AX84 BBS Recent Posts as of Sun, Aug 30th, 2015 @ 17:50:22 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com RE: Analog vs Digital Pedals?(by Darren Porter) Are you using a battery to power your reverb pedal or a wall-wart? These units eat batteries VERY quickly... as in they last less than an hour. Digital reverb is _very_ current hungry. Read the reviews for these pedals... the only 1 or 2 star reviews seem to be because they received one that just flat out didn't work. Nobody else really complaining about anything like you describe at all. Other negative complaints pretty much only about things like "just prefer pedal xyz over this one."Sun, 30 Aug 2015 17:44:55 -0400 http://ax84.com/bbs/dm.php?id=492029 RE: Dual secondaries as center tapped secondary(by AneirinE) Thanks Phil. Sun, 30 Aug 2015 16:46:29 -0400 http://ax84.com/bbs/dm.php?id=492028 RE: Dual secondaries as center tapped secondary(by PhilS) Yes, when out of phase, the windings are bucking each other (subtraction instead of addition) and you should get 0V or close to it. Glad you got it sorted.Sun, 30 Aug 2015 16:40:29 -0400 http://ax84.com/bbs/dm.php?id=492027 RE: Dual secondaries as center tapped secondary(by AneirinE) <div class="quoteblock" >On 08/30/2015 @ 16:21, trobbins wrote : You should be fine with your approach. The test is to measure 700VAC between the winding ends that you want to take to the 350-0-350 rectifier. : : Each of the 350VAC windings is likely to have 50-100 ohms dc resistance. : : The slight variation in VAC of the two windings may be alleviated by the winding dc resistance variation, and any actual outcome is just a low level of 50/60Hz ripple on the first main filter cap. This is normally of no consequence, and usually occurs in a normal PT due to winding resistance differences, and in valve diodes if you are using them. : : Ciao, Tim -- </div> Thanks Tim. Exactly the advice I required. Thanks!!Sun, 30 Aug 2015 16:29:12 -0400 http://ax84.com/bbs/dm.php?id=492026 RE: Dual secondaries as center tapped secondary(by trobbins) You should be fine with your approach. The test is to measure 700VAC between the winding ends that you want to take to the 350-0-350 rectifier. Each of the 350VAC windings is likely to have 50-100 ohms dc resistance. The slight variation in VAC of the two windings may be alleviated by the winding dc resistance variation, and any actual outcome is just a low level of 50/60Hz ripple on the first main filter cap. This is normally of no consequence, and usually occurs in a normal PT due to winding resistance differences, and in valve diodes if you are using them. Ciao, TimSun, 30 Aug 2015 16:21:23 -0400 http://ax84.com/bbs/dm.php?id=492025 RE: Dual secondaries as center tapped secondary(by AneirinE) <div class="quoteblock" >On 08/30/2015 @ 16:11, PhilS wrote : I don't think you are going about this the right way. You'll have to use trial and error to determine phase if the transformer isn't documented. Flush cut and heat shrink the 0 wires. Use the two 40 wires as the outer legs. Join the two 350 wires to form the center tap. Now you've cut out the "bottom" 40V between the 0-40 section of the winding and thereby subtracted 40 from 390. The only thing that gets grounded is the CT you've created. IOW, imagine like this 0-40-390|390-40-0 where you join the two 390's to form the CT. : : If it turns out this is out of phase (you'll know right away because voltage won't be anything like you expect), I'm reasonably certain you still cap off the zero wires, and then join one 390 to one 40 wire for the CT. IOW, you'll use 40-390|40-390. That still gives you 350 on each side of the CT. I wouldn't be concerned at all if there is a difference of 2V or less between the 2 halves. You might be OK with an even bigger spread. -- </div> Thanks! I actually did almost exactly what you suggested. Perhaps I didn't explain it well enough. The first way, measuring across the two 390s I got about 2v. When I reversed them, I got ~700v. So, the correct arrangement is 40-390|40-390. Pleased to hear that the 2v difference isn't to be concerned about. Thanks! Sun, 30 Aug 2015 16:20:45 -0400 http://ax84.com/bbs/dm.php?id=492024 RE: Dual secondaries as center tapped secondary(by PhilS) I don't think you are going about this the right way. You'll have to use trial and error to determine phase if the transformer isn't documented. Flush cut and heat shrink the 0 wires. Use the two 40 wires as the outer legs. Join the two 350 wires to form the center tap. Now you've cut out the "bottom" 40V between the 0-40 section of the winding and thereby subtracted 40 from 390. The only thing that gets grounded is the CT you've created. IOW, imagine like this 0-40-390|390-40-0 where you join the two 390's to form the CT. If it turns out this is out of phase (you'll know right away because voltage won't be anything like you expect), I'm reasonably certain you still cap off the zero wires, and then join one 390 to one 40 wire for the CT. IOW, you'll use 40-390|40-390. That still gives you 350 on each side of the CT. I wouldn't be concerned at all if there is a difference of 2V or less between the 2 halves. You might be OK with an even bigger spread.Sun, 30 Aug 2015 16:11:29 -0400 http://ax84.com/bbs/dm.php?id=492023 RE: Dual secondaries as center tapped secondary(by AneirinE) <div class="quoteblock" >On 08/30/2015 @ 13:47, trobbins wrote : Perhaps some more info would be of help. : : What are you going to do with the 350vac? Does it have to be one "350VAC", or can it be 350-0-350 ? What is the DCR of each winding? Are you loading the transformer heavily, compared to its ratings? Can you use the 40-0-40 spare windings? -- </div> Many apologies. I am using the 350v as the HV supply for a 100w amp with 4x6l6 in the power amp and 4 x 12ax7 in the preamp. Yes - I am basically trying to get 350-0-350 from two secondaries of 0-40-390. These two secondaries are both rated at 0.2A. The transformer is designed for a stereo amplifier and both of these secondaries are designed to be exactly the same (used as two bridge rectifiers for two amplifiers). So, what I've done so far (and since my last message) is tap off the 0 taps on both secondaries, ground the 40 tap on one secondary and ground the 390 tap on the other. This leaves the two remaining taps on the two secondaries as essentially a 350-0-350. I don't need the 40-0-40 winding so they are essentially capped off and not connected. I've figured out the answer to my question about phasing - i.e. both sides of the centre tap are now out of phase. However, it appears when I read the AC voltage of the two new "350VAC" taps they are slightly different. I.e. one is reading ~351VAC and the other is ~349VAC. Another detail is that I am reading these values without the valves installed - i.e. not much load at all. I suppose what I am concerned about is the two new 350VAC taps giving slightly different voltages. I am currently thinking that it is not a problem as the PS filter capacitors will charge to the highest voltage in one half of the cycle anyway and the other half of the cycle, despite being a couple of volts less, will help maintain that charge mostly (unlike a half wave rectification where the caps charge only during half the cycle and require more current). Am I wrong - do I need to worry about the slightly different voltages? Thanks! Sun, 30 Aug 2015 15:46:47 -0400 http://ax84.com/bbs/dm.php?id=492022 Power tube safety(by ColinD) I worked on an amp not too long ago. It was either a Rivera or a Jet City. I discovered that the power tube cathodes were floating unless a plug was inserted into the speaker jack. It seems like kind of a neat easy way to protect things if you forget to plug in a speaker cable. It the cathodes are floating there is no current through the tubes. I suppose it wouldn't offer any protection against a grossly mismatched speaker load, but does anybody here know how it's done? Colin_DSun, 30 Aug 2015 15:31:23 -0400 http://ax84.com/bbs/dm.php?id=492021 RE: New UK build Firefly, with problems...(by DarkV) Looking at your earlier posts, you said you were getting 207 volts AC between the two red wires out of the PT? That should be 380 volts AC. I'd check to make sure your AC in is connected correctly. <a href="http://www.hammondmfg.com/300series.htm#300wiring " target="newwin1440971425">[link]</a> Sun, 30 Aug 2015 14:47:51 -0400 http://ax84.com/bbs/dm.php?id=492020 RE: Analog vs Digital Pedals?(by Darren Porter) The tc hall of fame shouod work just fine in front of the amp. I know more than one person who uses them in front of a tube amp and i use a home-built reverb pedal that uses the same reverb engine and it works fine. Try and return the pedal. Probably decective. The difference between reverb pedal in front or in a loop might be different as far as you might prefer it one way over the other but it won't be as drastic as it sounds like "digital garble." Tc makes some of the finest pedals on the market. You likely have an oddball lemon.Sun, 30 Aug 2015 14:17:47 -0400 http://ax84.com/bbs/dm.php?id=492019 RE: Dual secondaries as center tapped secondary(by trobbins) Perhaps some more info would be of help. What are you going to do with the 350vac? Does it have to be one "350VAC", or can it be 350-0-350 ? What is the DCR of each winding? Are you loading the transformer heavily, compared to its ratings? Can you use the 40-0-40 spare windings?Sun, 30 Aug 2015 13:47:05 -0400 http://ax84.com/bbs/dm.php?id=492018 RE: New UK build Firefly, with problems...(by DavidmR) Sorry - back again...!! So, I terminated the yellow/green CT wire from the PT and took that to ground at the chassis, and left out R16 & R17... The good news is it's still quiet hum-wise, and measuring from either green wire at the PCB terminal blocks to ground I have 3.41vac, and across the two green wires I have 6.79vac - which ought to be enough? Doesn't seem to have cured the low output, though - it is still as quiet as the unplugged guitar. Any suggestions? It's heading toward 'expensive paperweight' territory, now....! Thanks again, David Sun, 30 Aug 2015 10:10:50 -0400 http://ax84.com/bbs/dm.php?id=492017 RE: Analog vs Digital Pedals?(by Kursad K) <div class="quoteblock" >On 08/30/2015 @ 04:17, Dave C wrote : Thanks Kursad ..... wow that was a detailed explanation. : : If I knew the TC pedal would sound great with an effects loop I would consider adding one. However, I wonder if power tube distortion could have the same effect albeit not as severe? Most of the demos sound very good so I'm assuming those are through an effects loop. : -- </div> Output stage distortion is not much of a problem because it's often a slight breakup on hard-hit chords. KursadSun, 30 Aug 2015 10:07:00 -0400 http://ax84.com/bbs/dm.php?id=492016 RE: New UK build Firefly, with problems...(by DavidmR) Thanks again DarkV, I think I'll try putting it to ground on the chassis, as the amp is currently super-quiet (hum-wise), and more relevant I've a) cut the CT wire too short to make it to V3, and b) no idea how I'd connect that to the PCB....! Many thanks, David Sun, 30 Aug 2015 09:27:10 -0400 http://ax84.com/bbs/dm.php?id=492015 RE: New UK build Firefly, with problems...(by DarkV) Connecting the filament CT to chassis ground and removing R16 and R17 should work, however that might put some noise in the circuit. <a href="http://ax84.com/media/ax84_m276.gif " target="newwin1440971425">[link]</a> The schematic shows the filament CT connected to V3a/b cathode. I'm just not certain if it's after R13 to ground, or actually between pins 3 and 8 on V3. Sun, 30 Aug 2015 08:59:15 -0400 http://ax84.com/bbs/dm.php?id=492014 RE: New UK build Firefly, with problems...(by DavidmR) Just looking into this some more... Is it as simple as taking the green/yellow centre tap to a ground on the chassis and leaving those resistors out, or am I missing something else...? Thanks again! David Sun, 30 Aug 2015 08:47:17 -0400 http://ax84.com/bbs/dm.php?id=492013 RE: New UK build Firefly, with problems...(by DavidmR) Wow! Thanks for the quick reply DarkV! Okay, so my problem is not having used the green/yellow centre tap, and as I understand your reply (bear with me, I'm a noob!) I need to hook this up whilst leaving the R16 & R17 resistors out? Any ideas how / where this centre tap wire would tie into the Rev 8 PCB build (and I'm now hoping I didn't cap it too short!!) Many thanks, David Sun, 30 Aug 2015 08:37:13 -0400 http://ax84.com/bbs/dm.php?id=492012 RE: New UK build Firefly, with problems...(by DarkV) It's impossible to measure resistors while they are in the circuit. All the other components will interfere with your readings, but you pulled them anyway and verified their values. Your power transformer has center tapped filament wires so you won't be using R16 and R17. The CT attaches to V3a/b cathode (I'm not 100% certain if it's before or after the resistor) and you wire the two green wires normally according to the schematic. I'm not a tube amp expert, but if the filaments aren't heating properly you're not boiling off enough electrons that make the tubes do their magic. Without the CT hooked up and R16/R17 installed you might be halving the filament voltages. I'd fix this first and then see how everything works. Sun, 30 Aug 2015 08:19:03 -0400 http://ax84.com/bbs/dm.php?id=492011 New UK build Firefly, with problems...(by DavidmR) Hello All, Hoping someone can save me from pulling all of my hair out...! I have just finished a PCB8 build of the firefly, per the John Calhoun guide except for a Hammond 369EX for UK 240v mains, and the 125A wired at 1&4 for a 16ohm Marshall MX112 cab. What little sound I get out of sounds awesome, with a great tone, but it's so quiet! It's hard to hear above the acoustic sound of the strings... I've read through all of the troubleshooting threads I could find here, and have checked resistors and voltages which all checked out except: 1) the V3b - R13 voltage is 3.2v, not the 8.8v on the schematic. 2) I'm only getting 3.4v at the green heater wires, which should be 6.3v Ref. 2), when I was checking the resistors, I read 50ohm across both R16 and R17, which should be 100ohm, though when removed I get the correct reading. Whilst they were out, I tried the heater (green wire) volts again to see if these were pulling it down and got (valves out!) 65v on one and 67 on the other. Is this because o the 369PT? I wired it up without using the gn/ylw centre tap which was capped and stowed, as I read a post from some years back here saying this wasn't required. I'm getting 240v at the mains, 207v on the red PT wires (with 0v on the red/ylw) and everything else seems to check out... My Electronics knowledge is very limited (Mech Eng) hence this build as a first attempt, so I'm at a stage where I'm a bit lost and would welcome any help! Thanks in advance ;) David Sun, 30 Aug 2015 14:47:51 -0400 http://ax84.com/bbs/dm.php?id=492010 RE: Dual secondaries as center tapped secondary(by AneirinE) OK - I measured across the two non grounded secondary taps. My reasoning is that if they were in phase the reading would be 0vac and if they were out of phase the reading would be 700vac. The reading was around 2vac, which makes me think that they are in phase but the two windings are slightly mismatched. So...still keen for advice on my first post but also whether I should swap the taps of one of the secondaries around? And, should I be concerned about this apparent winding mismatch? I'm guessing it would mean that the center tap in this arrangement isn't exactly in the center? Is that a problem? Thanks. Sun, 30 Aug 2015 05:23:06 -0400 http://ax84.com/bbs/dm.php?id=492009 Dual secondaries as center tapped secondary(by AneirinE) Sorry if this has been asked before. I can't seem to find a definative answer. I have a PT with dual secondaries each with 0-40-390. I want 350Vac to rectify using silicon diodes in a full wave rectifier set up. I have grounded the 40v taps of both secondaries and sealed off the 0v taps. The two remaining 390v taps should then give me 350vac. I've used these two taps as each side of the full wave rectifier. Do I need to worry about phase here? What happens if the two sides are in phase? How do I ensure that they are out of phase (if required)? Thanks! Sun, 30 Aug 2015 16:46:29 -0400 http://ax84.com/bbs/dm.php?id=492008 RE: Analog vs Digital Pedals?(by Dave C) Thanks Kursad ..... wow that was a detailed explanation. If I knew the TC pedal would sound great with an effects loop I would consider adding one. However, I wonder if power tube distortion could have the same effect albeit not as severe? Most of the demos sound very good so I'm assuming those are through an effects loop. Sun, 30 Aug 2015 04:17:55 -0400 http://ax84.com/bbs/dm.php?id=492007 RE: Firefly OT(by steve25) the hammond 125 OT spec for the build is already over kill for a 1 watt amp, and a PT with 235/235 used thru that recto tube will end you up wil 30 to 40 volts or so too much B +!Sun, 30 Aug 2015 03:54:01 -0400 http://ax84.com/bbs/dm.php?id=492006 RE: Firefly OT(by Audiosalvage) Sorry, this is a Firefly REV3 build.Sat, 29 Aug 2015 21:34:45 -0400 http://ax84.com/bbs/dm.php?id=492005