AX84 BBS Recent Posts as of Fri, Jan 30th, 2015 @ 15:17:00 http://ax84.com Amp Parts and KitsDoberman Music Products, LLC offers a variety of parts and kitshttp://shop.dobermanamps.com preamp/power amp matching(by wizardineminor) I am looking to start up a project which I would like to be a dual channel amp. I play in a "stoner rock" band with songs ranging from clean(ish) funky style to straight up doom. I know there are plenty of dual channel amps out there and some would be very much up to the task of covering that range, but I am set on building. I am an electronics student, and have built a number of pedals. My technical knowledge about amps specifically is by admission pretty limited (but I'm constantly researching). Finally to the point...What types of modifications would need to be made to the PREamp of a schematic if one wished to have a power section that was twice as powerful? For those interested in more details the two amps I would like to combine are the 5f6-a Fender Bassman to handle the cleans and some effects (I plan on putting an fx loop in there) and a vintage Orange probably the OD120. Two very different amps, I know. The biggest hurdle I envision is the fact that one is a 50 watt 6L6? amp and the other a 120 watt EL34 amp. I would like the final project to be somewhere around 100 watts and to run EL34s. Would I be able to take an existing 100 watt EL34 power amp schem and power the two preamps as they were in the originals or would massive amounts of preamp modification be needed to avoid disaster? Sorry for the novel of a question. If anyone has an answer I would be very grateful since search engines seem to have no idea what I'm trying to ask!Fri, 30 Jan 2015 14:46:38 -0500 http://ax84.com/bbs/dm.php?id=488848 RE: Coolest new product at NAMM(by Stephen Keller) <div class="quoteblock" >On 01/30/2015 @ 06:31, Merlin wrote : It's just a VFD. </div> That's the speculation, but no one in a position to know is saying that for sure. The Korg press release says Noritake applied their VFD expertise, but it's not clear that means the NuTube is a VFD or has the same structural flaws of a VFD when hacked for amplification. Do you have a citation that authoritatively says these are just re-purposed VFD? StphFri, 30 Jan 2015 09:21:10 -0500 http://ax84.com/bbs/dm.php?id=488847 RE: Which one of these layouts do you prefer?(by Stephen Keller) <div class="quoteblock" >On 01/30/2015 @ 06:37, Merlin wrote :...unless there's one in your junk box dying to be used. </div> Even if it's dying to be used, I think any self-respecting 5AR4 would prefer to wait for a proper push-pull build where its talents would be better exploited. StphFri, 30 Jan 2015 06:59:39 -0500 http://ax84.com/bbs/dm.php?id=488846 RE: Coolest new product at NAMM(by RickTown) Well that's disappointing. I thought it was something new. So much for innovation...Fri, 30 Jan 2015 06:59:10 -0500 http://ax84.com/bbs/dm.php?id=488845 RE: Are Siemens capacitors any good?(by Henrik Lidbjrk) Ok, yep that's pretty much my view as well, at least for now.Fri, 30 Jan 2015 06:47:57 -0500 http://ax84.com/bbs/dm.php?id=488844 RE: Edit(by Merlin) <div class="quoteblock" >On 01/30/2015 @ 06:37, Merlin wrote : I <b>wouldn't</b> bother with a valve recto in a single ended amp, </div>Fri, 30 Jan 2015 06:38:25 -0500 http://ax84.com/bbs/dm.php?id=488843 RE: Which one of these layouts do you prefer?(by Merlin) It's small, but eminently possible. Form should follow function; the layout will be more definable when you know what sort of caps you want to use, as they're the next largest component. Don't put them too close to anything hot. With that in mind, I would tentatively start with layout 2, but move the EL34 a little further away from the cap. I woudl bother with a valve recto in a single ended amp, it's just more heat, unless there's one in your junk box dying to be used.Fri, 30 Jan 2015 06:37:34 -0500 http://ax84.com/bbs/dm.php?id=488842 RE: Are Siemens capacitors any good?(by Merlin) Plastic is plastic, it doesn't make any difference in a valve amp. A few ceramic treble caps might make a difference, but where pastic is concerned, I woudln't sweat it.Fri, 30 Jan 2015 06:33:02 -0500 http://ax84.com/bbs/dm.php?id=488841 RE: Coolest new product at NAMM(by Merlin) It's just a VFD. Nobody use VFDs anymore, which is why they're desperately trying to appeal to a new market. I've been hanging on to a couple of them (one from an VCR another from a calculator) for quite a while now, meaning to try them out in a pedal. Still haven't got round to it!Fri, 30 Jan 2015 06:31:05 -0500 http://ax84.com/bbs/dm.php?id=488840 RE: Which one of these layouts do you prefer?(by Henrik Lidbjrk) Yes, I shall do the headphone trick when and if I start building this amp. But does the size of the chassis seem resonable at all for an amp operating at 10-15W?Fri, 30 Jan 2015 02:42:25 -0500 http://ax84.com/bbs/dm.php?id=488839 RE: Coolest new product at NAMM(by Henrik Lidbjrk) Haha, that's interesting! They may have some success with it at Korg, but I suspect its impact will be limited. DIYs like us love tubes for their shiny glass, red glow and vintage sound and feel (power consumption be damned) and mainstream buyers want tubes because of, well, the same things (even if it's just a 12AX7 making no audible difference in an otherwise solid state microphone preamp).Fri, 30 Jan 2015 01:59:10 -0500 http://ax84.com/bbs/dm.php?id=488838 RE: Are Siemens capacitors any good?(by Henrik Lidbjrk) Okay! Yeah, there's a whole load of electrolytics in the same storage, but I never used those because of their age. (There's also tons of carbon composition resistors there, which is nice because I have access to pretty much every R value imaginable, but I try to use modern materials if possible.) By the way, in what positions (in an amp circuit) generally do capacitor types and brands make the biggest difference, if any at all? I'm thinking about using as many of those Siemens caps as possible but perhaps substitute a few of 'em for Mallorys in certain places if that seems resonable.Fri, 30 Jan 2015 01:51:11 -0500 http://ax84.com/bbs/dm.php?id=488837 Good source for Hammond 125E transformer in Toronto?(by sajyho) Hi guys, Hope you're OK.I'm livin in Asia and my brother who lives in Canada wants to come and visit us, so I asked him to buy a Hammond 125e for me, but unfortunately he doesn't know where to buy one! Can you name a good and cheap source in toronto so I can tell him? Thanks SajadThu, 29 Jan 2015 23:58:55 -0500 http://ax84.com/bbs/dm.php?id=488836 RE: Max capacitance on a 6AX5GT???(by trobbins) Note that a key condition you are falling short on is the effective series resistance - it needs to be increased. That said, when I input the datasheet conditions: 350V, 50R, 10u, 3.2k resistive load, I get 123mA rms, I also get higher repetitive peaks than expected. I used a resistance, as the datasheet would most likely been based on that type of load. A little further investigation and it became clear that the old adage "shit in, shit out' is at play. Whoever entered the diode characteristics has stuffed up. The datasheet shows a plate V-I curve very close to that of a 5Y3GT, which has a model vfac parameter value of 0.000354. I then used 0.00032 to get pretty close to 125mA loading and 350Vrms, but also gave a steady state peak of 420mA. Lowering the output load current from 125mA to 100mA then gives ok steady state peak, and you can go up in cap with only a gradual increase in steady-state. Going up to 100uF still keeps the transient peak below 2.5Apk. Going to your PA circuit, and using 300V, 40R, 110mA, 20uF then the steady peak is right on the 375mApk limit - so I guess you do need tobe carefull in changing parameters - especially the max load current (eg. during cranked operation). Sometimes its not the OT CT ripple that is a major hum contributor, due to push-pull attenuation. And it is more beneficial to use a small inductor in with (or replacing) R28 to get the screen ripple down further.Thu, 29 Jan 2015 23:39:03 -0500 http://ax84.com/bbs/dm.php?id=488835 RE: Are Siemens capacitors any good?(by Merlin) I have some of those (1uF 400V) which I use from time to time, mainly because they're quite small. I've never had any problems. Plastic caps stay good for years, maybe forever, unlike electrolytics.Thu, 29 Jan 2015 23:07:12 -0500 http://ax84.com/bbs/dm.php?id=488834 RE: Balanced line out(by Merlin) <div class="quoteblock" >On 01/29/2015 @ 17:28, quayhog wrote : Can most any line level transformer be hooked up backwards? What about center taps and shielding which is usually on the schematic as on the secondary.? Do I have any concerns there? </div> Yes, any transformer can be used backwards if you're happy with the resulting turns and impedance ratio. You also must not apply excessive voltage (which shouldn't be a problem with line-level signals, it's more likely with power transformers). Centre taps and shileds don't make any difference unless of course you <i>need</i> a CT on one winding or the other.Thu, 29 Jan 2015 23:05:44 -0500 http://ax84.com/bbs/dm.php?id=488833 Emulating tube rectifier sag with solid state components. (by Alva Goldbook) Has anyone ever tried doing this and had found successful results?Thu, 29 Jan 2015 20:04:35 -0500 http://ax84.com/bbs/dm.php?id=488832 Coolest new product at NAMM(by RickTown) <a href="http://www.korg.com/us/news/2015/012212/ " target="newwin1422649024">[link]</a> It's just a teaser but damn!! I really want to see a datasheet and play with a few of these. As Hedley Lamarr once pronounced: "My mind is aglow with whirling, transient nodes of thought careening thru a cosmic vapor of invention."Fri, 30 Jan 2015 09:21:10 -0500 http://ax84.com/bbs/dm.php?id=488831 RE: Balanced line out(by quayhog) Can most any line level transformer be hooked up backwards? What about center taps and shielding which is usually on the schematic as on the secondary.? Do I have any concerns there? I'll have the amp on my workbench next week. Upgrade power supply caps and three prong AC cord. I'll make sure the 7591 are biased correctly. Then I'll tackle the line out mod.Thu, 29 Jan 2015 17:28:02 -0500 http://ax84.com/bbs/dm.php?id=488830 Are Siemens capacitors any good?(by Henrik Lidbjrk) I have access to an old surplus supply of components, which includes many old Siemens capacitors, exactly like these: <a href="http://www.garrettaudio.com/My%20Pictures/DSC05804.JPG " target="newwin1422649024">[link]</a> They're probably from the 1960s-1980s some time. Anyone know if they are known to sound good / work well in tube amps? I've used two or three and those worked fine. Just wondering if they're considered good compared to for example Mallorys and Orange drops.Fri, 30 Jan 2015 06:47:57 -0500 http://ax84.com/bbs/dm.php?id=488829 RE: DC heaters?(by John Hynes) Yes - typically, you'd want to use a regulator as well, and you'd just float the regulator on a reference voltage - in other words, a 12V regulator referenced to 100VDC so that you'd be applying 112VDC to one side of the heater and 100VDC reference voltage to the other. You'd want a pretty stable reference as well... -John Thu, 29 Jan 2015 09:53:27 -0500 http://ax84.com/bbs/dm.php?id=488828 RE: Which one of these layouts do you prefer?(by RickTown) I would say that since it's a small blank chassis, use the headphone trick to find what works best for low noise between your transformers then arrange your layout around that. Any of the above layouts might be great or terrible depending on your specific trannys.Thu, 29 Jan 2015 09:12:22 -0500 http://ax84.com/bbs/dm.php?id=488827 RE: WIMA Capacitors - These will be fun to turret board!(by Stephen Keller) <div class="quoteblock" >On 01/29/2015 @ 08:30, scr4mbles wrote : I figured that! I'll need to just suck it up and solder longer wires onto them I guess! </div> Soldering longer leads to these is going to result in mechanical stresses being transmitted into the leads in a way that the WIMA component designers likely did not account for. To avoid failures due to mechanical shock, make sure you affix any caps so modified firmly to the turret board with some hot glue or zip ties. StphThu, 29 Jan 2015 08:58:32 -0500 http://ax84.com/bbs/dm.php?id=488826 RE: WIMA Capacitors - These will be fun to turret board!(by scr4mbles) I figured that! I'll need to just suck it up and solder longer wires onto them I guess! Yep, made sure to order 400V+, should be all good:) Thu, 29 Jan 2015 08:30:55 -0500 http://ax84.com/bbs/dm.php?id=488825 RE: Triple single ended, palmer drei(by Stephen Keller) <div class="quoteblock" >On 01/29/2015 @ 04:55, RoyGuitar wrote : <div class="quoteblock" >On 01/29/2015 @ 04:21, steve25 wrote : : it likely runs all 3 tubes in parallel like can always be done with a big enough single ended OT to handle the total wattage and all 3 tubes are always fired up but for the level of audio drive signal set to them by the 3 controls! : : No magic here! : -- : : </div> : : Thankyou very much! : : So this means a primary impedance is chosen which would suit all of the tubes? : : Roy -- </div> You can select a reasonable compromise in some cases as Steve25 suggests. For really dissimilar output tubes a multi-tapped OT primary might be necessary to present different impedance ratios to each tube. For example it could be tough design challenge to settle on a reasonable compromise that works well when paralleling, say a KT88 and an EL84. Of course such tubes have vastly different output levels, so getting an even balance could be pretty difficult as well. The Palmer Drei avoids that sort of problem by using parallel El84, 6V6GT and 6L6GT tubes, which are relatively similar with respect to desired load. I'd start designing around a 3.5-5K load and see how that shaped up on paper. Perhaps one of these single-ended Hammonds would be up to the task: 1630SEA: 30W 3.5K 135 ma. 1628SEA: 30W 5K 120 ma. StphThu, 29 Jan 2015 08:28:44 -0500 http://ax84.com/bbs/dm.php?id=488824