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DIY 5f1 Champ. Scavenged transformers.

Discussion in 'Amp Tech Center' started by Ronno25, Jan 29, 2021.

  1. Guitar-Rocker

    Guitar-Rocker TDPRI Member

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    The cathode bias will pull your B+ down to a useable range with the original PT. Also adjusting the bias value of the cathode bias will change the plate. The original, B+ with the dropping resistors shown had the plate at 310V and the screen at 240V. Readjust those dropping resistors.

    Original was cathode biased at 270R with a plate of 310v. The 5F1 is 470R with a plate of 340v, I think
     
    Last edited: Jan 31, 2021
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  2. D'tar

    D'tar Friend of Leo's

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    I have not checked spec sheets but I would think you have plenty of heater current available to add a tube for a vibrochamp.
     
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  3. Ronno25

    Ronno25 TDPRI Member

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    I've made some progress. After going through a few chassis options I settled on a thrifted one - dimensions 9x4x2. It's tiny. A little longer than a standard 5f1 chassis but .75" shorter. I had to be creative with where I drilled. The chassis was also missing a 2x1 square out of the back so I had to bolt on another piece of steel. Somehow it all fits... I've wired up the board, input jack, and volume. I plan to finish the rest today. 0b963f27-5fcd-4f58-907c-e8de56a2f3fd.jpg b80f8263-d0d8-4db7-a190-a9f4ab0712d2.jpg 60eb1f31-d802-458a-97d0-6eca211058ad.jpg 8f43e096-d935-49f5-a3a1-b7e7503ba9d6.jpg f48465aa-82b3-4f5c-bfc1-d5fcf0b6bc35.jpg
     
    Last edited: Feb 8, 2021
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  4. Ronno25

    Ronno25 TDPRI Member

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    Finished it up. I haven't fired it up yet though. It's not perfect looking but it feels good to have drilled the chassis myself and persevered through the struggle of getting everything to fit in the tiny chassis, especially with this massive power transformer. I'll take some voltages tomorrow and post them up here. Hopefully the amp runs first try. If anyone sees any wiring issues please let me know! Oh - I left an extra hole in the chassis to do either a switchable cathode bypass cap or a switchable NFB. Any tips on what's best? unnamed-2.jpg unnamed-3.jpg unnamed-4.jpg unnamed-5.jpg unnamed-6.jpg unnamed-1.jpg
     
  5. Ronno25

    Ronno25 TDPRI Member

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    Okay so I've taken some readings without 6v6 installed and using a lightbulb current limiter:

    Power transformer is listed as 340-0-340

    Filaments 2.84vac and 2.8 vac
    5v line 4.63vac
    B+1 410vdc
    B+2 390vdc
    B+3 345vdc
    V2p1 0vdc
    22k NFB resistor 1.5vdc
    V1p6 238vdc***********these seem high
    V1p1 244vdc***********these seem high
    V1p3 1.49vdc

    Why am i getting 0vdc at V2p1? There appears to be a discrepancy between the robrob layout and the mojotone. I went with the mojotone. It looks like my voltage may be grounding out at V2pin1? Do I need to clip the connect between V2 p8 and p1? I added an artificial center tap on the 6v6. Could this be causing problems?

    Also my voltages on V1 p1 and p6 seem high. Is this a problem?

    5F1_Layout_large.png
    a750b907-0203-4c9a-b16d-1144662abc31.jpg
     

    Attached Files:

  6. Guitar-Rocker

    Guitar-Rocker TDPRI Member

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    V2 pin 1 is not internally connected on a 6V6 (note not all octals are the same). Some even use that empty pin on a 6V6 as a connection point for things such as a connection point for the grid resistor to anchor that resistor and not have it hanging up in the air.

    Much like tacking a screen resistor across pins 4 & pin 6
     
    Last edited: Feb 9, 2021
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  7. Ronno25

    Ronno25 TDPRI Member

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    I removed the wire between V1 p1 and p8. I also noticed a short between V2p2 and p3, which I fixed.

    I took more voltage readings with ALL tubes in and NO limiter.

    Filaments 3.15 / 3.15
    5v secondary: p2 8vac p8 6vac
    B+1 402
    B+2 363
    B+3 317
    V2 p8 23.4
    NFB 1.48
    V1p6 208
    V1p1 212
    V1p3 1.52

    Questions:

    My filaments and 5v secondary seem off. What does this mean? Any fixes?

    Anything else seem out of whack?

    I recognize that my voltages are higher than typical, this must be due to the higher voltage secondary on the PT I'm using.
    ----------------

    I plugged a guitar in and let it rip. Sounds pretty good and is in the ballpark of what I'd expect for a tweed champ circuit. I watched the power tube for red plating and didn't see any, though I've never seen it in person and am not really sure what it looks like.
     
  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    When in use a light bulb current limiter will throw the voltages off. The amp without the 6V6 in will throw off the voltages downstream.

    The filament VAC measurement should be made across the pins. One probe on 6V6 pin2. The other probe on pin7. (12a_7 pins4/5 and pin9) When the 6.3v heater winding is measured to ground only half of the voltage registers.

    Check the 5VAC heater on the rectifier the same way. A probe to each yellow wire. Those voltages seemed off with your measurements. Try again.

    The plate dissipation is on the high side. Watch for red plating. Some amps run this hot without trouble. Look at the hot 6V6 in the dark or dim light.
     
    Last edited: Feb 9, 2021
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  9. Ronno25

    Ronno25 TDPRI Member

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    That makes sense. I rechecked the heater filaments and 5v secondary. Filaments were 6.4 VAC and Rectifier secondary was 5.22 VAC. So they check out. Thanks!

    I watched the 6v6 and compared the look of it to a silverface champ I have. It looks about the same, so I don't think it's red plating.

    How do I calculate the plate dissipation, and if it is high how do I lower it?

    I'm also wanting to know how to lower voltages in the amp. I created another thread for that question: https://www.tdpri.com/threads/lowering-5f1-voltages.1062662/
     
    Last edited: Feb 10, 2021
  10. timfred

    timfred Tele-Meister

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    Your plate dissipation is
    23.4V/470Ohm= 49.8ma

    Plate sees 402-23.4=378.6V

    378.6V x 0.0498A = 18.8 W

    But you can assume that the screen dissipates a few ma so really it’s probably somewhere between 17 and 18 W. Target is 14W but there are lots and lots of Champs running higher PD like you have.

    You could try a bigger cathode resistor but if it’s not redplating I’d just run it.
     
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  11. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Agree with @timfred

    The larger cathode resistor should drop the plate dissipation (cool the bias). Changing the cathode resistor too far can make the amp sound wrong ime. I think you would be chasing your tail to try to get this amp down to 100%PD using just the cathode resistor.

    Lowering the B+ would be a better option imo. That said, if you are happy with the way it sounds now, there is no need to change anything. The good sounding 5F1 circuits usually run way high PD. As timfred noted the bias numbers include the screen current. The last 6V6 I measured had 18% screen current. Subtracting 18% would bring the numbers on your 6V6 to just over 100%PD.

    If you have a 500R cathode resistor handy, you might install it. I wouldn't go larger than that. OMMV.

    A jj 6V6 may handle as much as 19W. This would be another option to put your mind at ease.
     
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