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1945 Hammond CV Restoration

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    1945 Hammond CV Restoration

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    Part 2 coming...

    #2
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    Other folks have commented on how the 5-tube preamp is kinda a rat’s nest. I have to agree somewhat but it’s not as bad as other “rat’s nest” designs I have seen. Particularly old AM/FM radios often have atrocious wiring. This at least has some good wire dressing towards the base, and some method to the madness. I have to guess that since there is no B+ power supply, AC EMF and ripple noise is less of a problem than in preamps where B+ is filtered on-site. (Totally conjecture. Feel free to lecture me if this is an off-kilter judgement)

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    So far I have removed the preamp, rheostat, matching transformer, and preamp base. Next up is removing the manuals and generator. They’ll get a thorough cleaning and restoration they deserve.

    On, and the swell base is cracked and missing a big chunk. Who knows how that happened? Maybe just an accident while moving it. Planman54 has been so kind as to send me a CV swell base from his old CV cabinet. Click image for larger version

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    More to come…

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      #3
      Are those silver caps under the clamps original? My Northern had wax Aerovox Canada caps in there. And the brown ones were clamped to the body as well, IIRC. They were all... what, .02s? I think? Wire dressing was pretty similar.

      I upgraded my amp to the latest revision, and there was a noticeable improvement in overall gain, but the 6J7 microphonics got worse. I plan to shock-mount my amp. The factory rubber grommets were almost complete gone, so I removed them, but now I know what they were probably for.

      Wes

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        #4
        I'm not really that fond of the vibrato preamp's design. They were trying to build a more complicated preamp on the same footprint as the two-tube preamp, and the wiring harness gets in the way of repairs, making them much more time-consuming than they would be otherwise. Having the volume control upstream from the input means that 100% of preamp noise appears at the output. It pulls more current than the earlier preamp, so B+ voltage is a bit lower than is ideal for the tube choices.

        While it's true that it's not as bad as some amps to work on, I'd still say it's a odd piece of work.

        All in all, I think the AO-10 was a huge improvement in just about every way.
        I'm David. 'Dave' is someone else's name.

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          #5
          Wes,

          These are silver caps, and they definitely look original. "Fast" brand, which is the brand stamped on the 0.5 / 0.5uf oil cap as well.
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          Their internal makeup of the silver caps is a mystery to me. I'm happy to be enlightened but otherwise I plan to replace them since they are mostly coupling caps. I'll test out the 0.5 / 0.5 oil cap hoping it's still OK, as they typically are.

          This is a type F preamp. The schematic does not have any specific notes about type F, but I do see notes for "Type D and below". Hmm. Wes what type is yours?
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          My 6J7 socket is marked "6J7GT", but the actual tube installed is a black-painted metal 6J7. Is there any need or benefit to using a glass type 6J7 as opposed to the metal type?

          Also, I have no shock mounting on the 6J7GT socket. Click image for larger version

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          David, you mentioned B+ voltage being lower than ideal. Eventually I'll get a stand-alone B+ supply, just so as not to be held hostage to a tone cabinet. Is there an ideal B+ voltage I should set the supply to?

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            #6
            I think 200VDC is a good target under load.

            I'm a bit more partial towards these amps after having a completely successful recap for my model AV. Keeping trek II as a backup, I was happy to hear the fantastic tone and low-noise from the original preamp and it is a keeper! FWIW I did a full recap except for the dual 0.5 which tested well & no shorting. I think I kept everything else original. No microphonics issues and it came with some really nice Amperex full glass envelope 6SJ7's.

            Don't forget about the mica cap inside the 6J7 housing.

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              #7
              I know I can replace the mica caps including the ones with dot (domino) markings, but should I? I’ve typically left them be if they aren’t toasted, leaking, or otherwise causing problems.

              I bought a Heathkit C3 last year and it was DOA. It’s PT smoked upon first use even with all dials turned down and no capacitor connected, and being warmed up slowly with a VariAC. The seller had done a piss poor job of “restoring” it. Caps not soldered, and in wrong place.

              So I’m eventually going to get some kind of capacitor leakage checker as well as a decent LC bridge.

              I have and use an ESR meter as well.

              But David is right about the rat’s Nest. By the time I get down there to lift a leg or remove the mica caps on some of these tube sockets I may as well replace the cap.

              Comment


                #8
                The shock mounting is between the amp and the amp shelf. Rubber grommets - notice how big the holes in the chassis are compared to the #8 screws?

                My amp was approximately revision J, but it had that extra cap on the input which made the amp REALLY bright.

                Those silver Fast caps are probably high-quality paper/foil caps in metal housings. So Hammond USA used better caps than Northern Hammond, which were just paper/foil in wax.

                I found that I was removing a LOT of parts just to dig for the bottom-most coupling caps, so I just said "screw it" and replaced everything in the amp aside from the sockets, iron, and oil caps. It sounds great now. IIRC I replaced the mica caps with equivalent film caps. I could be wrong about this. I think David mentioned at one point that those ones had given him and Harvey (or Sal?) grief at one point.

                I do know I replaced the electrolytic in the rheostat box with a film cap.

                Wes

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                  #9
                  Either my preamp never had shock mounts or they have long since been removed. The screws that mount the preamp were not loose to begin with. Maybe that was a later feature or unique to our neighbors to the north?

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                    #10
                    Both Sal and I had vibrato preamp repairs come back to bite us because we didn't replace the 100pF mica caps on the 6SJ7s. They really shouldn't fail there with only 1.2V on a 400V capacitor, but they do.

                    While it is true that most mica caps are very stable, these appear to deviate from the usual pattern.

                    In fact, I recently heard of another batch of mica caps from India that are causing problems as treble caps in a current-production guitar amp.
                    I'm David. 'Dave' is someone else's name.

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                      #11
                      Good question. What is the diameter of the mounting holes in your preamp? Mine were somewhere around 1/4", maybe 3/8", quite a bit larger than the #10 screws would need. I measured them, but I've lost the measurement. Grr.

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                        #12
                        Originally posted by Wes View Post
                        Good question. What is the diameter of the mounting holes in your preamp? Mine were somewhere around 1/4", maybe 3/8", quite a bit larger than the #10 screws would need. I measured them, but I've lost the measurement. Grr.
                        They are larger but only due to a keyhole shape. The spot where the screw torques is snug. I see no evidence of past gooey rubber.

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                          #13
                          On all the CV preamps I can recall, they are screwed directly to the bracket. You loosen the screws, slide, and remove. No shock mounts.
                          I'm David. 'Dave' is someone else's name.

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                            #14
                            Keyholes. Interesting!!

                            I thought Northern was buying amp chassis from Chicago, but I don't recall the keyholes, just round holes, like on my (Chicago) two-tube preamp. Maybe they were rolling their own? The font is the same for the label stamps.

                            I know my Belleville CV wasn't unique, as I've seen at least one other Northern unit like that online. I should check my other CV, which was made in the Montreal plant. This is a small manufacturing variance, but it tells me somebody was trying to solve some problem. I'm *guessing* it was microphonics. When I still had ratchets, I could hear the lower Bb drawbars move, through 3 different sets of tubes. They would kind of "zing" when the pedal was floored and the room was quiet.

                            My two-tube preamp has shock mounts on one of the tube sockets. These were made of different rubber, and have lasted through the years.

                            Muckleroy, when you're done, would you mind posting the max output level you can achieve across the G terminals for tone 25? I had to make some changes to get rock-and-roll gain. Curious if my troubles were localized to this unit, or the design.

                            Wes

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                              #15
                              My last two vib amps had popping noises when selecting vibrato depth. The top mica was culprit both times. I've always repaired amps with a feeling that, well, in this case 70 year old capacitors should be changed out at a minimum. Always exceptions, but I wouldn't spend too much time considering keeping the micas in the same way as the chassis mount oil caps. To each his own though.

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