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Power transformer getting too hot,low plate voltage

Discussion in 'Amp Tech Center' started by J-Turn, Mar 25, 2020.

  1. J-Turn

    J-Turn TDPRI Member

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    How should I do that?
     
  2. J-Turn

    J-Turn TDPRI Member

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    It sounded like the amp you might choose if the devil challenged you on guitar instead of fiddle lol
    Actually still sounds pretty good!

    I did red plate one el84 in the middle of the process, but I didn’t mention it because I had other known working tubes and didn’t want to “muddy the water”.
     
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  3. Wally

    Wally Telefied Ad Free Member

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    you got a lesson on the old truth that an amp can sound fantastic right before it burns down due to excessive current draw due to mis-biasing. As for biasing, there are a number of methods. Check Rob Robinette’s site for a step by step de#cription. Read Aiken Amp’s Tech Info/White Pages on biasing. There will be a good description of methods and some in-depth technical analysis.....same at The Valve .wizard site. The sort story for the time being is that if the tubes are not redplating and the PT is not hot, the amp is safe to play. I congratulate you on your persistence, and I am happy we did not give up while thinking that the PT was shot before we established that it is in fact still functioning.
     
  4. J-Turn

    J-Turn TDPRI Member

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    I have definitely learned a lot from this project. Thanks to you guys who stuck with me! I feel like I truly understand about half of what I did lol
     
  5. elpico

    elpico Tele-Afflicted

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    Hehe don't worry it gets easier. Just a lot of new terms at first.

    The fix you went with will be fine to play and hear the circuit and decide what you want to do with it in the future. If you decide you want to keep the circuit long term then I'd suggest it's worth it to do the third option at that point, which was build a better bias supply. This one does waste about 4 watts (for no reason now that the preamp is gone), which isn't huge, but is still heat you'd just as soon not have in the chassis and transformer long term.

    Have fun with it.
     
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  6. J-Turn

    J-Turn TDPRI Member

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    I would be interested in hearing about how you would install option three?
     
  7. J-Turn

    J-Turn TDPRI Member

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    I watched this video, and was wondering if this would be something similar that I could do?
     
  8. elpico

    elpico Tele-Afflicted

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    You could do anything from the minimum changes needed to building a completely different amp in there so I guess the answer is "it depends".

    Minimum changes to remove the power wasting bits would involve removing R702 and the resistors you added to balance it, flipping the bias diodes and cap around to generate -30V instead of +30V, and adding a voltage divider to drop/adjust that -30V down to the approx -15V you want.

    Your amp does have a bias wiggler trem though so if that effect is important to you then you'd have some more work to do to get that mixed in right. And it looks like this trem is just an on/off effect with fixed speed and depth, you'd probably want to mod that too if you intend to use it much.

    So I guess the question is how important is trem to you? If you don't need it then then basic changes above are fine. If you want a good trem too then you'd either be modding this one or rebuilding a new one (possibly like the princeton circuit you linked above)
     
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  9. J-Turn

    J-Turn TDPRI Member

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    So I played the amp for about an hour last night and it sounds interesting so I think I’ll Keep the tone like this for a while. I do want to get that extra heat out, because after an hour it was very warm (nowhere near as hot as it was after 15min before). I would like to take your advice on flipping the voltage. I have a few questions.
    Voltage divider, are you talking about a pot or a fixed value resistor, or is it an either or? And would that go in the exact place R702 was?

    You mentioned the trem in an earlier post and I didn’t know what you were talking about. Playing it last night, I did seem to notice the slightest something going on. I have no controls for it. I literally only have one volume knob. On the schematic I see R708 listed as “cps speed”. Is that where I could tie in?
     
    Last edited: Apr 4, 2020
  10. elpico

    elpico Tele-Afflicted

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    No the center tap would go straight to ground now, no replacement for R702. Here I'll draw it:

    [​IMG]

    That should give an adjustment range of about -9 to -18. If that turns out to be not enough you can change the values of the 5k resistors a bit.


    I didn't realize you were missing the controls.The trem on/off switch would've gone across R714. Closing the switch bypasses that 1meg and turns the trem on. And yep, R708 looks like the one to make adjustable for a speed control. The schem doesn't give a value for it but from it's neighbours I imagine it's around 100k?
     
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  11. Paul G.

    Paul G. Friend of Leo's

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    A few years ago, I was playing in a roadhouse, there were some pretty bumpy roads on the way. The band was cooking, and my guitar sounded sooo good! The drummer gestured me and yelled "Is your amp supposed to be smoking like that?"

    It was a bad connection from the bias resistor and cap to the cathodes and I lost bias. If it wasn't for Crash's eagle eye, it could have been a disaster.
     
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  12. J-Turn

    J-Turn TDPRI Member

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    I finally got the parts to be able to make this circuit. Hooked it up and now have 311v pin 7&9 el84
    And -11 for neg bias at R703&r704
    Is this good?
    I can dial in -15 but that puts the + up to 346.
    Also the chassis seems amplified now. If I tap it I hear it through the speaker.
     
  13. elpico

    elpico Tele-Afflicted

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    Cool, that should probably be enough range. The bias voltage itself isn't of much interest, what you want to know is how many watts the power tubes are running at. To calculate that you need two numbers, the plate voltage and the plate current. You have the first one, now you need the current. There are several ways to measure this. I believe somebody posted some links for you earlier, but if not a search for how to bias a guitar amp should get you there.
     
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  14. elpico

    elpico Tele-Afflicted

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    I should've added that you want to set the bias to the most negative value at first and work your way up from there, so leave it at the -15 end for now not -11.

    And I overlooked your point about the "amplified chassis". That's called being "microphonic", an electrical component turning a physical vibration into an electrical signal. All tubes do this to some extent, so do some capacitors, and even some guitar pickups. If you've ever seen the old "talking into your pickup" move in the middle of a solo that's what's going on there. In certain tubes it can be strong enough to become a problem so if it's really bad you can tap on each tube with a chopstick and identify which socket you might want to try a replacement in.
     
  15. J-Turn

    J-Turn TDPRI Member

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    I set the bias to -15 and measured the plate current 30ma & 25ma with a plate voltage of 343 = 10.29watts & 8.57watts
    I found I had the ax7 go microfonic
     
  16. Wally

    Wally Telefied Ad Free Member

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    343X.03=10.29/12=85.75% of max plate dissipation
    343x.025=8.575/22=71.4% of MPD

    Low for cathode biased, imho, but usable....unbalanced but if it isn’t humming, it is usable.
     
  17. J-Turn

    J-Turn TDPRI Member

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    Is the imbalance coming from my output transformer readings of 246 & 280ohm.
    Any suggestions for tweaking to get the best/optimal performance?
     
  18. Wally

    Wally Telefied Ad Free Member

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    output transformers do not usually have balanced resistance measurements in the primary. The most imbalance I have ever seen was in a Peavey OT that I ordered from Peavey. When it came, I took some resistance readings and had to call Peavey to double check. They said indeed that was the way it was designed. The two sides were far apart.....like 45ohms and 150ohms or something like that.

    I would think that the tubes are not closely matched.....since they are the old tubes, correct? I don’t know if I would worry about it as long as it doesn’t hum and you like the sound.
     
  19. D'tar

    D'tar Friend of Leo's

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    Swap the tubes position and recheck.

    This is still fixed bias yes?
     
  20. J-Turn

    J-Turn TDPRI Member

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    I think it still is since I didn’t directly put resistor to the cathode.(?) someone please correct me if I’m wrong.
     
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