AX84 BBS New Threads as of Wed, Apr 23rd, 2014 @ 16:10:09 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="newwin1398283810">[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? Wed, 23 Apr 2014 13:54:58 -0400 http://ax84.com/bbs/dm.php?id=484244 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="newwin1398283810">[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 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 Wed, 23 Apr 2014 15:24:32 -0400 http://ax84.com/bbs/dm.php?id=484210 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!!Wed, 23 Apr 2014 13:37:13 -0400 http://ax84.com/bbs/dm.php?id=484188 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 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 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 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 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 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 October Club blown transformer?(by John Bgron) Hi guys, I finally got around to finishing my October Club amplifier. It worked (almost) fine when I first powered it up but V2 started red plating. I powered down, removed the power tubes and started taking a few measurements then the fuse blew, and it kept blowing each time I turned the amp on. I disconnected the power transformer to test it but now it trips the safety switch in the house. Is it safe to assume the power transformer is blown? Many thanks, JohnTue, 15 Apr 2014 22:48:43 -0400 http://ax84.com/bbs/dm.php?id=484138 Output problems(by winghamwill) Hi, i have finnished building my first amp, an SEL and it sounds awesome!! So loud, no buzz, its just great, although, when recording last week i had a problem where it suddenly cut out, as we were stuck for time i changed amp at the time. Since then i have done a bit of investigation and the 8ohm output works fine, its just the 4 ohm, (i didnt put a 16 ohm output in). When i play through the 4 ohm i can hear a buzzing theough the output transformer almost to the sound of what im playing on my guitar? Is this just a fault with that output on the transformer or something different? Any ideas? Cheers WillMon, 14 Apr 2014 03:36:12 -0400 http://ax84.com/bbs/dm.php?id=484123 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 SajadWed, 23 Apr 2014 15:31:38 -0400 http://ax84.com/bbs/dm.php?id=484114 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 unknown mod function(by Gorhrut) before i begin, i am a newb, and most likely will not understand what u are saying unless it is written in very simple terms. ok, i received some mod advice for my preamp and dont really understand what it does. this is the advice: " the last big mod is the LED mod. Cathode (short leg) goes to ground and the anode connects to the resistor in the gain stages." the resistor he is talking about is v1a cathode resistor which is 820 ohm and has a .68uf bypass next to it. what effect will this have on the sound/what is the function? am i supposed to run the led in parallel with the resistor or after it in series? (ive seen people replace the resistor and bypass for LED bias and ive seen leds used on the plate but what will it do in this situation?)Mon, 14 Apr 2014 05:33:01 -0400 http://ax84.com/bbs/dm.php?id=484088 Fender PA 135, Help Needed(by mroweitall) I've got a Fender PA 135 from 1979 that I scored from a local facebook post. I live too far away from any repair technicians and I don't know much about tube amplification, but I DID run with 3 out of the 4 power tubes missing. (I have a 16 ohm 12" eminence speaker plugged into one of the "8 ohm minimum" speaker outputs.) It worked for a couple of days until the other day when it quit. I took it apart and noticed that the 12ax7 preamp tube, (which is responsible for gain in all 4 channels), it wasn't lit (glowing) nor was preamp tube v1-v4. All the power tubes light, as well as v5 and v6 of the preamp, but I've never been able to get any reverb from it, (other than shaking the whole head, then I hear the spring bounce a little.) Lost with what to do. At first I thought maybe the little reverb drive transformer needed replaced, then when I noticed the preamp tubes weren't lit, I don't know what could cause that...any ideas?Thu, 10 Apr 2014 08:18:39 -0400 http://ax84.com/bbs/dm.php?id=484086 Low voltage amp 26A7-GT(by JaapK) Another attempt of me to get a good sounding amp with a double pentode, 26A7-GT. Low voltage. And must say: maybe this time with success.. This is what it is now: self split power amp 26A7 and 3 gain stages. First stage EF86, than two stages 12SX7. Power supply: PT secondaries 2 x 25V/2 x 600mA. One is used for filament 26A7. The other is doubled so the B+ rail is around 65V. With -13.5V at the katode the 26A7 is around its max but stays alive. Power out: around 1 Watt? don't now exactly. Datasheet gives 500 mW @ 26.5 V. First results are quite oke! Gonna do some tweaking and try a tone stack somewhere. Am thinking of using maybe a 6J5 pentode instead of the EF86 making it a all octal amp. Or a pentode that runs at filament of 12V so I can put it in series with the 12SX7. Well if anyone has ideas or so, please let me know. I can post the schematic and maybe some simple clips if someone is interested. JaapSat, 12 Apr 2014 07:44:32 -0400 http://ax84.com/bbs/dm.php?id=484078 SEL - James Peters Mod(by Dave C) Has anyone tried the James Peters mod in a SEL? I'm running an EL34 in the power stage and started by changing out R19-> 5K and removing C12. This is often touted as the sweet spot on the second stage for a JCM800. In my case it just killed the shimmer,snap and grunt that I had going with the stock schematic. I left it like this for a few days and then put it back to the original configuration. I never made it to the second mod which is to reduce attenuation on R16 and R17. Has anyone noodled around with this mod? I'm guessing it may be a bit different when pushing an EL84 as intended. Sun, 13 Apr 2014 07:54:00 -0400 http://ax84.com/bbs/dm.php?id=484070 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 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 Adjustable fixed bias resistor substitutions (by deiseldave) Hello, I am building a Mark Huss-esque Plexi6V6, except (unlike Mark's) my PT had a 45V bias tap. So I planned on using the adjustable fixed bias scheme shown on the EL34world site (10K pot with 27K going to ground). The problem is, I just realized I forgot to order a 27K resistor... The closest values I have are a 22k and a 33K. Can I sub one of these? Thanks. Sun, 06 Apr 2014 08:45:26 -0400 http://ax84.com/bbs/dm.php?id=484047 power supply question(by water) This is an embarrassing question. But I am confused at a very fundamental level. I have read many tutorials and some info stored on the forum here. I think I have a basic grasp of how it all works...but I have a question about the PS Caps. When you first read about how tubes work, the explanation is pretty much the same. A filament heats the cathode, the cat is made from or coated with a material that releases electrons when it gets hot. These electrons gather in a Space Charge between the cat and the grid. As an AC signal hits the grid, it will cause more (or less) electrons to flow to the positive plate. As the grid gets more positive, more electrons flow. As the grid becomes more negative, less electrons flow. There is never any mention of power supply caps...just that the heated cathode creates the electron space charge. So my question is...do the filter caps "add" anymore electrons.? Or do they just store and release electrons that are created by the cathode via thermionic emission.? I realize the caps are part of the B+ supply at the plates. I hope you guys can understand what I am asking. Thank You Sat, 05 Apr 2014 20:12:17 -0400 http://ax84.com/bbs/dm.php?id=484015 Hum, maybe from power supply?(by KnaveWave) So after tinkering around a bit with my tw express + Merlin VVR amp I've decided to track down its horrid hum issue. This is my 2nd and 3rd builds (made a pair of em), and both have the same issue. Basically the amp is functional and sounds nice, but has an unacceptable amount of hum and tendency towards oscillation. Hum is present w/o any input connected, and gets louder as volume increases. I've been picking off what I assumed would be the likely culprits one by one: tidied up the heater wiring to the first two tubes, got rid of the ground bus across the front pots and reworked grounding so that there are two chassis ground connections (amp circuit to a lug next to the input, and a mains ground immediately next to the IEC socket), switched to isolated input/output jacks, and re-routed the mains 120VAC from fuse to power switch so that it tracks the sides of the chassis (before this is was a straight shot across the top of the power supply). The only change that gave any noticeable improvement was the mains re-routing. After this change, there are certain settings (I think its about a 3 on volume and low treble) where there is very little hum, but for the most part hum remains unacceptable. I finally decided to break out my scope and see if it would help. After a bit of probing I noticed the following output taken immediately off of the B+5 connection from the power supply (symptom is there for all B+ outputs). <a href="http://i67.photobucket.com/albums/h315/Richard_Troxell/BPlus_zpsb17fa079.jpg " target="newwin1398283810">[link]</a> From what I can tell, it looks like blips of 60hz AC peaks are making it out of the power supply. My theory is that this is directly feeding into the signal path and causing the hum. For grins, I've also disconnected all pre/power tubes, and am getting the same reading. Does this seem plausible? Any debugging suggestions based on the waveform? Sun, 13 Apr 2014 12:08:08 -0400 http://ax84.com/bbs/dm.php?id=484013 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