Re-wiring old PA amps for guitar

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Paul S, Oct 19, 2004.


  1. Paul S

    Paul S Member

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    Hi,

    has anyone here re-wired or modded old PA amps for guitar use? My grandfather has a few old PA heads in the basement, a small Bogen, and a couple of larger Stromberg-Carlsons.

    I guess the first order of business is to put in 1/4" inputs and outputs, but I'm wondering about what else to do. I've heard that you can do interesting mods with the multiple inputs (phono, mic, etc).

    Anyone here done this, or can you point me to any good web sites?

    Thanks,

    Paul
     
  2. fullerplast

    fullerplast Senior Member

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    These can sound really good Paul, it all depends on what you have. I've personally heard and played through an old Stromberg Carlson that had little done besides the input jack and it sounded great, with nice breakup characteristics.

    You're probably going to find some dried up caps and may need to do some creative replacement of obsolete parts, but nothing too drastic. Try and reverse engineer the preamp circuit and see how it compares to a modern guitar amp. Chances are good you will not be too far off.

    I don't know of any web sources for this offhand, but you may be able to find schematics of your old amp if it was a reasonably popular one. Fun project...
     
  3. Paul S

    Paul S Member

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    Looks great--thanks, ceterisparibus! Thanks also to fullerplast for replying.

    Paul

     
  4. hasserl

    hasserl Member

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    Hey Leftee, how's it goin? Good to see you over here, I just joined the group.

    Paul, I've done this also, it is a great way to go. You've get a great head start with a chassis and transformers, and depending on the amp you might just rewire it for guitar, or do like i did and gut it and start all over.

    RCA Clubmaster

    Sounds great! clip
     
  5. Leftee

    Leftee Guest

    Hey buddy! Good to see you over here! :D
     
  6. Geetarpicker

    Geetarpicker Member

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    I really like the (late 40s?) Operadio 1A30. Just got my second one. I first heard one of these cranked with guitar over 25 years ago, and remembered it ever since. 2 channels, Anode cap metal preamp tubes, tube rectifier, twin 6L6G output. Lighted control panel, opposing rotation on chicken head controls, goes to like 24! hahaha... Nice gain and overdrive. Sounds a bit like a JTM45, really.
     
  7. Paul S

    Paul S Member

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    Thanks for the reply and links, Leftee, and for the pics and info Hasserl (btw, we communicated a bit on the Carvin forum about modding Nomads, and such).

    I ended up purchasing another Nomad with your stage II mods, and it's great! No one talks about Carvin tweed amps here, but for me, especially with your mods, they way outperform Peavy Classic 30's, Blues deluxes, etc. I even bought another Belair, just to do hear the same amp w/ a 2—12" setup. Nice work!
     
  8. Andy H.

    Andy H. Member

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    If you are lucky enough to come across a little old PA amp they can really surprise.
    I picked up a TOA HA355 valve amp*, rated at just 10 watts, which sounds fantastic for guitar. I just brightened up the two mic inputs with a 100 and a 220 picofarad cap, and love what it does. Both clean and driven it's really very nice. (John Phillips heard it and said it reminded him very much of an old 20watt Marshall in character. Considering the prices of those new reissues, I think I did pretty well for GBP25...!)

    *There is no sign of this model on the (pretty comprehensive) TOA website. It looks very '70s to me. Kind of like a scaled down very plain THD Univalve. Grey top and bottom grilles, and side plates, brushed steel front with 4 chromed knobs. Unassuming in looks, but not in sounds - if anyone spots one, make an offer for it!

    Andy.
     
  9. hasserl

    hasserl Member

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    Hey Paul, very cool to hear about your modded Carvins! They really do rock. I swear by my Nomad, I call it my Secret Weapon, because it is so unassuming. We used to call cars like this a Sleeper, that is they didn't look like much but were capable of blowing the doors off unsuspecting rich kids in shiny Camaros or Mustangs.

    Have you done anything with either of the the PA's yet? The best place to start is with a schematic of the circuit so you know what you have to work with. If you cannot find a schematic anywhere the next best thing is trace one out yourself, which if you haven't done so before is a real good excersize. Start at the input and work your way all the way thru. Then start with the power supply and trace it all out. By the time your done you will have a great understanding of the circuit as it is now. Then you can decide whether you can simply swap in a 1/4" jack for the input, or if you need to do some more serious rewiring. Take a look at the tube assortment and compare to other amps and go from there.

    Just a note though, sometimes these old PA amps have some problem components that can cause you to spend a lot of time chasing down oscillation problems or some other problem, in which case it's almost easier to gut them and start over with fresh components. As long as the trannys are good that's the main thing.

    BTW, if you've visited the Carvin bbs much you probably know leftee from their too! (as well as other boards, he does get around :D)
     
  10. Leftee

    Leftee Guest

    I used to get around. These days I've cut my Internet time waaaaay back. :D

    I'm not on the Carvin BBS these days.
     

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