cr_XD Posted July 19, 2007 Report Share Posted July 19, 2007 hi, i´m studying schematics in order to build a preamp and have come accross this one. the second tube U2/1 U2/2 in the schematic appears in a new configuration for me, can somebody explain what difference exists between the first stage and the second. i´m used to seeing both stages like the first one. thank you Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted July 19, 2007 Report Share Posted July 19, 2007 This is a pretty standard long tail phase splitter using a pair of triodes connectetd as a kind of differential amplifier. Signal goes in the U2/1, and feedback goes into U2/2. The two outputs drive the power tubes with opposite polarity. Most people would think of this as part of the power amp. hi, i´m studying schematics in order to build a preamp and have come accross this one. the second tube U2/1 U2/2 in the schematic appears in a new configuration for me, can somebody explain what difference exists between the first stage and the second. i´m used to seeing both stages like the first one. thank you Quote Link to comment Share on other sites More sharing options...
JoeAArthur Posted July 19, 2007 Report Share Posted July 19, 2007 Yeah, just a different way of drawing it. Quote Link to comment Share on other sites More sharing options...
Geo Posted July 22, 2007 Report Share Posted July 22, 2007 (edited) It's also confusing because they take the output of U2/1 after the 9.1k, apparently to try and balance the two outputs of the phase inverter. If you're just building a preamp, leave out the phase inverter and attach your output jack after C6. The input jack looks strange. I don't know why they used a stereo switching jack. The power supply is also puzzling. What's with that extra feed from point A? I guess they want a DC voltage to reference the heaters? Seems awfully complicated. If you're just building the preamp section, you can use a much simpler power supply. Edited July 22, 2007 by Geo Quote Link to comment Share on other sites More sharing options...
cr_XD Posted July 22, 2007 Author Report Share Posted July 22, 2007 if i wanted to add a whole tone stack, i could replace the tone pot with a more complicated one (thre pots like marshall or fender do?) and also have Quote Link to comment Share on other sites More sharing options...
cr_XD Posted July 22, 2007 Author Report Share Posted July 22, 2007 if i wanted to add a whole tone stack, i could replace the tone pot with a more complicated one (thre pots like marshall or fender do?) and also have a second master volume where you suggest to cut the preamp, after c6 right?? does it make sense. Also, it seems sensible to me that an external preamp input could be added after c6 before the master volume to use an external preamp (like my gt-8) and use only the power amp. This makes sense because lately i've been doing so with one of my amps and it seems to me that it is more relevant to my ears to have a valve power stage than a valve preamp. does this have sense to anybody else?? however i always rely on valves for crunch, but i usually end up having a clean sound and a high gain sound... i plan on getting one of theese at some point, but for now i've settled on the better documented valve junior for a first contact with poweramps. ;P Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted July 24, 2007 Report Share Posted July 24, 2007 Power supply: You mean the point labeled 35V? Yes that is to bias the heaters up. It is well worth doing because it reduces the hum significantly. It matters most for the first stage in the preamp. It's also confusing because they take the output of U2/1 after the 9.1k, apparently to try and balance the two outputs of the phase inverter. If you're just building a preamp, leave out the phase inverter and attach your output jack after C6. The input jack looks strange. I don't know why they used a stereo switching jack. The power supply is also puzzling. What's with that extra feed from point A? I guess they want a DC voltage to reference the heaters? Seems awfully complicated. If you're just building the preamp section, you can use a much simpler power supply. Quote Link to comment Share on other sites More sharing options...
cr_XD Posted July 24, 2007 Author Report Share Posted July 24, 2007 Power supply: You mean the point labeled 35V? Yes that is to bias the heaters up. It is well worth doing because it reduces the hum significantly. It matters most for the first stage in the preamp. Then it's not adviseable to leave out right?? Quote Link to comment Share on other sites More sharing options...
cr_XD Posted July 24, 2007 Author Report Share Posted July 24, 2007 in this one, would the break point between the preamp and poweramp stage be after c3?? the second valve stage looks to me like the phase inverter of the previous, am i right?? Quote Link to comment Share on other sites More sharing options...
Geo Posted July 24, 2007 Report Share Posted July 24, 2007 (edited) Power supply: You mean the point labeled 35V? Yes that is to bias the heaters up. It is well worth doing because it reduces the hum significantly. It matters most for the first stage in the preamp. Then it's not adviseable to leave out right?? It's not necessary. The usual practice is to connect a 100ohm resistor to each leg of the heater winding and ground the resistors. If the amp is grounded properly, it will be quiet. The 35v supply and the method I described do the same thing--reference each half of the heater supply to a stable point. Just as a note--one amp I built has heaters with 2x100ohm to ground and is dead quiet until you get up past 3 o'clock. After that, the noise you hear is unavoidable because the amp is single-ended. "if i wanted to add a whole tone stack, i could replace the tone pot with a more complicated one (thre pots like marshall or fender do?) " Yes. You'd do better to treat the tone control AND volume control as a unit and replace it with a Fender-style tone stack and volume (for example in something like this) http://schematicheaven.com/fenderamps/prin...gz34_aa1164.pdf If you want a mid control, replace the 6.8k that connects the tone stack to ground with a pot, something like 10k-50k. Note that the Fender tone stack has a big gain loss. You can reduce this by increasing the size of the mid pot/resistor, but either way the Fender stack will lessen the overall gain in the preamp. That's probably not an issue, though, since this is a stand-alone preamp and not a preamp inside a complete amplifier. It will still provide plenty of voltage gain to the input of your amp, though it may not produce "hi-gain distortion" on its own. You might want to add another 12AX7 if that's what you're looking for. But that gets into other things (you have to cut the signal back down between each triode stage). "and also have a second master volume where you suggest to cut the preamp, after c6 right??" Yes, add a 1meg pot AFTER the capacitator. "Also, it seems sensible to me that an external preamp input could be added after c6 before the master volume to use an external preamp (like my gt-8) and use only the power amp." That should work. For EL84's in the poweramp, I don't think you'd need more than 10v of drive out of the phase inverter, and I believe that type of phase inverter provides a gain of 25, so you would want 0.4v output from the preamp to drive the poweramp to full output. That's all off the top of my head, probably wrong. "for now i've settled on the better documented valve junior for a first contact with poweramps." Probably a good plan... but poweramps aren't any more difficult than preamps. "in this one, would the break point between the preamp and poweramp stage be after c3?? the second valve stage looks to me like the phase inverter of the previous, am i right??" Yes. Your second schematic has a preamp identical or very close to the first one (just a better-drawn schematic in the first one). If this doesn't make sense, I can draw you a schematic or two. Edited July 24, 2007 by Geo Quote Link to comment Share on other sites More sharing options...
cr_XD Posted July 25, 2007 Author Report Share Posted July 25, 2007 That should work. For EL84's in the poweramp, I don't think you'd need more than 10v of drive out of the phase inverter, and I believe that type of phase inverter provides a gain of 25, so you would want 0.4v output from the preamp to drive the poweramp to full output. That's all off the top of my head, probably wrong. great, cos i've just won a peavey classic 30 in ebay (valve junior will have to wait). and i was looking at the schematic only to realize that the effect loop on it doesn't have a bufffer at the return. It is just fed into the power stage. Is there any advice against unbuffered effect loop. i'm sure i will not have any problem with my current rig, as i have already been using it with an unbuffered loop without problems. but besides signal level, what may be an issue?? Geo: lot of thanks for your explanations. Quote Link to comment Share on other sites More sharing options...
JoeAArthur Posted July 25, 2007 Report Share Posted July 25, 2007 (edited) Power supply: You mean the point labeled 35V? Yes that is to bias the heaters up. It is well worth doing because it reduces the hum significantly. It matters most for the first stage in the preamp. Then it's not adviseable to leave out right?? It's not necessary. The usual practice is to connect a 100ohm resistor to each leg of the heater winding and ground the resistors. If the amp is grounded properly, it will be quiet. The 35v supply and the method I described do the same thing--reference each half of the heater supply to a stable point. Just as a note--one amp I built has heaters with 2x100ohm to ground and is dead quiet until you get up past 3 o'clock. After that, the noise you hear is unavoidable because the amp is single-ended. Well, it's actually more than referencing the heater supply to a stable point. The theory is that the standard practice sets up a condition where the cathode is positive with respect to the heater which causes it to act like a small plate drawing electrons from the heater causing more hum. Referencing the heater to a positive voltage higher than the cathode makes the cathode negative with respect to the heater, preventing the attraction of hum causing electrons. A properly twisted heater supply and balancing the two sides to ground through resistors (or a wire wound pot) is usually all that's needed. "and also have a second master volume where you suggest to cut the preamp, after c6 right??" Yes, add a 1meg pot AFTER the capacitator. You will also have to use an additional capacitor after the master volume pot leading to the phase inverter otherwise you will mess up the biasing of the phase inverter. I'd suggest doubling the size of C6 for both. great, cos i've just won a peavey classic 30 in ebay (valve junior will have to wait). and i was looking at the schematic only to realize that the effect loop on it doesn't have a bufffer at the return. It is just fed into the power stage. Is there any advice against unbuffered effect loop. i'm sure i will not have any problem with my current rig, as i have already been using it with an unbuffered loop without problems. but besides signal level, what may be an issue?? It's not unbuffered - it just doesn't have a separate stage to buffer the effects. The effects loop is serial and the effects are inserted into the signal path. Buffering is provided by the emitter follower transistor before the loop and the gain stage at the input to the power amp. Additional buffering wouldn't provide any benefit. You may be thinking of a parallel effects loop that allows you to blend wet and dry signals. A parallel loop needs some sort of extra buffering to prevent the output of the effects loop from being fed back into the input of the effects device. This type of serial loop places that mixing/blending burden on the effects device, not the amp. Edited due to way too many quotes!! Edited July 25, 2007 by JoeAArthur Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted July 25, 2007 Report Share Posted July 25, 2007 On the raising the heater voltage: Joe is right about the effect being different from just estalishing a solid reference. However, it is best to use the 100 ohm resistors and also connect their "center point" to an elevated voltage. I have measured both effects separately and together. In the real world I would agree that you do not really need to do both, unless you have extremely low hum pickups. Quote Link to comment Share on other sites More sharing options...
Geo Posted July 26, 2007 Report Share Posted July 26, 2007 Well, it's actually more than referencing the heater supply to a stable point. The theory is that the standard practice sets up a condition where the cathode is positive with respect to the heater which causes it to act like a small plate drawing electrons from the heater causing more hum. Referencing the heater to a positive voltage higher than the cathode makes the cathode negative with respect to the heater, preventing the attraction of hum causing electrons. Interesting. Thanks for explaining--I hadn't heard that before. Quote Link to comment Share on other sites More sharing options...
cr_XD Posted August 14, 2007 Author Report Share Posted August 14, 2007 ok, after studying schematics and missunderstanding everything XD, i've settled myself to build a soldano x88r. it's a rather ambitious project, and i´ll be spending my money on this instead of buying an v-twin pedal (as there was no need for it, just curiosity). I tried to register on the sloclone page but i am still waiting to get my accound activated, which is a week now, so if any of you know a moderator there and could help i would really appreciate it. on the other hand i´m looking at the schematic and the layout and find that the tranformer they use is different. 369Bx in the first one and 370cx in the second. Which should i use?? 369bx gives 150-0-150 in the secondary whilst 370cx gives 275-0-275. In my case they start with a 3 because in europe we need the universal series. another issue is with the parts list. I'm sure there is one already done, and i don't want to miss anything as i have to get a lot of things shipped from germany, so i would like to have a glance at it if any has it, please, let me know. I know probably this is all discussed on the sloclone forums. but i really can't have access, so if any of you could help me thank you, as soon as i have all the parts i will try to make a tutorial or sthg Quote Link to comment Share on other sites More sharing options...
curtisa Posted August 18, 2007 Report Share Posted August 18, 2007 When I built my X88R clone, the schematic I had didn't give me the secondary voltages on the transformer, so I just assumed a voltage and went with it. The 275-0-275 Hammond transformer you list there will probably get you close enough - should give you about 380VDC for the plate supply, which is roughly what mine works on aswell. The 150-0-150 is probably a little bit low, although you could bump up the supply voltage by configuring your power supply as a voltage doubler. Probably more trouble than it's worth though. I'd go with the 275-0-275 Hammond 370CX if I were you. Had a bit of a look at that SLOclone forum - it looks very empty! No registered users, and a maximum of 14 users online at any one time. I'd suggest it's probably dead! Quote Link to comment Share on other sites More sharing options...
cr_XD Posted August 21, 2007 Author Report Share Posted August 21, 2007 yes, i eventually got my password and posted a few times. I was actually really excited with making a pcb with pcb sockets like it had been done by someone (who was not able to publish it) for free. But i am really discouraged now because of the poor feedback. It looks like a great project, and maybe i was waiting for the same behaviour as in this forums which i think it is great. I don't know what i will do. if there is no one interested in an open project like that i don't see a point in doing it. btw, what are youre reassons against or in favour of pcb only designs. i see it as the easiest way, and since it is only a preamp maybe the statements are different to what it has already been heard off in other discussions. byes. Quote Link to comment Share on other sites More sharing options...
cr_XD Posted August 21, 2007 Author Report Share Posted August 21, 2007 The 275-0-275 Hammond transformer you list there will probably get you close enough - should give you about 380VDC for the plate supply, which is roughly what mine works on aswell. The 150-0-150 is probably a little bit low, although you could bump up the supply voltage by configuring your power supply as a voltage doubler. Probably more trouble than it's worth though. Eventually i think i will get the transformer done to my specs in late septemeber. It is cheaper than buying the hammond. However,. given your comment, i think i will ask for the same specs. Do you think is reassonable, or should i ask for other voltages. Thanks Quote Link to comment Share on other sites More sharing options...
curtisa Posted August 21, 2007 Report Share Posted August 21, 2007 btw, what are youre reassons against or in favour of pcb only designs. i see it as the easiest way, and since it is only a preamp maybe the statements are different to what it has already been heard off in other discussions. I don't really have any problems with PCB versus point-to-point designs. Provided it's done well, a PCB-made amp can perform as well as any point-to-point amp out there. The whole PCB's are inferior to point-to-point argument is a load of twaddle IMHO - it's more down to the quality of production and layout that makes the difference. My X88R clone is a point-to-point job, however I didn't build all 3 preamp stages, so I had less to squeeze in. I only built the third channel, and then did some clever switching to make it a two channel preamp sharing the same tone controls. Separate gain and output controls were added to give me a little bit of individual separation of the two channels. The only reason I went point-to-point was that I had all the parts available to start with, and I didn't want to waste my time designing and etching a custom PCB. If you're building the X88R with all three channels, and you have the PCB layout handy, build it on the PCB. Point-to-pointing all three channels of the X88R will be mighty tricky. Some pics of mine if you need some ideas: http://i153.photobucket.com/albums/s222/ac1176/Preamp1.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp2.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp3.jpg http://i153.photobucket.com/albums/s222/ac1176/Preamp4.jpg Eventually i think i will get the transformer done to my specs in late septemeber. It is cheaper than buying the hammond. However,. given your comment, i think i will ask for the same specs. Do you think is reassonable, or should i ask for other voltages. Entirely up to you. If it's cheaper to get a custom-wound transformer than to buy one off the shelf, get the custom wound one. Be aware that you need to also specify the current ratings of the secondary windings when custom ordering. The only other thing you may want to specify in your custom order is for a dual winding or tapped primary - if you intend to travel to a country that has a different mains voltage you can reconfigure the primary winding to work on 110V/220V/250V etc Quote Link to comment Share on other sites More sharing options...
cr_XD Posted August 23, 2007 Author Report Share Posted August 23, 2007 wow, impressive work. it looks really clean, i guess you are using to transformers because one is for the heaters right? on the other hand, i have two set-ups in two different places, so maybe i'll start with a copy of yours... do you have any further info?? if it is the third channel of the x88r the values should be the same and the layout is just in the pictures. The only thing i would not be able to figure out is how you figured out the second channel. thank you for your time and help! Quote Link to comment Share on other sites More sharing options...
curtisa Posted August 23, 2007 Report Share Posted August 23, 2007 wow, impressive work. it looks really clean, i guess you are using to transformers because one is for the heaters right? Not quite. I'm using two transformers because I didn't have a proper multi-secondary transformer handy to start with. All I did was use a 240/12V transformer to step down from the mains, and then feed the 12V output into a 240/20V transformer wired backwards - so you step down the voltage and then step it back up again. This yields a secondary voltage of 144VAC, which is then fed into a voltage doubler circuit, giving about 400VDC for the plate supply. The step down-step up technique is quite common, retains your galvanic isolation from the mains (important!), and cheaper than getting a custom transformer made. The only drawbacks are you should never exceed the rated voltage on ANY winding, you can't expect much current from the HT winding (so it's only good for preamps), and you need the space to fit two transformers. The 12V output of the first transformer also feeds the heaters, which are DC regulated for low noise. on the other hand, i have two set-ups in two different places, so maybe i'll start with a copy of yours... do you have any further info?? if it is the third channel of the x88r the values should be the same and the layout is just in the pictures. The only thing i would not be able to figure out is how you figured out the second channel. thank you for your time and help! I'll have to dig out the schematic I was working from and hand draw the modifications I did to build mine - I sorta designed it on the run. Leave it with me for a few days and I'll see if I can draw it up. Fundamentally the third channel was built as is. It's the "clean" channel I added that required the changes. If you want true independant operation (eg separate gain, bass, mid, treble and output controls) you're better off building the X88R as it is presented than building mine. Quote Link to comment Share on other sites More sharing options...
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