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Thread: Question about Tube Output Power

  1. #1
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    Question about Tube Output Power

    Hypothetical situation: I take my head that has 4 6L6s and replace them with tubes that have, say, double the power, like 4 KT88s. Would my amp's overall output power double? Could this have any adverse effects on the amp? Would there be any adjustments necessary, besides biasing?

    Sorry if this is a noob question, i'm still fairly new to tube technology.

  2. #2
    Senior Member Wilder Amplification's Avatar
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    Quote Originally Posted by hells.saints View Post
    Hypothetical situation: I take my head that has 4 6L6s and replace them with tubes that have, say, double the power, like 4 KT88s. Would my amp's overall output power double? Could this have any adverse effects on the amp? Would there be any adjustments necessary, besides biasing?

    Sorry if this is a noob question, i'm still fairly new to tube technology.
    Tubes don't "have power". They control load current...basically they're a "current control valve" (now you know why certain countries still refer to them as "valves").

    That being said, power output of an amp is governed by the OT primary load impedance, the power supply voltage across this load impedance, and the amount of current the power transformer can source. The tubes just control this current.

    The whole "KT88/6550s have double the power" is something that got misconstrued by the fact that they can dissipate more power. Given the correct operating environment and surrounding circuitry, you can easily design an amp that will get double the power out of 1/2 the tubes (100 watts per pair). But the power supply has to be designed for it and the OT has to provide the proper load for it.

    To summarize, if your power transformer cannot source the current to make double the power at its rated voltage output, you're not going to get double the output power no matter what tubes you put in your amp.
    Jon Wilder
    Wilder Amplification

    Quote Originally Posted by m-fine
    I don't know about you, but I find it a LOT easier to change a capacitor than to actually learn how to play well
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    I doubt if any of my favorite players even own a soldering iron.

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    Senior Member Enzo's Avatar
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    And to put that point in even simpler terms, would you expect your table lamp to get twice as bright if you used a heavier extension cord to plug it into?
    Education is what you're left with after you have forgotten what you have learned.

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    Irrespective of hikes in output power, you may find that the bigger tubes have a harder dynamic response & the amp generally breaks up a little later, this may still be advantageous even if you don't double the power output.

    The most important factors when swapping from 6L6/5881 to EL34/6550/KT88/KT90 are that all these tubes draw more heater current (1.2A per tube for EL34, 1.6A per tube for the others, compared to 0.9A per tube for 6L6/5881). You need to make sure that your PT can handle the extra heater load (heater voltage & B+ voltage will frop if not). Likewise, at typical 6L6 voltages 6550/KT88/KT90 might take 55-60mA per tube at idle, again, if this loads up the PT voltages will drop, may also burn out the OT.

    A Fender twin typically has a 6A rated 6.3VAc winding, you can already see that the 4xKT88 draw more than this, so you would need to add a auxilliary filament transformer.

    Subbing 4x6L6 for 2xKT88 just requires rebiasing and an impedance match. 6550 require a larger screen grid resistor (as do EL34, which is typically not a drop in sub for 6L6) than 6L6, @ 1K 5W.

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    On a related note, if you have an amp that should be making around 100 watts RMS (four JJ KT77's with 470 plate volts into a 1750 ohm load) but is only making around 46 watts RMS, where should you start to troubleshoot the problem?

    I have a problem like this in an amp project that I've been working on. It started life as a Sovtek MIG 100U, and I kept the power transformer because it had the correct voltages and was running a quad of EL34's, so I figured it should keep up. I modified it into a clone of a VOX AC100, with a second channel with higher gain in addition to the stock Vox single channel. I'm using the same phase inverter as VOX used, a 12AU7 seesaw variant of the paraphase. To mix the channels, I added another triode (half a 12AU7) in parallel with the first triode in the phase inverter. They share a plate resistor and cathode resistor, and the second triode in the phase inverter has it's own plate and cathode resistors for better performance. The VOX circuit had both halves of the phase inverter sharing the cathode resistor.

    I seem to have a problem in the phase inverter area as I only get around 23v AC signal out of the first half of the phase inverter, and this is with a 55v signal coming out of the high gain preamp. The bias is -35v in fixed bias and around 29v in cathode bias. I pulled the output tubes and get about the same signal swing without them in there, so I don't think the output stage is affecting things. I built the stock Vox circuit with the single triode and a Fender LTP in an outboard chassis and subbed it one at a time into my amp in place of my phase inverter circuit, and I get around the same performance...which is not enough. The supply voltage for the phase inverter is around 410v, so there should be plenty of signal swing. The signal looks good on the scope too. I've checked and verified all of my wiring several times, and checked parts values and everything appears correct. The amp is perfectly stable with low hum and good tone, no oscillations that I can hear or see on the scope....yet I still have low power.....does anyone have any ideas of what kind of things I should check or where I should go from here?

    Thanks,

    greg

  6. #6
    Senior Member Wilder Amplification's Avatar
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    Quote Originally Posted by soundmasterg View Post
    On a related note, if you have an amp that should be making around 100 watts RMS (four JJ KT77's with 470 plate volts into a 1750 ohm load) but is only making around 46 watts RMS, where should you start to troubleshoot the problem?

    I have a problem like this in an amp project that I've been working on. It started life as a Sovtek MIG 100U, and I kept the power transformer because it had the correct voltages and was running a quad of EL34's, so I figured it should keep up. I modified it into a clone of a VOX AC100, with a second channel with higher gain in addition to the stock Vox single channel. I'm using the same phase inverter as VOX used, a 12AU7 seesaw variant of the paraphase. To mix the channels, I added another triode (half a 12AU7) in parallel with the first triode in the phase inverter. They share a plate resistor and cathode resistor, and the second triode in the phase inverter has it's own plate and cathode resistors for better performance. The VOX circuit had both halves of the phase inverter sharing the cathode resistor.

    I seem to have a problem in the phase inverter area as I only get around 23v AC signal out of the first half of the phase inverter, and this is with a 55v signal coming out of the high gain preamp. The bias is -35v in fixed bias and around 29v in cathode bias. I pulled the output tubes and get about the same signal swing without them in there, so I don't think the output stage is affecting things. I built the stock Vox circuit with the single triode and a Fender LTP in an outboard chassis and subbed it one at a time into my amp in place of my phase inverter circuit, and I get around the same performance...which is not enough. The supply voltage for the phase inverter is around 410v, so there should be plenty of signal swing. The signal looks good on the scope too. I've checked and verified all of my wiring several times, and checked parts values and everything appears correct. The amp is perfectly stable with low hum and good tone, no oscillations that I can hear or see on the scope....yet I still have low power.....does anyone have any ideas of what kind of things I should check or where I should go from here?

    Thanks,

    greg
    What's your screen voltage and idle plate current?
    Jon Wilder
    Wilder Amplification

    Quote Originally Posted by m-fine
    I don't know about you, but I find it a LOT easier to change a capacitor than to actually learn how to play well
    Quote Originally Posted by JoeM
    I doubt if any of my favorite players even own a soldering iron.

  7. #7
    Noodle of Reality Steve Conner's Avatar
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    Hey Greg,

    Check the power tube grid waveforms on a scope. Can the PI drive the grid voltage right up to 0V? If so, the power tubes will clip it with grid current there, so the PI is doing as much as it can be expected to.

    If it doesn't make it to 0V then you need more PI output. With 35V bias, you need 70V p-p of drive for each grid, which an AC voltmeter would see as 25V RMS.

    Now, what about the screen voltage and screen resistors? If the screens aren't getting enough juice, you can drive the control grids as hard as you like, but the output will still be lacking. KT88s need a bit more than EL34s.

    Next, what about the power supply, does it sag excessively under load?

    If you put EL34s back in, does the power output go up to what it should be?

    Etc.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    Quote Originally Posted by Steve Conner View Post
    Hey Greg,

    Check the power tube grid waveforms on a scope. Can the PI drive the grid voltage right up to 0V? If so, the power tubes will clip it with grid current there, so the PI is doing as much as it can be expected to.

    If it doesn't make it to 0V then you need more PI output. With 35V bias, you need 70V p-p of drive for each grid, which an AC voltmeter would see as 25V RMS.

    Now, what about the screen voltage and screen resistors? If the screens aren't getting enough juice, you can drive the control grids as hard as you like, but the output will still be lacking. KT88s need a bit more than EL34s.

    Next, what about the power supply, does it sag excessively under load?

    If you put EL34s back in, does the power output go up to what it should be?

    Etc.


    I'm not sure how to tell if the PI is driving the grid voltage up to 0v? I think I was getting about 20v AC out of the plate of the first half of the PI when I had a clean signal coming into it, and the second half of the PI was a couple volts less than that. Those numbers go up as I run the preamps up more of course, but then its distorting the signal past the RMS power level where I should be checking for output power of course.

    The screen voltage is about 460v or so I think and the plate is at 470v. I have 1k 5w screen resistors, and I'm using KT77's, not KT88's. I haven't tried EL 34's in it yet, but have a set of used old stock Mullards that I can try. I doubt if those will make a huge difference however as a JJ KT77 is pretty similar output power to an EL34.

    The power supply doesn't seem to sag a whole lot under load. I forget what it was and will have to measure it again tomorrow, but it wasn't going down lower than 400v on the screen when loaded if I remember right. I post what it is tomorrow. The power transformer was running a quad of EL34's at 485v, and three 12AX7's in the Sovtek. In my amp, I have an EF86 and 12AX7 in one channel, and a 12AU7 and 12AX7 in the second channel, and a 12AU7 seesaw paraphase inverter, and then the quad of KT77's. In place of lots of 12AX7's and 12AU7's, I've used a couple 12DW7's so my overall tube count is lower. With the extra tubes over the Sovtek, the filaments are still at 6.3v, so no issue there.....and the B+ drops to 470v in my circuit instead of the 485 in the Sovtek. I'll gret some numbers for you tomorrow and thanks for the help!

    Greg

  9. #9
    Senior Member Old Tele man's Avatar
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    ...assuming the power supply is providing sufficient Vp and Vs, here's the basic equation for tube/transformer PP output power:

    Po = rp' * (Vg*gm)^2

    ...where:

    Po = output power (at primary-side of OT)
    rp' = effective plate load resistance (tube and reflected load)
    Vg = control grid rms signal voltage
    gm = tube average transconductance

    ...the only "tricky" part is rp', the effective plate load resistance combination of tube rp and reflected speaker load Zo back through OT.

    ...notice that the equation is simply: Power = I-squared x R, expressed in vacuum tube variables, ie: Ip = Vg*gm.
    ...and the Devil said: "...yes, but it's a DRY heat!"

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    Quote Originally Posted by hells.saints View Post
    Hypothetical situation: I take my head that has 4 6L6s and replace them with tubes that have, say, double the power, like 4 KT88s. Would my amp's overall output power double? Could this have any adverse effects on the amp? Would there be any adjustments necessary, besides biasing?

    Sorry if this is a noob question, i'm still fairly new to tube technology.
    As mentioned already could replace 4 6l6's with 2 kt88's- but it probably won't be worth it. Many larger tubes need higher voltages and higher plate impedances to be at their best.

    Why do you need more power? 100 watts of tube amp into an efficient cabinet should be enough to deafen you. If you're trying to use it for bass a higher voltage large tube (kt88) amp would be the way to go anyway.

    jamie

  11. #11
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    I decided to post a new thread rather than continue to hijack this one. Sorry hells.saints about the hijack.

    Thanks, here is the new thread if you can't find it.

    Greg

    Amp should be 100 watts but low on power, why?

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