Page 1 of 2 12 LastLast
Results 1 to 35 of 38
Like Tree7Likes

Thread: Ampeg R12-R Reverberocket power transformer replacement

  1. #1
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82

    Ampeg R12-R Reverberocket power transformer replacement

    Hi everybody

    I have just bought a vintage Ampeg R12-R in need of repair. The first thing that need to be sorted out is the power transformer - h.t. secondary winding is open

    Drop-in replacements are available in the U.S. from fliptops.net and Mercury as well, but I couldn't find anything here in the UK or EU and importing from the U.S. would be quite expensive

    I have been looking at the Hammond catalog and although they don't list a replacement for the Ampeg R12-R there is one that suits a Tweed Deluxe that seems pretty close in ratings and it fits the chassis without drilling additional holes

    https://www.hammfg.com/files/parts/pdf/290BEX.pdf

    I am not sure wether or not this would be suitable for the Ampeg R12-R

    The tube rectifier is a 5Y3 so 3A are OK

    The other tubes are 3x 6SL7 at 300mA each, 1x 6SN7 at 600mA and 2x 7591 at 800mA each. This makes a total of 3.1A which is slightly above the transformer rating

    Changing the 7591 output tubes to 6V6 would drop the required filament current to 2.4A but honestly I would love to keep the 7591 tubes if possible

    The secondary h.t. i know nothing about, since it's open I cannot measure it, and I couldn't find any specifications online. I have tried emailing Mercury and see if they are kind enough to share that information, but I also thought it might be worth to ask for help here as well, hopefully there are some Ampeg experts around

  2. #2
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    4,183
    That Hammond looks just about right for your Ampeg. If I'm not mistook, Mouser has a presence in the UK and they have a selection of Hammond iron in their catalog, that should save you a lot of shipping & customs cost. We're not much Mercury fans on MEF. Prices are sky high, plus you'll have to ship & pay duty - on the inflated price. OW!

  3. #3
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    Hammond are easy to get here in the UK and they usually are my choice but in this case I was thinking the 6.3V rating might be ok for 6V6s but not so ok for the 7591s and despite the common trend seems to favour the 6V6 in this amp, I was really looking forward to trying out the 7591 version.

  4. #4
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    27,819
    Carefully note the wires locations. You plan to replace the transformer, so remove the old one. Now you are at the what is there to lose stage. Take off the end bells and see if PERHAPS the reason it is open is repairable. The wires that it is wound with have to join the insulated wires that exit the thing. That joint may have failed on your HV wires. Also, since the winding wire is small gauge, it may have broken, and one place that can happen is right at the terminal for the insulated wire. If it snapped there, you might be able to scrape off the enamel and solder a patch wire up to that terminal.

    Seems worth a few moment effort.
    Justin Thomas likes this.
    Education is what you're left with after you have forgotten what you have learned.

  5. #5
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    Thanks for the tip. I am definitely gonna try that.

    However I must say the power resistors look like they got pounded quite a bit, and also the output tube sockets are a bit loose, electrolytic caps have never been replaced and now more than 50 years old. The guy who sold me the amp has given me the invoice / report from previous service and they reported a burnt area on the pcb and I can see they cleaned it next to the power resistors.

    I am really afraid something must have taken the poor transformer down, but it will be worth trying to see if the wire is broken at the terminals

    I have located a company in the UK that can rewind the original transformer at a decent cost. It's just slightly expensive than what mouser charges for the Hammond, but won't deprive the amp of the 7591 tubes
    g1 likes this.

  6. #6
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    Got the transformer back from the doctor. I have spent around 80GBP for the rewind, well cheaper than buying a new transformer

    I have been thinking of adding a fuse on the ht secondary in order to protect the power transformer from similar failures

    I am not entirely sure if a fuse between ht centre tap and ground would be safe enough for a tube rectifier circuit

    If I am not mistaken I remember diode rectifier need two fuses, one each for each ht winding leg, is that the same for a valve rectifier ?

    Since I don't know the exact rating of the power transformer, how can I figure out the correct fuse current rating ?

  7. #7
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    As i dig deeper I am discovering new things about this amp

    The 5Y3 tube that was in the amp when I received had one shorted section. This is probably what caused the power transformer to fail

    My amp has 5Y3 tube rectification and 7591 power tubes. There are labels inside the cabinet with tube configuration and the schematic says it's version R-12R-A. This is basically a transition model between R-12R (6V6 and 5Y3 rectifier) and R-12R-B (7591 and diode rectifier)

    I have noticed the can multi section capacitor is actually the one Ampeg used for the diode rectifier version. this means the first cap section after the rectifier tube is 60uF

    From my understanding this would be 3x times the value recommended for a 5Y3 rectifier, which is 20uF. I suppose 60uF is not optimal and might have shortened tube life, hence the power transformer fault

    It seems Ampeg got in the new batch of capacitors for the diode rectifier but still had leftover stock of 5Y3 rectifiers, and the R-12R-A was born.

    My idea would be keeping the 5Y3 rectifier, so I am probably gonna replace the filter capacitors with 20uF, like the previous 5Y3 versions of this amp
    Last edited by beatnik; 07-26-2017 at 03:40 PM.

  8. #8
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    27,819
    From my understanding this would be 3x times the value recommended for a 5Y3 rectifier, which is 20uF. I suppose 60uF is not optimal and might have shortened tube life, hence the power transformer fault
    Over the recommendation for sure, however, the amp worked for what 40 years? I don't think we can say this for that re the cap size.
    Education is what you're left with after you have forgotten what you have learned.

  9. #9
    g1
    g1 is offline
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    8,585
    You could add protection diodes to the rectifier tube. This will prevent your new transformer from damage if you get another shorted rectifier tube.
    Certified Dotard

  10. #10
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    yes the diodes would save the power transformer in case the rectifier is shorted. they probably also increase tube life ?

    but me thinking that even with the diodes there are other possible failure modes that can damage the power transformer, by too much current draw. and the fuse would cover a lot of possible scenarios, at least as far as the pt is concerned.

    i am still wondering wether a single fuse from ht ct to ground (like early Marshall JTM 45) would cover most faults or two separate fuses on each end of the ht winding would be safer

    the next tricky part will be getting the right fuse rating

  11. #11
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    the repaired power transformer has arrived and i have been doing some more servicing on the amp.

    the power tubes cathode resistor looks a bit worn out. i am thinking it would be better to substitute it but I cannot find a 140 ohm resistor of at least 10W rating, closest values are 130 and 150 ohm, I am wondering if these could work without upsetting the bias of the power tubes ?
    Attached Thumbnails Attached Thumbnails img_0105.jpg  
    Last edited by beatnik; 07-31-2017 at 01:30 PM.

  12. #12
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,027
    That's a 10% tolerance on that 140R, anyway... What's it measure? If it's bad, then I'd err on a little cooler, but either should work.

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  13. #13
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    27,819
    Where did you look? My first stop is always Mouser, and I found these right away.

    http://www.mouser.com/ProductDetail/...i1XBTa6fmkU%3d

    Given the choice you offered, I would also pick the 150 ohm as it cools the tubes a little bit.

    And don't think of it as upsetting the bias. Bias is just a more or less thing anyway. Consider that cathode bias has no adjustment, and anything from hot tubes to cool ones will get plugged in there, so right from the factory that resistor was just approximate.
    Education is what you're left with after you have forgotten what you have learned.

  14. #14
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    11,667

  15. #15
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    thanks for your replies

    i think i will get a 150 ohm resistor with through hole leads, that's easier to fit on the circuit board than the chassis mounted one

    stay tuned

  16. #16
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    27,819
    The flip tops one has wire leads.
    Education is what you're left with after you have forgotten what you have learned.

  17. #17
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    the flip tops one would be perfect but unfortunately shipping to the UK is too expensive for just one resistor

    i am giving some attention to the power supply capacitors

    the 6V6 version schematic shows an additional 20uF on node D. This cap is not present in the 7591 versions. I have attached the schematics for reference

    Is there a reason behind this change or Ampeg just wanted to save some money on capacitors ?

    I am planning to replace the capacitors with the following values

    node A : 20uF
    node B : 40uF
    node C : 10uF
    node E : 20uF
    node F : 20uF

    Do you think it would be better adding the additional capacitor on node D or it's not really necessary ?
    Attached Thumbnails Attached Thumbnails ampeg-r-12-r-schematic-original.jpg   ampeg-r-12r-original.gif  

  18. #18
    g1
    g1 is offline
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    8,585
    You must also consider the change to the resistor after that node. It's not just the cap that was changed.
    Certified Dotard

  19. #19
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    you are right, on the 6V6 version that resistor is 10K , on the 7591 version with tube rectifier it's 47K instead

    i would expect more voltage drop with the 47K resistor, but I don't understand why the missing capacitor

  20. #20
    Old Timer Tom Phillips's Avatar
    Join Date
    Jun 2006
    Location
    California, USA
    Posts
    3,336
    Quote Originally Posted by beatnik View Post
    ...the power tubes cathode resistor looks a bit worn out...
    My comment is that the resistor doesn't look "worn out." We don't always go by the look. I suggest that you measure the resistance as previously suggested and replace only if it is out of tolerance. The sandbox housing is not the active part of the resistor so it doesn't matter if it looks bad. (Yours does not look burnt up) Inside is a wire wound resistor which will continue to give good service if it tests OK and the solder connections are good. The 10W resistor actually looks to be in better shape than just about every other part in the photo you posted.

  21. #21
    Senior Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    27,819
    It might help if you put your location as UK in your profile. It impacts the advice we deliver.
    Education is what you're left with after you have forgotten what you have learned.

  22. #22
    g1
    g1 is offline
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    8,585
    Quote Originally Posted by beatnik View Post
    you are right, on the 6V6 version that resistor is 10K , on the 7591 version with tube rectifier it's 47K instead

    i would expect more voltage drop with the 47K resistor, but I don't understand why the missing capacitor
    The caps at point A and B are also smaller in the version with the cap at point D. That could be a factor.
    The circuits fed by point E are also different. Everything is inter-related. The capacitor is just part of several changes, you can't just separate it from everything else.
    Certified Dotard

  23. #23
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    OK I see. I thought it's recommended to have a filter capacitor on every power supply node.

    This is just my second amp restoration project. I am learning a lot from your advice

    So would you confirm the cap choice list on my previous post is fine ?

    Basically the only change compared to the original schematic is the first cap after the rectifier is 20uF instead of 60uF, but standing to the 5Y3 datasheet this is the recommended maximum capacitance value

  24. #24
    g1
    g1 is offline
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    8,585
    I think your choices are fine, but you may notice the difference going all the way down from 60 to 20uf at the first node.
    The data sheet I have lists 32uf as the max. capacitance for 5Y3. I've seen many examples using 40uf with no major issues.
    The 60 they used was probably ok with the tubes of years gone by, but I wouldn't trust a modern production tube into that.
    I think you could go at least to a common 33uf for your first node, but your choice of 20uf (or common 22) should be fine.
    Certified Dotard

  25. #25
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    Thank you.

    I have found a JJ multisection cap that would do the job for a decent price and it's 20+40+20+20uF
    Justin Thomas likes this.

  26. #26
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,027
    The more the CE can caps prices go up, the more JJs I buy... the only thing I don't like about the JJs is that the diameter is so much larger than the old Mallorys. But, still not worth 3x the price - I don't generally get to work on stuff that needs that level of restoration!

    Justin

    Edit: then again, they fit nicely inside the chassis, too.
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  27. #27
    Old Timer olddawg's Avatar
    Join Date
    Dec 2007
    Location
    Carlsbad, CA and Kona, HI
    Posts
    2,315
    Quote Originally Posted by Justin Thomas View Post
    The more the CE can caps prices go up, the more JJs I buy... the only thing I don't like about the JJs is that the diameter is so much larger than the old Mallorys. But, still not worth 3x the price - I don't generally get to work on stuff that needs that level of restoration!

    Justin

    Edit: then again, they fit nicely inside the chassis, too.
    Or you can hollow out the old cap can and stuff it with modern capacitors that are much smaller than ancient ones. I've done that a few times.
    Justin Thomas and Ted like this.

  28. #28
    Old Timer
    Join Date
    May 2006
    Location
    Chill-Ville, VA
    Posts
    2,027
    Well, I could, but I'm not sure I trust my handiwork that much! Have you seen my amp builds? :P
    But, that's probably a worthwhile skill for all of us to practice at...

    Justin
    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
    "Sort of like not checking for toilet paper before taking a dump. ." - Chuck H -
    "All I ever managed to do with that amp was... kill small rodents within a 50 yard radius of my practice building." - Tone Meister -

  29. #29
    Old Timer olddawg's Avatar
    Join Date
    Dec 2007
    Location
    Carlsbad, CA and Kona, HI
    Posts
    2,315
    Quote Originally Posted by Justin Thomas View Post
    Well, I could, but I'm not sure I trust my handiwork that much! Have you seen my amp builds? :P
    But, that's probably a worthwhile skill for all of us to practice at...

    Justin
    It isn't that tough. I usually use a Dremel tool with a grinding wheel. But it can be done easily with a pair of dikes and a file. Just make sure you save the bottom plate and take your time. I just make the appropriate connections to the appropriate components, wrap it with electrical tape (and sometimes a cardboard insulator), and put it back together with 5 minute epoxy. Then let it set with a clamp or rubber band or something over night. I'm not impressed with modern can caps that are available.
    Justin Thomas likes this.

  30. #30
    Ted
    Ted is offline
    Senior Member
    Join Date
    May 2007
    Location
    London
    Posts
    289
    Farnell is my first choice for components in the UK. They are a serious component supplier and don't charge premium prices just because something is going in an old amp.
    The have a minimum order price of 20.

    Sometimes specialist stuff for Ampegs has to come from Fliptops, but for most parts there is are alternatives if you use a little imagination. For multi-section can caps I use Hotrox who supply both F&T and JJ parts.

    I am not particularly hung-up on using original form-factor electolytics. Modern made-by-the-million high temperature caps are reliable and cheap

    Wirewound resistors go on forever. They are designed to get hot. They can look a little tatty after 50 years. But if it works, it works.
    g1 likes this.

  31. #31
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    hi guys

    just wanted to update you with the situation about this amp

    i have got the transformer re wound by a specialist company here in the uk. great service and affordable as well

    i have replaced all electrolytic capacitors and a couple of resistors and put in a fresh set of tubes. i used jj 7591s for the power amp

    the amp works nicely and all the voltages seem to be in the ballpark, but I am now struggling with a hum problem. the hum is present with nothing on the input, and regardless of the potentiometers settings

    without the preamp tubes the hum disappears, but if I put in the phase inverter it comes back. i have tried three different 6sl7 tubes, so I doubt it's a tube issue. also tried another 5y3 rectifier just in case, but no difference

    i am wondering if this can actually be a ground problem. right now i have all the 0V points tied together and they go to chassis at a single point, on the input jack sleeve

    i think that originally the amp had the 0V signal split in half and one side went to chassis at the power supply filter can capacitor and the other half grounded at the input jack sleeve

    i didn't think that my modification to the grounding would cause any problem. what is your opinion on this matter ?

  32. #32
    Ted
    Ted is offline
    Senior Member
    Join Date
    May 2007
    Location
    London
    Posts
    289
    An amp of this era will probably never be totally hum free. But it shouldn't be obtrusive. Some older ampegs didn't have switching jacks on the inputs and hum like bastards until you actually plug something in. Even if they do have switching jacks corrosion sometimes measn they don't work properly - a bit of switch cleaner on a trip of paper dragged between the contacts may fix it.

    But the grounding arrangement you describe is almost certainly not the original one.

    Try grounding the cathode resistor of V1 (and the power electrolytic for that stage if its not part of the multicap) to the input jack and all others to the tab on the multicap (if there is one)








    Quote Originally Posted by beatnik View Post
    hi guys

    just wanted to update you with the situation about this amp

    i have got the transformer re wound by a specialist company here in the uk. great service and affordable as well

    i have replaced all electrolytic capacitors and a couple of resistors and put in a fresh set of tubes. i used jj 7591s for the power amp

    the amp works nicely and all the voltages seem to be in the ballpark, but I am now struggling with a hum problem. the hum is present with nothing on the input, and regardless of the potentiometers settings

    without the preamp tubes the hum disappears, but if I put in the phase inverter it comes back. i have tried three different 6sl7 tubes, so I doubt it's a tube issue. also tried another 5y3 rectifier just in case, but no difference

    i am wondering if this can actually be a ground problem. right now i have all the 0V points tied together and they go to chassis at a single point, on the input jack sleeve

    i think that originally the amp had the 0V signal split in half and one side went to chassis at the power supply filter can capacitor and the other half grounded at the input jack sleeve

    i didn't think that my modification to the grounding would cause any problem. what is your opinion on this matter ?

  33. #33
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    what you are pointing out is actually true. the input jacks are switching type but they are not meant to ground the input when no cable is plugged in. the hum is also present when the guitar is plugged in though.

    the amp had been serviced before (quite poorly) so I am not sure what was the original ground arrangement. When I got the amp part of the circuit was grounded at the capacitor can, and the section closer to the preamp grounded at the input jack sleeve. Since the new capacitor had the ground tab isolated, I decided to join everything to the input jack sleeve.

    the modification you suggested requires quite a lot of changes to the wiring, but it makes total sense to have the more sensitive input circuit grounded separately. do you suggest to separate just the first triode cathode resistor or the second triode as well ? (they are part of the same V1 tube, although the second half seems to be part of the tremolo circuit)

  34. #34
    g1
    g1 is offline
    don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    8,585
    Where is the center-tap of the HV winding grounded?
    Certified Dotard

  35. #35
    Member
    Join Date
    Nov 2008
    Location
    London UK
    Posts
    82
    the ht and filament center taps are joined on the multisection filter cap, and from here they are tied to chassis

    i have moved v1 filter capacitor to this point as well (before it was grounded at the input jack sleeve) - this lowered the hum a little bit although I am not entirely happy and would like to improve things

    right now there are 5 different points where the circuit is grounded to chassis :

    1) the portion that is grounded to chassis at the input jack sleeve is shared between V1 first and second triodes - should i try only the first triode on this point ? (the second triode cathode is grounded at point 5 described below)

    2) tone capacitor grounds on the potentiometer shell - this is definitely stock wiring, but maybe would be more correct moving this at the input ground ?

    3) tremolo speed control grounds at the potentiometer shell

    4) tremolo intensity control grounds at the potentiometer shell

    5) the rest of the circuit ground signals are joined together and tied to chassis at the multi section filter capacitor negative terminal, this includes the h.t. and filament centre taps

    do you think this layout could be improved ?

    I forgot to mention that instead of 60uF for the first filter capacitor I have used a 20uF, since that's used on the previous revisions of this amp, with 6V6 power tubes, I assumed it would have been fine

Page 1 of 2 12 LastLast

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. pignose g40v power transformer replacement
    By pontiacpete in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 11-12-2015, 01:51 PM
  2. Must find replacement power transformer!
    By Skoked in forum Maintenance, Troubleshooting & Repair
    Replies: 9
    Last Post: 07-07-2013, 06:04 PM
  3. B-15N Power transformer replacement
    By Billyaxe in forum Maintenance, Troubleshooting & Repair
    Replies: 27
    Last Post: 06-01-2013, 11:40 PM
  4. Very low output on '65 Ampeg Reverberocket R12-R-T
    By drasp in forum Maintenance, Troubleshooting & Repair
    Replies: 12
    Last Post: 04-29-2010, 02:49 AM
  5. 1966 Ampeg reverbrocket R12-RT reverb tank
    By rocknronnie in forum Maintenance, Troubleshooting & Repair
    Replies: 1
    Last Post: 08-22-2007, 04:53 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •