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Davis 201A PA Conversion

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  • #76
    So the weekend is back and it's time to work on the amp again.

    So I wired up three switches for the channel "blending". So it works like this: For each of the 3 channels, a wire is run from the grid of the preamp tube to one side of the switch and then the other side of each switch goes to the 1/4" jack for the guitar. So turning on a switch sends the signal to the appropriate channel. This works correctly but if you flip the switch while the amp is on, there's a pretty unpleasant pop, which I was kind of worried about. Is there an easy way around this? The switches are actually at the back of the amp, so I could just be sure to turn the amp off when I switch channel configurations, but it'd be nice if I could do it without the pop.

    Sadly, I'm not noticing much thickening of the tone either. I think the LED biased channel and the regular resistor biased channel sound a little too similar. May need to change the biasing a bit. It might make sense to rewire one of the sockets (2 channels) for a 12AX7 as well. That ought to give me a slightly different tone than the 6EU7s as well as allowing a little tube rolling with both sockets (as I currently only own three 6EU7s and they're the ones that came with the amp and the only rolling I can do is swapping tube sockets).

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    • #77
      I'm wondering if this is why everyone uses LDRs in their channel switching. Kinda surprised nobody has chimed in on this yet.

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      • #78
        Originally posted by Pdavis68 View Post
        I'm wondering if this is why everyone uses LDRs in their channel switching.
        The grid leak biased channels will have the bias voltage on their grids, AC couple the grids and put a 1M resistor to ground from the inputs then all the switching will be at ground potential.

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        • #79
          Thanks for that Dave, but I may need a bit of hand-holding on this, simpleton that I am. By AC coupling, you mean to put caps between the channels? If so, what value of cap? I assume it matters since I'm guessing certain values would simply pass the guitar signal between all the grids?

          The switches are all DPDT (I'm just using one side). Should the ground be on the input jack or on the off side of the DPDT switches?

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          • #80
            It would help to have a schematic of the switching. I hate to follow descriptions and then answer to circuit changes with more descriptions. Especially when you may need to read through all those descriptions to describe further refinements. AAargh!

            With a downloadable picture I can tack in changes and submit them more easily. You also get more participation this way because NO ONE like to do the other way.
            "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

            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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            • #81
              Originally posted by Chuck H View Post
              It would help to have a schematic of the switching. I hate to follow descriptions and then answer to circuit changes with more descriptions. Especially when you may need to read through all those descriptions to describe further refinements. AAargh!

              With a downloadable picture I can tack in changes and submit them more easily. You also get more participation this way because NO ONE like to do the other way.
              Well, this was a good exercise because apparently I screwed up and forgot to replace one of the original 2.2M grid leak resistors with a 1M. :-(

              And I believe originally U1 had 2.2M grid leak that I may have cut off. I may have gotten a little confused when I was doing that work. Obviously I need to do some fixing... But this is how it's currently set up.
              Attached Files
              Last edited by Pdavis68; 08-09-2015, 01:45 PM.

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              • #82
                I'd want to have the unused inputs grounded. Can you add the cathode circuits so we can see how the tubes are biased?

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                • #83
                  Well, this is just embarrassing... I obviously got channels 1 & 3 mixed up while I was working on them.

                  So this is how it's currently set up:

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                  This is how it will be after I fixed that first channel (Can I have a grid stopper on channel 3)?

                  Click image for larger version

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                  So where exactly would you place the resistors to ground the unused inputs? Wouldn't the grid leak resistor ground them (well, with the exception of my stupidity leaving channel 1 without a grid leak... lol.)

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                  • #84
                    Well, thanks, adding that grid leak fixed the popping sound. Works like a charm. Now I just need to get some spray to clean out the pots. A couple of them are scratchy.

                    Put together a 1x12 cab and stuck in the Magnavox speaker I pulled from a Conn organ (mentioned in another post) and it doesn't suck at all. Another successful weekend of amping... Still have a few things I want to do, but I'm in an awesome mood now! Thanks Dave & Chuck!

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                    • #85
                      Originally posted by Pdavis68 View Post
                      Well, thanks, adding that grid leak fixed the popping sound. Works like a charm. Now I just need to get some spray to clean out the pots. A couple of them are scratchy.
                      If cleaning them doesn't help, be sure to check for DC on the pots (where it shouldn't be). Since there were some errors already located, and the amp is old so there may be leaky caps, it's a definite possibility.

                      Originally posted by Pdavis68 View Post
                      ...but I'm in an awesome mood now!
                      Sorry to rain on the picnic

                      On the up side... I've tried a couple of old Maggie 12" loud speakers. Not woofers and not anything newer with foam surrounds and such. Just plain old 50's and early 60's "loud speakers". They sound damn fine for guitar amps Newer speakers are more efficient, but that's not always a priority.
                      "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

                      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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                      • #86
                        At least one pots pops in the same place when you turn it, so it's just got a bad spot in there. I suspect the coupling caps are okay. Someone else had done some work on this amp relatively recently before I bought it and they completely recapped it (except for filter caps).

                        Next step, turn the "phono" pot into a master volume!

                        After that, I need to actually check the biasing of the PI (I keep putting that off) and I'm toying with the idea of putting a pot on the NFB.

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                        • #87
                          Well there goes my happy weekend. Thin I blew the OT. ARGHHHHHH... Put the master volume in, it didn't work well at all (I think the pot was kinda bad). Removed it, put the tubes back in and sat the amp on the cab, left the room for a bit, came back, plugged in the amp turned it on, strummed the guitar a few times, nothing. Figured I had disconnected the grid wires or something in the power tubes. Started pulling tubes and noticed the speaker cable hanging unplugger. Plugged it in, everything was really staticy and the guitar when it played was pretty quiet. Recooked grid connections (that's where the mast volume went) and turned it back on and the static was ridiculously loud.

                          Can't believe I did that. I suck at this. Guess that's $30 at least down the tubes (lol. Just caught the pun.)...

                          I may need to learn to wind transformers...

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                          • #88
                            The OT may still be OK. Pull the power tubes and do some winding continuity tests. Then plug in and take some voltage readings. I'm not saying it's NOT blown. But see about it at least before getting drunk and grumpy tonight.

                            I can tell you this. I've seen many OT's survive the "I was working on the amp, forgot to plug in the cabinet for a sound check and didn't notice until I got no sound!" I may have done this to MOST amps I've owned at one time or another. No failures yet.
                            "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

                            "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

                            Comment

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