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Thread: Fender Champ Power Output Rating

  1. #1
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    Fender Champ Power Output Rating

    Hi there, first time posting in this section. My question has to do with power ratings for a single 6V6 amp such as a Fender Champ. I worked on a similar amp recently, a Harmony 8418 made by Valco. I calculated the power dissipation of the power tube to be just over 10 watts, which seemed reasonable since 6V6's are rated for 12 watts max dissipation. What confuses me though is that the literature on the Fender Champs says they are 5 watt amps. Am I missing something here? Why would they design the Champ to output only 5 watts if they had up to 12 watts available from the power tube?

    B.L.

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    You are confusing tube dissipation with the amp output power..Typically output power will be something less than 60% of the tube dissipation.
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    Quote Originally Posted by bobloblaws View Post
    Hi there, first time posting in this section. My question has to do with power ratings for a single 6V6 amp such as a Fender Champ. I worked on a similar amp recently, a Harmony 8418 made by Valco. I calculated the power dissipation of the power tube to be just over 10 watts, which seemed reasonable since 6V6's are rated for 12 watts max dissipation. What confuses me though is that the literature on the Fender Champs says they are 5 watt amps. Am I missing something here? Why would they design the Champ to output only 5 watts if they had up to 12 watts available from the power tube?

    B.L.
    Oh, OK. I thought it was pretty much the same thing. I know how to calculate tube dissapation, is there a way to calculate output power? Or do you have to look at the output waveform on a scope?

    A Deluxe Reverb cranks out 11 W per 6V6. Is the difference because one is class A and the other class AB?

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by bobloblaws View Post
    Why would they design the Champ to output only 5 watts if they had up to 12 watts available from the power tube?
    5 watts - from 400 or 500 Hz on up. You'll be very disappointed at the puny power of the mighty Champ at 100 Hz. Like 1 watt or less, clean sine wave. Of course you can measure some distorted waveform and find a larger power figure but any way you look at it, the power is very limited at lower frequencies.
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    Senior Member Enzo's Avatar
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    Output power is what audio power it can put into a load. A crude method would be to measure the signal voltage across the load (speaker) and use Ohm's Law to calculate power.
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    So why is the output power of a Deluxe Reverb, for example, so close to that of the tube dissipation while the Champ is roughly only half? Something to do with efficiency and/or amp class?

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    Senior Member Old Tele man's Avatar
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    Po ≈ (Zo')*(gm*Vg)^2

    where:
    Po = Output power, watts(avg)
    gm = tube transconductance, A/V
    Vg = grid-signal AC-drive voltage(*)
    Zo' = effective plate load impedance, Ω

    NOTE: (*) with PP circuits: (1) assume the bias voltage is the limit of the AC-drive voltage (ie: Class-AB1) and (2) Zo = Zoo/4 roughly.
    Last edited by Old Tele man; 10-15-2017 at 02:24 AM.
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    Quote Originally Posted by bobloblaws View Post
    Something to do with efficiency and/or amp class?
    Yes. Class AB is much more efficient than class A.

    Amplifier Efficiency

    http://www.electronics-tutorials.ws/...r-classes.html
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    Supporting Member Jazz P Bass's Avatar
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    Also, I believe that you are missing the relationship that the output tube has with the output transformer.

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    Right, that makes sense to me now, thanks all.

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    Quote Originally Posted by g1 View Post
    Yes. Class AB is much more efficient than class A.

    Amplifier Efficiency

    http://www.electronics-tutorials.ws/...r-classes.html
    Right, that makes sense to me now, thanks all.

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    Quote Originally Posted by Jazz P Bass View Post
    Also, I believe that you are missing the relationship that the output tube has with the output transformer.
    As in "reflected load" etc.? I read up on that awhile back, I would have to refresh my memory.

  13. #13
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    Quote Originally Posted by bobloblaws View Post
    Oh, OK. I thought it was pretty much the same thing. I know how to calculate tube dissapation, is there a way to calculate output power? Or do you have to look at the output waveform on a scope?
    Preferred method, simply because itīs "real World" is to drive amp to just clipping, checking that on a scope screen , and do the Math.

    Now when you are *designing* an amplifier, when there{s nothing to turn on and measure yet, just some drawings and calculations on a piece of paper, the regular method is to calculate it using the tube curves, available from the datasheet. (thatīs what they are there for).

    Here{s an example, not exactly of a Champ, but similar principles apply:


    Details to be considered:
    1) the (incredibly useful) curves , showing:
    Current
    plate voltage
    grid voltage
    dissipation
    etc. , in all combinations.
    personally I like "graphic" design using curves instead of "digital" design using simulators any day of the week, because although the latter can crunch numbers "apparently" very precise (as "0.5384% distortion", "102.257 mA current" ... REALLY????) but graphics show you "all data at once".

    Ok, in the example you have a tube with 300V plate , 100mA plate, both at idle, and designer drew a 4000 ohm load curve.
    See that plate voltage can swing from 0V (nominally, actually some 60/80V minimum "saturation voltage") to twice +V or 600V and current can swing from 0mA to 200 mA (again , nominal, actual tubes might have difficulty reaching that, specially if worn)

    So here you have 300V peak swing, 100mA peak swing, so 300*0.1=30W peak Watts (yes, it{s actually that simple) so 15W RMS possible.

    Now I said that actual peak voltage swing will be less, some 300V minus 80V saturation voltage so 300-80=220V peak or 73% what an "ideal" unobtanium tube would achieve.
    By the same token, current swing will also be 73% of ideal value, so 73 mA peak.

    So letīs recalculate using real world tube values (which are also shown in the datasheet, they are the bent *real* curves) 220V peak * 73mA peak=16W peak , so 8W RMS ... which matches very well with what we can get from a real world "Champ", using an EL34.

    By the same token, you will find the 6V6 one can provide some 50W RMS.

    As you see, amps *can* be designed, and very accurately, just with curves, pencil, paper and calculator

    By the way, whay Leo and others dis, no doubt.
    Not many copmputers available way back then

    Nor calculators, the calculation tool used by Engineers was the mighty slide rule:


    EDIT: now I notice I had only a very quick look at the graph, did all after it in my mind (since I do it *often*) and thought idle current was 100mA.
    Itīs actually 80mA so that value should be used instead, lowering end results a little, but too lazy on a Mothersīs day Sunday afternoon to rewrite everything with corrected values, in any case the procedure is correct, Math is correct .... just does not exactly apply to the graph I pulled from the Net.

    The *method* is the one I showed in any case.

    EDIT2: maybe this will clear OP doubts:
    *idle* power dissipation is 300V * 80mA=24W, yet we only get less than 8W RMS
    terrible but true
    Last edited by J M Fahey; 10-15-2017 at 07:59 PM.
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    Wow, that is a lot to digest. I come across references to the tube curves quite often but I don't know much about it. I will definitely study this info when time permits.Thanks!

    - B.L.

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    push pull amps are like making toast, you press the lever down and the toaster starts to heat up, the toast is done and it pops out and the toaster cools off,
    well, a class A amp like the Champ is like having the toaster on all the time, maybe not as hot, but always on, the tube does not like this, it will protest after a while, so we have to limit the plate dissipation of the tube a bit so it does not fail.

    this means we can not get as much juice from the Champ as we do in the Deluxe Reverb.
    In the Deluxe Reverb, the tubes get to cool off every half cycle.

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    Quote Originally Posted by cjenrick View Post
    push pull amps are like making toast, you press the lever down and the toaster starts to heat up, the toast is done and it pops out and the toaster cools off,
    well, a class A amp like the Champ is like having the toaster on all the time, maybe not as hot, but always on, the tube does not like this, it will protest after a while, so we have to limit the plate dissipation of the tube a bit so it does not fail.
    That is an interesting analogy. But in the example I gave, my amp was biased so the 6V6 was dissipating about 10.3 watts which is about 85% of the max 12 watts. That is more than the conventional 70% for push pull amps. Robinette's bias calculator (https://robrobinette.com/Tube_Bias_Calculator.htm) indicates that 90% is safe for cathode bias amps and 100% is safe for class A fixed bias. The general thrust of the other replies appears to be that it is a question of efficiency and not less dissapation that results in the lower output. Or am I confused by your use of the term "plate dissapation".

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    at 10 watts Class A i am afraid you are going to be buying a lot of tubes,

  18. #18
    Supporting Member eschertron's Avatar
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    The latest Champ, the reissue 600, runs with 360vdc on the plate and -22vdc bias, for ~338vdc anode-cathode potential. Schem here: Champion 600

    Using an online tool, I read 15.5W total dissipation. This includes plate and screen; if screen is 20% then plate is ~12.4W and the screen is the other 3.1W. To answer the OP, this 90% continuous plate dissipation has nothing to do with audio output power. The output power using the schem's indicated 7vac is about 5W. Cranking the amp will get significantly more.
    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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    Quote Originally Posted by eschertron View Post
    Using an online tool, I read 15.5W total dissipation.
    I get similar (15.9W) with Robinettes calculator.



    Quote Originally Posted by eschertron View Post
    ...if screen is 20% then plate is ~12.4W and the screen is the other 3.1W.To answer the OP, this 90% continuous plate dissipation has nothing to do with audio output power.
    I understand now that I had originally confused tube dissipation with audio output power, but what value constitutes 90%, and of what? Max dissipation for that tube is 12W, no?



    Quote Originally Posted by eschertron View Post
    The output power using the schem's indicated 7vac is about 5W.
    I'm not trying to be nit-picky, I just want to understand your points accurately. Did you mean 3.35vac? That is what I see on that schematic at the output, TP13. There is a reference to 7vac at TP5.



    Quote Originally Posted by eschertron View Post
    The output power using the schem's indicated 7vac is about 5W. Cranking the amp will get significantly more.
    My understanding was that when the output spec is listed at 5W it refers to the max output. Or does it refer to distortion free output?
    Last edited by bobloblaws; 10-18-2017 at 06:01 AM.

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    Quote Originally Posted by cjenrick View Post
    at 10 watts Class A i am afraid you are going to be buying a lot of tubes,
    The online calculators seem to indicate that 10W plate dissipation is well within the acceptable range for a 6V6. If I'm missing something I'd really like to know what it is.
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  21. #21
    Supporting Member eschertron's Avatar
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    Quote Originally Posted by bobloblaws View Post
    I understand now that I had originally confused tube dissipation with audio output power, but what value constitutes 90%, and of what? Max dissipation for that tube is 12W, no?
    The number I'm using is 14W, although I have seen ONE source that calls out 12W. Not sure the reason for the discrepancy. Maybe the GT designation indicates 14W?


    Quote Originally Posted by bobloblaws View Post
    I'm not trying to be nit-picky, I just want to understand your points accurately. Did you mean 3.35vac? That is what I see on that schematic at the output, TP13. There is a reference to 7vac at TP5.

    My understanding was that when the output spec is listed at 5W it refers to the max output. Or does it refer to distortion free output?
    Yes, TP5 is where I'm getting the number. In the online calculator I used, 7vac (or about 20vpp) at the grid of the power tube resulted in ~5W output power. I didn't check, but I assume this is about the limit of clean power. Or it 'just happens' to be the voltage at the grid with the design input signal. With the 6V6 slammed, you'll get 7 or 8W of snarling distortion.

    edit: I think it's up to the maker to assign a distortion figure to their rated outputs. For Fender, it's generally a clean signal. For some others, not so much - presumably because the user won't play the amp clean. Why advertise an amp as an 80W amp when it can be advertised as 150W?
    Last edited by eschertron; 10-19-2017 at 02:13 AM.
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    Quote Originally Posted by Steve Conner View Post
    If it still won't get loud enough, it's probably broken.

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