AX84 BBS Recent Posts as of Tue, Jul 22nd, 2014 @ 05:12:10 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com RE: Preamp with integrated graphic eq: Signal ground Vs. Virtual gro(by EeroNiemela) Thanks for the reply! I do indeed have a cathode follower and the heaters are elevated to reduce the cathode-heater voltage in that stage. However, I failed to mention that I'm powering the heaters from a 6,3 V tap (no centre tap) on the PT, and the 12,6 V tap is a separate one. The only other thing the 12,6 V supply would be powering beside the Eq would be the channel switching relays. I drew a quick schematic about what I'm getting at: <a href="https://dl.dropboxusercontent.com/u/66888777/2014-07-22-1225.jpg " target="newwin1406020335">[link]</a> What seems obvious is that I must not connect the negative pin of the regulator to ground. Can I connect the eq to the power supply like this, or is there something I've overlooked? And could the relays cause switching noise, if they're supplied from the same power as the eq?Tue, 22 Jul 2014 04:04:39 -0400 http://ax84.com/bbs/dm.php?id=485610 RE: Preamp with integrated graphic eq: Signal ground Vs. Virtual gro(by Ardencaple) Hi The first thing you need to realise, is that the circuit you show uses both positive and negative power supplies. That is, the op amps are provided with a supply of +9V / -9V OR +4.5 / -4.5. So the signal ground is halfway between these. The signal ground does need to connect to the amp (and chassis) ground at ideally, only one point. As you have pointed out, you need to ensure that this signal ground sits at the mid point of the DC supplies for the op amps. This is particularly true if you are using the heater supply. If the winding is a centre-tapped winding (6.3 - 0 - 6.3), then this corresponds to the dual battery version of supplies shown on the schematic, so you can connect the centre tap to the system ground, and this will provide a correctly balanced supply. However, there could be other issues. If your amp uses any stages that have cathodes at high voltages above ground (such as Cathode Follower stages), then it may sometimes be necessary to connect the heaters to a higher voltage in order not to exceed the heater / cathode voltage limit. Plus there may be other hum issues. The point I'm making is that you are trying to make the heater supply perform two jobs, and it may not be possible to optimise both .. I'm not saying it won't work, but you should be prepared for some head scratching at some point. Grounding can be tricky .. In any case, I think you should build the circuit separately, with an external test supply (for example two batteries!), before integrating it into an amp. That way, you will know that the circuit itself works. Incidentally, you are not using the term 'virtual earth' correctly. A virtual earth is a signal summing point that is forced to be at 'earth' potential by the circuity, even though it is not connected to an earth point. Look up how an inverting op-amp amplifier with resistive feedback works: the negative input of the amplifier is at 'virtual earth'. Tue, 22 Jul 2014 03:14:22 -0400 http://ax84.com/bbs/dm.php?id=485609 Preamp with integrated graphic eq: Signal ground Vs. Virtual gro(by EeroNiemela) I''ve been tinkering with the concept of adding a graphic eq to a tube preamp (ala Mesa). The Graphic Eq schematic is this here: <a href="http://www.generalguitargadgets.com/pdf/ggg_eq6b_sc.pdf " target="newwin1406020335">[link]</a> It seems to me that this is a very typical GEq design, essentially the same as eg. the EHX GEq pedal. So what I'm basically trying to accomplish is building this circuit inside the same chassis with a tube preamp. I intend to power the eq with a regulated supply off the 12,6 V tap of the PT. This supply would also power channel switching relays. What confuses me though is the grounding. The signal ground would be the usual ground lug connected to the chassis near the input jack. But what about the virtual ground of the Eq circuit? I fear that simply connecting the virtual ground to the chassis ground lug might cause a problem by making the two grounds "fight" eachother somehow. Then again, as can be seen from the schematic, the Eq circuit combines the two grounds anyway, so I can't really see a way of isolating the virtual ground from the signal ground either - the two will have to meet somewhere. So I guess my question boils down to this: Will connecting the virtual ground to the signal ground of the preamp (which is also shared by the HT (B+) filter cap grounds and safety earth connection) cause issues? Tue, 22 Jul 2014 04:04:39 -0400 http://ax84.com/bbs/dm.php?id=485608 RE: Inexpensive P1 type amp kit, any interest?(by jason d) If you could do that for $50 I'm pretty sure there would be a lot of interest, especially for 1st time builders.Tue, 22 Jul 2014 01:41:56 -0400 http://ax84.com/bbs/dm.php?id=485607 RE: Graphic EQ slide pot value(by EeroNiemela) Thanks for the replies! I have been studying the circuit and opamps (which I'm new to) and it does seem like the slide pot value is not critical.Tue, 22 Jul 2014 01:37:25 -0400 http://ax84.com/bbs/dm.php?id=485606 RE: Cap coupeld reverb help(by RickTown) Welcome to the group. Use an image sharing site to post your pictures then post a link here for us to follow. It's important for you to post a gut shot or two because it could be a layout issue that won't be on the schematic. My first guess is a ground loop from grounding the reverb stages in the wrong spot. Only gut shots will tell us for sure.Mon, 21 Jul 2014 15:54:49 -0400 http://ax84.com/bbs/dm.php?id=485605 FS: Epi VJ Tweed Deluxe Head Project And Nevoibe Project(by Javaman) I have the following unfinished projects for sale : 1. Epi Valve Junior Conversion Tweed Deluxe head From PPwatt.com . This is a very Clean Build That needs debugging . It is Complete but does not power up. Comes with Tubes, Custom Face and rear plates, Power Cable, and a Beautiful Tweed Covered Epi Head Cab. If you have the ability and the time you can have a sweet Fender tweed Deluxe amp in a very small Lightweight package for the price of a pedal. $250 OBO 2. This is a completely functioning Neovibe Board that needs to be finished wiring into a brand new Pedal Parts Purple sparkle Enclosure. $100 OBO I do not have the means or time to complete these projects and need to purchase band equipment. Feel free to ask any questions PM me or email at Jotova@aol.com Plenty of good deals here and on TGP Ebay ID for a reference: Jotovibe Thanks ! JT [img] <a href="http://i60.tinypic.com/167ta52.jpg[/img] " target="newwin1406020335">[link]</a> [img] <a href="http://i59.tinypic.com/kb2fl4.jpg[/img] " target="newwin1406020335">[link]</a> [img] <a href="http://i59.tinypic.com/4t3ayc.jpg[/img] " target="newwin1406020335">[link]</a> [img] <a href="http://i62.tinypic.com/wi4olh.jpg[/img] " target="newwin1406020335">[link]</a> [img] <a href="http://i60.tinypic.com/33vbxhk.jpg[/img] " target="newwin1406020335">[link]</a> [img] <a href="http://i61.tinypic.com/4lt5hw.jpg[/img]" target="newwin1406020335">[link]</a> Mon, 21 Jul 2014 14:34:15 -0400 http://ax84.com/bbs/dm.php?id=485604 Cap coupeld reverb help(by reverberocket) Hello guys first time poster here. I have some trouble with my reverb I build in my amp. I build a reverb like a ampeg superb reverb in my tweed amp. It sounds nice but if i close the pot of the reverb it starts humming real loud. Does somebody have some ideas. I will try to get my schematic on here as soon i figur out how this works. Also thanx for all the information I already got here. I learnd a lot but I am still on a bumpy roadMon, 21 Jul 2014 15:54:49 -0400 http://ax84.com/bbs/dm.php?id=485603 RE: Where to get parts?(by RickTown) It's nice living 20 minutes from AES. I walk up to the window and walk away with my box of goodies! Spoiled...Mon, 21 Jul 2014 08:31:59 -0400 http://ax84.com/bbs/dm.php?id=485602 SEL Effects Loop Noise(by DethRaid) I've added Mark Lautizer's effects loop (schematic at <a href="http://pelvis.home.xs4all.nl/ax84/HO-FX+NFB.pdf " target="newwin1406020335">[link]</a> ) to the SEL I built, but I get a rather high E flat ringing out when I'm not actively playing my guitar, even when nothing is plugged in to the effects loop. My dad and I think that something in the circuit is causing my tubes to oscillate, although neither of us know why. Do any of you have any ideas about how I can get rid of this noise? Here's some more details: A few years ago, I built the SEL from the kit you used to be able to order from Doberman (no clue if you still can). A few days ago, I added in Mark Lautizer's effects loop (schematic at <a href="http://pelvis.home.xs4all.nl/ax84/HO-FX+NFB.pdf " target="newwin1406020335">[link]</a> ), with the minor modification of a 2.7K and 3.9K resistor in parallel in place of the 1.5K because I forgot to order a 1.5K resistor. I attached the 4 ohm tap from my output transformer secondary to the point labelled "from Output Transformer Secondary" since I had read on this forum to just try different taps from the output transformer to see which one works the best. When I powered on my amp and flicked the standby switch, I was greeted with a very loud screeching noise, even though all my volume and gain and tone controls were set to zero. I then attached the "From Output Transformer Secondary" to ground, which results in a rather high E flat ringing out when I'm not playing my guitar. I've found that I can lower the pitch by turning up the mid knob, and get rid of the noise entirely by turning the treble know to zero, but that's no fun. I can also get rid of the noise by turning my guitar's tone knob to zero.Mon, 21 Jul 2014 07:05:33 -0400 http://ax84.com/bbs/dm.php?id=485601 RE: Where to get parts?(by Stephen Keller) <div class="quoteblock" >On 07/20/2014 @ 20:38, Darren Porter wrote : I've never experienced a minimum charge at AES. I recently did a $14 order with them. </div> It has been a while since I last ordered anything from AES. Perhaps they've changed their policies. StphSun, 20 Jul 2014 20:48:33 -0400 http://ax84.com/bbs/dm.php?id=485600 RE: Where to get parts?(by Darren Porter) I've never experienced a minimum charge at AES. I recently did a $14 order with them.Sun, 20 Jul 2014 20:38:17 -0400 http://ax84.com/bbs/dm.php?id=485599 RE: Where to get parts?(by Stephen Keller) <div class="quoteblock" >On 07/20/2014 @ 15:19, Darren Porter wrote : Something to consider... AES I think still has their 10% off everything sale going on and at the price of ordering everything you need you will get free shipping. : : I have had VERY good experiences with AES... Mouser, not so much. </div> Agreed AES's <a href="http://www.tubesandmore.com " target="newwin1406020335">[link]</a> 10% off is a good deal right now. I have also had good experience when ordering from AES, but I generally don't order from them unless I have a lot to purchase because of their minimum charge and $100 entry fee for free shipping. On the other hand, I've had no trouble with Mouser. StphSun, 20 Jul 2014 20:06:28 -0400 http://ax84.com/bbs/dm.php?id=485598 RE: Where to get parts?(by Darren Porter) Something to consider... AES I think still has their 10% off everything sale going on and at the price of ordering everything you need you will get free shipping. I have had VERY good experiences with AES... Mouser, not so much.Sun, 20 Jul 2014 15:19:29 -0400 http://ax84.com/bbs/dm.php?id=485597 RE: Vibrato/Phasor design using Nuvistor tubes(by CChurchill) Ahhh...... Sweet, sweet, impedance. Forgive me if I tumble down this rabbit hole for a while. It feels like I'm beginnng a drawing, using both hands at the same time; each hand starting at opposing edges of the page, with the total picture becoming becoming clearer as they work towards joining in the center. Is it normal to see formulas and phase angles running through my mind when I close my eyes? In any case, I really appreciate you guys pointing me in right direction. Great stuffSat, 19 Jul 2014 22:58:47 -0400 http://ax84.com/bbs/dm.php?id=485596 RE: Grid leak resistor in Single Ended amp(by JKCavitt) If you rely on vol/tone pots for your grid leak, make sure you don't use a capacitor to tweak the tone after the tomestack or MV (e.g. .022 uF from tone/vol pot to the grid). You'll loose the grid leak that way. BTW don't ask how I know this.... Best practice IMO is to supply a fixed grid leak resistor. Sat, 19 Jul 2014 02:07:21 -0400 http://ax84.com/bbs/dm.php?id=485595 RE: Where to get parts?(by Stephen Keller) <div class="quoteblock" >On 07/18/2014 @ 18:21, LincolnR wrote : : One quick question - while looking at your BOM, I noticed that you selected a 5 amp fuse, yet the schematic calls for a 1 amp fuse. Did you have some reason for that, or was it just a goof? </div> Its just a screw up on my part. The only values I deliberately changed were the various bumps in resistor wattages. StphFri, 18 Jul 2014 19:15:02 -0400 http://ax84.com/bbs/dm.php?id=485594 RE: Where to get parts?(by LincolnR) <div class="quoteblock" >On 07/16/2014 @ 13:44, Stephen Keller wrote : : Hm..., : : I just took a look at that Mouser P1 BOM; somehow or other it has been truncated. I'll have to recreate it. Unfortunately, I won't have time to do that for awhile. The one that appeared earlier in that thread (with a different size chassis) appears to be intact (at least the dollar figure is about right). : : <a href="https://www.mouser.com/ProjectManager/ProjectDetail.aspx?State=EDIT%20&ProjectGUID=df806595-7faf-4984-b782-8a8394ceee67 " target="newwin1406020335">[link]</a> : : Stph -- </div> Stephen, One quick question - while looking at your BOM, I noticed that you selected a 5 amp fuse, yet the schematic calls for a 1 amp fuse. Did you have some reason for that, or was it just a goof?Fri, 18 Jul 2014 18:21:43 -0400 http://ax84.com/bbs/dm.php?id=485593 RE: Graphic EQ slide pot value(by Mitch Markin) I've seen this circuit before and it used 50K pots so the value doesn't seem to be overly critical. The frequency of each band is set by CA, CB, and the 1K and 220K resistors. Fri, 18 Jul 2014 16:29:53 -0400 http://ax84.com/bbs/dm.php?id=485592 RE: What should I use for the shielded wires?(by sam7arlo) This is all starting to come together and make sense. Thanks for the help!Fri, 18 Jul 2014 16:29:43 -0400 http://ax84.com/bbs/dm.php?id=485591 RE: 2204 build questions(by Audiosalvage) I.ve heard of them but right now I have a bridge rectifier in there to increase a low B= to start with. Besides I'm getting 5.7v at the filaments. If this doesn't work out, I'll look for another PT.Fri, 18 Jul 2014 16:13:09 -0400 http://ax84.com/bbs/dm.php?id=485590 RE: What should I use for the shielded wires?(by JeffW) You only tie the shield to ground at one end or you end up with an antennae. There are differing ideas about using signal ground or chassis ground. I use signal ground but would not hesitate to try both to see if one is quieter in a given situation. There have been some good discussions on the subject so you may want to do some searching on here.Fri, 18 Jul 2014 15:22:10 -0400 http://ax84.com/bbs/dm.php?id=485589 RE: What should I use for the shielded wires?(by sam7arlo) When using RG-174 do you have to tie the shielding to ground? If so, at one or both ends?Fri, 18 Jul 2014 14:57:56 -0400 http://ax84.com/bbs/dm.php?id=485588 RE: What should I use for the shielded wires?(by sam7arlo) Sounds like good advice. Thanks!Fri, 18 Jul 2014 14:38:40 -0400 http://ax84.com/bbs/dm.php?id=485587 RE: What should I use for the shielded wires?(by sam7arlo) Thanks. That is interesting. Though I want to stick to the plan, avoiding upgrades, alternatives, etc. this time, since it's a first build. Have a great weekend!Fri, 18 Jul 2014 14:37:41 -0400 http://ax84.com/bbs/dm.php?id=485586