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Tweed Deluxe: Managing the Heat

Discussion in 'Amp Tech Center' started by gabasa, Feb 28, 2021.

  1. gabasa

    gabasa Tele-Meister

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    Hey all, I just built a 5D3 type amp with one channel, in a Harvard chassis, you might have seen my recent thread on it. It turned out great and I'm really happy with how it sounds and responds to my playing, but its getting a bit hot after an hour or two of playing.

    Actually, the top of the chassis and the PT get quite hot. The details:
    • The tubes are 6AT6, 12AX7, 2x6V6 (current Gold Lion), NOS 5Y3
    • The PT is from Victoria, the same one they use for their 5E3 builds
    • I subbed the first dropping resistor for a filter choke
    • The second dropping resistor is 22k in this amp
    • 470 Ohm screen resistors
    • The 6V6 cathode resistor is 270 Ohms
    The first thing I'm thinking is that maybe I shouldn't have used a choke, as the inductive reactance is way lower than the 10k resistor that's normally in its place.

    Would you swap it out for the 10k resistor called for in the 5D3 schematic as a first step to managing this? Or, if the voltages are good in the preamp, up the value of the cathode resistor?
     
    Last edited: Feb 28, 2021
  2. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    Voltage chart would help here along with bias numbers.
     
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  3. D'tar

    D'tar Friend of Leo's

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    2X6v6 cathode biased 100% will be a marshmallow cooker by design.
     
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  4. Dacious

    Dacious Poster Extraordinaire

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    That's how they is. Around 70% of the power entering from the wall cord is dissipated by radiation. My Superchamp rated 17 watts is rated 90 watts consumption on the back panel.
     
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  5. gabasa

    gabasa Tele-Meister

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    I get the marshmallow cooker thing, but it still runs quite a bit hotter than my 5E3.
    The 6V6 screen voltages are a little bit higher than the plate voltages.
    Here are some quick measurements:

    Heaters: 6.6V

    270 Ohm Cathode Resistor: 23.6V

    5Y3 secondary: 375V

    Both 6V6 tubes measure out the same:
    • Pin 3: 368V on plates
    • Pin 4: 371V on screens
    • Pin 8: 23.7V on cathodes
    V2 (12AX7):
    • Pins 1 & 6: ~198V on both plates
    • Pins 3 & 8: ~1.5V both cathodes
    V1 (6AT6):
    • Pin 2: 1.8V on cathode
    • Pin 7: 178V on plate
    If you can pick at this, I'd really appreciate it. Thanks for the help in advance.
     
    Last edited: Feb 28, 2021
  6. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    That’s running at about 110% - I’d cool that down a bit. Do you have a 300 ohm resistor for the cathodes of the 6v6’s?
     
  7. gabasa

    gabasa Tele-Meister

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    No, but I can get one in the morning. Do you think that 300 is high enough? Would 330 be overkill?
     
  8. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    Try 300 first, I like to run my cathode biased amps between 95 and 100%, but not over. While running over can sound great, you will cook tubes and components.
     
  9. gabasa

    gabasa Tele-Meister

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    @dankilling, thanks for the suggestions. I'll grab a 300 Ohm resistor tomorrow and try it out.

    If you were me, would you also install a dropping resistor instead of the choke, or leave it as is for the time being?
     
  10. dankilling

    dankilling Tele-Afflicted Silver Supporter

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    I’d make one change at a time until it’s right.
     
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  11. gabasa

    gabasa Tele-Meister

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    Thanks. I'll solder in the resistor tomorrow, then report back the new voltages.
     
  12. 2L man

    2L man Tele-Meister

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    Yes lower the Screen voltage adding a series resistor! Very high Screen voltage seems to make bias current react more to cathode resistor change.

    lt seems very common that way higher Screen voltages are used on instrument amps than tube datasheets succest. Obviously its because of better distorted sound?

    I don't fully understand how all goes there sound vise but Screen current increase rapidly when anode voltage approach zero...
     
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  13. gabasa

    gabasa Tele-Meister

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    Thanks for the comment, I was also thinking the same thing. A 5k or 10k dropping resistor should put the screen voltages nicely lower than the plate voltages.

    However, I have a choke in it. I was hoping to keep it in there!
     
    Last edited: Mar 1, 2021
  14. 2L man

    2L man Tele-Meister

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    Installing additional series resistor I think you can leave choke there as well but perhaps it cause choke effect lessening. If you have room you can improve filtering installing a capacitor too and then you have LCRC or RCLC filter.

    On my 5F2-A I have choke before B1 but I wanted to lower power tube voltage 10V so I added a 270 ohm series resistor and 4,7uF poly cap which are only on OPT/power tube anode circuit. For the B2 I also changed first resistor bigger when power tube did not drop that line as much anymore.
     
  15. Nickfl

    Nickfl Friend of Leo's

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    Do you have screen resistors in there personally I think it's a good practice to throw a pair of 1k or 1.5 k screen resistors on these older designs that didn't include them, that'll lower your screen voltage is a little bit but especially help keep them from overheating when over driven. I don't know if this will necessarily make your amp run physically cooler but it should help and it'll definitely make your tubes last longer.
     
  16. gabasa

    gabasa Tele-Meister

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    I installed 470 Ohm screen resistors when I put the amp together. I agree, screen resistors are always a good thing.

    I just picked up 300r and 330r resistors to try out on the cathodes. Once I do, I'll remeasure the voltages and see where things are at. I may or may not swap the choke out with a dropping resistor afterwards.

    I have a 5k/5w Mills wirewound lying around and a 10K on the way from DigiKey, to have some values to play with. The 5D3 used 10k and the 5E3 used 5k, so either value should work. I like the Mills resistors because they're compact.
     
  17. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    As you try things, I would try the @Nickfl suggestion of 1k5 screen resistors.

    You might repurpose the 270R bias resistor. Put it in series with the choke, rather than removing the choke.

    Do you have an idea of the mA rating of the power transformer?
     
  18. gabasa

    gabasa Tele-Meister

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    Upped the value of the cathode resistor to 300r and according to @robrob 's website I'm at 97.9% tube dissipation.
    Check out the screenshots.

    Screen Shot 2021-03-01 at 7.38.19 PM.png Screen Shot 2021-03-01 at 7.38.55 PM.png
     
  19. gabasa

    gabasa Tele-Meister

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    I'm up for trying anything right now.
    Not sure of the mA rating of the transformer, it's Victoria's standard 5E3 PT that they use in their 20112 amps.

    At this point, at 97.9% dissipation on my 6V6s, would there be any reason to remove my choke and install a dropping resistor in its place?

    I'm going to play the amp for a while now and see if the higher value cathode resistor makes any difference to the chassis and PT heat.
     
    Last edited: Mar 1, 2021
  20. gabasa

    gabasa Tele-Meister

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    Well... it still sounds great, I can't really hear much difference with the 300r cathode resistor.
    After about 20-25 minutes of playing, the chassis and PT are comfortably warm ... but not burning hot.

    I'm open to trying out any or all of the above suggestions, but I'm going to play around with it for a few nights and see if this really made a difference to the amount of heat before I try another change to the amp.
     
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