AX84 BBS Threads with new messages as of Fri, Jul 03rd, 2015 @ 15:24:46 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits Parallel Single Ended Amplifier(by JKCavitt) I finished this project last weekend - installed into a cab I built: <a href=" " target="newwin1435951486">[link]</a> It is a parallel, singled-ended amplifier. I used two Hammond 125ESE OTs and a 270Hx PT. Preamp is based on the Hiwatt DR103. Poweramp has two cathode biased EH 6CA7s. Sounds nothing like the Hiwatt as it is single ended, cathode biased, and no NFB or ultraliner transformers. However, it has much more gain that I expected. I thought with the lossy tonestack it might provide lower gain gain than my previous SE amplifier <a href=" " target="newwin1435951486">[link]</a> but it has much more gain. First time I have tried a plate fed tonestack since the P1eX I built around 10 years ago. This one is a real rocker. I added a switch to select between the late 60s and early 70s V1 bias - basically split or shared cathode bias. The 70s setting sounds a bit brighter, but other than that the difference is subtle. I usually leave it in the late 60s position. Tonestack is interesting - mids are much different than the F/M/V tonestack. I can get close to Martin Barre's tone on Aqualung, but nowhere near Pete Townsend's tone at Leeds. Maybe I just need to fiddle with it more.Fri, 03 Jul 2015 11:37:17 -0400 6922 phono preamp idea(by Stevo) I've been wanting to build a phono preamp for some time. From time to time I'd look for a decent project, but hadn't come across a full project that was ready to build, so I've been reading, and come up with a topology cobbled together from various sources. My schematic is here: <a href=" " target="newwin1435951486">[link]</a> and I'm wondering if anyone would care to take a look and offer any kind of criticisms. By way of explanation, the circuit consists of three gain stages of about 26 dB voltage gain each, followed by a cathode follower to drive the following load. The RIAA eq is accomplished in two steps. The first step is the 50hz lpf and 500hz shelving between stages 1 and 2. The 2122hz lpf is between stages 2 and 3. I've taken output impedance of the 6922 into account for the resistor values coming off the plates for the filters, but if anyone notices some glaring error, I'd be appreciative. I was going for something simple, but effective, and intend to build it with a very well filtered power supply. Any comments are welcome. Thanks.Fri, 03 Jul 2015 08:00:22 -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. Fri, 03 Jul 2015 05:07:15 -0400 Standalone based on Core 50W PP build(by Darren Porter) I've been waiting for the AES 10% off sale and now that it's here and it's summer (lots of free time) I am finally going to build this. I'd like for this amp to serve both as my test-bed for stand-alone preamp builds and as my amp for my live rig. I will probably make it either into it's own head or rack-mountable so I can also rack-mount a preamp into the rig as well and just carry one box. So, I want to do a 50 watt stand-alone power amp based on the October Stage/Core 50W PP Standalone projects with the following features: 1) KT77 output tubes with an eye towards swapping 6V6's in for when I don't need 50 watts - which will honestly be most of the time. 2) Reverb and effects loop built into this power amp so I don't have to build them into every preamp I build in the future. 3) Possibly a cathode/fixed-bias switch. I have a couple of questions about the topology and power supply... Should I go input jack => 12AU7 parallel input stage => effects => PI/AMP -or- Should I go input jack => effects => 12AU7 parallel input stage => PI/AMP I'm thinking the latter but not 100% sure why. For the power supply, this is a weakness of mine. I really don't have much of a clue doing power supply circuit design. The effects loop and reverb design I am looking at uses a total of three 12A_7 tubes. Looking at the standalone 50W power supply, would I just hang a few more nodes off the end for a B+5, B+6 and B+7 node for the three extra tubes? What value of filter caps should I use? 47u or 22u? Do I need to lower the value of R3 so more voltage gets down to those tubes? After I figure out that power supply issue I think I can put a parts order together and get this rolling! Thu, 02 Jul 2015 06:00:29 -0400 Book recommendation (by nonzero) Hey kids (and grownups), I am looking into making some of my own effects pedals to go with all these homemade amps, but I don't know where to start. With the amps, this site and Merlin's books set me on the correct path. Does anyone have suggestions as to what material I should read to start building useful circuits right away? (theory is great, but useful schematics would be even greater).Wed, 01 Jul 2015 17:18:22 -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 AES July 2015 - everything 10% off(by Darren Porter) FYI.... 10% off all items at AES Wed, 01 Jul 2015 09:28:36 -0400 Solid State FX loop.(by Alva Goldbook) I've been thinking of including an FX loop on a project I'm working on, but the more I think about it, the more it seems to me that a tube driven FX loop is a bit of a waste, and plus I would think that a solid state FX loop would be more transparent and can easily work off of a far lower impendence. I think I'll probably have to get a PCB made for this amp anyway, so I figured it would be better to just use a solid state FX loop. Problem is I don't know how to approach such a design, and I'd rather learn how to build a good quality loop than buy a kit from someone else. Any ideas? Wed, 01 Jul 2015 00:36:29 -0400 Parts selection for a new build(by GeorgeG) What brands do you guys like for: Caps Resistors Pots Switches Transformers That will result in a good sounding build without wasting money chasing the last 5%? Thanks, George Tue, 30 Jun 2015 19:07:06 -0400 Custom chassis(by Gruszie) Just wondering if anyone would be interested in purchasing custom sized chassis? I am considering buying a box and pan brake to make chassis for my builds and was just wondering if any one out there would be interested. Is seems that all the custom metal shops charge a fortune for custom sized chassis and thought it would be cheaper in the long run to just buy a box brake and possibly sell a few as well. I figure price could be around $30-40 depending on size. If there are a handful of interested buyers out there I just might purchase a box Brake and start making my ownTue, 30 Jun 2015 14:54:10 -0400 (diagnoses Help), I was playing my amp when..."pop"(by proton45) Hello, and thank you in advance, for any help you may give... I was playing my amp (as I have been doing for a month(?), about), and all of the sudden I heard a "crackle & a pop", and my V2 preamp tube burned out. I (foolishly) replaced it, and "pop", the same thing happened again. I opened up the amp, and tested a few of the power resistors, and I checked a few of my solders (everything seems ok). The amp is basically an "SEL", with the Raffler mod, and EL34 powered. Then I took a few voltage readings...the V1 plate resistors are both about 330VDC, and (all) each of the filtering capacitors are all about 330VDC. These are the quick readings I took, and what I might expect, if I had powered up, sans tubes...the V2 12AX7 is (no doubt) fried again, but I'm not sure about the other tubes... These are (obviously), the kind of readings I might see, without tubes, but Im not sure where to start my shake down. Any advise I could get, in diagnosing this problem, would greatly be appreciated. Its a little expensive to "trial and error" my way through a box of 12AX7 Thanks again for any help...Charlie Mon, 29 Jun 2015 13:49:18 -0400 First build of Jet-A(by Ben Sykora) The first sample of Jet-A has been constructed. Pictures are located here: <a href=" " target="newwin1435951486">[link]</a> <a href=" " target="newwin1435951486">[link]</a> <a href=" " target="newwin1435951486">[link]</a> The assembly went really smooth, amp function is right, but I encountered a resonance issue... At first I was worried about board layout and I even cut some traces in order to re-route grounds with copper tape. But none of that helped. When I dis-mount the O.T. and move it away from the input tubes, that seems to fix the resonance issue. I've never run into O.T. interference issues before - but then again, I've never mounted the OT next to the input circuitry before either. Maybe that wasn't the smartest move. My next step is to prototype up an new chassis layout and make sure the amp functions correctly. In the meantime, I figured you guys would like to see the progress. cheers Ben Mon, 29 Jun 2015 09:49:49 -0400 Mosfet Phase Splitter?(by Jeph88) Working on an amp. Using the LND150 as the input and recovery after the TS. Came across CarlB's post about using a IRF820 as a phase splitter, cathodyne. Done some bench tests and it seems to work really well. However, I am curious if anyone think the LND150 or the ZVN0545A would be better suited. Also can anyone give me some tips on calculating current draw for these fets? Mon, 29 Jun 2015 05:54:42 -0400 LED Biasing(by Hank Cohen) So I've seen lots of mention of LED's as a substitute for cathode resistors and I think I get the general principles; LED's or indeed all diodes have a predictable near constant voltage drop independent of current up until they vaporize anyway. And this constant voltage drop can be used to replace the cathode resistor and eliminate the need for a cathode bypass capacitor. But is there a concise writeup somewhere of how this LED biasing should be done? Something remedial, something for someone who understands the arithmetic of Ohm's Law but still puzzles over why low resistance = high load. I've already searched the forum archives but nothing tutorial seems to pop out. Thanks,Mon, 29 Jun 2015 00:29:52 -0400 Would like feedback on clean preamp idea(by Darren Porter) If anyone would be so kind to take the time and look over my idea and give some feedback, I would really appreciate it. I'd like to hear what others think before I take the time to actually build it. Thanks! <a href=" " target="newwin1435951486">[link]</a> Sun, 28 Jun 2015 21:02:31 -0400 Mosfet Phase Splitter 2 ideas?(by Jeph88) <a href="" target="newwin1435951486">[link]</a> Sat, 27 Jun 2015 05:59:38 -0400 October Studio Transformers - help/input wanted(by Martin Schrder) Hello all, I wish to build an October Studio and would like your input/opinions/suggestions regarding transformers. I have tried but so far failed to aquire the skills allowing me to select the right transformers on my own. My approach for want of a better word is to see what I can get and match this against what appears to be on the schematic. Apologies in advance if there is already a thread where I might get this information - I haven't found it. So here are a number of options and questions regarding transformers for an October Studio build - I am thinking no fx-loop, possibly quad output. * Output Transformer: Hammond 125C or 125D. <a href=" " target="newwin1435951486">[link]</a> At 8W and 10W respectively I think these should be OK. Note that I haven't really decided yet if I want quad output or not so is the 125D simply the better choice or doesn't it really matter? How do people feel about the quad output option? Is it worth the extra chassis hole so to speak? * Choke: Hammond 156G or 157J. <a href=" " target="newwin1435951486">[link]</a> I've seen a build somewhere (probably Joe Kavanaugh ) with the 156G so I guess that is sufficient. The schematic doesn't specify the inductance (and also I don't really know which of the specifications for a choke are the more important) but looking at the october schematic I wonder if 157J is a better choice? (I note though that the 2W p-p schematic features a 156L...) * Power Transformer: This is where it gets difficult - at least for me. I have a number of options. ** Hammond 370CX or 370DX. <a href=" " target="newwin1435951486">[link]</a> and <a href=" " target="newwin1435951486">[link]</a> respectively. The CX looks right with the DX being slightly more powerful - perhaps unnecessarily so. ** 80VA toroidal. <a href=" " target="newwin1435951486">[link]</a> tube-town use these (albeit a slightly lower power one) in their kits. I've picked this one because it close to the spec on the schematic (coil tapped at 260-0-260V it is still slightly below what appears on the october schematic... though also slightly above the 2W p-p schematic). It is cheap by my standards and if useable could be a choice for this and future projects of mine. ** igmt-m18tmb: <a href=" " target="newwin1435951486">[link]</a> This is really intended for a marshall 18W. While I do not have the specs for this exact transformer I expect it to be close to this <a href=" " target="newwin1435951486">[link]</a> . My reason for picking this is it is made by a german 1-man company, he made all the transformers in my JTM and I wish to send any additional business of mine his way if I can... and if I can persuade him to produce a stand-up version. As far as I can understand they all meet the demands for heater and signal current. So which is the right choice? - Simply buy the 370CX? - Could the toroidal be OK or is the lower voltage going to be a problem? - Could the igmt work as well or would the 290 volts be too difficult to drop? Would the high current rating make it even more difficult? Or would it not really matter all that much? Obviously any and all opinions are welcome. Cheers, MartinThu, 25 Jun 2015 23:44:18 -0400 Fixed bias delay using bridge rectifier(by AneirinE) Hi, I've built an amp using an ac-coupled bias supply feeding from a full wave bridge rectifier for the HV - like a Marshall 4203 artist bias supply. It works really well. I have one concern though- it takes a little while (about 2 seconds) for the bias supply to reach the -ve voltage required for the bias supply. As a result, at switch on, the tubes are dissipating much more current than they are supposed to. I'm using an el84 power section and I've read this turn on current to be double the current for the 70% recommended dissipation for the tube. I've read on here that this shouldn't be a problem if the tubes are cold. But, what if they are warm - I.e. the amp is turned off then on again? How can I safe guard for this? The only thing I can think of is to find a relay that works only at the voltage bias to switch in the HV supply. Does anybody have any other suggestions? Or dont I need to worry about it? Thanks! P.S. I haven't used a standby switch.Thu, 25 Jun 2015 23:17:20 -0400 Another Tube Reverb Question (by Alva Goldbook) Well, not so much a question, as I'd like the opinion of the board. So I'm working on a reverb circuit. I'd like to get the kind of reverb that's as rich as found on blackface amps. These circuits are typically driven by parallel 12AT7 triodes with a 12ax7 triode on the recovery side. The amp I'm working on already has 4 pre-amp tubes without the verb. (3 pre-amps, plus 1 for a tube driven FX loop) I'd hate to end up with 6 pre-amp tubes and just leave one half of a 12ax7 unused. To avoid this, I'm kicking around two ideas. A) Use a parallel pair of 12AT7s to drive the reverb, and use the recovery triode from the FX loop for the verb return. Or B) Use a 12DW7, and drive the reverb pan with the single 12AU7 side of the 12DW7 and use the other half of the 12DW7 (a 12AX7 triode) as the recovery. Each approach has it's significant down sides. Approach A would mean that a normal reverb control would also effect the FX return levels, while Approach B would provide the reverb pan with less current than a pair of 12AT7s would provide. Thu, 25 Jun 2015 19:16:57 -0400 Tube amp project(by Audiosalvage) <a href=" " target="newwin1435951486">[link]</a> Here we have a nice 1-12 combo amp project ready for your amplifier chassis. Features wood covered in tolex type material with vintage grill cloth, 12" no name speaker, handle and feet. Inside width is 14.25" and panel opening is approx. 10" x 2". This cab is not perfect, there are some flaws and is sold as you see it.Thu, 25 Jun 2015 13:59:34 -0400 Views / Watchers Bids Price Time left Actions Tube Amp Cabine(by Audiosalvage) Here we have a nice 1-12 combo amp project ready for your amplifier chassis. Features wood covered in tolex type material with vintage grill cloth, 12" no name speaker, handle and feet. Inside width is 14.25" and panel opening is approx. 10" x 2". This cab is not perfect, there are some flaws and is sold as you see it. <a href="" target="newwin1435951486">[link]</a> Thu, 25 Jun 2015 13:54:21 -0400 Who wants to run the kit business?(by Chris Hurley) Hi Folks, I'm looking for someone to take over the AX84 kit business. I haven't had time to focus on it for a while now and I think the time has come to shut down the Doberman end of things. I'd like to work out something where kits can continue to be available though, if I can find the right person to take it over. If you're interested in talking about it, send me an email with your thoughts. Best regards, ChrisThu, 25 Jun 2015 13:01:56 -0400 Hi gain/Low gain input jacks?(by Alva Goldbook) I'm tinkering with this idea to build a JTM45 type amp, but it seems like a waste to have a bright input and a normal input. I thought it would be more interesting to make one the bright input jack and just have the other input run in series with the bright in order to have a higher gain channel. The idea isthat I could use an A/B box and have channel switching without fussing with relays. The 2203 circuit basically did this, but it was wired so that you could only plug in one channel at a time, which would make channel switching moot. So I was thinking it would be better to just insert the lower gain input after the first gain stage. But the problem I suspect with that is that you'd turn the low gain jack into an output and you'd send dangerous high voltage out to the guitar. I've been trying to figure out a solution to this, but I'm not having much luck. Any ideas? Here's a block diagram to better illustrate things. <a href="" target="newwin1435951486">[link]</a> Thu, 25 Jun 2015 08:11:53 -0400 220k grid resistors...pentodes in parallel(by Jeph88) Do the power tubes in a Fender Twin react differently with the 220k control grid resistor vs say the Super Reverb? If so does anyone know how to calculate ?Wed, 24 Jun 2015 04:18:51 -0400 Oscilloscope voltage rating(by Darren Porter) I got my Rigol DS1054Z today. yay! Now about voltages and measuring... This is from the Q&A on the vendor's website... <div class="quoteblock" >Q: Could you confirm the maximum input voltage for the DS1054Z, using a 1x probe? A: The Rigol DS1054Z comes with the RP2200 probes. In the 1x mode, the maximum input voltage is: 150Vrms. When in the 10x mode, the maximum input voltage is: 300Vrms. This is approximately equal to 425 VDC, if you consider the peak value. However, its always advised to use a DC Blocking capacitor when working with tube amplifiers with plate voltages over 300 Vdc. </div> So am I safe probing around the 20-watt push-pull amp with just the 10x probes it came with? Would that answer also mean I should probe after the coupling capacitor? I do have some 100x probes coming.Tue, 23 Jun 2015 21:45:06 -0400