AX84 BBS Threads with new messages as of Sun, Apr 26th, 2015 @ 09:49:57 Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kits Under $50 Over-Ear Headphones(by Douche Baggins) Just purchased a Status Audio HD One headphone <a href=" " target="newwin1430056197">[link]</a> last week. I researched a lot of under $50 headphones before deciding on this one. There are several reviews on line, almost all are positive and mine is as well. Very neutral stock setup and need EQ to bring them to life. When EQ'd properly sound great with very good passive noise cancellation. What tipped the scales compared to others in it's price range is that they are made in the USA..Brooklyn, N.Y I have no affiliation with the company in any way. Sun, 26 Apr 2015 09:23:39 -0400 Hookup Wire Choices(by KenInVic) I'm looking at some different lines of wire from Mouser to use for hookup inside the P1 I am building. Two are 18AWG and are Alpha series 1857 and 3075. The other two are 22AWG and are Alpha series 1855 and 3071. Any reasons to choose one over the other in each of the gauges? I have some 18AWG TEW wire that is just too unweildy to use as it's more for wiring appliances. BTW, I'm okay with PVC ($$$) and have a personal preference for stranded. Here are some Mouser #s for the two 18AWG 602-1857-100-04 602-3075-100-04 Here are some Mouser #s for the two 22AWG 602-1855-100-02 602-3071-100-03 <a href=" " target="newwin1430056197">[link]</a> Thanks in advance.Sat, 25 Apr 2015 20:49:08 -0400 Poweramp Cathode Bypass W/ Bias Check Resistor(by lkrasner) When putting a 1 ohm resistor in series with the bias resistor to facilitate checking of the bias current, should the bypass cap fully bridge BOTH resistors, or is it ok to leave it only across the larger bias resistor?Sat, 25 Apr 2015 18:25:47 -0400 Using an old SE transformer(by DaveW) I have been given an old partridge transformer. As far as I can tell it is a single ended unit and looks to be quite high power. It has one secondary which has a resistance of 44.8 ohm, the primary has a resistance of 413 ohm tapped at 376 and 265 The unit it came from originally had two EL34 valves and looked like a PA chassis (homebuilt). If I put 7.6V AC to the secondary I get 59.8v across the full secondary. This calculates to a ratio of 7.83 so a 16R speaker would give a 998 ohm anode impedance. Is this a bit low for a pair of EL34s? Could it be a 100v output transformer? I would like to use this any suggestions what valve/valves would this work well with? Thanks, DaveSat, 25 Apr 2015 07:50:05 -0400 JCM 800 voltages(by mategenomics) Hi guys! I am building JCM 800 100w clone. Schematic is wired and I proceeded to check the voltages against table. (Note : I inserted only preamp tubes as I measured voltages). Heaters are all good. For bias I have 44V tap on my mains tranny. Behind BIAS diode I get -69V (I omitted the 27k feed resistor as the voltage is already quite low). I get -37.3 to -43.4V on pins 5 on power tubes. Quite narrow range but I think I can manage that by swapping higher resistance BIAS pot (I have 50k, so maybe 100k would do that?). However I get some weird measurements on various pins compared to the table as following : V1 : 1-270 2-0 3-3 6-247 7-0 8-210 V2 : 1-199 2-0 3-2 6-352 7-199 8-210 V3 : 1-373 2-165 3-329 6-368 7-165 8-330 V4 : 1-0 (on all power tubes) 3-515 4-510 5- -43(for all power tubes) 6-510 8-0 (on all power tubes) V5 : 3-512 4-0 6-0 V6 : 3-514 4-0 6-0 V7 : 3-511 4-0 6-0 I double checked the wiring and possible shorts but nothing unusual. Still I think voltages on V2/V3 are too high for preamp tubes and there is the missing reading on pins 4 and 6 ond V5/V6/V7. Do you have a clue about what is wrong? Thanks in advance :)Sat, 25 Apr 2015 07:41:09 -0400 output section(by jmpj46) I am building the lead 2 amp brian schmittling version on the output jack the yellow wire goes to the +and- the - goes through the 220r 5watt resistor and the middle lug with the black arrow goes to + on both jacks what does that do only one i ever use is the + and - Sat, 25 Apr 2015 05:29:03 -0400 P1 (EX Sort of) on a record!(by Frank Tone) Hello all! I have an EP out now that I'm quite proud of, and it features and amp that I built with the help from many members of this forum back in 2003! I've built many amps since then, but the P1EX is still my go to recording amp! You can hear the amp on track 1 "Keep the good times coming" at: <a href=" " target="newwin1430056197">[link]</a> Or at my website under "music" <a href=" " target="newwin1430056197">[link]</a> About the amp: It's a 12v version of a P1EX (12ax7 and 12v6) and the voltages are a little lower(about 250v on the plates of the 12v6). So it's more in line with the original P1, I suppose. Also, I changed the plate resistor on the first gain stage to clean it up a bit and give it some chime, which I think it excels at! I try to keep this self promotional stuff to a minimum, but thanks for listening! Thanks again Chris and everyone who's ever answered a question! Fri, 24 Apr 2015 21:15:51 -0400 Seeking long 1/8" / 3.5 mm stereo jack(by Paul Fawcett) I need help from the brain trust here. I'm looking for a panel mountable 1/8" / 3.5" stereo input jack (this is for plugging an iPod level device into a hi-fi project I'm working on). The problem that I'm facing is that the faceplate material for this amp is a full 1/8" thick aluminum. On all the jacks I've found so far, the bushing is just to damn short to poke all the way through and still let me get a couple of turns of the fastening nut onto the threading... I need a jack with a longer-than-standard bushing. Anybody? Fri, 24 Apr 2015 19:38:33 -0400 Ca Dreamer Fuse Blowing(by Jon Endres) Built a CA Dreamer a couple of years ago, very close to Dave Zeryck's 2.6 version whose schematic is available here. Worked and sounded great for 2+ years, but then started blowing the 250mA fuse on the output side of the rectifier tube. At first it was just blowing on occasion, but now it will blow about 50% of the time. It ONLY blows when switching from standby to on (i.e. connecting the rectified high voltage to the filter caps and the rest of the circuit). If it hasn't blown by the time it's all powered up, it's fine regardless of how long it's powered and how hard it's played. It will also often blow the fuse if the standby switch is left on and the unit is simply powered up. The DC voltage will start to rise as the rectifier starts to conduct, and then kaflooey. I originally tried replacing the power tubes, no improvement. It will now blow the fuse even with all tubes removed, so I'm getting close to concluding that it has to be one or more of the filter caps, probably that 40uf monster that hangs right off the rectifier, or maybe the 20uf downstream after the choke. The other two filter caps come after 10K resistors, so they couldn't draw enough current to blow a fuse. The high voltage also passes through the primaries of the output transformer, but unless they're intermittently shorting to ground, without the power tubes inserted they're essentially open. Am I thinking correctly, and what/how should I test next? I've got fuses with higher current ratings (e.g. .315, .5A), but I'd like to keep this non-destructive if possible. The caps aren't bulging, and respond normally to the stupid ohmeter 0->infinity test. THANKS FOR ALL SUGGESTIONS! Jon I'm assuming that the Fri, 24 Apr 2015 06:42:44 -0400 JCM 800 volume/gain relationship(by Audiosalvage) I'm hearing all the praise for JCM 800 sounding it's best at near full volume. I think mine sounds great at lower volumes getting more and more woofy and less and less juicy as MV increases. In other words, I can get a nice VH tone with bright staccato attack just under drum volumes. If I were to set my MV high and gain lower, my sound would be so clean and brittle, lead solos would be painful and difficult to play. Thu, 23 Apr 2015 18:34:31 -0400 grounding(by jmpj46) I am building the ax84 lead 2 under the associate page looking at the schematic you ground the yellow/red wire from transformer.If you look at his club schematic for a reference he don't have a board layout for the lead 2 the yellow /red wire goes to the board - side then he has the board - going to the input ground.If you look at the sel schematic coming off the board - side it says add ground lug if needed if i do that it blows the fuse so if use the red/yellow wire to ground does that ground the whole system because it will blow the fuse if you run it to chassis ground thanks.Thu, 23 Apr 2015 14:11:35 -0400 How many channels for 2X12 tube guitar amp project?(by roberts67) Hello! I have an empty 2x12 Vox cabinet, a bunch of speakers and a Motorola 6V6 3 channel console amp. So I want to put together a tube guitar amp and know enough not to get killed. Each channel on the Motorola has 2 6V6 tubes and 1 12AX7. The output transformers are for 8 ohm speakers. Should I - 1. Have a mono guitar input and split to 2 channels and run each channel to an 8 ohm 1X12 speaker? or 2. Have a mono guitar input to 2 push pull pairs of 6V6 tubes and run the combined output into 2 4ohm speakers wired in parallel(to get 8 ohms)? 3. Have a mono guitar input to 1 push pull pair of 6V6's to 2 4ohm speakers wired in parallel(to get 8 ohms)? Any direction appreciated. RobertThu, 23 Apr 2015 14:09:35 -0400 Resistor Clearances(by KenInVic) At what wattage rating do I need to ensure I am giving room for air circulation between a resistor and the eyelet/circuit board? Is there a rule of thumb? Other than for the heat concern, I would rather set the resistors flat against the board for the added mechanical stability.Thu, 23 Apr 2015 06:18:40 -0400 Parallel Single Ended Amplifier(by JKCavitt) I finished this project last weekend - installed into a cab I built: <a href=" " target="newwin1430056197">[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="newwin1430056197">[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.Wed, 22 Apr 2015 20:57:50 -0400 My latest amp project(by Paul Fawcett) Hey all, just wanted to share a quick picture of my latest amp project: <a href=" " target="newwin1430056197">[link]</a> A schematic will follow, but the amp itself is a 4x6V6, and is a real merger of new and old. It takes cues from the tweed era and old Gibson Scout type circuits, but has many tweaks that lend a much more modern sound (it's fixed-bias, uses higher voltages -- 400V on the plates, more filter capacitance, makes use of LED biasing, etc. There's even a MOSFET lurking in there). The plan with this one was to build it just from stuff I already had lying around the shop (ok, ok, there is a LOT of stuff lying around the shop!). It only has three preamp tubes, but manages to incorporate both tube reverb and tube tremolo. The breakdown is as follows: V1. ECC832/12DW7 the high gain 12AX7-like half of this triode is for the amp input gain stage, the 12AU7-like half drives a single tube reverb. The reverb uses an "F-series" high impedance tank. Tone stack is a simple tweed style one knob. V2. 12AX7, one half is second gain stage, the other is reverb recovery. V3. 12AX7, One half is a cathodyne phase inverter, the other half is the low frequency oscillator for the tremolo. There is no cathode follower for the LFO, instead I used the scheme I described in this thread <a href=" " target="newwin1430056197">[link]</a> involving a MOSFET to blend bias voltage with the oscillator output in order to accomplish power tube bias shift trem (works great!) The awesome faceplate was made for me by AX84's own MasonAtom (Mason Wolak). Mason is the proprietor of TMI amplification <a href=" " target="newwin1430056197">[link]</a> and I can't recommend his work more highly. The picture does not do it justice.. The lovely gloss material I chose is so perfect and reflective that it's almost impossible to get a straight-on shot without seeing reflections in the material, and the laser engraving is super high quality. If anyone happens to be in the Richmond, VA area, I'll be taking this amp and a few other creations to the Richmond Area Gear Expo (RAGE) that is taking place at Sound of Music Studios on Broad St., tomorrow 18 April 12-4PM. I'd love to see some AX84 folks there! <a href="" target="newwin1430056197">[link]</a> Wed, 22 Apr 2015 20:13:24 -0400 next build(by bullpeters) i wrote a long post then deleted it Built a 4-4-0. but its gets "hot"to soon. the clean to mean thing is not real wide i love to build another 6sn7 output stage, will an october be "that"more versatile? squeezed into a little epi junior combo (found at tip) with a new head? i want some thing for the living room that does more than "bridge of sighs". plus the kids complain about all the "Noise"(distortion) my question october more versatile than the 4-4-0? more clean less mean? loath to put a SE EL84 back into what was an SEL84 etc cake and eat it situation i guessWed, 22 Apr 2015 19:51:26 -0400 Cell Phone Video Recording Phasing(by Douche Baggins) Would like to know why when recording a video using a cell phone they tend to cause phasing in the audio. Phasing seems to get worse when recording an amp that has heavier distortion as compared to clean or mild gain amp. Here's a couple clips I recorded recently: Clean: <a href=" " target="newwin1430056197">[link]</a> Distortion: <a href="" target="newwin1430056197">[link]</a> Wed, 22 Apr 2015 19:43:24 -0400 Hot Air Rework Station(by FredB) I got a job building and testing satellite antennas and matching circuits that require soldering surface mount components, while this is possible with a regular soldering iron, I would like to explore the use of a hot air rework station to do this. So, I was wondering if anyone here had any experience using one, and if so what brand and model would that recommend to get and/or to avoid.Wed, 22 Apr 2015 00:29:09 -0400 VVR3 White noise issue(by Leroy from Oz) Hi All I've just installed a Hall VVR3 into a 6v6 plexi and i'm getting a loud crackly white noise/hiss when the pot is rotated above about 75% of the pots rotation. The noise is not while i'm rotating the pot, it happens after I've stopped. When i rotate the pot back below 75% the noise stops. It's power amp only VVR. I've used this exact vvr3 board in a previous install without issues. I'm thinking it may be a bad pot. I don't have a dual gang 1M log pot handy so i'd need to order one to check. I'm just wondering if any one has experienced similar issues or has any suggestions. Thanks. LeroyTue, 21 Apr 2015 20:29:44 -0400 Clean Preamp, any audio demo?(by ImNotThrasher) Hi, im looking for a clean amp, for months this "Clean Preamp" was catching me, some friends says that I need to build a Fndr Twin, I like this too, but the AX84 just looks more "original" and versatile. So I need information from who build it, if it can compare with other amps or any info or opinion of this preamp, I was searching for audio demos or videos and I didnt find anything, im looking this for clean and for using it with my distortion pedals, I know that this one has a lot of headroom, but I never hear it, any help?Tue, 21 Apr 2015 19:39:10 -0400 Selectable bypass caps in class A power section?(by Alva Goldbook) If I'm remembering this correctly, if you have a power section that is cathode biased, and you use a lower value bypass cap (say 47uf) then won't you get kind of a natural compression? But if you raise the value of the bypass cap to around 220uf (or maybe more) that you'll get a response that is closer to a fixed biased amp? I was thinking that it would be pretty cool to be able to switch between two values, but I'm wondering if this would cause problems with flyback voltages or something like that. Has anyone ever tried this?Tue, 21 Apr 2015 19:10:16 -0400 Dagnall D2505 OPT(by Lord of the Livingroom) Hi all, Wondering if anyone knows the specs for this OPT? I'm currently building a JTM45 clone, and picked up one of these at a reasonable price. I believe it's from a JTM45 re-issue amp that had KT66's in it. I am unable to find any information about it's primary impedance anywhere on the web. And the new Dagnall website people haven't replied to my request for information. Does anyone know anything about it?Tue, 21 Apr 2015 09:26:18 -0400 22k PA line Transformer(by bullpeters) In Australia, you can buy real cheap Pa line transformers for El84, 6V6, push pull amps. Does any other Australians know a source for PA line amps with a 22k primary? Or basically anything that would reflect properly for a 6SN7 push pull out put stage. Checked the web but got nothingTue, 21 Apr 2015 03:12:27 -0400 Voltage Divider Q?(by vicrattlehead) Been reading up on some things and a question came to me about voltage dividers: If the voltage divider ‘rule’ is: Vout=Vin*R2/(R1+R2), where R2 is the path to ground, why are large-ish value resistors used in amps like the HO (just for example)? Wouldn’t a pair of 10k resistors (just as a random value) have the same 50% signal reduction as a pair of 220k’s (like what’s used between stages on the HO)? According to that formula it’s the ratio between the 2 resistors that determines the voltage reduction, not the values. Is there some inherent benefit to using larger values? I’m trying to get a better grasp on a lot of these concepts so I can better understand how and why my amp works like it does. I am admittedly weak on electronics theory though, so any help would be greatly appreciated. Sat, 18 Apr 2015 18:14:37 -0400 1x12 Cab Batten Screw Locations(by cpsmusic) Hi Guys, Finally got some time to put together the 1x12 cab. I notice on the diagram that the location of the screws that hold the long battens are different - one screw is inset 2 inches while the other end is inset 2 1/2 inches. Any reason for this? Cheers!Fri, 17 Apr 2015 09:34:28 -0400