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About dude

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  • Birthday 02/14/1990

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  1. Awesome, that's more-or-less what I was thinking (minus the grid stoppers, so thanks for that). Alright, I should be able to order some parts today. Hopefully I can get started soldering this thing up later this week. Thanks for all your help.
  2. How important is the value of the grid resistors on the EL34s? Since I'm cathode-biasing my power tubes, I'm thinking I can probably just replace the 220k grid resistors that normally go between the grid and the negative bias voltage supply with a dual-gang 220k (or higher) pot for my PPIMV. Is that okay or is the grid current pretty big on these beam power pentodes? Also, for my 6au6 preamp stage: I'm thinking I'll run the first of the two bootstrapped as a triode, and then run the second with a high plate voltage in pentode mode (mostly because this is what's provided on the datasheet). My theory is that I'll get a bunch of gain from the first stage that will distort on the relatively low-gain second stage and give me some nice, tasty pentode distortion. That might be totally wrong though. EDIT: Actually its probably safe to run them both as pentodes. Datasheet shows the transconductance getting higher with lower grid#2 voltages (more "pentode-ey" operation). Granted I'll have a lower plate resistor, which will reduce the gain and max voltage swing somewhat, but I ought to be fine, I think.
  3. Very cool. I'm not going to use any active filter controls (just the variable negative feedback knob), so that's okay with me. Though if I have room when I'm punching holes tomorrow, I'll just put in a PPIMV and a regular MV.
  4. Nearly! Need to calculate my bias resistors for the two 6au6 stages and then order my components. I think I'll punch some holes in my chassis this weekend though. EDIT: I was toying with the idea of putting my master volume AFTER the phase splitter (use a dual gang pot), but I'm not sure how well that would work with negative feedback. I guess if the feedback isn't as intense as it would be for an op-amp, then adjusting the open-loop gain should still affect the closed-loop gain reasonably well. Does that sound reasonable?
  5. Man I'm off today. You're right. It would just mess up the biasing on the phase inverter (when you turn the MV all the way down).
  6. Oh oh oh. Right. My mistake. Good catch. That would have been an unpleasant mistake. Popping fuses left and right.
  7. The tone stack preceding the master volume (right before "A") should block DC from the stage before (its the same circuit as the big array on the left). The 10k grid resistor was there based on sims I ran without the grounding cap. In fact, all of the weird mods I made to the phase splitter were essentially because I forgot to add that cap. So, yeah, I'll just use a traditional LTP and be done with it. I'm already doing enough weird things with this amp as it is.
  8. Aha! I was missing the cap to keep the grid of the second triode at AC ground. That's pretty important. Thanks for running that. I think I'll go with the standard, and upon further consideration I AM going to add negative feedback. I may make it variable with a pot, though. I've got a switch on my old project amp, and the ability to change the amount of feedback gives you tons of variability in tone.
  9. I'm not sure which resistor you are referring to here. I started with the more traditional circuit with a 1M on each grid and a 100k/78k on the respective plates, but I ran into a few problems as I tweaked it. The first was a high frequency roll-off (bad on the positive triode, REALLY bad on the negative triode). The second was a pretty unequal output amplitude (I think I was seeing more like a 3dB difference - about double the amplitude). I know the circuit I started with is pretty tried-and-true (especially since you knew the dB difference off the top of your head), so there was probably something flawed with my simulation (might have been the transience in the capacitors. I only simulated the first 20ms most of the time), but I'm intrigued enough by this setup that I think I'm going to try it. If it is horrible, I'll just copy the standard Fender/Marshall/every-tube-amp-made-in-the-60s power amp and be done with it. In the sims I ran, this LTP circuit has a gain of only about 2-3 (which is fine), but it is pretty closely matched accross the full audio spectrum. It also can provide a pretty huge voltage swing on the output (~60v pk-pk each), so I should have a decent bit of headroom, I think, though I don't have a lot of frame of reference here. I assume this is because at 100k they are horribly matched with the output resistance of the EQ in front of them, and I will suffer a huge gain loss as a result? Yes, I will increase them. Is there any reason not to make the jump all the way up to 1M? Then I could use only one value of pot across the board.
  10. Alright, I've knocked together a more final schematic here. Looks like I forgot to calculate the bias resistances and such. Well, that can be taken care of later: I did a factor-of-ten modification to the RC values in the EQ, and it made a huge difference. The boost and EQ now have a very good response, and the controls seem to behave as they should. I get a (normalized) +5dB at max, -5dB at min, and 0dB at center (log). We'll see how it acts in practice. The long-tail phase splitter took me a lot of fiddling and simulation, but it seems fairly good now. The signals are very close to balanced, and I should be able to have a lot of clean headroom if I want it. I'm using EL34s for the power tubes, and I am cathode biasing. I may add a switch to change bias resistors so I can swap for 6L6 conveniently later. No negative feedback for now. On my other project amp I have a switch to turn feedback on and off, and I really like the sound without, so I'm going to start there and add it in if I decide I need it. Overdrive will work better with feedback. I picked the values for the bias and grid #2 resistors based on an EL34 datasheet. I'm going to look a bit more into this, though, because I'm not convinced these are correct. And because I want to understand everything. (time to read the power amp section of the Audio Cyclopedia again ) I dropped a gain stage, as the music I've been playing lately has been more in the "crunch" range and less in the "lead" range of overdrive. Lastly, I'm moving the first EQ to before the preamp volume (like on a fender or mesa) and putting the main gain stages (2 pentodes) after. This will put the two EQs firmly before and after the gain section. I don't THINK there will be any problems putting the preamp gain immediately following the EQ with no buffer, but I could be wrong. Any thoughts on the design are, as always, deeply appreciated. I'm about ready to order my components (the only thing I haven't got yet) and get building! EDIT: I forgot to draw in a 1M series resistor before each band's 1M pot on the EQ. There should be one on each band.
  11. Throw in an adjustment on the negative feedback and you've got yourself a pretty cool amp. Might have to try that next. Something lower-wattage this time. I've got some power dual triodes sitting around that I REALLY want to use in a push-pull power section.
  12. Okay, well I'll need to mess around with some values and see if I can get something that works well. Though honestly what I have now looks alright. I'd rather have a flatter response though. Ansil, I like that circuit a lot. When I was reading up on vactrols I started thinking of ways to use them to shape the amp strictly from electrical control. I had a dream once of building a modeling amp that simply rearranged the circuit inside to model different famous amps. Hook a microcontroller up to a bunch of conveniently-placed vactrols, and get instant switching from Fender cleans to Marshall Crunch to Vox Bark. Of course you might need a motorized door on the back of your cabinet too EDIT: and yes, I'm using the built-in 12ax7 model, however good that may be. I used to have a link to a page that had SPICE models for a LOT of tubes (even the obscure TV scanner pentodes and such I've got lying around). I'll have to see if I can find it.
  13. Yes, the "10" is 10%. The Pots are 100k. If I increase the R values of my filters I'll probably have to go with 1M pots instead. I can also reduce the output impedance by splitting the 150k cathode resistor into something large on top and something small on bottom, and then bypassing the larger, upper resistor with a good-sized capacitor. Trouble there is that if I do a 150k resistor on top and a 1k resistor on bottom, I need about 4u for the bypass, which is a little silly. But, I just ran a simulation and it works alright. I'll try increasing the filter R values tonight. I expect that will have better results. Plus, using smaller C values will make getting the right parts a bit more convenient. I'd probably be able to use ceramic caps all around. The reason I didn't try that before is because I have a bunch of 100k log pots sitting around, but oh well, I can order some 1M.
  14. Thanks for the replies, guys. Good call on the input resistor, curtisa. I'll change that. Yeah, the 5-band EQ will probably be a little weird. I'm too enamored with the idea of a 5-band EQ in front and back to stray from it this time. I toyed with the idea of throwing some op-amps in and having proper second-order, buffered, active filters in there, but I decided to keep this one "all tube", because, you know, "all tube" is the most important thing when it comes to guitar tone. Just look at Tom Scholz. (/sarcasm) Here's the schematic: The simulation looks mostly good. With the pots all set to 10% (halfway on log pots, theoretically anyway), the response of the filter portion without the follower is very flat, and adjusting the individual band levels produces pretty okay results. The cutoff between bands is definitely what you'd expect for first-order filters (read: really not sharp), but it should work okay for my main intended use: limit the input frequencies to a reasonably narrow band, and then keep the output frequencies mostly flat, but with an upper-mid peak. Could I do this with static filters? Absolutely. Do I want to be able to fiddle endlessly with 10 filter knobs to shape this tone exactly? Yes. Yes I do. Now, the problem arises when I put the follower in front of it. On its own, the follower's output has a nearly perfectly flat response with a subtle low-end roll-off. When I hook up my EQ circuit (which also has a flat response) as shown above, however, the response of the whole system is not flat. It looks roughly the same as the response of the 5 band eq on its own (same low and high cutoff), but there is a gradual decrease from low to high of something like 3 or 6 dB. The 150k cathode resistor should be too big to be a factor, so my only thought is that the cathode resistance or capacitance must be at play here. I'm not sure exactly how either of those are modeled on a tube. Any ideas? Also, Ansil, you always bring in the coolest filter ideas. It's late and I'm having trouble following your descriptions above, but I'd love to chat about them later. EDIT: V2 is ~300v
  15. And here's a (bad) photo of my current schematic (without values for the most part) for your viewing pleasure. After some consideration I'm doing a one-channel amp. I have the vactrols I need to do channel switching, but I want to keep it simple for now. Well, sort of simple. I've got an input buffer/gain stage driving a preamp volume control. Then we've got a cathode follower driving a 5-band eq for a tone stack. Then another gain stage driving a pentode gain stage. I'm thinking having the brunt of the distortion happening here is going to give me some really tasty pentode saturation. We'll see. Finally, I've got another cathode follower driving another 5-band eq (post-distortion). Then a master volume. I put another gain stage next in case the previous EQ kills my signal volume too much, but I don't think it is necesarry so I'm going to remove it. Then of course comes the phase splitter and power section. I'm doing a long tail phase splitter in case I want to add negative feedback now or in the future (I'll play it by ear based on sound). The buffered EQ stages have approximately 0-1 dB of gain. This is a lot lower than the traditional TMB Fender-style tone stack, which has more like 7dB of gain. Also, the pentode (6AU6) has about half the gain of a 12ax7. I was worried I wouldn't have enough gain in the preamp, but I think I'll be okay. I've got basically the equivalent of 2.5 12ax7 gain stages, which is roughly the same as a JCM800 (plenty of gain). ALSO, the 5-band EQ has a gain of 0 when the bands are all at 10% (centered on a log pot). If I need more, I can always turn them up to get an additional 20dB.
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