Restoring a 1964 Ampeg Reverberocket

mgreene

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Okay, progress!

First, thanks everyone for reviving this thread and chipping in a lot of helpful information. I'd left the Ampeg for a few weeks to work on other projects, but seeing all the feedback inspired me to put the R-12Rb back on the bench the past couple of days.

So, the amp is now completely rewired and works (including reverb), except the output is very low. I made a number of changes to the wiring/ schematic I posted earlier in this thread... after finding another copy of an original schematic (not frayed and missing bits like the one in my amp!), and being able to test voltages to identify where the remaining problems are.

One big problem I realized on power up: Those original 7591s are toast, and so is one of the 6SL7s -- the one that was in the phase inverter position.

- Was getting very low voltages in the P.I. (each plate should be around 210-215 VDC but both were reading around 100 VDC. Swapping out the 6SL7 for another one for the reverb brought the voltages up to where they should be, but now the reverb barely work. Obviously, I need another 6SL7.

- Another of the 6SL7 and the single 6SN7 are very microphonic... lots of dinging and ringing when you tap on them or the chassis. I've read microphony is typical in these tubes. Probably I'll just hunt up a set of 6SL7/ 6SN7 and replace the bunch.

- The 7591s were both bad. Very intermittend output and the familiar spits and crackles of bad power tubes. I wanted to keep going, so I installed a new set of 6V6s (as suggested by a member above), and swapped the cathode and grid connections to the pins, to accommodate the different pinout. Probably I'll hunt up some 7591s and can always convert back, but adding the 6V6s eliminated the noise issues.

- As mentioned to Nick, above, I installed an artificial centre tap -- 2 X 100 ohm resistors to ground from the indicator light -- which eliminated the hum.

The low output is still a concern. I'm worried about the output transformer at this point (plate voltages, B+, rectifier, etc are at, or close to, voltage on the schematic), but am going to check over all my wiring once again in case there is still something I overlooked. Working inside these old Ampegs,with their cramped little chassis, is like building a ship in a bottle -- so it gets tough to see what he hell is going on inside there and get the DMM and scope leads clipped where they need to go, without shorting against neighboring components.

Here's a pic of the amp in voltage test mode...

View attachment 609080

Octal preamp tubes are notorious for being microphonic - you may have to go through a few to find some non-microphonic ones.

Tip - look on ebay at lots of old stock 6SN7 and 6SL7s from a good seller - if you can get them cheaply enough, its a good way to get a bunch at one time.

I have a M-15 that came to me with a computer fan cut into the back and the B+ caps detached and hanging - I am afraid that the power xf in that one will be toast too.
 

flyswatter

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Good tip, mgreene. I'd like a few octal preamp tubes on hand anyway as about once a year my shop sees an old Ampeg -- there's a handful of owners in my area, and I'm sure a supply of 6SL\n7s won't go to waste.
 

radarman

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Nice work fly.. I have a 63 on the bench with mucho 60hz hum. This one here has been worked on and the ps filter caps were replaced with that JJ cap can and a couple of others.. both the others mounted over where the PT wires exit the PT, so I can't see if this one has a filament winding center tap or not. It does have the SS rectifier. Im surprised you have any room in there to add the two 100 ohm center tap resistors on the lamp. Mine has no room at all, but the artificial ct can be added anywhere on the filament string. You didn't say... did your PT have the 6.3v center tap? Also can you describe the updates you made to the schematic you made and posted please? thank you

Also my amp has two resistors going from the rectifier to the nearest power tube pins 2 and 7 which is the filaments. Does your amp have this.. or did it have this? I traced it out and it goes to the cathode res/cap for the power tubes. So it is elevated filament center tap.

I'll be replacing all the tubes to eliminate them as the source of the hum.
 
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flyswatter

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Nice work fly.. I have a 63 on the bench with mucho 60hz hum. This one here has been worked on and the ps filter caps were replaced with that JJ cap can and a couple of others.. both the others mounted over where the PT wires exit the PT, so I can't see if this one has a filament winding center tap or not. It does have the SS rectifier. Im surprised you have any room in there to add the two 100 ohm center tap resistors on the lamp. Mine has no room at all, but the artificial ct can be added anywhere on the filament string. You didn't say... did your PT have the 6.3v center tap? Also can you describe the updates you made to the schematic you made and posted please? thank you

Also my amp has two resistors going from the rectifier to the nearest power tube pins 2 and 7 which is the filaments. Does your amp have this.. or did it have this? I traced it out and it goes to the cathode res/cap for the power tubes. So it is elevated filament center tap.

I'll be replacing all the tubes to eliminate them as the source of the hum.


Hi Radarman,

No, mine does not have a center-tap for the heaters, but did have room on the pilot light lugs to add 100 ohm resistors to ground. They can be added elsewhere in the filament supply -- maybe off of V1 would work better for you.

I'm guessing that your 60Hz hum is caused by lack of a heater CT. That's exactly what it was with mine and it went away when I added those resistors.

The updates to the schematic were mainly corrections to the letter coding between the preamp plates (A, E, F, etc.) and the filter caps. I had to guess my way through on the first draft because the original schematic I had had crumbled at the top. I later found a better copy -- and also after wiring the amp, I made the revisions. .... I did forget to post that, but will. Thanks for the reminder.
 

slider313

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Nice work fly.. I have a 63 on the bench with mucho 60hz hum. This one here has been worked on and the ps filter caps were replaced with that JJ cap can and a couple of others.

I'll be replacing all the tubes to eliminate them as the source of the hum.

Give this a look.


<iframe width="640" height="360" src="" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
 

flyswatter

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As promised, here is the revised schematic -- minor corrections but important ones from the one I posted earlier in the thread (when I was working from a damaged original -- so please ignore that version on the 1st page of the thread, I can no longer edit that post).

This version of the R-12Rb corroborates the original schematic inside my amp, another similar (albeit complete) scan of same found online, and my extensive journey through the amp itself. I also looked at Joe Piazza's schematics found everywhere online, but they had glaring mistakes. It was a bit tricky because not everything in my amp -- or on either version of the original schematic -- is exactly the same, but I think I've got it pretty close to the stock wiring.

The only changes I made were to add in a safety ground for the AC cable and put the aforementioned cathode "K" (heater CT connected to the power tube bias resistor/ cap) in parentheses (with a note), since none of the earlier schematics I've found agree.

Still open to corrections, if you find any to be made. If you download the image, it should open nice and big on your photo viewer.

Ampeg revised 2019-06-05.jpg
 
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radarman

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The pins for V1 are switched on my amp. The oscillator has pins 1, 2 & 3 and the first amplification stage has pins 4, 5 & 6.
 

timfred

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How well would you say the tone control works on this amp? Tone change through the full range? I don't find much difference between 5 and 10.

If you look at the various Ampeg R12-R schematics, while the preamp and tone control schematics all have the same layout, the values of the bypass caps, coupling caps, and tone caps vary quite a bit.

At first I assumed it was to adjust the circuit to the various 12AX7, 6SL7, and 6U10 tubes used in V1. But the slight differences between tubes wouldn't explain why your amp has, for example, a 100uF bypass cap on V1 while mine has a 0.01uF.

I've only played the one that I have, but I'd bet if you lined up all the Reverberocket models you would find that there's no one "Ampeg" sound.
 

radarman

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My amps V1 sections are reversed from your schematic..but the tone control seems to function normally, albeit like you say not much change from 5 to 10.. it seems to lose highs a little when the reverb is turned up.. the 100uf cap is on the schematic..
 

flyswatter

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The triodes on V1 were flipped on mine, too, and are shown that way on the Joe Piazza schematic, but both copies of the original Ampeg schematic show them the way I put them on mine. I swapped the wires on my V1 to reflect this.

Timfred -- I agree. There seems to be no end of variations on vintage Ampegs and no schematic is likely to be definitive in terms of all the particular amps out there. Like a lot of single tone control amps (5E3, etc), the tone control doesn't do much more than cut brightness. I usually just leave it at 12 o'clock and let the amp do its thing.
 

Schlumpfmeister

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I am strugging with the same amp right now:

1. Is my assessment correct that one of the can caps serves as power tube cathode cap? There is a grey cable going from pin 5 to the can. Unfortunately, on mine, it is disconnected, therefore I am not certain. Also, the can cap values would add up for the filter section. However, the left most brown cap seems to serve as the B filter cap as it is connected to pin 4 of the power tubes by way of a red cable daisy chain.

2. I am wondering about the function of the rectangular (replacement) cap in the middle of the picture. The lower leg connects to a mass point, the upper leg by way of a black cable to the hot side of the power switch. This would imply an artificial ground, then it would be a "death cap" and to be eliminated, in particular as the chassis is well grounded by way of an (original) 3 prong cable?

With the disconnected cathode cap and an incorrect PI 6SN7 tube I am no longer surprised the amp doesn't generate very high output, in particular on the bass side.

For the record: my V1 is also flipped, relative to the above schematic.

Thanks for any advise!

Best Andreas, from Germany
 

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Schlumpfmeister

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I am strugging with the same amp right now:
Best Andreas, from Germany

Adding one more thing - mine seems to still have the original filter caps, some resistors and coupling caps have been replaced though. There is no 120hz hum, it does hum however if no instrument cable is plugged in. I suppose due to some ground loop?
 

BobbyZ

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Probably best to a new thread. Otherwise people will be throwing out answers for the original amp in question.
That said from the photo it looks like original electrolytics the paper wrapped multi cap and I see a cathode bypass cap on the board that I believe is original.
Metal cap can normally has the power tube cathode cap, it'll list something around 25uf at 25 or 50 volts on the can. Rest of the sections will be like 400 volts or so.
Biggest trouble with Ampeg back then was they changed things a lot and schematics even in the amp probably often don't match. Both of my 6v6, 5y3, octal RRs are different, the schematics inside are different and don't match either amp, or the schematics online.
No big deal if the amp is original and you can read everything. If someone's been inside changing things it gets interesting.
I'd change all the electrolytic caps, everyone, pay particular attention to how they are grounded from the factory and do it exactly that way. I've seen where guys think they're going to improve on the grounding and they fail.
Ampeg engineers knew what they where doing.
The first one I recapped went from being noisy to so quite I didn't think it was working at all, until I hit the string a the guitar. Lol
 

Schlumpfmeister

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Adding one more thing - mine seems to still have the original filter caps, some resistors and coupling caps have been replaced though. There is no 120hz hum, it does hum however if no instrument cable is plugged in. I suppose due to some ground loop?

Resolved - The way the 3 input plugs are set up does not allow for grounding input of nothing is connected. I will change that.

Probably best to a new thread. Otherwise people will be throwing out answers for the original amp in question.
That said from the photo it looks like original electrolytics the paper wrapped multi cap and I see a cathode bypass cap on the board that I believe is original.
Metal cap can normally has the power tube cathode cap, it'll list something around 25uf at 25 or 50 volts on the can. Rest of the sections will be like 400 volts or so.
Biggest trouble with Ampeg back then was they changed things a lot and schematics even in the amp probably often don't match.
No big deal if the amp is original and you can read everything. If someone's been inside changing things it gets interesting.
I'd change all the electrolytic caps, everyone, pay particular attention to how they are grounded from the factory and do it exactly that way.

Thanks for the advise, but maybe we are about done ...

Yes, it is pretty much original but:
- coupling caps for V1-1 and V1-2
- 2 oscillator caps
- echo pot

Unfortunately, the sticker on the can cap is gone.

I have measured all relevant voltages: anodes, kathodes, and grids. It seems that all caps are still working well. It doesn't hum if something is plugged in, so for now I intend to keep it as is other than relevant fixes.

I am still wondering about that rectangular cap connecting ground from the eyelet board to the hot side of the power switch? If it was an artificial ground it would be irrelevant as the eyelet is well mass grounded, the amp originally as a 3-prong power chord!?

Ah, and for the record: Mine does't have the heater center tap, at least I so far was unable to identify it anywhere.

Schematics wise, mine fits well with the above schematics but:
- flipped V1-1 and V1-2 (including the voltages)
- addition of the strange death cap
 

King Fan

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So no new thread, no more advice needed, you’re not going to replace the paper caps or cap can or old electrolytics? but you’re still wondering about that ‘rectangular cap in the middle of the board'? Do you mean this?

101D1556-319C-46E7-A3D7-46C50834EAD0.jpeg
 

Schlumpfmeister

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Yes this one. By now I am certain this is a death cap and I will remove it.

Why should I change the old electrolytics proactively if there is no hum and all voltages are ok?

I am still considering it, the can cap is a problem as there is no easy replacement (at least on short notice in Germany). One option I am contemplating is emptying it and putting normal axial electrolytics into it. Modern parts are much smaller, some of them could be put onto the eyelet (for instance at the death cap opposition).
 

King Fan

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Ah, from the photo I thought it looked like a wirewound resistor. But I can't see it in 3D or trace the wires, of course. I was also puzzled that your cathode cap appeared to be disconnected from the cathode? But... our smart friend @BobbyZ knows a ton about Ampegs. If he's not worried, I'm not. I did like his idea about new caps.

I like your idea about re-stuffing the cap can. I've done it; works perfectly, looks vintage. A *ton* of great cap info on this antique radio site; much of it applies equally to our old amps:

https://antiqueradio.org/recap.htm

For destuffing, he has links and discussion well down the page under "To Hide or Not to Hide".

You might also read his description of old reliable and unreliable cap types (like paper) and his discussion of cap testing -- you need a specialized cap tester that applies full voltage.

There *is* a school of thought that if it sounds OK you don't need to replace caps. It is very much a minority opinion on this site, though, since new caps can only sound the same or better, but as far as I know leaving the old caps isn't usually dangerous -- except for old filter caps, which we are told are a risk to your vintage PT.
 

Schlumpfmeister

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Thanks for the info!

Quick update:
  • The above logic makes a lot of sense, regardless of current perfect function an old filter cap presents a latent risk --> I will replace all filter caps and the few electrolytic caps otherwise used
  • I found a source for the correct CE Mfg can cap (60/40/25-50v/10), so I will just order that, it will cost a bit more than my can conversion, but I can keep the wiring as is then, and I am quite worried about ground loops which I hope I can prevent from occuring otherwise
  • The death cap is on its way out
  • There is also a mechanical side to the project:
    • I removed the old (replacement) tolex and will have it re-tolexed with the correct material. Preparing the housing (predominantly, removing the old glue remnants) turns out to be nightmare
    • A past owner had the original C12R speaker replaced with a RCA PA type, together with 2 Dynacord HM10 treble speakers. I will close the baffle holes for the treble speakers and replace the RCA with a period-correct C12Q
This turns into a much bigger project, and more expensive than expected, I hope I will get it back from the amp with a decent sound.
 

BobbyZ

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Thanks for the kind words @King Fan unfortunately I'm not that smart, missed the disconnected wire for instance.

@Schlumpfmeister Glad you're replacing the electrolytic capacitors. In my experience old ones like these can look and test good but break down in the bass frequencies, really sounds like speaker going bad. Real common in low powered amps like these or Princetons, everything seems fine but the old speaker. Old speaker gets replaced with a new more sensitive (louder) one, then often the amp doesn't get turned up as far and the problem seems to go away.
(In a larger amps like a Super or Twin Reverb the things will sound fine with filter caps that are leaking. They don't get turned up enough to push them)
Then eventually the old cap leaks, blows up or just starts drawing more current, bigger fuse goes in until the power transformer is the "fuse".
In amps that don't get much use they're even worse, because none use is bad and not being used they'll still look like new.
So when I do an old amp it gets every electrolytic replaced. The cathode bypass caps really don't hurt anything if they blow but I've had so many issues go away with a fresh set. Noises that get blamed on resisters is one.

In the States your amp wouldn't have had a grounded cord, other countries might have been ahead of the US in that regard. We didn't get them until around 1970 or so. Then of course a lot of them got cut off so you could still plug in to old outlets. :)
 

BobbyZ

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Oh if that amp uses a 5y3 rectifier tube, use 20uf filter caps. Ampeg went overboard on filtering with the 5y3 shortly before they just went to solid state rectifiers. One of my RRs had the bigger filter caps and a replacement 5y3. The other had 20uf and the 5y3 still says Ampeg on it, I assume it's original. I did that one with a 20uf can from FlipTops. The other one I restuffed, done that on several cans but that one was the worst to get apart, full off tar or something.
 




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