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NAD - SVT for me!

Discussion in 'Amps and Cabs [BG]' started by Bugeyed Earl, Mar 29, 2018.

  1. Outstanding, thank you so much!!
  2. anderbass


    Dec 20, 2005
    Phoenix. Az.
    Well I'll be dang, your right. Just checked my 72 blueline SVT, Its the exact same way. The enlarged hole must be a carry over from the earlier SVT's that had a fixed speaker cable. The hole needed to be large enough for the 4-pin XLR style plug to fit through...
    bobyoung53 likes this.
  3. Made some progress on this beast: replaced a handful of resistors and electrolytic caps, replaced the little trimmer with a proper hum balance pot, cleaned up some ugly wiring, jumpered a couple of baked traces, and installed a new AC cord.


    Some of the interesting things I found along the way: a previous tech installed a couple of replacement electrolytics (the big wad of electrical tape epoxied to the chassis was one of these jobs,) but left the old caps in the circuit. Same thing with some resistors, a couple were dead, so the tech just piggybacked a new one (not necessarily of the correct value) onto the back of the board. Seems like a pretty lazy approach.

    I thought the preamp board was the least messed-with, but I had a closer look today and found a big crack that had been repaired with some jumpers - this was done a bit better than the other work, I'm guessing by a different tech than piggyback guy, so I didn't mess with them.


    It's alive!!!

  4. Wasnex


    Dec 25, 2011
    That's coming along nicely. Any concern about the carbon between R26 and R37 causing a cross circuit short across the plate B+ supply paths. Probably not, but if there is concern for this, the transformer lead could be lifted from it's original junction with the board (leaving it connected to the black jumper wire) and you could break the trace at the junction of R30.
    Bugeyed Earl likes this.
  5. That's a great point, thanks for mentioning it! It cleaned up pretty well, but I didn't think to check for conductivity. I'll do that the next time I get a chance. The carbon doesn't look like it reaches the resistor traces, but it's hard to be certain.


    Wasnex likes this.
  6. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Those little burn spots are not to be of concern...they just make your tones warm :D

    Glad to see you giving this head so much love, Bugeyed. I'm sure it will kick much ass when it's done.
  7. Well, I have been aiming for a more "smoky" tone...
    bobyoung53, Son of Wobble and JimmyM like this.
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    These old amps always seem to come with some issues. The trace bounded by the lines of R27 and R38 could be brittle. Part of it has a bite taken out. That trace carries the plate resistor current. I would reinforce it with a soldered over wire. At the very least clean it and put a layer of solder over it to prevent the copper from oxidizing.

    This tyoe of damage occurs when plate resistors fail. THey are designed to do this to protect the amp so it is importance to not install a higher than 5W replacement. Use a flameproof resistor. THey should be mounted a bit off the board so that the heat does not cause damage. Often people use cement block resistors. Mount them sideways, when the resisto blows flames come out where the glue is, not the cement. By pointing that side away from the board, you buy some insurance.
    bobyoung53 and Bugeyed Earl like this.
  9. Yep, I've already jumpered that trace with a solid wire. It wouldn't have gotten baked if the previous tech hadn't mounted the resistors on the back of the board, so that's aggravating, but it's fixed now... I've got the plate resistors replaced also:


    I haven't messed with the diodes D15-20, but that might be something I'll look into in the future if I decide to update the amp to the later spec. I have some 12DW7s on the way to replace the 12AX7s that are in V1 & V3 of the preamp, and a set of power tubes are also on order. Looking forward to hearing this amp at its full potential :)
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The top is looking way better than it did. It really has cleaned up well. I find that cleaning an old amp is relaxing.

    I often leave the D-15-20 components in place even though they are not needed. I often change the other rectifier diodes. I’m in there and they are inexpensive.
    bobyoung53 and Bugeyed Earl like this.
  11. Do you usually use 1N4007's for the diodes, or are there upgrades you prefer? I've got a bag of those around here somewhere.
    beans-on-toast likes this.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Loud uses 1N4007's in their current SVT's for the diode bridges. That's what most people use in these vintage amps.
    Bugeyed Earl likes this.
  13. Wasnex


    Dec 25, 2011
    Those original screen resistors look sweet! The screen supply traces for my early blueline are a total mess and I know that last owner really cranked the amp. Obviously the resistors have been replaced many, many times...in fact one of them was scorched when I received the amp. I guess it's true, the 6146 equipped amps really don't like to be driven hard.
    Bugeyed Earl likes this.
  14. Looking forward to seeing the finished work!
    Bugeyed Earl likes this.
  15. daveman50

    daveman50 Supporting Member

    Feb 24, 2007
    Westchester County NY
    This is a labor of love. Can't wait to see it finished and that you get some love back from the amp.
    Bugeyed Earl likes this.
  16. I've made some progress with this head: the carbon in the crater was bugging me, so I excavated it with a dental pick until I no longer had conductivity in the pit. It didn't bridge to any of the surrounding traces, but I figured I'd clean it up anyway. A handful of tubes arrived, and I was able to power up on the lightbulb limiter, and we have tone!!
    The power and standby switches were both sketchy, so I replaced those, but everything else was looking good, so I gave it full power.

    The amp had a bad hum that quieted down greatly once I set the bias, and I have a big load resistor on the way so I can set the the phase inverter balance. In the meantime, I've loaded it up with new Tung Sol 6550's and started messing around with preamp tubes. I settled on some vintage GE 12DW7s, and a GT 12AX7 in V2. I don't know if there was much audible difference between the tubes, I was testing them in my garage where I've been working on the amp, and it's hard to pick up on those subtleties while tools are rattling off the shelves and bikes are falling off their hooks, etc. It's no surprise that an SVT is loud, but this is the first time I wondered if my 412 cab or I would suffer internal damage first.

    There's still a bit of a weird out of phase sound on channel 1 that I need to figure out, I'm not sure where that's coming from.

    Thanks to everyone for the well-wishes and tech info, this has been a fun project so far!
    Last edited: Apr 29, 2018
  17. I had some Fender fabric laying around, so I made a replacement for the missing baffle until I can find some proper stuff (does anyone have a line on 70's Ampeg repro grill cloth?)

  18. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The grille cloth for your amp is not available as far as I know. The pre ‘67 cloth is available.

    BTW, it helps to glue on a layer of black fabric store felt on the baffle board. Remove the material wherever there are frame cutouts. The felt adds a padding under the grille cloth that makes it look better.
    Korladis likes this.
  19. Wasnex


    Dec 25, 2011
    I don't think tube rolling makes much difference in SVTs. I suspect that 12DW7 don't vary much from manufacturer to manufacturer but it could be the circuit as well.

    I have rolled GEs and a Raytheon 12DW7s and the biggest difference was as slight difference in gain. I think the GEs were brightest, followed by the Raytheon, followed by the JJs...but the differences were so small that I put the JJs back in. Since then I have also ordered some Philips and Mullards...but I haven't rolled them yet.

    Regarding the phasiness...The crack in the preamp board looks like it might extend all the way to the junction of R19. Perhaps another broken trace along the length? Do all the tones controls and selectors behave is expected? Other than that, without some sort of device to inject and monitor audio along the signal path, I wouldn't have the slightest idea how to troubleshoot this problem other than looking for obvious shorts and open circuits. I look forward to learning what the problem is.....your doing great :thumbsup:.
    Last edited: Apr 29, 2018
    Korladis and Bugeyed Earl like this.
  20. Thanks for the tip, I'll try that if something more suitable comes along.
    beans-on-toast likes this.

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