltage insulation rated for 1000V. What are the downsides to this set up? Also, for the power supply, I would like to be able to vary the B+ up to 460V(give or take), as well as provide a range of B- voltages. I will probably use a separate dedicated transformer for my heater supply. What are my best options for this? I also want to build different modules for experimenting with different rectifiers and filter supply combinations. Your advice and experience is, as always, respected and appreciated.Wed, 22 Jul 2015 13:24:20 -0400 7/2 Outage(by Chris Hurley) Sorry for the outage folks- The server had an issue after a reboot. I need to move it to different hardware one of these days. Sat, 11 Jul 2015 03:55:17 -0400 Parallel Single Ended Amplifier (by T Kaye) Hi J. just read your Parallel single ended amplifier post,,,love the pictures,,,wow super nice job,,i've been thinking about building one but I don't have a schematic to go by,,I was wondering if you would share yours with me,,,thank you,,Terence,,,Wed, 01 Jul 2015 16:20:22 -0400