AX84 BBS Threads with new messages as of Wed, Apr 23rd, 2014 @ 02:03:56 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com JTM45 clone startup problem(by ormen) Hi, first of all I want to apologize if this will be a lengthy post but I just want to make sure I explain the problem I have in the best possible way I can. I just recently started to build a JTM45 replica. It's based almost entirely on the schematic and layouts found on metroamps with some modifications like the grounding layout and a few more which I will mention later. Metroamps site is unfortunately dead as you probably already know. So here's link to the documents of what I'm building: <a href="http://www.speedyshare.com/SMpAg/JTM-45-KIT-V2.1.pdf " target="newwin1398233036">[link]</a> The file is quite big because it's a "how to" guide but you can find the board layout at page #17 and #18 and the schematic at page #31 (which is not applied to 100% on the layout as you might notice). Anyway now when you know what amp I'm talking about, here's the problem that I've encountered. Let's start with the voltages I measured before, this was first with no tubes, then I got everything as it should be. I'm using another PT so the HV secondary is 330 VAC which is correct, the heaters for all the tubes including the rectifier is also correct. Since the PT I'm using is not the same as the original I had to figure out how to connect the power lamp which also has a resistor built in (I don't know if the lamp on the original design has that), so I used the 120 VAC connection where you connect two cables together from the PT and connected that to the lamp because it's rated for 220 VAC which is 10 VAC less than the lowest voltage I could get otherwise. I'm not 100% if I did this properly but it seems to work out fine as voltage reading goes, which is: With rectifier and preamp tubes in only. Starting on the preamp tubes. V1 #1] 221.5 VDC #2] N/A #3] 1.65 VDC #6] 223 VDC #7] N/A #8] 1.65 VDC V2 #1] 188.1 VDC #2] N/A #3] 1.110 VDC #6] 325.5 VDC #7] 188.1 VDC #8] 189.1 VDC V3 #1] 249 VDC #2] N/A #3] 42.2 VDC #6] 243.8 VDC #8] 42.2 VDC And on the power tubes sockets without the KT66's installed. V4 & V5 #3] 445 VDC #4] 445 VDC #6] 445 VDC Heaters are in range on every single place and the rectifier also have correct voltage readings. The bias circuit seems to be working fine, I'm getting -37.9 to -49.3 VDC to pin #5 on the sockets and I'm using 5.6K grid stoppers which is an added mod to the original design. I have to bias pot adjusted to the most negative voltage possible to start with. To my understanding all voltages seems okay with exception from V3 cathodes which seems to be a bit on the low side and the screen grids on V4 & 5 seems to be high, I get the same reading as on the plates, I should get about 5 VDC lower? I have the 470R resistors connected. I have gone over everything at least five times and made sure nothing is wrong and checked for shorts and I've found nothing which seems to be wrong. I've seen that this amp has some slight difference on the screens, like on the schematic, which has a 1k resistor added, which the layout doesn't have and I have built it to the layout with some mods which I have mentioned. So here is what happen when I plugged the power tubes in and as soon as I flipped the standby on a loud hum comes from the speaker and V4's tube lights up as hell and the HT fuse blows. It's like it gets invaded with a big current? As far as I've been able to see it looks like it's only on V4 and I've tried switching the tubes and it seems to be on that particular tube socket. It Looks like that I'm not saying this as absolute truth just that it really looks like that, it happens fast so it's not so easy to see with the amp upside down. I'm using another impedance selector switch, it's not the same as shown in the documents. I'm using one which has a shaft which you attach a knob to. Maybe I haven't installed it correctly? I do get sound from the speaker though. As I mentioned, a loud hum. Anyway, I have really spent a lot of hours to really try to check so that everything is wired according to the layout with exception for another grounding which also is intact. I've built the ax84 PP 50 watt and lead preamp as a first project last summer and that amp worked on the first start up so I do know how to follow instructions. Apart from that my experience is very limited and I have no ideas how to troubleshoot this further and thats why I need your help. Does anyone have any advice how to troubleshoot this? Tue, 22 Apr 2014 21:37:57 -0400 http://ax84.com/bbs/dm.php?id=484244 Single Ended Negative Feedback(by tkwink) I was looking through some schematics last night, and stumbled upon a 5F6 (not A) Bassman schematic. I noticed that the negative feedback and presence pot were connected between the Mid pot and ground in the tone stack. Would this be an effective way of providing NFB in a single ended amp that does not have a driver stage after the tone stack (like AX84 core projects)? <a href="http://www.prowessamplifiers.com/schematics/fender/bassman_5f6_schem.pdf " target="newwin1398233036">[link]</a> Just curious, TravisTue, 22 Apr 2014 18:31:45 -0400 http://ax84.com/bbs/dm.php?id=466114 October Bright Volume Noise(by John Bgron) Hi guys, My October Club has developed an annoying noise/buzz which gets louder as I turn the Bright Volume pot. I've swapped all my preamp tubes with a known working set and done the chopstick test around the wires of all the preamp tubes. I've also reflowed most joints in the area and on the ground bus. Nothing I've tried so far has had any effect on the noise. I'm really not sure what else to do other than replace the pot. The amp has worked flawlessly for a few days. Can anyone point me in the right direction? Many thanks, John Tue, 22 Apr 2014 18:14:52 -0400 http://ax84.com/bbs/dm.php?id=484210 Adding Tremolo to P1-Ex(by Jacoby Smalls) Hello dear amp enthusiasts, Recently a friend gave me an old P1-extreme that I've begun rewiring. I converted the first 12A_7 into a parallel input gain stage, (thanks Merlin for the idea!) followed by a tilt tone control and gain control that feeds one of the second 12A_7's triodes as the second gain stage, feeding a master volume and EL84 PA. That all sounds just absolutely fantastic. I'm more familiar with the fender-ish 6V6 SE amps, and didn't realize how amazing EL84s sound in this type of topography! My question: I've rewired the second triode of the second 12A_7 tube to act as a tremolo. I 'stole' the design from an old Garnet SE amp. It has just one triode acting as an oscillator, with the plate connected to the 1st lug of the MV control. I can't seem to get it to work, though. At first I thought that maybe my frequency corners established by the RC networks just weren't in the audible range (I tweaked Garnet's values to fit easily available components) but now I'm thinking maybe I'm missing a second connection back to the main circuit path. any tips? has anyone tried this before? I've been combing the web for info on tremolos and other schematics that are similar to this one and haven't found much. Any leads would be much appreciated. Regards, JacobyTue, 22 Apr 2014 17:56:09 -0400 http://ax84.com/bbs/dm.php?id=438229 ECC88 / E88CC linearity(by DanGu) Hi! An old friend of mine asked me to build him an hifi SE amp using 300B tubes. So designing it I thinked to drive that big triode with an SRPP stage. My choice dropped to the E88CC looking at those grid curves. So LTSpice comes again with ayumi models gently provided by jazbo8 here, but can't find the model for that triode. So I used the ECC88 model, and in simulation worked unexpectedly well! My question is Are E88CC and ECC88 the same triode? On the net I found infos that are equivalents and on ebay these are selled sometime as the same tube, but datasheets does not agree. ECC88 exibits lower anode voltages than E88CC Can u help me to figure out if its the same tube or not? If not A tube model would be appreciated :) Thanks guysTue, 22 Apr 2014 11:25:57 -0400 http://ax84.com/bbs/dm.php?id=484172 Anode and cathode resistor effect on frequency response(by foulowl) Hi folks, Looking for a good rough rule of thumb with regard to how anode and cathode resistors affect frequency response, if they do at all. (for example, increasing X will boost high end, etc.) Thanks!!Tue, 22 Apr 2014 03:10:16 -0400 http://ax84.com/bbs/dm.php?id=484188 Feature films which show valves / tubes:(by Merlin) For some trivia I thought it would be fun to collect a list of feature films in which valves / tubes make an appearance! Can you add to the list? Here are the films I know of so far: Coneheads (1993) (As I recall they make a device to communicate with their home planet, and the "final component" is an octal) City of Ember (2008) (Features a time-locked suitcase. The timer is made from nixie tubes. Of course?!?) Ordinary People (1980) (I *think* there is a valve radio in the shrink's office). The conversation (1974) (There is a valve hifi amp in the main character's apartment. It nearly gets smashed at the end!) Infernal Affairs (2002) (A high-end valve hifi amp plays a crucial role in the bad guy's undoing!) Mon, 21 Apr 2014 23:30:16 -0400 http://ax84.com/bbs/dm.php?id=422771 Existing PT good enough?(by Dustin LeBlanc) Hey everyone, I am considering transforming my Blackheart Little Giant into a Comanchero. I am trying to do some cost estimating on the project to ball park what it will take monetarily before I decide to put the amp under the knife. I know that I will need a new OT along with other parts, I am hoping to reuse as much as I can from this combo. The speaker, cab, chassis, existing tubes, and as much other stuff as I can keep are coming along for the ride. The biggest items on the purchase list are transformers. I am trying to get specs on the existing PT and I pulled the chassis and here are the numbers: 0-100V-120V-240C <- Input/primary voltages I think? 5V-0 6.3V-0 275V-0-275V - secondary/ output voltages? I dont see amperage listed at all and that seems to be my main concern. The comanchero calls for 220-0-220@125mA / 3.15@4a Without being able to see the amperage on the PT, how do I know if I can reuse it? I assume it can be tested in some manner, though I dont know if I have the tools yet to do so. Any pointers on this? If I can reuse this one, it would save a lot on the project. I have heard that these amps came with decent iron and the specs on the PT seem to indicate that but I dont want to misjudge the requirements. I obviously have a lot to learn still, my only projects thus far have been some mods to this amp, building a boost pedal and modifying an OD pedal. I am eyeballing the Camanchero because I want a PP amp, not SE (the blackheart seems to be a darn fine SE amp as is). I am looking for something with the tight responsiveness of a PP amp as it seems small SE amps are too spongy/muddy for me. Future plans for the amp could include an effects loop and/or reverb down the road if that matters at all. Thanks!Mon, 21 Apr 2014 16:48:29 -0400 http://ax84.com/bbs/dm.php?id=484219 Stand alone reverb!! Love this thing!!(by Zambo) <a href="https://www.youtube.com/watch?v=ABGlD4lLIzA " target="newwin1398233036">[link]</a> Just built this and love it! Never had one or tried one but im a believer. Used the standard schematic for the g15 but instead of the 6k6 power tube I used a 12at7 with 1k cathode bypassed with 22uf . seems to work just fine.Mon, 21 Apr 2014 07:12:33 -0400 http://ax84.com/bbs/dm.php?id=484213 JCM 800/Laney GH100L mods and questions(by Branimir Bozak) Heh, I know type of topic has been beaten to death around these forums, but i'm having problems searching (my bad english for example...) ;) Anyways, I own a Laney GH100L, it's a sort of JCM800 knock off (schematic: <a href="http://www.freeinfosociety.com/electronics/schematics/audio/laneygh100l.pdf), " target="newwin1398233036">[link]</a> and recently I had a chance to try a JMP Super Lead 50 from the 70s (not the reissue) and noticed how while it has too extreme channels- the bright and and dark one - you can combine them for a more balanced sound -> I like the way it has a flappy bass response, and a bit of a fuzz character to its tone, while Laney was more "tight" tone, similar to JCM800. So I figured I could mod my Laney a bit to get that kind of tone. While every mod I encountered involved more gain -> I'm not interested in that in any way since Laney has enough distortion capatibilities for my liking and also an extra gain stage that can be switched on for even more cascaded preamp distortion available, which I never use, and on the "regular" channel I have gain around 2 o'clock and pushing the output of the amp harder (i play in a louder downtuned band). So amount of distortion is not an issue for me... The main question is, and I forgot about some of the theories because I haven't been in amp moddin/building diy thing for a couple of years hehe, is : how does the cathode resistor and capacitor influence the gain stage? If i reckon usual way of adding distortion and tweaking the bass response was through those resistors and capacitors, and I notice the JMP has a different value for the dark channel ( <a href="http://www.dreamtone.org/images/1959%20JMP%20Super%20Lead.gif) " target="newwin1398233036">[link]</a> , so I thought I might give it a try, maybe it'll lower the amount of the distortion in the preamp and push the tone towards more bassier, fuller tone. Anyways I would like to know what does the cathode resistor and capacitor do to the preamp gain stage. Thanks, BranimirMon, 21 Apr 2014 06:22:38 -0400 http://ax84.com/bbs/dm.php?id=427133 P1eX clean for jazz(by Michael McCarthy) I have in mind building a clean p1eX for jazz. It doesn't have to be perfectly clean when dimed, but I would like it to be clean at least through the first half of the gain control on the first stage. I'm thinking of the following changes from a standard p1eX: 1. Allen TO11C output transformer, since the Doberman is not available. 2. Moving the tone stack to between the first two stages. 3. Adding a 0.022uF coupling cap following the second stage, before the master volume control. 4. Using the Merlin bone ray tone stack. 5. Placing the tone stack right between the first stage coupling cap, and the R21 gain attenuator. 6. Using a 5751 instead of a 12AX7. Here is a schematic: www.mccarthystringedinstruments.com/file_warehouse/20130521_BAS.pdf I have a few questions: There are some positions of the bone ray controls that have no capacitor in line, or no resistor in line, so I assume that the stack has to go right where I described it, and not obviate anything there. Are both of those assumptions correct? I do need another coupling cap after the second stage, right? Will this design, with the tone stack moved, and a 5751 instead of a 12AX7 have plenty of clean headroom? Is there anything else I might consider to go in a clean direction? I'm not interested in much distortion; maybe a little bluesy crunch, but that's about it. Sorry about that, Zaphod, et. al. I would really appreciate any comments from the fine people here. Thank you very much!Sat, 19 Apr 2014 22:49:53 -0400 http://ax84.com/bbs/dm.php?id=477253 Marshall Superlite TMB(by peterg) I am looking into building the above mentioned amp based on this schematic <a href="http://gerkules.16mb.com/tubeshematic/images/18watt_Superlite_TMB.jpg " target="newwin1398233036">[link]</a> Classic Tone manufactures transformers for this amp but I'm hoping to use Hammond. Hammond is manufacturing a match (1750PA) for the Classic Tone 40-18037 output transformer which will be for sale at the end of February. They are currently designing the exact match for the Classic Tone 40-18035 power transformer but it will not be in production until the end of April. Does anyone know of a current Hammond PT model that will work? Sat, 19 Apr 2014 16:42:01 -0400 http://ax84.com/bbs/dm.php?id=483103 Mark L's fx loop(by sajyho) Hi guys, sorry to bother you again, I'm trying to add a fx loop to my HO. I've tries some of 12au7 designs but none worked well. I've read on this forum that Mark L has a nice design using 12ax7, but all the links to the schematic are old and broken. Does anyone have the schem, if have please post a link. Thanks SajadSat, 19 Apr 2014 11:24:15 -0400 http://ax84.com/bbs/dm.php?id=484114 8" Celestion(by Peliass) Hi guys, I'm looking for a Celestion Vintage 8 or a Super 8. They're supposedly new but I can't find them anywhere! Does anybody have either of these speakers and if so, would be willing to sell? Also, any other 8" speakers that anyone is willing to sell? Thank you. Sat, 19 Apr 2014 03:29:00 -0400 http://ax84.com/bbs/dm.php?id=416577 Deluxe 57 reissue OT fault(by Frankleslie) I just recently had a Fender Deluxe 57 reissue in for repair. The guy who owned it said it had a strange "bubbling noise" when he played, and only occasionally. That was it. I started measuring all voltages and they where all pretty normal. The suprise came when I put a signal through it, ca. 3 watts and it started clipping, it should have been over 12W. After all other sources of fault was eliminated I changed the OT to a Magnetic Components type 40-18022, a 5E3 copy and the amp now sounds great and power is restored. Now, the really strange thing here is that the Mercury Magnetics OT that was installed showed no outer signs of damage, no smells, no arcing when it was tested in the amp. Even more strange is that when I measured the impedance ratio, it was ca 8K into 8 ohms, wich is very much as expected. I did not use high voltage AC, just a normal signal generator so as to measure the turns ratio. So it appears to be nothing wrong with it, but in service, it fails. Have any of you out there experienced something similar? I have read about the MM OTs just dying like this one. It also needs to be said that this player looks after his amps very well and he never plays fully cranked. Glad for any kind of feedback.Fri, 18 Apr 2014 14:26:55 -0400 http://ax84.com/bbs/dm.php?id=484057 Transformer Bass Response(by foulowl) Generally I have been advised that guitar should be eq'd with significant bass rolloff. Certainly it depends on what you're playing of course, but I have found generally that that is advised. Why then, the demand for oversized transformers with extended bass response? Edcor, Mercury Magnetics, etc? Just trying to understand. Thanks!Fri, 18 Apr 2014 13:21:22 -0400 http://ax84.com/bbs/dm.php?id=484010 Time to MOD the Hi-Octain(by Curran Murphy) OK- I have had my Hi-Octain running for over a year now, and I have built a train wreck/ac30, I want to figure out if I can add another pre-amp tube to this beast, and gain the heck out of it. When I bought this kit, I got the upgrade output tranny with it. This is a fun little amp,and I am want to figure out if I can also up the output tube into a SE el34 or bigger? Fri, 18 Apr 2014 11:52:32 -0400 http://ax84.com/bbs/dm.php?id=484107 Transformer selection for se el34(by kayakman) I am a newcomer to your community. I have had some positive exposure to Mercury Magnetics transformers. Thought of using there SE-3K OT and there F-CHMP-FS-P 362V-362V 53BiasB+6.3V 5v rectifier.(ss) My goal is to build an P1 Extreme ala EL34 with an input choke L1/C1/L1/C1 with a VVR. Does anone have experience with these transformers Is an input choke array overkill. I used to work on old Dynacos and Altec theatre amps. My son needs a tube amp, so I thought this would be a nice projectFri, 18 Apr 2014 08:29:01 -0400 http://ax84.com/bbs/dm.php?id=484067 New guy(by MrMDK) New guy here. I broke my left wrist in two places about 5 years ago. I tried every way I knew how to play guitar again and failed. Until last week. Sitting on my neighbors couch I was able to play guitar again for the first time in 5 years. I have been building back up every day. My fingers are ready to bleed. I am back with a vengeance. I sold off all my equipment to pay bills. So I am going to rebuild my equipment. This time I am going to build my equipment including my guitar. I live in Austin TX and have access to the Techshop. I can build pretty much anything out of wood or metal but electronics are a new hidden magic to me. I really am looking forward to being a part of this forum and learning from you guys. As for what I am going to build. I think I want to go 80s black label Fender and a 70s style marshal. Any and all help would be appreciated. If you need any custom wood work done I have access to cnc wood and I do cad design and will be willing to help or trade. Happy to be here. Fri, 18 Apr 2014 04:15:28 -0400 http://ax84.com/bbs/dm.php?id=484164 pentode or triode/3K or 5k for EL34(by kayakman) I am unsure if this amp is operating an el34 in pentode or triode I have an additional question regarding best primary impedance for EL34 3K or 5K Thu, 17 Apr 2014 23:36:05 -0400 http://ax84.com/bbs/dm.php?id=484177 October Impedance Selector(by John Bgron) Hi guys, I got my October Club working yesterday and played it all night. It's a great amp, thanks so much for the project. I'm confused about the impedance selector, I'm using a Hammond 125E and I hooked it up to the rotary switch as described in the build document but I'm not sure which is the 4, 8 or 16 ohm output. Can someone please explain with reference to the pole numbers on the switch? Many thanks, JohnThu, 17 Apr 2014 16:25:10 -0400 http://ax84.com/bbs/dm.php?id=484165 Possible replacement for polywood(by sajyho) Hi guys, I'm heading to build an ax84 1x12 cab. Unfortunately I can't find polywood anywhre, but I have access to MDF in every possible thickness, so my questions are: 1. can I use MDF in buffle and if so in which thickness? 2. if no, what is possible replacement for polywood in the baffle and back pannel? Thu, 17 Apr 2014 07:19:27 -0400 http://ax84.com/bbs/dm.php?id=483608 Trace Elliot Amp Repair(by boblove) This is the only area that I think this belongs. If this is out of bounds as far as forum rules go, let me know as well. So here goes. I have an old (older) Trace Elliot GP7SM 300 Bass amp head. All Solid state, non- tube so I'm a bit far out of my comfort zone on this one. It's non-working and I would like some coaching trouble shooting this one. I'm researching an accurate schematic and will have the chassis out of the enclosure and in the cradle tonight for pictures. The amp was puchased new from a Trace dealer circa 1996. It ran fine for about 2 month at which point it stopped working. It went in for a warranty repair and came out and ran for about a week and went back in and back out and in. The details are sketchy because it was so long ago but basically here's what I remember. The tech said some thing about a blown output transistor and having a hard time biasing the output transistor or some thing to that effect. I remember hearing alot about Trace amps of the time having problems. I don't recall specifics, meaning if it was design or poor part choices. I can't seem to find any old info on these amps about upgrades and such. Anyone here game to help me out with this ? Maybe some tech help for an adjustable amp cradle.Thu, 17 Apr 2014 07:04:36 -0400 http://ax84.com/bbs/dm.php?id=484144 EL34 vs EL84(by Mrwylde) Hey everyone, I am new to this site and have studied the H.O. build now for a couple of months, and am ready to ask some questions. 1. Can I run either the EL34 or the EL84 with the stock Hammond 269EX pt? 2. How do I bias the EL34 if I parallel the sockets?Wed, 16 Apr 2014 15:30:01 -0400 http://ax84.com/bbs/dm.php?id=483697 Traynor YCV40/50 power tube setup(by Andyman) I compared the scheamtic of the traynor YCV-40 and its british cousin the YCV-50B. One uses beam tubes (6L6GC) and the other uses the pentodes (EL34). But they have the same power supply.. 410V plate, 390 ohm screen dropper and small 1 watt 220 ohm screen resistors. I think they share transformers so the primary impedance is similar.. The grid bias in those amps have some sort of self-regulating function. But I would think that 1W 220 ohm screen resistors (with that small 390 ohm B+ dropper) would be a little small for EL34 tubes which screens can draw a lot of current when driven hard.. The YCV40 with its 6L6GC tubes with a modest 410V on the plates, 4K primary impedance and 40w output power are not super stressed, since the beam tubes draw much less current than the EL34 pentodes in the YCV50 Todays Marshalls with EL34 tubes are always fitted with 1K 5 watt screen resistors. Did Traynor miss something with their EL34 power stage design... ??? Wed, 16 Apr 2014 08:45:27 -0400 http://ax84.com/bbs/dm.php?id=484157