1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

What happenned there?

Discussion in 'Shock Brother's DIY Amps' started by marshman, May 6, 2009.

  1. marshman

    marshman Poster Extraordinaire Silver Supporter

    Posts:
    5,364
    Joined:
    Feb 1, 2007
    Location:
    SE PA
    Sorry this ran so long...if you're in a hurry, try the last paragraph...

    So, few years (like 6) ago, my 100 watt 1970s all-tube bass head went FZZZT on me while at home, practicing. Ok, it happens, bust out the backup and carry on. http://www.univox.org/pics/amps/1051.jpg

    At the time I knew which parts were the tubes, but that's about it. I extracted the 4 6L6GCs and stuck in a quad of EL34s I had lying around and fired it back up...bupkus. So, I stuck it under the bench and ignored it.

    About 2 years ago I got it out (I now know how to tell caps and resistors apart!) and looked it over. It 'works' again, but is obviously not healthy. I haven't a clue where to begin, so I take it to my local tech.

    Now, when the amp konked out, it was loaded with a trio of NOS 12AX7s and a reltively new set of JJ 6L6GCs. I played it in a 5-piece band with a heavy-handed drummer, a 55 watt Mesa and HRDx. I can literally dime every knob, back off the treble & presence a touch and I get beautiful, vibrant clean tone. Shouldn't have worked like that, I know, but it did.

    Now, I've gotten it back, 'repaired'. And it does seem to function properly now, which it didn't before. But now it's a guitar amp. If I crank it, it distorts, even with a tele. Tech tells me that all the tubes are plenty strong (though one 6L6 is a touch microphonic). Kinda bummed I don't have a bass amp anymore, but I'll tweak for that later.

    Here's the funny part...just for reference, the amp has a control panel that pretty much mirrors a Tweed Bassman: 2 norm inputs, 2 guitar inputs, vol1, vol2, bass/mid/treb/presence. And, cause it had never occured to me before, I tried jumpering the channels, and I noticed something funny... If I turn either volume up all the way, I get loud. I then bring the other volume up and get a little more volume and a "fattenning effect"...but when the 2nd vol hits about 7-8, the amp gets noticeably quieter! If we assume that the knobs "go to" 10, and add the two together, any 'sum' over 17 or so seems to yank the rug out from under the thing. Both on 8, really loud...both on 9, much quieter.

    Any ideas?
     
  2. drgonzo2

    drgonzo2 Tele-Meister

    Posts:
    210
    Joined:
    Jul 22, 2007
    Location:
    Daventry, U.K.
    Could be any of a number of things that have caused the change in headroom you've described (drifting resistors, iffy p/s caps, opt issues, etc). What did the tech do to get it back working?

    The volume dropping when the two volumes are dimed could be a valve issue (try swapping V1), but without the amp in front of me I can't really say.

    Sorry if this isn't much help... G
     
  3. marshman

    marshman Poster Extraordinaire Silver Supporter

    Posts:
    5,364
    Joined:
    Feb 1, 2007
    Location:
    SE PA
    I noticed that a few of the caps on the board had been replaced with Orange drops at the first trip to the shop, but the problem was still there...in the end, it seems he completely rewired the ground circuit, he says, as the previous wire was just not really up to the task. The filter caps looked the same as when I dropped it off, though it seemed he'd written the specs on them, but I'm sure if they'd been weak, he would've replaced them.

    I've already swapped out the NP 12AX7s for NOS 12AY7 in V1 and 12AT7 in V3/PI in the quest for bass perfection, but hadn't noticed if the volume drops were going on before I made the swap. I'll swap them around some more and pay attention tomorrow.

    Thanks for the suggestions.
     
  4. celeste

    celeste Friend of Leo's

    Age:
    59
    Posts:
    2,905
    Joined:
    Jun 24, 2006
    Location:
    Maryland
    If I had to with as little info given, I would guess that because the"fix" was reworking the ground wiring, there was a high resistance to one of the preamp tube cathodes. That would have cooled it off and given you a bit more headroom.
     
  5. Tim Armstrong

    Tim Armstrong Super Moderator Ad Free Member

    Age:
    61
    Posts:
    19,376
    Joined:
    Mar 5, 2003
    Location:
    Austin, Texas
    I'm just another schlubb who doesn't know much about amp circuits, but the volume drop kinda sounds like a phase issue between the two channels. But... that would tend to happen all along and get worse as you brought the volume up...

    Hmmm....

    I got nothing!

    Tim
     
  6. marshman

    marshman Poster Extraordinaire Silver Supporter

    Posts:
    5,364
    Joined:
    Feb 1, 2007
    Location:
    SE PA
    Well, for those of you that understand such things, I swapped some GT 12AX7/7025s back into V1 & V3, and it still does the whole 'seem quieter' thing when both volumes get up real high that it did with the 12AY7(V1) and 12AT7 (PI) in there...as you might imagine, the lower gain set sounds better with the bass, but both sets sound pretty good with a tele...I don't have any other guitars out of their cases right now, so I can't tell you about that.

    Since it didn't really help matters much for bass, I un-jumped the channels. Just out of curiousity, is there a 'right way' to jump those things, or do you just need to plug into one channel and run a patch cord from that one to the other?

    Plugged into my Peavey 1820 it sounds pretty good, but flubs a little when you get really deep, especially when I detune down to D. I'm gonna try it out with my Carvin 4x10 tomorrow, that's a pretty heavy-duty cab.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.