Fender Super Tweed Build

Discussion in 'The Squawk Box' started by Cjsinla, Sep 23, 2017.

  1. D'tar

    D'tar Senior Member

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    Z on your plug is the hot, should go to the fuse. Swap them around. I've seen a few of these power cords that do not conform to any standard as far as color codes.
     
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  2. Leña_Costoso

    Leña_Costoso Senior Member

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    Since the amp once worked, take it a step at a time.

    Do we have 5vAC on the yellow wires to each other?
    If yes, we have rectifier voltage available.
    Are yellow wires on correct pins?
    Do we have HIGH VOLTAGE AC on the red wires?
    If yes, you should have B+ going on.
    Are the red wires on the correct pins?
    Is the correct TUBE in the rectifier socket?

    Do we have preamp filaments lighting up?
    If yes, heater circuit must be ok, pilot light should light.

    Then its a matter of bias. Do we have minus DC voltage on the ungrounded side of the 56k "bias set" resistor?
    If yes, then the bias is working.

    If they're all yes, check other things, like speakers, guitar input, that sort of thing.
     
  3. Leña_Costoso

    Leña_Costoso Senior Member

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    My bad, yes, the wires are reversed on the cord, but... it still should work, with hum. I went by convention of the colors, but Cjsinla mentioned colors going to the blades on the plug. If what he said is correct, then its backwards.
     
  4. Cjsinla

    Cjsinla Premium Member

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    Right, they are reversed, z is not going to the fuse.
     
  5. Cjsinla

    Cjsinla Premium Member

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    No. W goes to the brown, Z goes to blue. So I did have W (brown) going to the fuse.

    Center tap for B+ is red/yellow. That is going to ground with the orange wire.
     

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  6. Cjsinla

    Cjsinla Premium Member

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    Ok. It’s alive. The switching seems to have worked but in the end, the brown wire is still hooked to the black and the blue wire is still hooked to the white. Not sure why it wasn’t working in the first place. Here’s a picture for comparison you can see that the white and blue wires are now hooked to the fuse and the brown and black wire are hooked together It just has the hot hooked to the fuse first in this new scheme.

    The amp definitely has more clean overhead with this power transformer. It was starting to break up at 2 on the bright channel with the Weber tranny and now it’s breaking up at 4 on the bright channel with the Mercury tranny. Also, the faint squeeling noise that used to happen when I turned the presence down to zero has disappeared with the new power transformer.

    Special thanks to @Leña_Costoso and @D’tar for all your help.
     

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    Last edited: Nov 30, 2017
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  7. Leña_Costoso

    Leña_Costoso Senior Member

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    Could be those higher plate voltages....
     
  8. Cjsinla

    Cjsinla Premium Member

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    Where do I check the plate voltage?
     
  9. D'tar

    D'tar Senior Member

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    [​IMG]

    Black probe to chassis red probe to pin 3 on 6l6. left hand in pocket.
     
  10. Cjsinla

    Cjsinla Premium Member

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    Right. What kind of voltage should I expect for my Super?
     
  11. D'tar

    D'tar Senior Member

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    If I were to guess around 410-420vdc


    I recommend a complete voltage chart for future reference. It would be nice to compare voltages from the old PT to the new PT. All tubes all pins, voltages at each node and wall voltage. If anything goes wrong you will have an idea which part of the circuit is at fault by comparing voltages to your records.
     
  12. Leña_Costoso

    Leña_Costoso Senior Member

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    By calculation, and using the MM chart in this thread, I think we're gonna see 440ish vDC, maybe more, on those plates, assuming 120vAC input to the PT

    D'tar is correct in the 410v rating, which is what is shown on the schemolayo.

    Those voltages are usually a little low as measured today. The SOP for determining those voltages was to set an intern at the drawing and a working chassis with a VTVM and measure all the voltages. It was totally dependent on the voltage available at Fender's plant on that particular day (and quite possibly what machinery they were running at the time). The voltages on the drawings are just sort of for reference in repair.

    The Weber transformer was lower in voltage, so a 40vDC boost will make a difference, and... bringing this up again... we have no idea what the bias of the output is.
     
  13. Leña_Costoso

    Leña_Costoso Senior Member

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    I forgot to add, just read from the standby switch to ground. That's close enough for government work.
     
  14. Splattle101

    Splattle101 V.I.P. Member

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    I used a Mercury in my tweed Super build (thread here). It's the model that's designed to deliver vintage voltages on the secondaries with modern wall voltages hitting the primary.

    For the bias supply I copied the circuit from the blackface Super Reverb so the amp would have an adjustable bias supply and I'd avoid the aggravation of guesswork or calculations re component values. I also installed a couple of matched 1 Ohm resistors on the cathodes. It all worked beautifully. :thumb:

    When I biased it up for the first time, I set the bias pot to -40 V just because that's what's on the 1950s schematic. I know that's not how one is supposed to bias an amp, but it seemed a reasonable value to start from. Gotta start somewhere, right?

    I then measured the anode voltages. They were exactly 410 VDC. Like absolutely bang on! I was gobsmacked. :shock: Long story short, calculations of current draw showed the 5881s to be right in the middle of their happy place, so I left it as it was.

    This is another example of the accuracy of the Mercury transformers that I mentioned earlier. I am absolutely sold on these things.
     
  15. Cjsinla

    Cjsinla Premium Member

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    To measure what?
     
  16. Cjsinla

    Cjsinla Premium Member

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    I told Patrick at Mercury that his product performed better than my other one and he said that they need time to break in? It will sound even better after 20 or 30 hours of playing? Does that sound right?
     
  17. Splattle101

    Splattle101 V.I.P. Member

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    I wasn't aware of that, no. I didn't know inductors had a 'break in' period. It's not an area of audio or electronic lore that I like to get into too much. People get quite irrationally attached to their views about breaking in speakers or tubes or caps. It's nearly as bad as arguments about standby switches!
     
  18. Leña_Costoso

    Leña_Costoso Senior Member

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    The main B+ voltage.

    FWIW, wall voltages have not, yet have changed. The median voltage has been, still is 117vAC, plus or minus ten percent. In 1952, you might have had 122vAC or 124vAC or... 115vAC or 110vAC at the wall plug. We find the same thing today, only with a little more consistency toward the plus end of things. Wall voltage at me place is 119vAC on average, but I've measured it at 121vAC (according to my Fluke meter at least). A get at least as anal about ham radio amplifiers as I do with guitar amps.... and ham radio amps run at very high voltages. Expecting on in soon that runs 2800vDC out of its power supply. You look at the input of 120v, and if it varies much, you're 2800vDC is all over the place (and your amp won't perform).

    Why am I saying this.... because there is no such thing as vintage voltage. Its just what they happened to see, with the VTVM they used that day, in the way the kid measured it, with the voltage available on some bench at Fender at the time they set the intern to his task.

    About 1996 or so, I was hanging in a gun range in Florida, and got talking to a guy who noticed my "Fender" t-shirt. Older guy (than me at least), decent shooter, likable guy. One thing in conversation became another, and in only a few sentences he told me he worked there while in school. I was almost awestruck. And he was one of the interns who did those measurements. He would take the layout, measure the voltages. Recheck the voltages. Bring the layout into the drafting department (aka, the drafting table) and accurately find the spots on the schematic that matched the layout. Ever wonder why in some of the schematics the voltages are clearly wrong? I know they are in one of the blackface Bassman amps... amazing growing voltage in the amp as it should be shrinking. Why dat? Because somebody got done with whatever in the shop, and turned off the power hog machinery, and wall voltage climbed in the middle of testing!

    Look at the old pictures of the assembly lines for amps. Ladies at wooden benches. Fender was a bit of a ramshackle "get 'er done" place in those days in their original plant in Fullertone. There are videos of guys building guitars in a "factory" that looks more like a 2nd rate cabinet shop by todays standards.

    But.... fast forward to the so-called vintage craze, and you have folks who set their gunsights on anything "vintage" and companies all too eager to satisfy consumer demand.

    What I can't understand is this: If you want a vintage amp clone, and get an actual "clone" transformer, you get the wrong voltage, since its likely your seeing upward of 117vAC at your plug. So do you not have a clone if you get an altered transformer, that gives you what you think you ought to get according to a drawing made under questionable circumstances, sixty years or so ago?

    Or.... do you just go get a nice Variac and run the clone amp at 112vAC or so? Don't worry, the heaters will take it. RCA and GE designed heaters to run ... (guess what)... plus or minus ten percent. Funny how that all works out, huh?

    Just the ramblings of an old guy. Take'm for what they're worth.
     
  19. Leña_Costoso

    Leña_Costoso Senior Member

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    Since we've been on the subject of high voltage....
    [​IMG]

    Those are 811a tubes, which run (in this amp) about 1950vDC on the plates.

    [​IMG]

    Considerably larger than your average guitar amp tube.

    The Ameritron AL-811 runs three tubes, grounded grid configuration, and they put out (claimed) 600 watts, RF. More like 500ish in the real world. The amp got recapped (easy, just unscrew!) and new tubes, as well as the infamous "meter protection diode" which blows whenever you arc inside a tube. This amp pulled enough current to twist the metal inside one of the fuse holders on the 120v input. They run two fuses there, one on hot and one on neutral, in case you want to rewire it to 240v operation. Same 1950vDC on the plates, just a little more stable (usually).

    Anyway, that was today's amp project. The next amp inbound does about 1200watts RF and uses a ceramic tube, which is no glass envelope and imbedded cooling radiator. They run 2700vDC more or less by 50 volts or so.
     
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  20. Splattle101

    Splattle101 V.I.P. Member

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    Does the room smell like ozone when you turn that thing on? :D

    Regarding the voltages, my experience here in Oz is that the wall voltage can vary by more than 10% over the course of an evening in summer. It's just really crappy regulation combining with large fluctuations in load, as houses go from running nothing more than the fridge to full bore summertime evening loads as folks come home, hit the aircon, the TV, the computers, and start cooking dinner, etc. :rolleyes: It's old, old generation and distribution infrastructure that was great when it was built in the 1940s, but it's not had the capital investment to modernise. Story of our times, huh?

    Regarding power transformer secondary voltages, I have been unhappy with them on all but two brands of PT: Hammond and Mercury. Every other brand of PT has delivered secondary voltages that are just way too high. Chinese, Mojotone, Weber, Magnetic Components: all of them. I've seen 6.3 V heaters with 7.5 V on them, for example. A Weber PT that put ~490-500 VDC on the anodes of a Super Reverb running a GZ34!

    It seems to be a thing across the board (with the two honourable exceptions I've noted). I speculate that they're skimping on the primary winding to save some copper.
     

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