AX84 BBS Recent Posts as of Mon, Mar 02nd, 2015 @ 10:22:02 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com RE: BIAS supply(by mategenomics) Hello guys thank you for replies. Discussing this with another experienced builder I will use the 44V tap as this should be enough voltage as steve25 said. However some tweaking with resistor values will be needed, maybe I will fully drop feed resistor to BIAS diode.Mon, 02 Mar 2015 05:13:54 -0500 http://ax84.com/bbs/dm.php?id=489403 RE: resistor choice.(by Merlin) <div class="quoteblock" >On 03/01/2015 @ 16:49, Dan Moos wrote : My question is, are metal oxide resistors that bad,</div> For hi-fi yes probably, but for guitar they're OK. <div class="quoteblock" > and what exactly is the bad sound they have?</div> A bit more noise. <div class="quoteblock" >I am leaning towards metal film if I make a switch, but am willing to hear the arguments for carbon film. </div> They're basically as good as each other these days. The noise difference is usually negligible. Metal films are usually blue whereas carbons are usually brown, so use metals for a 21st century look and carbons for a 1970s chic. <div class="quoteblock" > how much margin is considered acceptable wattage wise in a quality amp? </div> x2 wattage rating, or more. Except in cases where you're really up against the wall and you can't afford to double the wattage rating, such as with dummy loads dissipating tens or hundreds of watts.Mon, 02 Mar 2015 03:48:33 -0500 http://ax84.com/bbs/dm.php?id=489402 RE: resistor choice.(by bustapr) the main argument I hear over resistor types is the amount of noise they generate. Wirewound are produce very low noise(only thermal which isnt a huge issue in amps), but are expensive, big, and dont come in bigger values. Metal film are the least noisy of the common resistor types and generally, are affordable, and come in just about every value and wattage rating you can find. CF are slightly noisier than MF. availibility and price is about the same as MF. Metal oxide are much noisier than the two. Carcon composition are too noisy. As a general rule, higher wattage rating means the resistor will be bigger which also means less heat in the resistor which leads to less thermal noise generated. a 1W resistor will be less noisy than the same 1/2W resistor, but also pricier.Sun, 01 Mar 2015 23:41:44 -0500 http://ax84.com/bbs/dm.php?id=489401 RE: resistor choice.(by PhilS) Sorry, I misread your question and thought this was something you'd know. IMO, MF are OK to use. I've got some decent builds with MF. I've used CF and they seem to be just fine, too, but the 1/2W ones have flimsy leads and I don't like that. I don't know what folks object to in CF. CC is rather expensive for my taste. I sometimes buy stuff for salvage and manage to get a few good ones and I use them when I can. I think the simple truth is that I'm past 60 and I probably can't hear like I could when I was 20. Or maybe I just don't know what I'm looking for in hearing the difference. <shrug> I know some folks are very picky about this topic. FWIW, not me. Hope that sheds a little bit of light.Sun, 01 Mar 2015 23:14:18 -0500 http://ax84.com/bbs/dm.php?id=489400 RE: Anyone built Brian's L2L?(by Denny Gracey) Thanks John - I connected with Brian thru his site; he suggested lowering the preamp voltages by increasing R3 to 10K - that did the trick. The L2L is voiced more differently than the SEL than I expected, and enough so that it's worth keeping both. I used turrets on the L2L so it's easier to experiment with different values vs the SEL where I used eyelets, and looking forward to doing so. Sun, 01 Mar 2015 22:34:54 -0500 http://ax84.com/bbs/dm.php?id=489399 RE: resistor choice.(by Dan Moos) I know how to determine the wattage in the part. The main gist of the question was the choice of material.Sun, 01 Mar 2015 21:51:07 -0500 http://ax84.com/bbs/dm.php?id=489398 RE: resistor choice.(by PhilS) Half-watt is more than adequate in nearly all spots. Exceptions are fairly obvious like the B+ ladder. Do the math...V=I*R and P=I*V are your friends here. You rarely get close to 1/2W. Sometimes you need 1W rated for plate load resistors to make the hiss go away. Another advantage of 1W and higher rated is the thickness of the leads. There's another thing to consider. Many 1/2W resistors are only rated for 300-400V! If you are building circuits above that voltage, you may want to go to 1W resistors for the higher service voltage spec or look for higher rated 1/2W.Sun, 01 Mar 2015 21:07:03 -0500 http://ax84.com/bbs/dm.php?id=489397 resistor choice.(by Dan Moos) I have never really considered resistor type choice, but now I feel it's time to get a bit more into component selection than I have been. In all my builds, I have been using two watt metal oxide throughout. Reason? Two watts is more then enough margin in most of an amp, and AES sells a handy kit with a full assortment. My understanding is that metal oxide is not ideal for audio. Metal film and carbon film seem to be available in kits of 1/2 watt assortments (yes, I like kits.) My question is, are metal oxide resistors that bad, and what exactly is the bad sound they have? I am leaning towards metal film if I make a switch, but am willing to hear the arguments for carbon film. I have no interest in carbon comp. Also, since metal film/carbon film come in 1/2 watt, how much margin is considered acceptable wattage wise in a quality amp? Mon, 02 Mar 2015 03:48:33 -0500 http://ax84.com/bbs/dm.php?id=489396 RE: Check Schematic and Power Supply Questions(by turtle441) Merlin, Thanks for the input. The HT filtering section was copied straight from what are purported to be the original Lightning schematics. I found a lot of debate on the ampages forum over where the supply to the output tube screens should come from. (2 different schematics are circulating the internet, with 2 different filtering designs, neither one of which appears to be perfect.) I'm probably going to experiment with running the power straight off of the choke vs with the extra filter as shown on the schematic currently and see which sounds better. I appreciate the other input as well. Hadn't seen the arc protection set-up before, but I'll definitely give that a spin.Sun, 01 Mar 2015 11:56:43 -0500 http://ax84.com/bbs/dm.php?id=489395 RE: Blues peramp motorboating(by sambo69) Glad to hear it - i was going to build a blues-pre, and got a little intmidated when i saw you have to mod the pinout to support the replacement tube. I don't have much experience building so didn't want to screw it up. Regards SamSun, 01 Mar 2015 11:50:10 -0500 http://ax84.com/bbs/dm.php?id=489394 RE: Blues peramp motorboating(by Celeste hall) <div class="quoteblock" >On 03/01/2015 @ 09:13, sambo69 wrote : Celeste, : : did you ever get this sorted out? I'm curious as to the fix for it... : : Regards : Sam -- </div> Yeah, I was foolish. Since 7199 are are long out of production and expensive when you can find then, I used another triode penatode combination, the Russian 6F12P. The two major differences I can find are the triode is higher Mu, more like a 112AX7 then 7199 which is like a 12AU7, and the p9n out is different. I did no6t do a good job of converting the pin out to the different tube. That was one of those moments when you discover why nature in her wisdom has given us palms with curvatures that fit our foreheads.Sun, 01 Mar 2015 11:42:50 -0500 http://ax84.com/bbs/dm.php?id=489393 RE: Want Fender Vaporizer Mods!(by Douche Baggins) Google> Blues Jr. mods> <a href="https://www.google.com/?gws_rd=ssl#q=blues+jr+mods " target="newwin1425309726">[link]</a> Notice the first site listed? Ask the man himself for a schematic. Bigger sound you ask? = plug into a bigger speaker cab..problem solved lol. As tedmich recommended a resonance control will help you to tighten/loosen (for lack of a better term) bass. Sun, 01 Mar 2015 10:48:13 -0500 http://ax84.com/bbs/dm.php?id=489392 RE: Check Schematic and Power Supply Questions(by Merlin) <div class="quoteblock" >On 02/28/2015 @ 20:18, turtle441 wrote : Now for the questions: : 1. Fusing the high voltage secondary - How are you doing this from a layout standpoint,</div> I thought most people used a panel-mount fuse holder? One for the primary, one for the secondary, although two is certainly better for the secondary (one for each leg). <div class="quoteblock" > : 2. Standby switch - All the schematics that I've seen for the amps this was based on don't have a standby switch. </div> Dont waste your time with a standby. If you need a mute switch, just add a switch to short the output valve grids to ground or something. <div class="quoteblock" > : 3. Extra stuff on the rectifier There's a couple of extra parts strung on the GZ34 rectifier. 2 Diodes running from pins 6-7 and pins 4-5. I'd read in Weber's book that he suggests something similar as a failsafe should the rectifier fail, so I'm assuming that's what that is. </div> Yes, SS diodes in series with each anode of the tube recto are a good fail-safe, and reduce the likelyhood of the tube recto arcing in the first place. <div class="quoteblock" >But, then, they also show a 0.01uF 3kV cap strung between pins 6 and 4. What the heck does that do? </div> It helps to quash flyback voltages when the power switch is flipped to off. It's not essential, but it's a nice addition (it's more common to use two caps, one from each leg of the secondary to the centre tap, to reduce voltage stress on them). Your HT smoothng s a little unorthodox. Normally you would run the output tube screens from the output of the choke, rather than from another RC filter. Also, the preamp supply is normally build as a cascade of RC filters rather than several filters all branching off the same point, although it will work. Use 1k resistors on the screens. 100 ohms isn't really enough. You'll probably want to add a bleeder resistor to ground after C2 (several megohms will do) to reduce popping. I'd strongly recommend an arc protection diode or neon for the cathode follower too. <a href="http://valvewizard.co.uk/CathodeFollower4.jpg " target="newwin1425309726">[link]</a> Sun, 01 Mar 2015 10:26:04 -0500 http://ax84.com/bbs/dm.php?id=489391 RE: BIAS supply(by PhilS) You may need to adjust resistor values to get the desired range. Taking the bias supply from the high voltage is likely the simplest way to do this. There are many Marshall schematics floating around that show this. Pick one and tweak it for your needs. Duncan's PSU designer is very useful for modeling this and a great opportunity to learn how the software works.Sun, 01 Mar 2015 10:18:33 -0500 http://ax84.com/bbs/dm.php?id=489390 RE: Blues peramp motorboating(by sambo69) Celeste, did you ever get this sorted out? I'm curious as to the fix for it... Regards SamSun, 01 Mar 2015 09:13:41 -0500 http://ax84.com/bbs/dm.php?id=489389 RE: Anyone built Brian's L2L?(by John Hynes) Brian has this posted at his site: <a href="http://ddawgamps.com/uploads/Dirty_Dawg_052709.pdf " target="newwin1425309726">[link]</a> -John Sun, 01 Mar 2015 07:21:47 -0500 http://ax84.com/bbs/dm.php?id=489388 RE: Check Schematic and Power Supply Questions(by turtle441) And one final one ( although not technically power supply related). Do you typically use switching or non-switching jacks for the out jacks from the OT? It seems like non-switching would be the simplest. But, I've got some extra switching jacks sitting around, might try to save a few bucks. Sun, 01 Mar 2015 07:04:21 -0500 http://ax84.com/bbs/dm.php?id=489387 RE: Amp Switching Circuit Popping(by Brutus) Hi Ted... I doubt they'll supply a schematic of the module itself. It's more likely to be a 'hookup diagram' on how to install it into common amplifiers. I'm sure they want to protect their imtellectual property, even if they did borrow it from here... <a href="http://www.ax84.com/bbs/dm.php?thread=353791 " target="newwin1425309726">[link]</a> Winky winky...Sun, 01 Mar 2015 04:27:37 -0500 http://ax84.com/bbs/dm.php?id=489386 RE: Two More Questions About 1x12 Cab(by Mitch Markin) The front and rear panels are recessed mainly for looks. It makes alignment less critical. You could mount them flush to the cabinet if you want. The front panel is recessed more to allow extra room for grill cloth. If you are covering the cabinet, you could screw on the battens from the outside. That way your screws would have 30mm of penetration into solid wood. Just countersink the screws and use wood filler to fill the exterior holes before covering the cabinet. Or you could screw your battens on from the inside as planned and drill holes 5mm deep into the battens at each screw location. Make the diameter large enough to clear the screw heads. That would give you 15mm of penetration into the cabinet. Either way glue will help a lot. It will do most of the work holding the battens in place. Sat, 28 Feb 2015 22:20:59 -0500 http://ax84.com/bbs/dm.php?id=489385 Anyone built Brian's L2L?(by Denny Gracey) I just finished Brian's L2L SE based on the original schematic posted here; did a bit of thread searches today and discovered that original schematic was prototype and Brian and others have made changes since (e.g. moved tone stack to follow 4th stage), but I can't locate an updated schematic. If anyone has built an L2L, let me know. Thanks! Sun, 01 Mar 2015 22:34:54 -0500 http://ax84.com/bbs/dm.php?id=489384 RE: Check Schematic and Power Supply Questions(by turtle441) One more quickie. It seems like the preferred grounding scheme is 2 separate star grounds... One for preamp immediately adjacent to input socket and one closer to PT for the center tap, poweramp, power filtering. Which one do you usually run the grounds from the phase inverter to?Sat, 28 Feb 2015 21:02:05 -0500 http://ax84.com/bbs/dm.php?id=489383 RE: parallel wire(by JB Angelo) Yeah I don't know why I brain farted with the voltage rating as far as insulation. Anyway thanks guysSat, 28 Feb 2015 21:01:07 -0500 http://ax84.com/bbs/dm.php?id=489382 RE: Burning?!(by RickTown) Sorry. When I say R5, I mean R7. Sat, 28 Feb 2015 20:21:07 -0500 http://ax84.com/bbs/dm.php?id=489381 RE: Burning?!(by RickTown) The 200v is about what's on v3 pin 3. It's a cathode follower and the voltage there will be really close to its grid voltage which is about the same as the plate voltage of the previous stage. Your power tube is cathode biased which means that it's input grid should have basically 0v on it. The current in the power tube is controlled by the combination of voltages on its cathode, plate, and screen. Putting 200v on the input grid will force WAY TOO MUCH current to flow through the tube. The weakest link in this situation is your screen and it's corresponding resistor (R6). R5 is also at risk but since it's only 1/5 of the resistance and also rated at 5W, it will fare much better than R6. C6 is parallel to the cathode resistor and when you created the huge surge of current in your tube you sent your cathode voltage soaring as well. This is because the more current you put through a resistor, the more voltage drop you create across it. This voltage exceeded the rating on C6 which ruined it. This is kind of a reader's digest version. I would suggest that you read through the P1 theory docs even if you already have. It explains this circuit very well and will fill in any gaps that I've left in my explanation. Sat, 28 Feb 2015 20:19:14 -0500 http://ax84.com/bbs/dm.php?id=489380 Check Schematic and Power Supply Questions(by turtle441) Hey guys, working on my first amp design/modification. Prior experience is with "paint by the numbers" type build. Basically, the amp uses a switch to make a 2-channel amp by using 2 different tone stacks. Was wondering if you guys could take a look at my schematic, as well as answer a few newbie power supply questions. Schematic's here: <a href="https://dl.dropboxusercontent.com/u/110475524/SL%20Combo%20for%20AX84.bmp " target="newwin1425309726">[link]</a> I think I've got everything in the design pretty much ironed out. There's one coupling cap value (C2) that I'm going to have to experiment with (the 2 production amps I based it on had 2 different values, where it was relative to the switch, I didn't have a good way to have 2 values) and I'm going to play some with the power filtering node to the power tubes as it seems like there was some controversy online about exactly how that was done. Now for the questions: 1. Fusing the high voltage secondary - I've seen a couple of posts here advocating doing this, either by putting fuses on the secondary leads, or by putting a fuse or resistor on the center tap. How are you doing this from a layout standpoint, mounting fuses next to the rectifier? I haven't seen this in the schematics of production amps that I've looked at (admittedly, mostly older ones), so I wasn't sure how it was usually implemented. 2. Standby switch - All the schematics that I've seen for the amps this was based on don't have a standby switch. I checked the manufacturer's website, appears the production models don't either. In doing some reading on Merlin's website, he makes a pretty good argument that if you've got a tube rectifier, it's unnecessary. How are you all handling this now days, do you not use standby? Have any of you implemented Merlin's suggested solution (the 1-2W resistor in parallel with the standby switch)? Right now I'm leaning towards no standby, but was curious as Merlin's solution was a new one to me. 3. Extra stuff on the rectifier - This one doesn't pertain to my schematic... yet. I've been researching some different schematics and layouts for the spitfire and lightning online. Looking at the ceriatone's layout (not posting a link as I'm not sure what the rules are... it is freely posted on their website but I know admins are strict about that stuff here). There's a couple of extra parts strung on the GZ34 rectifier. 2 Diodes running from pins 6-7 and pins 4-5. I'd read in Weber's book that he suggests something similar as a failsafe should the rectifier fail, so I'm assuming that's what that is. But, then, they also show a 0.01uF 3kV cap strung between pins 6 and 4. What the heck does that do? Sun, 01 Mar 2015 11:56:43 -0500 http://ax84.com/bbs/dm.php?id=489379