theprofessor
Poster Extraordinaire
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?
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.
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?
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.
