bluefoxicy Posted March 28, 2008 Share Posted March 28, 2008 Hey all. I have an EL34L JJ coming from Eurotubes, as well as a Turretboards marshallization kit and octal power amp board, and a Hammond 125ESE. A couple questions now come up... [*]How the heck do I bias the EL34L? What's my target wattage? Can I serialize a resistor with a 500k or 1M pot without tone suck? [*]Where do I get carbon-film resistors? The local shop sells flameproof metal-film 5% tolerance but I hear those attenuate the low-end too much, and I want 1% tolerance [*]Anyone opposed to using a Yellow Jacket to plug an EL84 into the power tube socket in the future? Link to comment Share on other sites More sharing options...
bluefoxicy Posted March 28, 2008 Author Share Posted March 28, 2008 http://yarchive.net/electr/resistors.html on the same topic, this is a wonderful discussion about resistors. Link to comment Share on other sites More sharing options...
m-theory Posted March 28, 2008 Share Posted March 28, 2008 How the heck do I bias the EL34L? What's my target wattage? Can I serialize a resistor with a 500k or 1M pot without tone suck? http://www.kbapps.com/audio/tubespecs/el34p02.html No need for a pot, really. Just dial it in "manually," and replace the tube with one that matches what they sent you this time. You should just buy them in matched pairs, and that way you'll always have a spare, and they'll always be biased correctly when you plug them in. Where do I get carbon-film resistors? I'm not a resistor expert, mind you, but I think you want flameproof in there. I used vitreous enamel, just because they look purdy and are very compact. Wirewound will work. No need for 1% tolerance anywhere in this amp, to be honest. It's not a terribly noisy circuit with carbon film. Anyone opposed to using a Yellow Jacket to plug an EL84 into the power tube socket in the future? No comment, as I have no experience with them. I've got some octal questions myself, but I'm not going this route just yet, so I'll save my questions for later. Link to comment Share on other sites More sharing options...
bluefoxicy Posted March 28, 2008 Author Share Posted March 28, 2008 I'm not a resistor expert' date=' mind you, but I think you want flameproof in there. I used vitreous enamel, just because they look purdy and are very compact. Wirewound will work. No need for 1% tolerance anywhere in this amp, to be honest. It's not a terribly noisy circuit with carbon film. [/quote'] Flameproof are metal film. I'm using the extra resistors inside the guitar itself. Link to comment Share on other sites More sharing options...
bluefoxicy Posted March 28, 2008 Author Share Posted March 28, 2008 http://www.angela.com/catalog/resistors/Resistors.html another resource I was using a while back... took weeks to find it again! Link to comment Share on other sites More sharing options...
m-theory Posted March 28, 2008 Share Posted March 28, 2008 Flameproof are metal film. There you have it! You know more about them than I! I thought you were referring to the rest of the amp when you mentioned metal film, though. I'm using the extra resistors inside the guitar itself. What for? Link to comment Share on other sites More sharing options...
bluefoxicy Posted March 28, 2008 Author Share Posted March 28, 2008 I have a special II (1 vol, 1 tone), push-pull pots (DPDT). Fender Telecaster uses an overwound bridge pickup to supply more power from there, and a high-pass filter across vol to let more highs through. I want to have the neck pickup go right through the DPDT when pushed, but go through a resistor when pulled (attenuate some signal into heat); and also have the vol pot's leads open when pushed, but shorted across a high-pass filter when pulled (allows part of the highs to bypass the vol pot). This involves two resistors. The wiring for this is pretty simple... The DPDT is two isolated switches. When pushed, 1-2 is shorted; when pulled, 2-3 is shorted. HF Circuit: |---{1150pF cap}--| | | (In)---{320K resistor}---(out) Bright switch side A (BA): -----Vol pot (hot out) | | |--HF--Vol pot (hot in) | | 1 2 3 Bright switch side B (BB): |-----Vol pot (hot in) | |-{120k resistor}-| | | | --------------| | | 1 2 3 | ---- Pickup (hot) As you can see, with the pot pushed, BA connects the vol pot hot out to nothing extra (there's an insulated wire hanging off the end); BB connects the pickup (hot) to the Vol pot (hot in) and has an insulated resistor hanging off the side (again, nothing). With the pot pulled, BA connects the HF circuit (a high-pass filter with a resistor and cap in parallel) across the Vol pot's hot in and hot out, allowing highs to pass (rolling volume down attenuates highs slower than normal). BB connects the pickup (hot) to the other end of that resistor instead, attenuating some of the signal as heat. I believe this will do just about what I want it to do... turn my LP into a Telecaster at a pull of a knob. I'll have the other pot switch both into single coil mode (too bad I can't have it also throw another 120k in parallel to the first for the neck pickup, to roll back the attenuation a bit). Link to comment Share on other sites More sharing options...
cGil Posted March 28, 2008 Share Posted March 28, 2008 Hey all. I have an EL34L JJ coming from Eurotubes' date=' as well as a Turretboards marshallization kit and octal power amp board, and a Hammond 125ESE. A couple questions now come up... [*']How the heck do I bias the EL34L? What's my target wattage? Can I serialize a resistor with a 500k or 1M pot without tone suck? How? Same way you bias an el84. You take voltage readings and compute the wattage dissipated. As far as where to bias it... JJ's Tube Data Sheet sez Wa(max) is 25watts. Stay below 90% of that and the tube should be okay. http://www.jj-electronic.com/pdf/E34L.pdf Fat chance of that in a VJr, however. I squeezed 18watts dissipation out of an EH EL34 with 350vDC at B+1 and a bias of 220R/3w. But the PT got kinda hot doing it like that and would need a fan to keep from frying. SEwatt's standard octal mod schematic recommends 300R - 330R rated for 2Watts minimum for EL34. That should run cooler all around. http://www.sewatt.com/files/sewatt/Octal.pdf [*]Where do I get carbon-film resistors? The local shop sells flameproof metal-film 5% tolerance but I hear those attenuate the low-end too much' date=' and I want 1% tolerance[/quote'] The regular 5% tolerance resistors work fine. Tube amps have rather large ballparks to play in, and that extra accuracy of a few ohms won't make even the tiniest bit of difference once the resistor value is more than one digit. 1/2watt is the factory minimum rating in most of the amp for safety, but 1watt carbon film is safer and more quiet. However, for the power tube's bias resistor or power rail resistors, you MUST use flame resistant resistors like metal oxide or ceramic wirewound for both safety and electrical code requirements. The bias resistor should be at least 2watts minimum to handle the surge at power on. R13 should be 2watts. R12 should be at least 2w, and R10 should be at least 3w (5watt is best here.) This is so the amp doesn't burn down the house. [*]Anyone opposed to using a Yellow Jacket to plug an EL84 into the power tube socket in the future? Never tried it. But if you just feel like taking a step back after all those octal mods... well, why not! Personally, I'd just get another VJr and keep the el84 in that one. But that's just me. :- Gil... Link to comment Share on other sites More sharing options...
bluefoxicy Posted March 31, 2008 Author Share Posted March 31, 2008 How? Same way you bias an el84. You take voltage readings and compute the wattage dissipated. As far as where to bias it... JJ's Tube Data Sheet sez Wa(max) is 25watts. Stay below 90% of that and the tube should be okay. http://www.jj-electronic.com/pdf/E34L.pdf Thanks Gil. I'm assuming a cold bias here isn't too bad' date=' as long as I keep the plate voltage above 250 (it's 350 by default...)? These things are rated to run at 800V, I'm sure I could happily load up the plate with whatever voltage I can get; not sure how the OT will take me raising B1 though. Moving R10 next to R12 (beyond B1) and R11 beyond C9 would max out B1 wouldn't it? Probably around 400V (450V rated caps huh?) because I'm sure Epi planned for R11 fails and C6 gets huge peak voltages without a fire starting.... [b']You know I just realized I'm not ENTIRELY sure what causes overdrive[/b]; could someone explain this?! All I got is when you push the tubes beyond a certain point they start overdriving. Do I want to get the bias higher and the plate voltage lower (within its operating specs, not starving the plate) to make it overdrive faster? For cooling, why not solder (yes...) a smaller CPU heat sink directly to the primary transformer? Thermal paste VERY THIN to give center contact, then a bead of solder around the edges to couple it via an extremely good thermal conductor. If you NEED a fan, get a small CPU fan and parallel it with the 6.3V tube filaments, use a small resistor to lower the fan speed to quietness, plus maybe a little bigger cap ;) Never tried it. But if you just feel like taking a step back after all those octal mods... well' date=' why not! Personally, I'd just get another VJr and keep the el84 in that one. But that's just me. [/quote'] Those adapters are supposed to reduce the power across the plates or something, but I don't know by how much. I've seen them labeled "POWER REDUCER" in fancy graphics like it's some kind of awesome technology (resistors?). Might be nice to switch the tone around from time to time though. Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 4, 2008 Author Share Posted April 4, 2008 Hey Gil, I think I'll drop in one of these: http://radiodaze.com/hammond01.htm HX-270DAZ is what I'm looking at. I think it'll handle the 25W bias. I'll tell you how it goes. I'm going to TRY to send about 400V out B1 and adjust R14 to 24+ watts. I ... THINK... I understand a little about how electronics and current flow work now. (I'm not studying electronics, I'm learning on the fly by asking random people questions). Link to comment Share on other sites More sharing options...
tunghaichuan Posted April 4, 2008 Share Posted April 4, 2008 A couple of things: Are you sure you want the HX-270DAZ? That PT is Z mount, meaning it requires a rectangular hole to be cut into the chassis. The X mount PT is much easier to attach to a chassis. (The Valve Junior's PT is X mount.) X mount requires two small round holes to feed the wires through and four smaller holes for the mounting bolts. There are diagrams of both Z and X mount transformers at the bottom of the Radio Daze page. The HX-270DAX has the same ratings as the HX-270DAZ, but mounts X style. Here is a way to figure current draw for your high voltage winding in an SE amp. Figure that the E34L is a 25 watt tube. The 270DAX & 270DAZ are rated at 260-0-260@90mA. So figure 260 x1.41 = 366v peak voltage. With current draw it will be a little less, say 330v for the sake of this argument. So take your maximum plate dissipation (25W) and divide it by 330V to get your current draw, which = 75mA. Hammond PTs are conservatively rated so you're probably OK as the 270DAX & 270DAZ are both rated for 90mA on the high voltage winding. If you wanted to play it safe, you could bias at 90%, or 68mA. tung Hey Gil' date=' I think I'll drop in one of these: http://radiodaze.com/hammond01.htm HX-270DAZ is what I'm looking at. I think it'll handle the 25W bias. I'll tell you how it goes. I'm going to TRY to send about 400V out B1 and adjust R14 to 24+ watts. I ... THINK... I understand a little about how electronics and current flow work now. (I'm not studying electronics, I'm learning on the fly by asking random people questions).[/quote'] Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 4, 2008 Author Share Posted April 4, 2008 A couple of things:Are you sure you want the HX-270DAZ? That PT is Z mount' date=' meaning it requires a rectangular hole to be cut into the chassis. The X mount PT is much easier to attach to a chassis. (The Valve Junior's PT is X mount.) [/quote'] Yeah I saw that like 2 minutes ago and was like "must investigate form factor before buying." But thanks for the heads up! Wouldn't want me to go doing something stupid now. Here is a way to figure current draw for your high voltage winding in an SE amp. Figure that the E34L is a 25 watt tube. The 270DAX & 270DAZ are rated at 260-0-260@90mA. So figure 260 x1.41 = 366v peak voltage. With current draw it will be a little less' date=' say 330v for the sake of this argument. So take your maximum plate dissipation (25W) and divide it by 330V to get your current draw, which = 75mA. Hammond PTs are conservatively rated so you're probably OK as the 270DAX & 270DAZ are both rated for 90mA on the high voltage winding. If you wanted to play it safe, you could bias at 90%, or 68mA. [/quote'] 1.41 I couldn't find that number on the net the other day, THANKS! Damn 366, oh well. I don't NEED 400, I just wanted to mess with the amp and put 400V on B3. I COULD buy something else with higher voltage and increase resistance at R12, of course. Mind you typical Class A operation is 265V there, but 800V peak rated (kick in the head when turning the amp on? Nah, 100uF C6 and C9 to buff that out). I can leave 340V there, it's no big deal. Theoretically if I did want 400V, HX-272BX would give 423 by your math. So 400V, 25W / 400 == .0625A, 400 / .0625 = 64K resistance needed. The plate has 15K resistance and I don't know what the OT has, I need about a 49K resistor minus the OT in R10 BUT I'm ignoring R11 (why is there a voltage divider here?!!?!?!?!); I would need a bigger one in R12 to bring B2 down to 310 (I suspect it'd be to bring B2 down to 400V, since it supplies the grid DIRECTLY instead of through the OT and should probably match whatever the plate is getting...) I know nothing about this stuff though. :( Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 6, 2008 Author Share Posted April 6, 2008 OK I'm thinking mostly stock, 310V across B1... uh. http://www.diycustomamps.com/valvejunior.htm Assuming a 310V anode and 9V cathode and I want 25W... 25 / 310 = .0806A 9 / .0806 = 111.66K I need about a 110K resistor for this. I'm thinking I can get a 250K 4W linear pot (they're an inch and a quarter in diameter) and run a (twisted? Who cares, it'll keep it neat) cable to it from the bias solder points, wired as a variable resistor, with around 50k in series with this to make it a 50k-300k adjustment. Put two insulated contact points next to it, one on each side of this circuit, to throw an ohmeter across. Meter it, dial it to 110K, and I'm happy. (of course I want to measure my ACTUAL voltages and do real calculations first) Link to comment Share on other sites More sharing options...
Bootlegger Posted April 17, 2008 Share Posted April 17, 2008 hey... new to all this.. and kinda a stupid question but, can i just put in my new JJ EL84 without biasing? i was told recently that with a single ended tube amp like this there is no need for biasing, that it was only needed for anything that had more than one power tube.. were they wrong? and is biasing hard? i want to get into amp modding and all that and eventually start building my own someday... Link to comment Share on other sites More sharing options...
tunghaichuan Posted April 18, 2008 Share Posted April 18, 2008 hey... new to all this.. and kinda a stupid question but' date=' can i just put in my new JJ EL84 without biasing? i was told recently that with a single ended tube amp like this there is no need for biasing, that it was only needed for anything that had more than one power tube..were they wrong? and is biasing hard? i want to get into amp modding and all that and eventually start building my own someday...[/quote'] Generally, you can just pull the old tube, and replace it with a new tube with the same designation without adjusting anything in a cathode bias amp like the Valve Junior. However, it is a good idea to check the bias. Modern production tubes are notoriously inconsistent, so even two tubes of the same type and same manufacturer may draw different amounts of current. You want to make sure that the new tube is under the maximum dissipation levels. If the tube is dissipating too much wattage, it will wear out prematurely. Having said all that, when Epiphone manufacturers VJs, they don't worry about setting the bias correctly at the factory. They just build them and plug in the tubes. If you want to build your own amps, starting off with checking the bias on a VJ is a pretty good place to start. It will teach you some fundamental concepts of vacuum tube amplifiers. tung Link to comment Share on other sites More sharing options...
Bootlegger Posted April 18, 2008 Share Posted April 18, 2008 Generally' date=' you can just pull the old tube, and replace it with a new tube with the same designation without adjusting anything in a cathode bias amp like the Valve Junior. However, it is a good idea to check the bias. Modern production tubes are notoriously inconsistent, so even two tubes of the same type and same manufacturer may draw different amounts of current. You want to make sure that the new tube is under the maximum dissipation levels. If the tube is dissipating too much wattage, it will wear out prematurely. Having said all that, when Epiphone manufacturers VJs, they don't worry about setting the bias correctly at the factory. They just build them and plug in the tubes. If you want to build your own amps, starting off with checking the bias on a VJ is a pretty good place to start. It will teach you some fundamental concepts of vacuum tube amplifiers. tung [/quote'] thanx a lot, i'll look into how to checking the bias and all that. my dad actually has a ton of old electronic books from the navy, like 14 volumes or something and they go over pretty much everything you'd want to know about this stuff. and there's always online forums like this one and FAQ's! Link to comment Share on other sites More sharing options...
bluefoxicy Posted April 18, 2008 Author Share Posted April 18, 2008 I shot my bias from the hip, i should really go in and bias my amp properly one day. I used Tung's reference numbers from the discussion on his how-to-bias-a-VJ page, which are probably wrong for an EL34... Link to comment Share on other sites More sharing options...
tunghaichuan Posted April 18, 2008 Share Posted April 18, 2008 thanx a lot' date=' i'll look into how to checking the bias and all that. my dad actually has a ton of old electronic books from the navy, like 14 volumes or something and they go over pretty much everything you'd want to know about this stuff. and there's always online forums like this one and FAQ's![/quote'] To get you started, here is a guide that I wrote on checking/setting the bias on a Valve Junior: www.diycustomamps.com/valvejunior.htm#vjbias It works for most SE Class A amps. Also register for www.sewatt.com and start reading the posts there. There is a huge FAQ there with lots of knowledge about the Valve Junior. tung Link to comment Share on other sites More sharing options...
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