Restoring a 1964 Ampeg Reverberocket

Discussion in 'Amp Tech Center' started by flyswatter, Feb 18, 2019.

  1. flyswatter

    flyswatter Friend of Leo's

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    Next Step:

    I went down the board, removing the old brown adhesive, testing each component, keeping the original parts that test good, and replacing the ones that don't. The original electrolytics and all but one of the old ceramic caps needed to go. Also found some earlier replacement resistors that were the wrong value so replaced those, too. Having the redrawn schematic on hand made the process easy. Next step is to remount the circuit board, rewire the tube sockets, and test all the pots and jacks.

    BEFORE:

    Ampeg board01 before.jpg

    AFTER:

    Ampeg board02 after.jpg

    Still need to fix the lead dress on the north side of the board but I'll do that when I redo the grounding and pots in the chassis.
     
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  2. Mudman32

    Mudman32 Tele-Meister

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    Great start!
     
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  3. Swampertech

    Swampertech NEW MEMBER!

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    Sandy Henry at Precision Design
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  4. flyswatter

    flyswatter Friend of Leo's

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    Good call, Bobby. As I restored the board I confirmed the brownies on the left were replacements, maybe pulled from a 70s Fender -- see those a lot in Twins of that period. They tested right on spec (0.05uf) so I left them in.
     
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  5. doctorock78

    doctorock78 NEW MEMBER!

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    I am following this intently... Have the same version (a '63 R12RB) that is giving me a hard time. Zero reverb, weak weak trem, weak and strangely distorted output... I know there is a great tone buried in there!
     
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  6. swampyankee

    swampyankee Tele-Holic Gold Supporter

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    Great thread!
    I have a 65 Reverberocket that is in great shape. I even replaced a non-OE speaker with a reconed Jensen of the correct model and era.
    Unfortunately I don't have the skills you do, flyswatter, so I'll have to send it out to have a crackle noises diagnosed and fixed.

    Sent from my SAMSUNG-SM-G870A using Tapatalk
     
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  7. tele_savales

    tele_savales Tele-Meister

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    Respectable work!
     
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  8. timfred

    timfred TDPRI Member

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    Great project, I'm wrapping up work on a 66 R-12-RT. The "K" is the center tap of the heater winding, I think connected to power tube cathode as a hum cancellation measure?
     
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  9. flyswatter

    flyswatter Friend of Leo's

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    Thanks for the replies and suggestions. I have the amp all wired up. I just had to postpone further work (my personal project) as my repair shop has been very busy. Will get back to the Ampeg soon.
     
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  10. flyswatter

    flyswatter Friend of Leo's

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    You may be right. The amp has a persistent hum and adding an artificial center tap for the heaters may be the cure. There's already one tap to ground from the PT but I'm unsure whether it is for the heaters or HT.
     
  11. mgreene

    mgreene Tele-Holic

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    I would compare your circuit to the 6V6 circuit and co
    I would compare your circuit to the 6V6 circuit and consider changing to 6V6.

    But I have had both - 6V6 and 7591 reverberockets and I think the 6V6 sounded better - clean and dirty. Plus 6V6 will be cheaper and more plentiful all day long.
     
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  12. BobbyZ

    BobbyZ Doctor of Teleocity

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    One good source for 7591s is free organs before they hit the landfill. Got a few good pairs that way but there's some lifting and I've got stuck with 6V6s, 6L6s and EL84s. :)
     
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  13. Nickfl

    Nickfl Tele-Afflicted

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    The "K" connection is for heater elevation. Referencing the heater center tap to the power tube cathodes instead of ground elevates it by the cathode to ground voltage (18v in this case) and helps reduce hum. You can do it with a real or artificial center tap. See Merlin for more: http://www.valvewizard.co.uk/heater.html

    According to your schematic that center tap should be coming from the heater winding. You don't have a HT center tap because your amp uses a bridge rectifier.

    Nice job bringing that old amp back to life! I restored a Reverbrocket 2 last year, though it wasn't as rough as yours, especially inside. Its also very cool that you took the time to draw a schematic and post it, there seem to be a lot of undocumented variations on these old Ampegs and it will definitely help someone out at some point in the future.
     
    Last edited: May 17, 2019
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  14. timfred

    timfred TDPRI Member

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    On mine the heater center tap is a yellow wire, in case that helps.
     
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  15. flyswatter

    flyswatter Friend of Leo's

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    Thank you, Nick! Before seeing your post, I'd actually figured out exactly this working on the amp the past couple of days. Yes, "K" references the heaters to an artificial centre tap by way of the 140R bias resistor (a bizarre way of doing it that'd I'd never seen previously). The reason "K" is gone from other schematics is Ampeg started referencing the heaters to ground in the more conventional way.

    I installed a pair of 100R/ 2W resistors from the indicator lugs to ground, and the heater hum disappeared.
     
  16. flyswatter

    flyswatter Friend of Leo's

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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...

    20190517_160110.jpg
     
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  17. BobbyZ

    BobbyZ Doctor of Teleocity

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    "Like building a ship in a bottle."
    Yeah that sums it up!
    The best thing I ever bought is those clamps that look like a pair of scissors, surgical clamps I guess.
    No idea why I went so long without those darn things! Did my first RR without them, trying to get my fat fingers in there plus a hot iron. I knew about those things but didn't know the cheap tool store in town had them for a buck or two. Got a half dozen now. :)

    If I was going to build one of these from scratch, I'd use tube sockets that drop in from under the chassis. That way you can do the board, pots, jacks, switch and sockets, then stuff the works in the chassis and only have a few things to solder in place.

    Those tubes get so microphonic that the only way to find the guilty one is just start replacing tubes one at a time with good ones. Of course sometimes your "good ones" are microphonic too.
     
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  18. flyswatter

    flyswatter Friend of Leo's

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    If working from scratch and not concerned about cloning the cosmetics, I'd go one further and drop the whole circuit into a DR-sized chassis..... considering that the R-12-Rs have about the same component count as a AB763 circuit in half the space. I'm enjoying figuring this amp out, but one thing that frustrates me is the whole question of lead dress is pretty much moot when everything is so crammed together and all the leads can only travel laterally up or down the chassis.

    Since I've already done the schematic for this amp (corrected version to be posted as soon as I transfer my handwritten corrections to the computer), it might be fun sometime to do a Reverberocket circuit in a roomier format.
     
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  19. BobbyZ

    BobbyZ Doctor of Teleocity

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    The thing that amazed me about the two RRs I've got is how quite they are with all that stuff jammed into such a little space. After I recapped my first one and fired it up I didn't think it was working till I hit a guitar string.
    The people that built them knew what they were doing!
    Although the tin foil on the back cover is pretty cheap and replacing that with aluminum furnace tape sometimes makes a big difference.
     
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  20. mgreene

    mgreene Tele-Holic

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    I got 3 pairs - the smallest ones work as heat sinks for soldering too.
     
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