Fender Super Tweed Build

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

  1. Oatie

    Oatie Member

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    Cool on your Super build, I have an original 1959 and I want to remind you the original Fender tweed Twins, Bassman, Pro, Super and Bandmaster all had 1 Meg LIN Vol Pots from the factory. All the PDF schematics show Audio Log 1 Meg Vol pots but they are all wrong. From 1956 to my 1960 tweed Twin, all the above amps (5881) have the LIN Vol pots. All the other tweeds have Audio Log pots, if you want to try some in your new build.

    [​IMG]


    The 1Meg LIN Vol pots above are from my April 1960 Tweed Twin 5F8/A . the taper is different on the LIN pots, but Fender used them in all the (5881) tube amps. The volume adjustment is good from Vol setting 1 all the way up. The sweep is smooth. Anyone with an original ampcheck your pots, Stackpole on the 1956 to about late 1958, then CTS pots after that for mist of the amps.
     
    Last edited: Dec 8, 2017
  2. Leña_Costoso

    Leña_Costoso Senior Member

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    Good information. The Fender drawings dont show either way.

    Other than the rotational position of the dial, the range is the same, so unless you're doing pinkie swells with the amp's knobs... does it matter?
     
  3. Leña_Costoso

    Leña_Costoso Senior Member

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    Hey I just noticed, but your Sperry SP-10A needs to have its movement's zero adjust set. The needle's not on "0", which throws all the readings off.[​IMG]
     
  4. Cjsinla

    Cjsinla Senior Member

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    I will check it out.
     
  5. D'tar

    D'tar Senior Member

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    :)
     
    Last edited: Dec 8, 2017
  6. Leña_Costoso

    Leña_Costoso Senior Member

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    No offense intended, D'tar. There are two adjustments. One is ONLY for ohms, and is as you describe. The other is for the ENTIRE MOVEMENT AND ALL RANGES. The mechanical setting of zero is necessary regardless of ohm zero. Ohm zero is only to adjust for the battery condition required ONLY for ohms readings.
     
  7. D'tar

    D'tar Senior Member

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    No offense taken. If I measure ohms I will zero ohms. If I measure voltage I will zero to the other end of the scale. Simply meant to bring attention to the adjustment of the meter due to the discrepancy of measurements. I shall attempt to be more clear in the future.

    So do you have a final voltage chart? How is this thing biased? Are you happy with the change. And will you be looking to upgrade to a MM O/T as well?


    @Leña_Costoso Thanks for the construction paper tip. I now have a sheet in my tool box :thumb:
     
  8. Leña_Costoso

    Leña_Costoso Senior Member

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    Thanks....I'm just a grumpy ol curmudgeon.
     
  9. Leña_Costoso

    Leña_Costoso Senior Member

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    I'm going to be replacing the bias pot in my amp next week. Finally... they shipped the audio taper one. And its a CTS not Alpha!

    So, what I'll do is get out my Fluke 83 and see what it reads for B+ at various points, filaments, and for the bias voltage at 40-42milliamps at idle.

    Went nuts today with a strange warbling buzz in my Super. An hour later I brought the amp into a larger room. No warble. WTFF? Into my office... no warble. Den... no warble. Back in the bedroom. Warble. The !@#$#@$ CEILING FAN!!!!!
     
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  10. brewsnale

    brewsnale Junior Member

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    Apologies in advance for any hijaking. . .
    Probably a stupid question but how would i begin a project like this witbout any electrical experience? ive been thinking abkut this for quit a while but never knew quite where to start.
     
  11. D'tar

    D'tar Senior Member

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  12. Cjsinla

    Cjsinla Senior Member

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    I didn’t have any experience either. I read up on some theory, how tubes and circuits worked. I also watched the amp building videos on the Tube Depot website. Then I started small, a Tweed Champ is an easy one. It helps if you have a good soldering iron and a multi-tester as well. I got one of those soldering setups that heat up very fast and have adjustable temperature control. If you get stuck the guys here are very helpful.

    Oh, yeah, I also got a free-standing magnifier with a light so I could see what I was doing clearly. My eyes are a little weak and it was much easier wiring the tube sockets with magnification.
     
    Last edited: Dec 11, 2017
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  13. Cjsinla

    Cjsinla Senior Member

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    I had that happen with my AC30, it was a fan on a table that caused my problem. It’s especially noticeable with the high notes.
     
  14. Splattle101

    Splattle101 V.I.P. Member

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    I'll see your ceiling fan, and raise you a Thermomix. :mad:
     
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  15. Oatie

    Oatie Member

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    Here is my 1959 Fender Pro Tweed with a new 1x12 baffle for the Gold ALniCo sound.

    I remove the original baffle w/ Jensen P15N and install either the 2x10 baffle or the 1x12 baffle.
    The tweed Pro is it has the exact same circuit as the 5F4A Super.
    The Pro has the 8 ohm OT, so I use two 16 ohm Celestion Gold 10" speakers to turn the Pro into a Super.

    In this pic I have a 3mica 5751 blackplate gold label Sylvania tube in V1, it raises the gain from 12AY7 ( 44 ) to 5751 ( 70 ) on that tube. Going to a 7025 RCA raises the gain to ( 100 ) and makes super gain.

    I need to install new filter caps in this amp to be safe and a new IN4007 diode from the PT.

    TO sum it up, if you want a 5F4 Super but cannot afford the price, buy a Pro and make a new 2x10 baffle for less than 1/2 the cost. The old stock Tung Sol 5881 tubes will last years. See how clean the repo 1x12 baffle looks painted black?
    Looks stock, vs. An unpainted natural Baltic baffle. My Super-Pro has the 3 baffles, 1x12. - 1x15 - 2x10 .

    You are really going to enjoy your build and speaker & tube rolling.

    [​IMG]
     
  16. Leña_Costoso

    Leña_Costoso Senior Member

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    The guy gets a lot of flack, but Dan Torres book Inside Tube Amps has a lot of simple basic easy math kind of stuff in it. And there's nothing glaringly inaccurate in it.
     
    Last edited: Dec 13, 2017
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  17. Cjsinla

    Cjsinla Senior Member

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    The Robinette link was filled with good information. I tried one of the 5e3 mods today, soldering a 470k resistor to the #7 pin of the phase inverter to cut down on second order distortion. Definitely added more clean overhead and smoothed out some of the flubiness. Do you know if that mod can be done to my Super?
     
  18. Leña_Costoso

    Leña_Costoso Senior Member

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    1. Get the amp's bias set on the coolish side, perhaps about 30ma at idle, per tube. That will make sure the amps output is running rather cleanly.

    2. Nix t3he 25uf cap on the 2nd tube. Best way is to probably pull the resistor and cap combination, and replace it with a fresh resistor. That will have the effect of decreasing the bass response and gain on that tube. You may find that you'll need "a little" capacitance there, as the response of no cap may be a bit too sterile. Try something in the order of 1uf at first, the maybe 4.7uf if you still need more.

    3. There is a 4.7meg resistor that acts as inter-stage "feedback" to reduce distortion. You can decrease the value there to something smaller, like 3.3meg or 2.4meg. The lower the value, the more negative feedback will be introduced, keeping things a little cleaner.

    4. Speakers, speakers, speakers

    5. Move the center tap on the OT to the higher voltage side of the choke if not already done.

    As always, ymmv
     
    Last edited: Dec 22, 2017
  19. Leña_Costoso

    Leña_Costoso Senior Member

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    I said I was going to get that audio taper pot into the bias circuit, and when I did the work today, replacing the "Alpha" (Alfalfa?) cheapo, I decided to take some pictures.

    [​IMG]
    Location of the pot, on the back side of the amp. Doofus me, I failed to Dremel a slot into the shaft. Good God, what a mo'fo it was to turn. But I got it. Next time I put tubes in, the chassis is coming back out so I can slot that thang!

    [​IMG]
    On the inside, there's the Bourns pot. Its a 250k AUDIO pot. You can see there is a little resistor there which is 8.2k ohms. The variability is from 255k to 8.2k, as the 250k pot isn't quite 250k ohms. That is in PARALLEL with the 56k pot that is standard "bias" set resistor. Someplace back on this thread, I did the math for everyone. The idle current ranges from a cool 26ma to a very hot 80ma. I've got it set to 39.8ma, which is as close as I could get without going into voodoo incantations for lack of a slot on the Bourns pot.

    I'm happy with that, as without the parallel (and variable) resistance, I was in the mid 20's for idle milliamps. A nice meaty 40ma or so, was totally noticable, even at bedroom volume, and I'm guessing because the amp was just so cold to start with. There appears to be plenty of leeway for all sorts of tubes.

    And, I think I said I'd measure the B+, just because. My wall voltage is 121.6 tonite. With all the tubes in place, bias set to idle at close to 40ma per 6L6, all controls set to zero, I had 397vDC on the plates of the 6L6's, and 390vDC, dead even on the screen grids. No complaints on the voltage, and that's with the MojoTone power transformer. My little diatribe on why Fender schematic voltages are to be taken with an educated grain of salt is someplace in this thread too.

    So... a parting shot!
    [​IMG]

    .....and its little cousin, the 1x12 Champ
    [​IMG]
     
  20. D'tar

    D'tar Senior Member

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    Both use the same type phase inverter so you may expect similar results, I opted for the pre pimv,
     

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