Fender Super Tweed Build

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

  1. Soul Tramp

    Soul Tramp Speaker Snob V.I.P. Member MLP Vendor

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    The link below is to a thread I did comparing Mercury Magnetics transformers to Magnetic Components. I concur with you findings.

    http://www.mylespaul.com/threads/transformer-talk.355415/#post-7419305
     
  2. Soul Tramp

    Soul Tramp Speaker Snob V.I.P. Member MLP Vendor

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    The orange wire is the heater center tap. Whether or not it's used depends on the design of your heater wiring schema. I haven't looked through all your posts so I don't know how you're wiring the heaters. I'm off to bed. If you haven't an answer by morning I'll look through the thread to see how you're handling the heaters.
     
  3. Cjsinla

    Cjsinla Premium Member

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    Thanks. Both greens go to the pilot lamp and then on to all heaters and the wires are not grounded with two 100k resistors like you see on some designs. So, that might mean that the orange CT goes to ground?
     
    Last edited: Nov 28, 2017
  4. Leña_Costoso

    Leña_Costoso Senior Member

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    The orange wire is the center tap for the filament supply. Ground it and you can ditch the 100 ohm "false ground" resistors at the pilot light where the green wires are. The other option is to just not use it, but I'd rather have the CT/orange than the 100 ohms resistors.

    The red/white is the bias. Goes to the board as you say. Keep in mind, the bias tap voltage is likely to be different, so your amp is going to run a little different until you adjust the bias supply to result in about 40ma current per output tube.

    Using a beefier PT will result in two things:
    1. Transients will be much tighter. MUCH tighter. Transients being the lowest thumps your guitar and playing provide. The Super being no slouch on bass tonality anyway... you'll have tight tight bass.
    2. Sag in the power supply will be reduced. Related to #1 above, but the overall voltage drop seen in A/B amps especially those with tube rectifiers will be reduced. The amp will lose some of its cry or wailing tonality when cranked.

    None of that is bad, just different. I've built previous Super's with Hammond iron, and those are noted to be overbuilt and then some. Can't say the tone was bad. Play an original Super and you'll get some 5E3 Deluxe wailing, if you crank it past 5 or 6 on the dial (with decent amounts of bass & treble applied).

    The 345v on the B+ winding seems a bit high for the "correct" Super voltage. For the 5F4 Super, something around 310-0-310 would have been just about dead nuts on. Again, higher voltage will give you a little more power, a little tighter low end. Bigger iron, better low end thump and less sag overall.

    Hope that helps!
     
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  5. Leña_Costoso

    Leña_Costoso Senior Member

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    That should be 100 ohm, not 100k ohms. And by all means ground the orange wire then. Providing the center tap ground will greatly reduce hum. The other way to reduce hum is to use a hum balance pot, like Fender did in silver face amps. That pot actually works pretty well, until it fails! It provides a "variable" false center tap, which can offset minor variation in transformer winding, and preamp tube filaments. But using the orange wire is good enough for most purposes.
     
  6. Splattle101

    Splattle101 V.I.P. Member

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    Great answer!

    It describes my experience mucking around with PTs in my 5E3. I went from the original Chinese PT, to a Magnetic Components PT, to a Mercury. It turned out the original Chinese PT was the same size as the MM, but not as nicely made or as accurate. Apart from the better sound of the MM transformer (already described by Lena Costoso), I was really impressed with how accurate the secondary voltages were. They were pretty much exactly what what the paperwork said they'd be, within a few %. None of the other PTs I've used (including Mojotone and Magnetic Components) hit the numbers so well. They're usually high, sometimes by 20%. Not the Mercury.

    I likes them. And given that the PT is the living, beating heart of the amp, I will never skimp on a PT again.
     
  7. Cjsinla

    Cjsinla Premium Member

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    @Leña_Costoso @Splattle101
    If i read this right, I hook the red/white bias wire to the board. Does that mean I must add a pot to adjust the bias?
     
  8. Cjsinla

    Cjsinla Premium Member

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    I have Tube Depot trannies in my 5e3. I had to use a 12ay7 tube to offset some of the flubbiness and increase headroom. Any idea where those Tube Depot trannies come from?
     
  9. Leña_Costoso

    Leña_Costoso Senior Member

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    The red/white wire replaces the bias wire on the Weber transformer.
    Here's the item to ponder:
    The bias tap voltages on those two transformers are more than likely different. Because of that, the bias voltage, and this the idle current on the 6L6's will also be different.

    All tube amps should have the bias set for the tubes they are using. Perhaps in Leo's time, tubes from one manufacturer were such that they had great consistency and a simple "one set voltage" would do.

    Eventually, Leo used an adjustable bias circuit. However, your bias circuit IS adjustable, even without a pot. You just need to change the 56k resistor to a value that results in the idle current being where you want it - typically between 35 and 45 milliamps at idle.

    Use a SMALLER value resistor to increase the idle current.
    Use a LARGER value resistor to decrease the idle current.

    How it works -
    The bias "tap" is an AC voltage. The diode in the bias circuit is "reversed" so that the rectified AC (which is now ripply DC), is negative to zero volts, or ground. The capacitor takes the ripple out. The 6.8k resistor helps with the filtering and provides protection for the bias circuit. The 56k resistor sets the actual resultant voltage at some point. When that 56k resistor is replaced with a smaller value, the voltage is closer to the zero volt point. When that 56k resistor is replaced with a larger one, the resultant voltage is further negative from the zero volt point.

    As we know, the control grid on a tube is the grid that stop the electron flow from cathode to plate (anode some folks call it). In order to decrease the flow of electrons, the control grid must be made a bit negative in respect to the cathode. That's why there is a negative bias voltage. Bias, just means "offset" by the way. If the bias voltage was hugely negative, the tube would completely shut off. No flow of electrons. If the bias voltage was at zero volts, the tube would be unrestricted in its electron flow and burn up. We set the "bias, or offset" so that the tube operates at a point where the wiggly signal on the control grid (from the phase inverter, in this example) is enough to allow electron flow in proportion to the signal. There's more to it than that... and we can begin to discuss amplifier class and things, but just know that 35 to 45 milliamps works ok. Too low, and you risk, or get, "crossover distortion". Too high and the tubes are working a lot harder - but - you might get interesting grind, at the expense of tube life.

    So.... its time for you to learn to measure, and set the bias point on output tubes. Scary? Only until you do it once or twice. Then its just another tube amp chore.
     
  10. Cjsinla

    Cjsinla Premium Member

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    Or, could I just take out the 56k resistor and put a pot there? This is the part of the circuit we are talking about, right? The red/white bias wire takes the place of the red/blue wire and connects to the 6.8k resistor. BTW, don’t I have to add some resistors from the terminals to ground on the power tubes to measure the idle current? @Leña_Costoso

    D554DC48-30E4-4862-8AB4-1086AA4BABD2.png
     
  11. D'tar

    D'tar Senior Member

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    A half size/value resistor and a 50kL trim pot in series should cover your needs.

    upload_2017-11-29_9-22-59.png
     
    Last edited: Nov 29, 2017
  12. Leña_Costoso

    Leña_Costoso Senior Member

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    Here's something that works pretty good on a Tweed super. Leave the 56k resistor in place. Drill a hole between the power transformer on the choke. Stick in a 250k audio taper pot. From the connection where the 56k resistor meets the 1N4007 diode wire a 10K or even 8.2k resistor to the wiper of the pot. Ground one leg of the pot. You'll have a range of 8.2 k true about 46k. Very very easy setup. You want audio taper so it doesn't Bunch up the resistance in a very small rotation. If you find that it does just unground the one leg of the pot and ground the opposite and instead. Done.

    If something happens to the pot, you always have the 56 K resistor as a fallback so the tubes don't go run away.

    Ez Pz!
     
  13. Cjsinla

    Cjsinla Premium Member

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    I just re-read your PM about that pot mod, did you do it yet?
     
  14. Leña_Costoso

    Leña_Costoso Senior Member

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    Yes. and no. Didn't get audio taper. Grrrrr!$#!#$@#$.

    Did get linear taper, and it "works" just fine, just a little more sensitive to rotation. But, its set and works just fine, hasn't shifted!

    The other thing you'll need to do, is get a way to measure cathode current.

    There are two easy ways. One is to build a bias probe type device. You can do this with NO RESISTOR inside it, and just measure milliamps directly. Parts easily had from www.tubesandmore.com
    PM me for details. Needs soldering, needs some epoxy to pot it all together, but they work great.

    The other way is to insert 1ohm resistors between cathode and ground. You can do this by undoing the existing ground at the cathode, and soldering some 1 ohm resistors to ring lugs. Put the ring lugs under the screws that hold the socket in place (or nuts as the case may be). Connect other end of the resistors to the cathodes. You measure "on the wire lead" of the resistor, to ground. Less chance to short a meter lead to something hot that way.

    Many folks online say you have to use 1 percent resistors. You can, but 5 percent will work good enough if that's what you have. Look at this way, you get 40ma current. The five percent resistor means you're off by 2ma. Well... its likely that your tubes are unmatched to at least that much. Does it matter if you're at 38 or 44 milliamps? In real terms no, unless you have golden ears and tend to crank it to the max. Yes, its better to get one percent, but the common five percent will do. What IS important, is to get two resistors that read the same. That is, if you're five percent resistors are .95 ohms and 1.05 ohms, don't use 'em. See if you can find two that read the same. Sorry to go against all the internet info and groupthink, but in practical terms... it makes not a hill o' beans difference.

    The meter you have is plenty accurate enough for these tests.
     
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  15. Leña_Costoso

    Leña_Costoso Senior Member

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    [​IMG]

    You can see here in the graph, how on the 50 percent rotation you're at 10 percent (or 90 percent) resistance.

    At zero rotatation, you're at 46,000 ohms.
    At 50% rotation, you're at 20,700 ohms
    At full rotation, you're at 8000 ohms

    That's smooth enough to be easily managed, and works ok. (example shown with 56k fixed resistor, 250k pot plus 8k in series with pot). You'll find a good spot in that range.

    With 56k resistor only, my tubes idled at 22ma.... exceedingly cold to the point of crossover distortion. At 33ma they got very smooth at high volume. At 40ma, same smoothness, just a little more compression.

    There is no "right" or "wrong". Its more about a tradeoff of compression on overdrive, vs tube life.

    I had KT66's in a Blackface Bassman that I reworked heavily. It was in a 2x10 combo, Celestion Vintage 10's as speakers. I ran those KT66's at 50ma at idle. They glowed at drive, sounded GREAT! Never had a problem with that amp. Wish I still had it.
     
  16. D'tar

    D'tar Senior Member

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  17. Leña_Costoso

    Leña_Costoso Senior Member

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    Please allow me to add...

    Good grief Charlie Brown, I've got an interesting amp on my bench. It develops close to 2000vDC on the plates (about 1950vDC) and puts out 600 watts on three output tubes. Had a tube short internally, plate to ... grid, or filament, or both. Took out a fuse, a fuse holder, grid resistors, and a diode. At least that's what I know of. Maybe took out more diodes in the power supply, dunno yet. Maybe caps too. The caps have screw connections to their board. Undo two Phillips head screws, and the cap is out of there. ITs got four of em.

    Its a linear amplifier for ham radio use. When we talk guitar amps, we've got more or less a fixed impedance in and out. For these amps, its variable impedance (and inductance) in, and variable out too. You have to "tune" two coils for grid current and plate current in order to keep it from tearing itself up during use.

    Parts should be in... in a few days. Uses 811A tubes, with anode (plate) connection on the top of the envelopes, connected with "ceramic hats".
     
  18. Cjsinla

    Cjsinla Premium Member

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    @Leña_Costoso @Splattle101
    I put the new transformer in but I think I wired it backwards to the AC. The blade with the W currently goes to the brown wire that connects at the fuse holder, and to the black 0v wire of the transformer. The blade with the Z goes straight to the white 125v wire of the transformer. I believe that’s backwards and I’m not getting any sound out of the amplifier and i was getting some funny noises so I turned it off. Any suggestions? If you look at the bottom left on the second picture you can see the three wires that come out of my power cable. The green and yellow one is the ground wire the brown and the blue ones are the hot wires.

    F891F2E6-D746-4490-B8D3-B35927657BE6.jpeg
    5C4B9675-C912-4A34-9C43-4241F9913E38.jpeg
     
  19. Leña_Costoso

    Leña_Costoso Senior Member

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    Normally, in the USA, on the power cord, the wider blade is neutral, and should be connected to the white wire in the cord. The smaller blade is "hot" or "line", and should have the black wire in the cord.

    International cords, use blue as neutral, and brown as hot.

    Normally, the "hot" or "line" goes to the fuse, then to the on/off switch. Neutral goes to the white wire on the transformer. So it looks like you have it correct.

    However... even reversed, other than hum issues, you should have had some sound. Reversal could have caused nasty hum.
     
  20. Leña_Costoso

    Leña_Costoso Senior Member

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    Where's the center tap for the B+ ? Usually its red and red as hot wires, and red with some color like yellow or blue as center tap.
     

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