NAD National Valco Tweed Amp Restoration Project

Discussion in 'Showcase' started by Manodano, Oct 10, 2018.

  1. Manodano

    Manodano Well-Known Member Supporting Member

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    My very good pal, the ultimate scroungehound, David Feinberg, retrieved this fabulous specimen from a pawnshop in some forgotten town on the edge of Arkansas near the Oklahoma border. David manufactures and then delivers factory built sewer pump stations to all sorts of out of the way places from his home base of El Dorado, Arkansas. Our engineer fathers did business together, and I've done a little business with him, but mostly we just scrounge old music gear up for each other and then settle up when he comes to Nashville on occasion.

    This time he hit a home run. Here's the Valco 51 with a National badge on front. Its basically Valco's version of an early Tweed Deluxe. This one is in almost totally unmolested and all original condition. Behold:
    IMG_2030.JPG IMG_2034.JPG IMG_2038.JPG IMG_2039.JPG IMG_2041.JPG
    [​IMG]
    [​IMG]

    Any of you boys good with date codes?
    IMG_2043.JPG
    IMG_2042.JPG
    I carefully took the back panels off so I could see whats inside.

    I don't think the back panels have ever been removed. I sense that no one ever really used the amp much. Clearly it's been stored in a reasonable environment.

    The speaker cone feels like it should, sort of crisp without being brittle, and sounds right when you rub it. Turns out its a Jensen Field Coil, which could be great if it's still intact.

    Tubes are all RCA, possibly original. The 5Y3 looks a little newer than the others, but its still real old so I can't tell for sure. Tubes are (metal) 6SQ7, (metal) 6SC7, (2) 6V6GT, 5Y3.

    The earliest Fender Tweed Deluxe used a 6SC7 for both preamp tubes, so this is probably real close to that circuit.

    The power cord has been messed with so that's got to go - it's brittle and has a funky plug on it. That's the only sign of monkeyfication on the amp.

    Over the next several days, the course of action will be to carefully remove the chassis and the speaker from the cabinet, clean out all cobwebs and dust without removing too much of the patina, give things a visual lookover for obvious scorched and burned parts, reassemble, then power up SLOWLY with my Variac combined with a current limiter.

    If there's no sparks, smoke, smells, or yells, somewhere around 70 volts the amp should start "working" well enough to plug in a guitar and see what happens.

    Its gonna be great!
     

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    Last edited: Oct 10, 2018
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  2. Manodano

    Manodano Well-Known Member Supporting Member

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    I spotted the spare fuse envelope stapled inside (common in Valco amps) but was disheartened to find it torn at the corner and empty. Then I noticed the torn paper corner and the spare fuse in a pile of fuzz in the front opposite corner of the amp. Its all 100% right there!
    IMG_2046.JPG IMG_2048.JPG
     
  3. devdem

    devdem Well-Known Member

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    Grrreat find! I would recommend following the path of wisdom and do not start it anymore. the caps are probably gone and God only knows what else. All the parts there, including transformers, are valuable, so don't risk blowing something out. Service it properly, save all the original parts, THEN start it up.
     
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  4. devdem

    devdem Well-Known Member

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    Oh and I think the 220304 number 3 in the 4th position denotes a year. So 53? 63? Cross reference it will tranny codes or pot codes.
     
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  5. mozz

    mozz Well-Known Member Supporting Member

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    That is the ultimate find right there.

    Could be a 43'. Field coil speakers weren't in use much after ww2.
    "Service it properly, save all the original parts, THEN start it up."

    Yes, there is no way, even with a variac, i would try to plug this in. My model 50 had ALL caps were bad. While you "may" be able to reform electrolytics, wax caps will be bad and you can't reform them. If it has this style white resistors, i'm gonna say 1943 not 53. There may be 2 or 3 schematics too, if you can't find the exact one, let me know, i have some 40's manuals.

    Field coil speakers do not sound the greatest, if you archive that speaker and use a modern one, you need a resistor to replace the field coil part of the circuit, let me know, i have some higher power resistors here.

    The white resistors, some were way out of tolerance to.


    supro20170701_155539.jpg
     
  6. andrewsrea

    andrewsrea Well-Known Member

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    Looks like a 1953. Cool find. If the Jensen works, it will be a first. They often developed small shorts. The speaker still works, but not well. And be careful - that is high voltage creating the magnetism on that speaker!

    I've worked on Valco's and Premier's of the same design (same factory - typical Valco / Supro operating plan: build common designs for anybody's brand). In those repairs, I typically swapped speakers, boxed and returned the original to the owner for posterity. They sound pretty good, kind of swampy. Good for Jazz & Blues, or greasy Rock.
     
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  7. jtcnj

    jtcnj Well-Known Member

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    Lost treasure found!!
    Most excellent!!
    Ditto on the electrolytic caps; dont power them and risk the rest.
    I know you save the original stuff on these but I would think you want a modern grounded 3 prong AC power cord and remove the death cap if it has one.
     
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  8. jimytheassassin

    jimytheassassin Well-Known Member

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    people usually pay extra for those kind of dust bunnies too! good score. im subscribed
     
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  9. pirate19

    pirate19 Well-Known Member

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    That is absolutely gorgeous--would love an old Valco like that
     
  10. RiverDog

    RiverDog Well-Known Member

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    ALL the mojo right there. Nice find!
     
  11. idiotsdelight

    idiotsdelight Well-Known Member

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    Damn you and your Feinberg all to hell.
     
  12. Manodano

    Manodano Well-Known Member Supporting Member

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    I'm making some slow progress. I set out to find a schematic, and soon found one that almost matched, which led me to believe someone had stuck the wrong preamp tube in the amp. Then I carefully disassembled the amp and lo and behold the preamp tube number is screened onto the chassis, so then I knew I had to find the correct schematic. If you zoom in, you'll see the left tube socket is etched "6SC7". The 5Y3 socket is marked the same way. I never saw that before.
    IMG_2059.JPG


    After a little more digging, I found this, which matches my amp (dang, that internet is a great thing!):

    gretsch_g6155.jpg
    The 6SQ7 preamp tube and the 6SC7 tube are are readily available and not too pricey, so I'm ordering those now, just in case. I'm too lazy to dig out my monster tube tester, and I'll want some fresh tubes, anyway. I never had an amp with either of those tubes, so I'm on new ground with those, both circuit-wise and sonically. I have a stash of 6V6's and 5Y3's, so no problem with spares, there.

    Here's a pic of the preamp/phase inverter side of the chassis. I was glad to see that I did not have those ancient white resistors, instead I found much newer looking carbon comp resistors and Sangamo caps. It looks like a factory wiring job, so if the amp has been rewired and updated sometime over the last 68 or so years, somebody did a factory spec job on it.
    IMG_2053.JPG
    I haven't spot checked any resistor or cap values yet with a meter, or made sure the output transformer is not burned open, or spot checked the power transformer voltages - that's all coming next.

    I'm taking andrewsrea's advice regarding the speaker. As I proceed for now, I'm going to set it up for a modern PM speaker. I have a stash of high-wattage resistors, or maybe a choke handy, so no problem, there.

    Later I may mess with the original electro magnetic speaker just to see how it works and what it sounds like. For now I don't want to chance having that speaker shorting out intermittently and creating havoc with the output transformer and power tubes. I'll set it up with my benchtop power supply and feed it a signal to see how its working if I ever have the time. My motivation is that I play a little lap steel, and maybe that speaker has some special lap steel mojo.

    I'll be installing a grounded power cord before anything gets powered up. The one on the amp is not original, and its all dry rotted and cracked, anyway.

    The power supply can cap is three 10uf/450 v sections. If I go ahead and replace it, does anyone have some recommendations regarding different (i.e., higher) cap values?

    Advice, comments, or concerns????
     
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  13. mozz

    mozz Well-Known Member Supporting Member

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    You can use all 22mfd there, won't hurt the rectifier. Does it have the big cardboard cap or is it a can? Tubedepot is my place of choice for caps. F&T.

    Few other things, that orange cardboard cap, you know you have to change that also no matter what. You may also be able to get a date code from that. Pots may also have a date code. I was hoping it was a 43', 1953 you must have one of the last field coil speakers being used in that time frame.

    If you look at the inputs, no matter what jack you use, you are loading down the pickups with 23.5k to ground, that's bad. I would rewire that, maybe typical fender deluxe or bassman type, just use 3 jacks, you would then have high-low. Use a 1meg to ground so the tube doesn't run away.

    As for the 6sq7, i have bunches, want a few let me know. Whether the 6sq7 will howl i don't know, they don't really get used too much in audio so maybe they are known to be microphonic. 6sc7 i can't part with becuase i build with them a lot, you can get Sovtek or JJ's i think? Most all metal 6sc7, will be microphonic, went through a bunch before i got one that didn't howl. The ones you want are 6SC7GT, glass tube, expensive, if you can find them. Do not always trust the socket markings, 90% correct, other times they use old stock. 6sc7 may not howl in the phase splitter but i have seen it do it, not as much as a V1 would.

    Picture of my tube inventory excel chart.
    image002 (2).jpg
     
    Last edited: Oct 13, 2018
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  14. Manodano

    Manodano Well-Known Member Supporting Member

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    That's a hell of a tube stash.

    I'll agree with your advice regarding the input/grid leak resistors.

    I am worried about microphonics from the 6SQ7. I don't think there's a glass equivalent.

    There's a glass envelope Sovtek 6SC7 in current production that sells for around for $20.00. I'm thinking there's a reasonable chance I might get a non-microphonic 6SC7-GT.

    The power supply caps are in a 3 section can. I'll probably replace it with a 20/20/20 450v cap can.

    The rest of the caps will get replaced with Orange Drops (Sprague Atom for the 25/25 bypass cap). I'm wondering if the 0.05 caps should be changed to 0.02?
     
  15. mozz

    mozz Well-Known Member Supporting Member

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    Since the first stage has a .02, i don't think you need to change the output caps to anything, .05 should be fine. Check them coupling caps for DC once you get it running, i have seen them crack, red ones, green ones, blue ones. I wouldn't swap to orange drops unless needed.

    Once you get it running, you can always tack a electrolytic across the output tubes cathode resistor, that will give it more balls.
     
  16. Manodano

    Manodano Well-Known Member Supporting Member

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    I had a couple of hours to work on the project today. First I worked on getting the grime off of everything. Cleaning the tweed is sort of a fools errand. I might end up making sure I have the grime off of it, and then

    I got interested in the speaker and the output transformer. The electromagnetic coil tested good - 1,000 ohms just like the schematic says, and the speaker measured a little under 4 ohms once I disconnected the output transformer. I'm putting the speaker away now.

    When I cleaned the funk off the tubes I noticed that one of the 6V6's was totally smoked. These are RCA blackplate tubes, marked and labeled identically, and one is clear and looks good. I'm pretty sure the other is NOT one of those blacked out glass versions, I think its simply burnt all to hell. So not I'm not sure what to think about the output transformer.

    I have the OPT totally disconnected from the circuit. On the three wires of the primary side of the OPT, I measured 1,750 ohms, 2000 ohms, and 250 ohms between the three combinations of wires. The secondary side only measured about 0.2 ohms - which is for all practical purposes, a dead short. What say ye about this, fellow amp mavens?

    I have a Tweed Deluxe output transformer laying around that I can use if I have to.

    I decided that I maybe better check the power transformer. I replaced the power cord with a grounded cord, clipped off the "death cap", and installed the 1000 ohm resistor that's replacing the coil in the speaker. While I was at it, I replaced the 68 year old power switch.

    I found a big, honkin' 25 watt 1000 ohm resistor in my inventory. Since I know this thang is going to radiate some heat, I suspended it thusly:

    IMG_2078.JPG IMG_2077 (1).JPG

    That was it for today. I'll see what kind of voltages the power transformer is putting out tomorrow.
     

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  17. mozz

    mozz Well-Known Member Supporting Member

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    Output trans may be fried. Primary center is B+
    From center to each leg, with tubes removed, should be the same, few hundred ohms each, maybe 1k each. They should be very close to each other. Secondary is gonna be real low, under 1 ohm.
     
  18. Manodano

    Manodano Well-Known Member Supporting Member

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    I fished a couple of push-pull 6V6 output transformers out of the boneyard and measured the resistances through the primaries and secondaries, and decided Mozz is probably right, the output transformer is too toasted to fool with. I rigged one of my surplus OPTs up to the amp for the trial run. Rigged my 1958 Voice of Music 15" speaker to that as well.

    I next measured the voltages coming out of the power transformer (no tubes inserted and no load). The 5v, 6.3v, and 335 volt sections tested fine. No sparks, smoke, or blown fuse.

    I inserted two "known good" 6v6's, a brand new 5Y3, and the stock 6SQ7 and 6SC7 since that's all I have available right now. I set up my variac, plugged my current limiter into that, then plugged the amp into the current limiter. Show time.

    Now let me tell you, at this juncture with an old amp, I am mighty cautious. I covered the can cap with a plastic bin just to be safe. Put on my glasses. Stood away from the bench. Then I eased the voltage up SLOWLY over several minutes. The amp started working at around 84 volts with the current limiter light bulb dim, indicating no catastrophic shorts. I worked it up to full voltage, with no problem. I removed the current limiter and again raised the voltage incrementally, and again, no problem.

    The amp is dead quiet: no noise, no crackles, no hum. The only circuit mod so far is the replacement of the field coil with a 1,000 ohm 25 watt resistor. I plugged in a Furrian and dangit, the amp was performing properly, given the rigged up test setup. Totally unbelievable. I can't wait to see how the new 6SQ7 and 6SC7 sound in the amp.

    I think my next course of action is to audition the various period correct output transformers I have stashed away and see if I can conjure up a happy accident. If not, I'll order up a new tweed deluxe style OPT. Same with the speaker - I have a stash of late 50's 12" alnico speakers that I want to audition. It may well be that the baskets of those old speakers are set up to mount an output transformer. If not, I'm musing over maybe just drilling some holes in the basket of whatever speaker I end up with and screwing the transformer to it. That'll solve the puzzle of how and where to mount the opt.

    I also came across a brand new choke in my boneyard. The big 1000 ohm resistor got extremely warm during my trial run. It was really throwing off some heat. I might swap it out for a proper choke. But mounting the dang thing is going to be a real problem.

    This is turning out to be fun project so far.



     
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  19. mozz

    mozz Well-Known Member Supporting Member

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    Great that it's working. Measure the voltage across the 1000/25w. Voltage squared divided by resistance equals your watts, you knew that. So if it's dissipating 10 watts, you need at least a 20w resistor, best to go 3 times to be safe. Very warm is ok, if it burns your finger, it's to hot. You know this too.

    Doubt you will find a choke with 1000 ohms, but i don't think it will matter too much, they aren't running the tubes too hot so a bit more voltage should be fine.
     
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  20. jimytheassassin

    jimytheassassin Well-Known Member

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    Nice project ! I’ve been enjoying reading the process here. The point about using a lightbulb limiter is a good touch. I’m making one now for my upcoming amp power ups. It’s handy that I have 90% of the materials around from household upgrades.


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