Need troubleshooting help w/ '63 Ampeg R12 Rocket...

kakerlak

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I've had this amp in storage since about 2000-ish. One of the power tubes shorted back then and reissues didn't exist at the time, NOS pairs were $100+, etc., etc. Anyway it worked flawlessly prior to the tube failing (one of the power tubes rattled if you had it in your hand and, sure enough, that tube eventually shorted out).

I just got new tubes in for it a couple days ago. It turned on, made sound (actually sounded good) but it was cutting out, popping, squeeling, crackling, etc. So I took tuner cleaner and sprayed down the tube sockets, scrubbed that around with the tubes, let it sit overnight. It is better now, and will sound good, quiet and consistent at low volume.

With the volume turned up to a point where the amp normally starts to distort, however, hard-hit notes will cause the same loud crackling/cutting out, pops, etc.

Secondly, the tremolo function does not work. I know the old Fender amps' tremolos always break b/c they have the optical flasher thing that always goes bad, but I don't see one inside this amp. Furthermore, when I click the foot switch, there is a momentary drop in the volume, like the tremolo should sound, but it only occurs when I hit the switch; the tremolo doesn't continue oscillating. (I did drop a nickel across the terminals on the switch, thinking the switch itself might be bad, but got no effect.)

So... this thing is pretty simple inside, circuit-wise and I can wield a soldering iron pretty well. What am I looking for to fix the two problems? I'd think caps, but, unlike the Fender caps that you visibly tell are bad (bubbles and discoloration), this thing has the metal can assembly, so I'm not sure of it...

FYI, tubes are brand new JJ 7591Ss and NOS brown base Tung-Sol 6SL7WGTs.

Here are pics of chassis insides, left to right and schematic, in case anybody "spots" something...

PICT1528.jpg


PICT1531.jpg


PICT1508.jpg


PICT1509.jpg


PICT1510.jpg


PICT1511.jpg


PICT1512.jpg


PICT1514.jpg


PICT1515.jpg
 

capnjuan

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Hi kakerlak; what's all the messy stuff; looks like syrup? Nasty condition adjacent to transformer / can cap / 5Y3 ... ??

Tremolo: 7 times out of 10, non-working trem is a function of worn out caps in the oscillator circuit; 3 red caps ... .05uf by schematic, 3rd pic from the top; they should be replaced value for value, all rated to 600VDC. The circuit also has a .1uf and a 25uf on the cathode ... I'd do those too ... they are not sacred nor do the affect tone, only the trem function; why do them if you don't know if bad? Because if it isn't one of the .05s then it's either the .1 or the 25uf ... besides, if you do them now, you eliminate as a problem now and in the future.

Snap/crackle/pop on hard notes;

Cathode caps: for the sake of convenience, we'll side-step the can cap; in addtion to the tremolo cathode cap, there are 3 more;
- 100uf/600vdc on front/preamp half of the 6SL7 tube
- 100uf/600vdc on the phase inverter/driver and
- 20uf/600vdc in parallel with the output tubes' cathode resistor; all electrolytic, all need to go.

The three blue electrolytics at the left end of pic 3 ... those get replaced value for value.

Pic 5; the 140 ohm resistor ... says 140 on it; supposed to be a bypass cap in parallel - mentioned above ... that's the 6V6 cathode resistor ... where's the 20uf bypass cap? If you don't find it, it needs to have one there.


Grunge: If this were my amp, I'd spend some time looking at the crud between the power transformer and the 5Y3 socket; you are looking for burned insulation or evidence of shorts or intermittents.

I'm going out of town this weekend; either post here or pm me if you need help ID'ing which part is which. CJ
 

kakerlak

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capnjuan said:
Hi kakerlak; what's all the messy stuff; looks like syrup? Nasty condition adjacent to transformer / can cap / 5Y3 ... ??

Tremolo: 7 times out of 10, non-working trem is a function of worn out caps in the oscillator circuit; 3 red caps ... .05uf by schematic, 3rd pic from the top; they should be replaced value for value, all rated to 600VDC. The circuit also has a .1uf and a 25uf on the cathode ... I'd do those too ... they are not sacred nor do the affect tone, only the trem function; why do them if you don't know if bad? Because if it isn't one of the .05s then it's either the .1 or the 25uf ... besides, if you do them now, you eliminate as a problem now and in the future.

Snap/crackle/pop on hard notes;

Cathode caps: for the sake of convenience, we'll side-step the can cap; in addtion to the tremolo cathode cap, there are 3 more;
- 100uf/600vdc on front/preamp half of the 6SL7 tube
- 100uf/600vdc on the phase inverter/driver and
- 20uf/600vdc in parallel with the output tubes' cathode resistor; all electrolytic, all need to go.

The three blue electrolytics at the left end of pic 3 ... those get replaced value for value.

Pic 5; the 140 ohm resistor ... says 140 on it; supposed to be a bypass cap in parallel - mentioned above ... that's the 6V6 cathode resistor ... where's the 20uf bypass cap? If you don't find it, it needs to have one there.


Grunge: If this were my amp, I'd spend some time looking at the crud between the power transformer and the 5Y3 socket; you are looking for burned insulation or evidence of shorts or intermittents.

I'm going out of town this weekend; either post here or pm me if you need help ID'ing which part is which. CJ


Wow, thanks for all the in-depth looking-over! It's not actually as bad as it looks in the photo; the flash really picks up any little dust specks. The the thing is actually pretty dust and corrosion free inside and there's no sign of insulation melting. That line of stuff across all the caps and resistors is also what's on the socket side of the chassis, near the can and rectifier tube. All I can figure is that it's some kind of varnishy stuff (it's not at all sticky) that the factory ran across everything, like paint-marking bolt heads on cars. It seems deliberately run across some part of every wire and component inside the amp.

The one thing I did notice is that the ends of the 1K/10w resistor are kind of fuzzy, like battery corrosion... The other thing some folks have mentioned over at the G&L forum (yeah, I like those too, to me, Guild:Gibson :: G&L:Fender) is the reputation for JJ 7591s to have smaller pin diameters. I did notice that one of the two power tubes wiggles around in its socket. I'm going to tap on it while the amp is on and see if that causes the sputtering, etc. If so, and probably even if not, I'm going to gently pry the pin sockets tighter and see if that fixes anything.

I hate to throw money at it and I'm always a little nervous around caps, but it might very well be in order. The tremolo would be kind of nice to have, though...
 

kakerlak

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capnjuan said:
Pic 5; the 140 ohm resistor ... says 140 on it; supposed to be a bypass cap in parallel - mentioned above ... that's the 6V6 cathode resistor ... where's the 20uf bypass cap? If you don't find it, it needs to have one there.

CJ

Regarding the reliability of this schematic: All the text on it is in type-set/printed font, but the places where the 7591s are listed is handwritten (the whole thing is printed, but they're handwritten onto the original from which the prints were made). The cabinet tube chart actually has 6V6 printed on it, scratched out by hand and 7591 written above.

Is it possible that this schematic was updated for a change in tubes? I believe these R12s were sometimes 6V6 amps, sometimes 7591s. If I recall correctly, these are not interchangeable tubes in that they pin out differently... So if they just cut out and wrote over the references to 6V6, I wonder if it's 100% accurate.
 

kakerlak

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OK, UPDATE: tightened up the pin receptacles in the loose tube socket and it has fixed all the crackling and popping. Amp goes flat out now with the same great old-time distortion it used to. :p

The tremolo problem remains, though. I guess I'm looking at replacing the caps on that circuit, then? I did swap the two 6SL7s around and it still doesn't work.

Should it mean anything to me that the tremolo does seem to "come on" the moment the footswitch is tripped, but just for a single/momentary sag in volume? I played around with it and the volume dip only happens every other time the switch is pressed (i.e. only when the circuit is closed and the function is "turned on") and the strength of the momentary volume sag is dependent on the intensity setting. So it seems the intensity function "works" but the volume dip never repeats. As though the "speed" function is broken. I don't know where I'm going with this, but I am happy the amp at least works now. Tremolo is super fun to mess around with, but not terribly useful otherwise.
 

capnjuan

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Hi kak: this is a clean version of your schematic: Ampeg Rocket 12a Schematic ... 7591s, 5Y3 ... this was/is the foundation for the Reverberocket series which also started with 6V6s/5Y3 and finished with 7591s/solid state rectifier. Good move with the sockets. Here are the 'get lucky' things that might straighten out your trem; clean the trem on/off switch, clean the controls, clean the hot post where the FS jacks in. If you've already swapped out 6SL7s, then that train has left the station. If these maintenance items don't do it, you'll have to get serious about it; your trem section below with the capacitors mapped from the schematic to your pic:

rocket12Atrem.jpg



That 6SL7 half ... it's the oscillator and it's oscillating whether you have engaged the trem or not. What it does is cause power fluctuations at the grid bias resistors ... the pitch isn't changing, only the volume. Anyway, the caps I've ID'd above are always humping whether the trem is on or not and they only have a limited useful life. The mission-critical capacitors are shown and were this my amp, I'd change those parts without thinking about. Good luck with your amp! CJ
 

kakerlak

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capnjuan said:
Hi kak: this is a clean version of your schematic: Ampeg Rocket 12a Schematic ... 7591s, 5Y3 ... this was/is the foundation for the Reverberocket series which also started with 6V6s/5Y3 and finished with 7591s/solid state rectifier. Good move with the sockets. Here are the 'get lucky' things that might straighten out your trem; clean the trem on/off switch, clean the controls, clean the hot post where the FS jacks in. If you've already swapped out 6SL7s, then that train has left the station. If these maintenance items don't do it, you'll have to get serious about it; your trem section below with the capacitors mapped from the schematic to your pic:

rocket12Atrem.jpg



That 6SL7 half ... it's the oscillator and it's oscillating whether you have engaged the trem or not. What it does is cause power fluctuations at the grid bias resistors ... the pitch isn't changing, only the volume. Anyway, the caps I've ID'd above are always humping whether the trem is on or not and they only have a limited useful life. The mission-critical capacitors are shown and were this my amp, I'd change those parts without thinking about. Good luck with your amp! CJ


Wow, thanks again! I really appreciate the thoughts and photoshopped diagram/photo combo! I think I can rule out the switch, as it does seem to be turning it on and off. (I've also closed the circuit with metal b/w the two terminals to bypass the switch and got the same single, momentary volume dip, so I don't think there's a problem with the terminals the switch's cord is soldered to. I guess that leaves cleaning the speed control for the heck of it, then replacing the components on that circuit, starting with the caps when the problem remains. :wink:

I'll let you know how things wind up, but at least for now I have a working little Rocket.

(oh, any way to best make sure I don't electrocute myself?)
 

capnjuan

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kakerlak said:
... oh, any way to best make sure I don't electrocute myself? ...
Pull the plug and drain the power supply caps ... the can cap ... it has three or four sections each section with it's own post or terminal. Take a screwdriver with an insulated handle and touch one hot terminal and let the shaft of the screwdriver touch the edge of the chassis ... it might spark ... but hold it there for 10 seconds or so ... repeat for each section. If you have a 5W / low value resistor ... say 50 - 1000 ohms; touch one resistor lead to one hot terminal, the other resistor lead to the chassis ... hold it there for 10 seconds or so ... repeat for each terminal.

Remember .... don't wet your finger when you put it in your ear .... :wink: J
 

kakerlak

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capnjuan said:
kakerlak said:
... oh, any way to best make sure I don't electrocute myself? ...
Pull the plug and drain the power supply caps ... the can cap ... it has three or four sections each section with it's own post or terminal. Take a screwdriver with an insulated handle and touch one hot terminal and let the shaft of the screwdriver touch the edge of the chassis ... it might spark ... but hold it there for 10 seconds or so ... repeat for each section. If you have a 5W / low value resistor ... say 50 - 1000 ohms; touch one resistor lead to one hot terminal, the other resistor lead to the chassis ... hold it there for 10 seconds or so ... repeat for each terminal.

Remember .... don't wet your finger when you put it in your ear .... :wink: J


Cool, that's what I used to do (screwdriver w/ insulated handle), but I'm more used to working on my Super Reverb and I never knew whether that was "proper." BTW, I did notice the largest red capacitor (nearer the pwr transformer) is labeled "Tiny Chief!" :lol:
 

capnjuan

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Actually ... the trick with the screwdriver is considered bad practice ... but if you don't have the right resistor ... it will work. It does stress the sections of the can cap ... but if you wanna know if love is real ... the can cap needs to go anyway.

Missing capacitor: I wouldn't be surprised if the original electrolytic bypass cap ... now not showing on your circuit board ... exploded spewing electrolytic paste all over the base of the 5Y3 ... that's probably where all the crap came from.

6V6cathode.jpg
 

Mark51

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Some of the Ampeg's have the 25uf/25v cap in the cap can, not on the board.

I also have a tremolo problem with a 1964 Superbreverb. It is basically a Reverberocket with a 15" speaker. It does the same thing. Hit the switch and you get 1 or 2 pulses then no more. I read that this can be caused by the caps gone bad, or, if the bias is set too high on the power tubes.

I'm just getting started on this amp but I did get a GA-40 amp working well and capnjuam was very helpfull with that one. Listen to him. In fact, when I get more into the SR I'll post more info and maybe he'll have some suggestions for me to try.

Mark
 

capnjuan

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Hi Mark; thanks for the vote of confidence ... most of the time I'm just a bad influence on people ... :wink: You might be right about some models putting the output tube cathode bypass resistor in one of the cans ... but I don't think this is one of those models.

I guess what I mean is that for the cap to be located other than adjacent to the cathode resistor, there'd have to be a connection from its + terminal to one (or the other) of the cathodes ahead of the resistor. After staring at Kakerlak's pic for far longer than good sense would justify, I can't find anything that would seem to work. Ok ... it's a little busy in there but still ... nothing stands out.

Kak02.jpg



I bought two different Gibson GA20Ts so I could get an idea of what all the GA40 / 5879 shouting was all about ... I found out :D Not the same amps of course but lots of good Gibson tone in both of them. Pic of our member mad dog's late '50s GA40 ... at one point he was threatening to sell it (I think just to bust my balls :evil: :wink: ... not sure if he still has it or not):

GA40_frontjpg.jpg



Happy to help any way I can. J
 

Mark51

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Capnjuan,

I brought this up ( cathode cap in the can) because I have a '63 Reverberocket and a '64 Superbreverb and both of them had the cap in the can right from the factory.

I was looking at your photo again. You can see the light gray wire from pin 5 on the right tube socket going over to the left tube socket. Then it goes a bit to the left and comes up to the resistor. I see another gray wire right below the left arrowhead you added. That is the one going to the cap can on both my amps.

Anyway. He'll need to add one to the board because you can't buy the cap can they came with anymore. I just did that to my RR. When I got that amp it was all original, even the tubes. Everything worked but had weak output. I changed all the electrolytics, cleaned the pots, checked the tubes ( all measured good on B&K 707) and gave it a try. I think it still has low volume and sounds a bit thin. I'll try to go over the voltages tonight then I am going to swap out the speaker with a new Weber and see what it does. Got to go to a jam tomorrow and I want to try it.

Later.

Mark

And don't worry. I'm beyond being influenced so my wife says.
 

capnjuan

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Well ... no matter how awkward it seems, it looks like that wire does run over to the can cap but anybody (E. Hull) who can fall in love with the 6U10 could easily conclude that everybody else; Fender, Gibson, YouNameIt was just missing the wisdom here. Must be a 4-section cap then, no? Thanks for straightening me out :)
 

Mark51

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I found that out when I went to Fliptops looking for caps for my Gemini 1 a while back. They have replacement cap cans and had alot of info there. Mine had a 60/40/10@450/20@25 can in it.

I swapped out the tired Jensen for a 12F150A that I had tried out in my PR.

Much louder and more punch. I guess everybody was right about needing to replacing the speaker in an old RR. I see many that say to use an Eminence C-Rex in Ampegs. I am going for low-medium volume chime, not cranked distortion. I may look for one to try.

Got voltage readings (will look them over tonight) and calculated power disipation. Running at 16 watts, 45ma per tube @348 volts. I'll leave it as is unless I see something in the voltages. Trem and reverb work great on this one.

Mark
 

kakerlak

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Hey, didn't realize this thread was back from the dead. Unfortunately, my amp is the opposite now... It powers on but only makes a loud hum. It will probably (sadly) be a long time before I get around to messing with it, but I really appreciate you guys arming me with so much info, though!

I was able to live with the no tremolo since the amp was working clean otherwise, but I guess whenever I do get around to working on it, that'll get fixed too.

So Mark/Capn, is the stacked cap Capn linked to on the "my amp broke" thread not a direct replacement after all?
 

capnjuan

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kakerlak said:
... is the stacked cap Capn linked to on the "my amp broke" thread not a direct replacement after all?
Hi Kaks; the answer is yes. The linked cap has 4 sections ... for guitar amps, these are more commonly 3-section; plates, screens, and preamp. Since this one has a 4th at 20uf, it can be used as a cathode resistor bypass cap for your 7591s.

The two cathodes are tied together and a 'hot' wire brought from one of the cap sections and tied to one of the cathodes ... the resistor can be tied to either cathode and run to ground.

Sorry about your amp; if you never got around to changing the can cap, chances are the hum is one or more of the sections going bad although there are lots of things in the amp that can cause hum. When you get around to messing with it, fire up this thread or the other. Mark knows a great deal about this amp and I'm sure would be happy to help as well.

J
 

63Polara

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Sorry to resurrect such an old post, but I have looked everywhere for help on my '59 Ampeg J12b Rocket's Tremolo issue, and this was the only place I have found a similar issue.
My Tremolo comes on momentarily when activated, but immediately decays and goes away after about 2 seconds.
The speed and intensity to appear to change when I adjust them, and keep re-activating the trem.
I have replaced all three .05us caps, the 25uf cap, and the .1uf cap in the circuit. I have also tried swapping the two 6SL7 tubes, with no changes at all.
Any ideas what to change next?
Would the 1M resistor be a potential problem? All that appears to be left in the trem circuit is resistors, so I'm at a loss...
Otherwise, it sure does sound awesome with my '63 Polara!
thanks
Steve
 

63Polara

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Re: Need troubleshooting help w/ '59 Ampeg R12 Rocket...

Well, I did figure out that the 1M resistor was actually the Intensity pot, and that tested good.
But I did find a 270k resistor that measured 1200k. This was the resistor that goes from the between the intensity pot and the power tube.
Can this cause a loss of trem? I would have figured a worn out resistor like this would cause it to be too intense, rather than having a loss of intensity, but I know very little on this stuff...
Any help would be much appreciated!
 

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Normally, you'll find that resistors drift higher in resistance until they burn open. I would replace them with the proper values and that should fix the problem.
A resistor that has that kind of drift is not going to work properly in any circuit. It's four times the value called for in the design. :wink:
 
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