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Tremolo on 1966 Ampeg Jet J12-D, and other tech notes

Discussion in 'Glowing Bottle Tube Amp Forum' started by theprofessor, Oct 2, 2020.

  1. theprofessor

    theprofessor Poster Extraordinaire

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    I posted here about a 1966 Ampeg Jet J12-D that I was considering. I decided to keep it. It sounds really, really good. Straightforward, fundamental tones, with a lot more body in the cleans than I'm accustomed to from Fender amps. Clean, middy girth that is still amazingly clear, and with a midrange that doesn't fatigue the ear as much as some of the upper-mids in Fender amps can.

    I didn't feel like messing with working on it myself, so I let my tech go over it.

    I got him to replace the can capacitor with a C.E. Manufacturing 40-20-20-20 @ 525V. There was some discussion in the above-linked thread about whether to separate the cathode bypass cap from the common ground in the can. I decided after talking with my tech, who A-B'd using the node on the can and hooking up the cathode bypass capacitor separately, to stick with the can for all the connections as was done originally. It sounds great. He told me that when he hooked up the cathode bypass capacitor separately, the amp was thinner and brighter and cleaner. Not as full sounding, and not as good. I took his word for it, and I think that after the can replacement, the amp sounds fundamentally the same as before (which is a good thing), but it holds together better and is, of course, now more reliable.

    My question now concerns the tremolo. On this amp, the tremolo is engaged by a switch on the pot which controls speed. There is no control for intensity. I am wondering how to locate which of the caps/resistors control the intensity. I might like to play around with values a bit. I'd like the tremolo to be more intense than it is. I have put the schematic in this post, along with a photo of the board where the tremolo string is. Can you tell me which caps or resistors affect the intensity, and what values you might suggest to increase it, relative to the schematic values?

    J-12D tremolo schem.jpg IMG_2463.JPG IMG_2584.JPG

    Other things:

    Tubes. The preamp tubes on this model are 6BK11's, which are now almost unobtainable, and commonly replaced by the increasingly endangered 6C10's. This amp came with two Ei "6C10" tubes, which have a taller bottle above the top mica than typical 6C10's I have read that these Ei 6C10's are in fact re-labeled 6AC10's. Whereas the three triodes of a 6C10 have gain factors of 100/100/100mu, a 6AC10's three triodes have gain factors of 62/62/62mu. I was able to secure two GE 6C10's to put in the amp. I cannot tell any difference in gain between the Ei's and the true GE 6C10's. In terms of tone, the GE's are more natural and fundamental sounding, with a bit more bass. I think they sound great. The Ei's sound good too, but they are unnaturally bright (not so much a problem with the brightness as with the "unnatural" part) and they are leaner. I do like the sound with the Ei's, and no matter whether they are 6C10's or (as is more likely) 6AC10's, it's nice to have a good-sounding backup pair.

    Output transformer. I suspected that the output transformer might have been changed when I looked at the amp in the store. I was right. It's a Mercury Magnetics Tone Clone of the OT-151-A that came in the amp in 1966. I can't see the numbering on the Mercury transformer with the chassis installed and haven't located any Tone Clone for that OT on the Mercury Magnetics page. But it sounds fantastic.

    Speaker. This amp came with an Eminence GA-SC64. It's a speaker I know I like, as I used one for a while in my SF Deluxe Reverb. It works really well tone-wise with this amp, and the amp is quite loud for a little 15W bugger. Quite. In this Jet design, at least, the 7591 power tubes are quite close to the speaker magnet. And the magnet on his GE-SC64 is substantial (38 oz.). My tech told me that at very high volumes, the magnet was interfering with the power tube performance and that he had traced it on an oscilloscope. Once he separated the chassis from the speaker and cabinet, the interference was gone. I'm not going to worry about it, as I'm never going to dime the thing, anyway. As I said, the amp is quite loud, even on "3."

    Shielding. The shielding on the inside of the panel was surprisingly thin. As in, very. And much of it was missing. I removed what was loose enough to scrape off pretty easily and then put my own shielding tape on top of it. Much better looking, at least.

    Hardware. One of the things with Ampegs is the scarcity of the clutch-head screws often used to secure the back panel and the chassis. They look to be chrome with #8 shafts and wood thread, around 1-1/8" long, and the clutch head takes either a 1/8" or 5/32" driver bit (it definitely takes a 1/8", but it's a little looser than I'd have thought). My amp was missing only one of these fasteners. Fliptops has these selling for $3.50 each, with the only shipping option at $8. I'm not going to pay that for one fastener, if I don't have to. And of course it's totally unnecessary, anyway. My tech put in a Phillips head fastener where the screw was missing, and it looks similar to the originals. But I'm kind of picky that way -- I like things to be just so, if I'm able. This hardware thing does seem to be pretty challenging with Ampegs. Even the old hardware stores with the slide boxes don't seem to have anything related to clutch head or anything really even close. The closest thing I saw as pocket screws that use a small square driver bit, but those were bronze-colored. My next door neighbor is a car mechanic and does lots of restorations on old cars. He said that clutch head screws like these were used on old Corvettes, such as those between '58-'62, but that finding that particularly length might be difficult. I've looked at Eckler's Corvette, and they only have machine thread (at least that I saw). I'm going to continue to hunt around a bit, but I'm not very optimistic about success in finding an exact replacement, except for at Fliptops.

    IMG_2587.JPG IMG_2588.JPG
     
  2. milocj

    milocj Friend of Leo's

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    Check a camper/RV store for those screws. They're very commonly used for those applications.
     
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  3. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Cool project!!! I think you've chosen wisely as usual, @theprofessor . Your hard question about the trem I'll leave to the smart guys. But that clutch-head screw is just the sort of fun, harmless detail I like to think about. BTW, you'll know this, but those look like clutch type A; there're also type G, which are butterfly shaped.

    I agree it may be hard to find wood-thread screws in Corvette catalogs. Fiberglass thread, maybe? :)

    Now, that Fliptops NOS screw for $3.50 plus $8 shipping? When I'm looking at an amp that's (to me) worth hundreds of dollars, I've been known to throw the occasional ten or twenty into some fun little detail. You'd want 3 or 4 of those against some future need (oh, yeah, as if I could find them when I need them in the future) so the shipping is a smaller part of the (larger) total. But looking at their catalog, I don't think the NOS version has that nice patina the screws do in your pic (very nice photos BTW).

    I like the idea these probably exist -- they're one of those burglar/vandal-resistant types. Sounds like @milocj is onto something -- he's a smart guy. And I'm pretty sure you could get pan-head *square* drive, but they'd be shiny and they're not 'just right.'

    My 'conserve v. restore' side says use the one patinated Phillips -- hey, this is an actual vintage amp; parts get lost; a replacement screw says it's *real.*
     
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  4. theprofessor

    theprofessor Poster Extraordinaire

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    You're too kind with your deference. I had possibly seen something with respect to the difference between type A and G clutch head, but had ignored it. I did not know that, and it explains why my type G bit drivers don't fit the type A clutch head screws well. So thank you for mentioning it!

    I am not at all opposed to going straight to Fliptops, if necessary. But I also like the idea of finding hardware overlap in muscle cars or mobile homes so that amp hardware like this doesn't seem so esoteric as something that only Ampeg used on some of their amplifiers. We'll see...

    Like you, I'm actually fine with the Phillips. I might like it even more if I had a whole set of them.
     
  5. Nickfl

    Nickfl Friend of Leo's

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    Try looking for clutch head sheet metal screws instead of wood screws. basically the same type of thread and probably much more common, looks like you can find those on eBay listed as parts for old Chevys, but not I'm not sure what length you need.
     
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  6. theprofessor

    theprofessor Poster Extraordinaire

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    Thanks for all the tips on the hardware. I do sincerely appreciate them. But before this becomes a thread only about hardware: is anyone able to pitch in on the main question of how to make the tremolo more intense?
     
  7. tubegeek

    tubegeek Friend of Leo's

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    @theprofessor

    I would normally focus on the 470K/10K voltage divider on the grid of two sections of V1. However, I have absolutely no idea what those two sections are doing - cathode tied to plate and other plate and other cathode grounded? Huh? I suppose it could have been being used as a back-to-back limiter/clipper but what the actual f? Two sections of a triple triode devoted to a signal diode limiter?

    It -was- 1966, and there -was- a fair amount of pharmaceutical LSD available in some circles, but still...

    I think that was Switzerland more than New Jersey in '66 though.

    The more I think of it the more I think that's what's going on there. In pedal effects you will often see yin/yang LEDs or silicon diodes used to clip a signal or limit it, and in signal oscillators sometimes too. This looks like it xould do the same thing, and if you adjusted that .4V bias at the shared grids it might possibly allow more oscillator through.

    Which might throw other stuff completely off at the other end.

    Was 2/3 of a compactron tube REALLY cheaper than an Intensity pot in 1966? Or was Ampeg just trying to put a clear distance between the Jet and Rocket models? (Rockets have Speed & Intensity knobs, & a hard-wired "doorstop" foot switch for the trem.)
     
  8. Nickfl

    Nickfl Friend of Leo's

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    RE: tremolo intensity the valve wizard points out that tremolo is very sensitive to tube aging and weak tremolo is often cured by simply replacing the tube. Which when you're talking about unobtainium tubes is unfortunate...
     
  9. theprofessor

    theprofessor Poster Extraordinaire

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    Yes, but I do have two very strong tubes in the amp now. And I'm still curious about making the tremolo more intense.
     
  10. theprofessor

    theprofessor Poster Extraordinaire

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    Well thanks for diving in and making an attempt, at any rate. It sounds at this point like I might just be leaving things well enough alone, and that the lack of an intensity pot means that even more of the function is thrown back to the tube itself. And as you say, if I were to change one thing on one or two triodes, it might affect other things elsewhere.

    No big deal. This amp sounds EXCELLENT. It's just a shock to me how good this little Jet sounds -- especially with traditional single-coils like Tele and Strat pickups. And I don't want to run the risk of changing that.
     
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  11. tubegeek

    tubegeek Friend of Leo's

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    You wouldn't kill anything if you tried, I dunno, lifting one end of the 10K and putting another 10K in series, just to find out what the heck happens.

    But that is a weird little subcircuit, no lie. Maybe @robrob has a clue what it does?
     
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  12. timfred

    timfred Tele-Meister

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    The part of the schematic that you circled in red is the tremolo oscillator and as long as the speed is good, no need to change anything there.

    Before doing any mods, I would check the values of the following components:
    1. 0.05 cap and 470K/10K resistors between the oscillator plate and booster stages (2/3 V1)
    2. 0.1 cap, 470K resistor, 0.01 cap, and 47K resistor that together feed the preamp signal (now modulated) from the plate of 1/3 V2 back into it’s grid.

    If all of those are OK, then what tubegeek says about upping the 10K resistor.
     
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  13. theprofessor

    theprofessor Poster Extraordinaire

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    Thanks very much. I'll do some looking under the hood next time I want to tinker with this and will report back.
     
  14. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Great idea, clearly. Seriously, I'm not smart about testing caps. :) *For our purposes here:* Is it adequate to test capacitance? In circuit? Or do we need to lift one end, or test for DC leak, or test for ESR, or???
     
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  15. jmp81sc

    jmp81sc Tele-Meister

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    I think the 1.5K resistor at the cathode of the V1 triode for the trem circuit controls the intensity. On the 5E9a and 5G9 Fender put a 250K pot there, usually in conjunction with other resisters. You could try jumpering in a pot to get to the intensity you like, measure the resistance then replace with a fixed value resister.
     
  16. jmp81sc

    jmp81sc Tele-Meister

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    Well after looking at the schematic more I can't tell what is going on ha ha. Better take anything I said with a large dose of salts. The 120K resister that sits by itself in the loop looks like a more likely candidate, as that would be midway up on a 250K pot.
     
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  17. BobbyZ

    BobbyZ Doctor of Teleocity

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    If I was going to do something with intensity I'd start by looking at other Ampeg schematics with a pot for it. Should be fairly apparent how they did it, maybe it's just a pot instead of a resister in one spot, or a pot and a resister.
    Then you could tack in a similar arrangement to experiment with, when you find the spot you like, measure the overall resistance and put a resister in close as you can get.
    Maybe it ain't that easy?
    Also trem circuits are kinda finicky. Make sure you know how to put it all back the way it is now, just in case.
     
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  18. theprofessor

    theprofessor Poster Extraordinaire

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    I've still got to look at the trem circuit and try out some of the proposals listed here and elsewhere. But in the mean-time, a note on tubes:

    I got a few GE 6C10's -- two from an ebay seller who tests and offers free returns, one from a guy on TGP who had purchased it from Brent Jessee but had never used it, and another from another ebay seller that was NOS and NIB ("Admiral" labeled).

    There were some variations among them, notably one with a slightly taller bottle. Though most of the label was rubbed away, you could see a remnant of the 188 date code on the side. Interestingly, not only did it have "6C10" on the top; it also had a dot date code there.

    With that tube in particular, the amp was a LOT louder. A LOT. It was also more brash sounding, and it induced some noticeable hum from the amp. When I put it either the NOS GE from Brent Jessee or the NOS "Admiral"-labeled one from ebay in the same position, the hum went away, and the amp calmed down. It was not nearly as loud, and it sounded much more natural -- much more like I'd expect an Ampeg to sound.

    Another thing: the two 6C10 tubes that worked well are exactly the same size. The 6C10 that induced hum and that has the date code on the top is slightly taller. All these facts lead me to believe that the 1961 hum-inducing GE "6C10" (which I've returned) is something other than a genuine 6C10. I don't know what, but different. The tonality of that one was more akin to the Ei "6C10" tubes that came in the amp (which are thought by some to be re-labeled 6AC10's), but the bottle is not as tall as those. Somewhere in-between those tall Ei bottles and the correct shorter, stumpier bottles of a genuine 6C10. I do like the tonality with the brasher-sounding tubes. It is more aggressive in a cool way. But for now, at least, I'm sticking with the two GE 6C10's that look identically constructed, sound identical, and behave the way I'd expect this circuit to behave.

    I cannot prove that the one that doesn't sound "right" isn't a 6C10. Perhaps there is some other factor, such as microphony, contributing to the sound I'm hearing (but it doesn't sound like a microphonic tube to me). Perhaps there is also a margin of tolerance for how tall the bottle is on these tubes. But these things make me wonder.


    This is what a 6C10 Compactron should look like, size-wise:

    IMG_2589.JPG

    IMG_2591.JPG

    Here is the one that sounds "wrong" in V1, next to one of my two 6C10's that sounds "right" in V2:

    IMG_2592.JPG

    IMG_2593.JPG
     

    Attached Files:

  19. mrriggs

    mrriggs Tele-Meister

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    What an interesting Tremolo circuit. It looks like the added resistors and capacitors at the first pre-amp stage create a local negative feedback loop. This feedback is shunted by the back-to-back triodes in the driver portion of the tremolo circuit.

    For an intensity control, I'd start by clipping a resistor substitution box across the 470k resistor in the voltage divider feeding the grids of the back-to-back triodes. Clicking in progressively lower values until a reasonable 'max' intensity is reached. Then wire a fix resistor of that value in series with a pot and hook them across the 470k resistor.
     
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  20. tubegeek

    tubegeek Friend of Leo's

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    OK that is an interesting look at the loop around the input tube. Literally never seen anything like this - do you suppose it's inspired by a test oscillator type circuit? At first I thought "why would they throw away so much gain right at the input?" But the 470K in the feedback loop suggests that the feedback fraction is very small, and the main function of the loop is to provide something that can be shunted to ground.

    I think.

    What a brain teaser!
     
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