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deluxe reverb amp with extra gain stage on chanel 1 to make it more like channel 2

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  • #16
    Also, keep in mind that the gain of the reverb (vibrato) channel will be a lot higher if the amp is built without the vibrato channel because the 50k load of the vibrato intensity pot isn't in place.
    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "If you're not interested in opinions and the experience of others, why even start a thread?
    You can't just expect consent." Helmholtz

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    • #17
      Awesome, thanks Tubeswell, thanks Chuck!

      Trial 1:
      Well, stole some time that I should have used to re-install the dishwasher, and other house repairs, took the failed attempt components out, switched the bias cap and resistor and plate resistor to the same values as in the preamp (on this 1/2 of 12ax7 that would have been tremolo). 820R resistor, 25v 25uf cap for cathode, and 100k plate. I had a trim pot + resistor installed from the failed attempt, left that in.

      With this configuration, output of the normal channel goes through a voltage divider that has a trim pot, into 1/2 a 12ax7 biased as above, then through another voltage divider 220k/100k into the mixing resistor.

      By poking around through other schematics, some calculations that are probably wrong, and some dumb luck. OK lots of dumb luck: sounds much better than before. Normal channel is a little more 'gainy' (just a touch) and should be in phase with the reverb channel. Even trim pot at max, volume isn't wildly out of spec compared to the reverb channel. Sound quality is pretty good. Noise, hum, about what they were before I started the hack fest.

      No sparks, no smoke, which is nice.

      Next trial, some fiddling with the mixing cap+resistor (dry to wet) on the reverb channel, as Leo suggested.

      Will also try the "both channels into extra reverb gain stage" suggestion after that.

      General question for you all: when you do these kinds of tone "tuning" experiments, its hard to do a side by side, for me anyway, it takes up to a week before I can get time to make the changes, then I forget what the exact tone was like. Do you record trials then listen to side by side? Recording is a whole other ball of wax.

      This poor amp chassis has been sitting on a stand on my desk for many, many months. Nice to get it making sound again! Can't thank you all enough for answering my many, often stupid questions.
      The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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      • #18
        Originally posted by Chuck H View Post
        Also, keep in mind that the gain of the reverb (vibrato) channel will be a lot higher if the amp is built without the vibrato channel because the 50k load of the vibrato intensity pot isn't in place.
        Chuck, thanks that would explain a lot. I could not figure out, on this poor hacked amp, why the gain was so different between the two inputs (normal vs vib). I looked for ages to find a wrong component, resistor, bad tube. This is what I asked for, I just didn't know it.

        Never liked tube vibrato, and after reading about the bias bumping going on, liked it less. My brother is a real guitarist, brought over his strymon box. Its got fantastic tremolo, a bunch of variations, and won't (that I know of) beat the OT bias.
        The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

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        • #19
          Originally posted by tubeswell View Post
          yes, one could replace one or more of the 100k plate resistors on the first two stages with between 150k to 470k (depending on whim), or reduce the cathode resistance (to get hotter bias and more tube current through the load)
          Thanks Tubeswell, i would like to try that, to get just a bit more gain on both channels. I got lost in the calculations the last time, back to the books and see if I can get it right. Wil try 150KR, since I have them, and it seems like not a huge change.
          The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

          Comment


          • #20
            Originally posted by mikepukmel View Post
            Wil try 150KR, since I have them, and it seems like not a huge change.
            As you go higher with the anode resistor, the load line flattens out a bit, giving the stage more output swing for a given input swing. Not a lot, not like a 470k will magically create 470k/100k = 4.7 times more gain! A few dB is all. For the high-gainers, that few dB more above the noise floor helps. For 150k from 100k, you might not notice it, especially if the stage is loaded down (haven't looked at the schem for your DLX). But go for it! Not a huge change, as you suppose, but will be a tweak in the right direction.
            If it still won't get loud enough, it's probably broken. - Steve Conner
            If the thing works, stop fixing it. - Enzo
            We need more chaos in music, in art... I'm here to make it. - Justin Thomas
            MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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            • #21
              Originally posted by mikepukmel View Post
              Chuck, thanks that would explain a lot. I could not figure out, on this poor hacked amp, why the gain was so different between the two inputs (normal vs vib). I looked for ages to find a wrong component, resistor, bad tube. This is what I asked for, I just didn't know it.

              Never liked tube vibrato, and after reading about the bias bumping going on, liked it less. My brother is a real guitarist, brought over his strymon box. Its got fantastic tremolo, a bunch of variations, and won't (that I know of) beat the OT bias.
              Disconnecting the vibrato circuit at the pot is part of several "SRV mods" for Fender type circuits to add gain and improve fidelity at either end of the spectrum. IIRC it's reported that Stevie had the vibrato disconnected at the pot in (ironically) his famous Vibroverb. Rendering it just a "-verb" I guess

              EDIT: He also had the "normal" channel tube pulled. This heats up the bias on the remaining vibrato channel by leaving only one tube running through the previously shared cathode circuit. It also raises the gain and changes feel a bit. But then it's also reported that he used a 5751 input stage tube, which would then decrease gain a bit. Anyway... Offered for your consideration. Certainly it was a very good sounding amp.
              "Take two placebos, works twice as well." Enzo

              "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

              "If you're not interested in opinions and the experience of others, why even start a thread?
              You can't just expect consent." Helmholtz

              Comment


              • #22
                not like a 470k will magically create 470k/100k = 4.7 times more gain!
                Don't forget the internal plate resistance (maybe 60k for an ECC83 with bypassed cathode resistor) which acts in parallel to the plate load (referring to the current-source tube model) and prevents much increase of gain with higher plate resistors. No sense to go higher than 220k.

                img013.pdf

                The theoretical (non-achievable) maximum no-load gain is given by µ (around 100 for a 12AX7).
                Last edited by Helmholtz; 08-21-2019, 08:21 PM.
                - Own Opinions Only -

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                • #23
                  Originally posted by eschertron View Post
                  As you go higher with the anode resistor, the load line flattens out a bit, giving the stage more output swing for a given input swing. Not a lot, not like a 470k will magically create 470k/100k = 4.7 times more gain! A few dB is all. For the high-gainers, that few dB more above the noise floor helps. For 150k from 100k, you might not notice it, especially if the stage is loaded down (haven't looked at the schem for your DLX). But go for it! Not a huge change, as you suppose, but will be a tweak in the right direction.
                  Thanks, will try that! As Im learning slowly, Im finding that smaller changes spending a little time to get a feeling for what they do, are better for me to sort out, than going hog wild.
                  The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                  Comment


                  • #24
                    Originally posted by Chuck H View Post
                    Disconnecting the vibrato circuit at the pot is part of several "SRV mods" for Fender type circuits to add gain and improve fidelity at either end of the spectrum. IIRC it's reported that Stevie had the vibrato disconnected at the pot in (ironically) his famous Vibroverb. Rendering it just a "-verb" I guess

                    EDIT: He also had the "normal" channel tube pulled. This heats up the bias on the remaining vibrato channel by leaving only one tube running through the previously shared cathode circuit. It also raises the gain and changes feel a bit. But then it's also reported that he used a 5751 input stage tube, which would then decrease gain a bit. Anyway... Offered for your consideration. Certainly it was a very good sounding amp.
                    Excellent, I can't wait to pop that V1 tube out and sounds *just* like Stevie! Heh. Ok seriously, very cool info, thanks. Oh my poor neighbors, the experiments Im going to try!
                    The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                    Comment


                    • #25
                      Originally posted by Helmholtz View Post
                      Don't forget the internal plate resistance (maybe 60k for an ECC83 with bypassed cathode resistor) which acts in parallel to the plate load (referring to the current-source tube model) and prevents much increase of gain with higher plate resistors. No sense to go higher than 220k.

                      [ATTACH]54814[/ATTACH]

                      The theoretical (non-achievable) maximum no-load gain is given by µ (around 100 for a 12AX7).
                      Awesome, thanks for the info and chart! 220k is a lot higher than the stock 100k (to me anyway, 220% seems like a lot) with changes that order, would you measure up everything, voltages and re-calculate the bias? How far can you go without risk of toasting the tube?
                      The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                      Comment


                      • #26
                        I'll draw up the schematic, and post it. Based on how it sounds and smells I don't think I did anything extremely stupid.
                        The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                        Comment


                        • #27
                          Originally posted by mikepukmel View Post
                          Thanks, will try that! As Im learning slowly, Im finding that smaller changes spending a little time to get a feeling for what they do, are better for me to sort out, than going hog wild.
                          I find myself making a list of changes, putting them in, and then trying to figure out which one *didn't* do what I expected

                          Originally posted by mikepukmel View Post
                          I'll draw up the schematic, and post it. Based on how it sounds and smells I don't think I did anything extremely stupid.
                          The anode resistor makes a wee difference in the dynamic behavior of the stage. A little gain, a little change to the clipping characteristics, etc. Nothing drastic. Like the difference between Hungarian paprika and Tex-Mex paprika on your deviled eggs. No way changing the anode resistor would make any part of the amp "melty". Well, maybe if one put in 150 Ohm instead of 150k!
                          If it still won't get loud enough, it's probably broken. - Steve Conner
                          If the thing works, stop fixing it. - Enzo
                          We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                          MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

                          Comment


                          • #28
                            Thanks E! Little tweaking is great. I learned the hard way, that a fender type thing isn't going to sound like a 100W marshall, and well we don't want it to
                            The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                            Comment


                            • #29
                              For some reason, back when, I had put an extra dropping resistor off the choke + cap to feed the reverb driver.

                              THis change, I just realized I used the wrong cathode resistor. I had a 1.5K on the bench but soldered in a 820R. Think I doubled or halved something. Dang.

                              Click image for larger version

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                              The only good solid state amp is a dead solid state amp. Unless it sounds really good, then its OK.

                              Comment


                              • #30
                                Excellent, I can't wait to pop that V1 tube out and sounds *just* like Stevie!
                                But then you'll also need 2 JBL E130-8s. At least that's what he used in 1986 when I had a chance to take a glimpse at his speakers and equipment from very close during a show in a German TV studio. He had two '64 Vibroverbs stacked with only the top one powered and connected to both 15" speakers.
                                Rene Martinez also told us that he used RCA 6L6s.

                                I liked his sound better than his later multi-amp setup with EV speakers.
                                Last edited by Helmholtz; 08-22-2019, 02:04 PM.
                                - Own Opinions Only -

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