1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice

Re-capping tube amplifiers

Discussion in 'Amplifiers' started by dozerman, May 8, 2018.

  1. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23
    ok folks, I may have a hum drum question for the considerable minds on this forum but here goes... when re-capping an amp, do I need to go back with the exact same caps and values in the filter caps or can the be played with? Say one had 4 100uf 450v strung together, could I use a couple 220uf or larger? Or does that mess things up within the amp? My understanding is that they smooth out ac noise and store HV.


     

  2. OldTech03

    OldTech03 Active Member

    Joined:
    Dec 15, 2017
    Messages:
    113
    Likes Received:
    75
    The only thing to consider is inrush currents against the bridge circuits it most likely won't be an issue but it is plausible that larger capacitance will increase the startup current surges. If you are rebuilding an amp then maybe along with larger caps higher current diodes would also be a good idea. Some other knowledgeable people on here may chime in.

    Oldtech03
     
  3. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23

    I have noticed when I do re-cap these older amps, most had 4007 diodes for the bridge. If I use larger amperage diodes, the HV transformer grunts really hard. I switch back to 4007 and all is well. Of course still a newbie to tube amps, I'm probably using the wrong diodes.
     
  4. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,430
    Likes Received:
    874
    The story that "more is better" is almost always partly true.

    Partly.

    The voltage rating is important.Your description of "strung together" suggests that they have been combined in series, or maybe series-parallel. Raising the voltage rating probably won't offer any useful result.

    Using a cap double the original capacitance can put stress on the power switch from the power-on surge. Charging the filter caps up from dead empty will create a surge current that's in proportion to the capacitance. Adding a surge resistor between the transformer's HV winding and the HV rectifiers to limit the additional peak current is the quick-and-dirty way, if there is room for it. The surge rating of your HV rectifiers will be the guideline to calculate the resistance value needed. The current drain the tubes pull from the HV will dictate the wattage rating the resistor will need, once you choose the resistance value.

    The 'step start' circuit used in the large Ameritron tube amplifiers will save your power switch from surge damage. Typical problem is that adding larger filters causes the contact points in the power switch to spot-weld in the "ON" position so that it no longer turns off properly. That's what happened in our Dentron MLA2500 when I doubled the capacitance of the HV filter caps. Seems to happen in D&A Phantom amplifiers, even with the stock-size filter caps. Learned to use a two-circuit 20-Amp per-pole switch with both sides wired together in that model.

    We have been buying chinese step-start circuit boards on Chinabay when we need to retrofit this feature in a large amplifier. The home-brew or "bowl-box" amplifiers almost never come with this feature in them.

    The small recitifier diodes found in some older sweep-tube amplifiers may look like a 1-Amp rated 1N4007, but some of them were a now-obsolete "2.5-Amp" rectifier. Same phyiscal size, more or less, but with the higher rating. That's what D&A used for a lot of their amplifiers. We standardized on the 3-Amp rated 1N5408 to replace those. If you had trouble using that one, I would suspect operator error, rather than blame the parts. I buy 1N5408 by the thousand, using them in everything from Pride DX300 power-supply boards to Browning radios.

    73
     
    #4 nomadradio, May 8, 2018
    Last edited: May 8, 2018
    ShadeTreeMechanic likes this.
  5. ShadeTreeMechanic

    ShadeTreeMechanic 808 On The North Side of Dover

    Joined:
    Oct 23, 2017
    Messages:
    406
    Likes Received:
    196
    Wow I am learning a lot. I am getting ready to re cap one of mine. Maybe the bigger diodes can pass more current?
     
  6. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23

    Yeah I'm learning myself. I've always replaced with what wasn't in there but it still expensive as hell to run all the same axial caps when you can string larger value radials and us each half the amount. I was just making sure my math was feasible or if the more capatance would screw with something else
     
  7. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,430
    Likes Received:
    874
    The radial-lead and "snap-in" type filter caps are a lot cheaper, and do the same job. The added labor to get them wired up in place of the older axial-lead part isn't free, but they'll do the job just as well once they're installed.

    73
     
  8. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23
    Lord I hate spell check or predictive text on this site. Reading my last response made no sense at all. This is the only site that does that on mobile.
     
  9. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23

    Well there it's is. I've not been using 5408 diodes. Operator error indeed. I'm recapping a Maco 300 and thats why those little 4007's have been popping. I will 'rectify' that when I get home.
     
  10. Onelasttime

    Onelasttime Well-Known Member

    Joined:
    Aug 3, 2011
    Messages:
    764
    Likes Received:
    322
    First I assume that this a cheaply aka poorly made CB tube amp since the words or phrase "strung together" was used! For power filter caps use the same thing as the OEM used or better. Do not do what CB'er's and I am using the term right now in a negative way to be clear do! You do not want crap-tastic disc caps in the unit in tuning sections and you do not want to ever use 2,3,4,6 caps where you should have and easily could have used a single cap! If you are going to try to go cheap on the caps just stop now put down the soldering gun or plumber soldering iron package the item well and sell it on ebay you will be doing yourself and the next owner a service! Also do not use cheap Chinese made no name of off brand caps. You might as well buy a Davemade and hire a neighbor kid to kick it around before you unpackage it!

    By the way "strung together" how? Paralleled or series or a combination of both? Go to either Mousers or Digikey and select the proper caps and put them in your cart than check out! That is what I do. I do sometimes by parts from other places but these guys do a good job weeding out any counterfeit parts and their stock is new enough that it is likely all going to be close to fresh. Not that you should not check each part before installing it but I do not think I have gotten a bad part yet "knock" on wood! LOL

    Generally you want to use the same type of cap in each place as the OEM until you know enough to know what you can and can sub out.

    "Cootering" Mircle Mechanic from Dukes of Hazard is not how you want to repair or build gear. The cheap disc caps need to be replaced with silver dipped mica in the tuning sections or doorknob's or the like because tube amps get hot and disc caps are about as stable in hot areas as ice cream is in hot area's!
     
    n8fgb likes this.
  11. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23


    @Onelasttime lol no I don't use craptastic caps. I don't go back with Mallory axials but I use ELUM brand radials and they seem to work just fine for my cheaply built amplifiers, and much more cost effective. I can replace those 4-1 over Mallory.

    My terminology of "strung together" seems to have folks beside themselves. I will watch that from now on. There were 4 caps in series @100uf a piece. My question was simply could I use a larger capatance capacitor to use less? Instead of 4 OEM Mallory @ $24 a piece, why not 2 220uf ELUM @$8 a piece? The difference in added capatance was my secondary question. As longboard as I kept the number even so I could still use the midpoint for my secondary/low side, Is there any appreciable difference?

    Thanks everyone for your comments. I don't mind being the punching bag for the folks that read these posts, needing help and actually learn something.
     
  12. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    1,430
    Likes Received:
    874
    Larger-capacitance filter caps will do three things.

    Filter out more of the AC 'ripple' that causes hum to be heard in an amplifier's output signal. There is a diminishing-returns factor. Once the ripple is below the level you can hear in the amplifier's output signal, adding more filtering makes little additional difference.

    Reduce the voltage "sag" under load. This is not too different from the rectifier-ripple issue. Once you reduce this to around 10 percent below the unloaded voltage, adding more caps makes a smaller and smaller difference.

    Make the initial turn-on current surge higher. Filling up those caps from empty draws a brief but large surge of current. The bigger the caps, the bigger the surge. There are design methods to cope with this, but it's part of the picture.

    A RF power amplifier is not the same as an audio power amplifier. Larger filter caps can influence bass response, especially really low bass frequencies.

    Not an issue for a RF amplifier.

    73
     
  13. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23


    Thank you for that @nomadradio. I was assuming larger capatance would filter better and assumed there would be more draw on start up but wasn't for sure. That explains the brief thump from the trannys when turned on. I did try to keep the numbers the same so I could use the secondary for the low side. With odd numbers you can't do that, again assumption. I'm not tech nor do I claim to be, but I'm not the type of person to roll over on something as simple as cap replacement. Thanks to all who have replied tho this thread.
     
  14. Onelasttime

    Onelasttime Well-Known Member

    Joined:
    Aug 3, 2011
    Messages:
    764
    Likes Received:
    322
    If a cap is being used to filter power as a supplier filter or blocking you are normally fine going larger assuming everything will fit. If you look at 99% of the junk seen in the CB world people will take an insane amount of caps when what they really needed was one cap of the correct value. Any time you use more caps to get what you need value wise you increase the possible failure points.


    1) Rubycon
    2) Sanyo
    3) Nichicon
    4) Mallory
    5) Panasonic
    6) Wima
    7) Cornell Dubilier
    8) Elna
    9) Fujitsu
    10) Nippon Chemicon
    11) Hitachi
    12) Vishay
    13) Kemet
    14)EPCOS
    15) Wurth
    16)Siemens
    17) MuRata
    18) OSCON
    19) BHC
    20) GE
    21) Ruby Gold

    I know I left some off but since I have been in the USA since around 1990 or 1991 or so these are what are easy to get and what is considered Tier 1 in the USA. ELUM are not even on my radar. Since I have zero experience with them I will not call them junk at this point but I would also not consider them to be Tier 1 at all either. I have some of all of the above currently. I am not talking audio grade or super specialty caps really. You can go off the deep end easy if you include specialty caps. A lot of audio guys go crazy exotic even on power filtering caps which I think is silly. Spend that Cash on exotics for the audio signal path not for power filtering! I did Google ""ELUM Electrolytic Capacitors" and I did not see anything that made me think they belonged on my short list of Tier 1 suppliers.

    In a solid-state radio you can easily get away with 85 C caps but in your amp and in power supplies you really should use 105 C or better and the longer the lifespan rating from the OEM the better since heat is the enemy and amps and power supplies often get hot compared to a radio. Vacuum tube devices even more so!

    Obviously for power filtering hi ripple current rating is super important.
     
    ShadeTreeMechanic likes this.
  15. dozerman

    dozerman hello, its me again

    Joined:
    Dec 16, 2013
    Messages:
    164
    Likes Received:
    23


    Thanks for the list of approved capacitors! if I were an audiophile i would use these some of these brands. Actually I do have a bunch of Nippon and nichion caps, but I got a great deal on the brand I mentioned. They fit nicely where the OEM caps did and perform well, subpar and all. After all, it is just cheap cb crap. Maybe once my feet are a bit wetter, and have a better understanding of the what's and why's, I may use those higher grades. Until then, I will learn with my wallet in mind. Thank you though for the list, it will help me in the future.
     

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!