Re: Correct measurement of repaired Guitar Amplifiers
- From: Mike Schway <mschway@xxxxxxx>
- Date: Thu, 27 Oct 2005 08:08:02 -0700
In article <1130422767.906510.210120@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Agent86" <maxnews@xxxxxxxxx> wrote:
> I am doing repairs of Guitar amplifiers but am in debate with a
> customer on the output power of an amp that has been fixed.
>
> It is a Fender Twin Reverb with Master volume with 4 x 6L6WXT new
> tubes.
>
> This is my power measurement setup and proceedure.
>
> 1. Connect 1KHZ sine wave generator to input but turned off.
> 2. Connect output of the amplifier to non inductive high power resistor
> matched to amplifier output.
> 3. Connect Osciloscope and True RMS Meter across the resistor.
> 4. Turn on the amplifier for 1 minute.
> 5. Turn up the signal generater until you see clipping on the sine wave
> and back it off a little.
>
> Use the RMS value of the output to the multimeter in the following
> formula.
>
> P=V^2/R
>
> In this this current case.
>
> V = 50V p-p
> VRMS = 17.675 VDC Aproximatly
> R = 4 Ohms
>
> So this would give
>
> P = 17.675 ^2/4
>
> P = 78.1 Watts
>
> I would expect about 100W RMS from an amp like this but there is no
> indication of any fault.
>
> Is my formula correct?
>
> The frustrating thing with Tube amps is the difficulty of getting
> schematics or specifications to match the exact amp you are working on.
> Valves were not in common use when I did my training and started
> repairs so I am having to learn what seems to be more intuitive repair
> and testing proceedures in Guitar amps. In all my other power amplifier
> repairs I have a power output value and a model number that matches the
> schematic most of the time and that is simple.
>
> I am too young to have exprienced Valve repairs but hopefully not too
> old to learn. Most of the time it is a no brainer but I get the odd amp
> I have difficulty with like in this case.
>
> I have had it put to me by an experienced repairer that I use the
> forumula above but instead of V being RMS it is Peak then at the end
> apply *.707 which in fact gives me over 100W but hours on the internet
> has always lead me back to my above proceedure. I must be doing
> something right as I rarely get complaints and when I do it is a job
> with intermitant fault I thought was fixed or the occational faulty
> output valve. Yes I match and bias valves and replace the complete
> output set at the same time.
>
> Any ideas and suggestions welcome.
>
> I don't however need to know complex power calculations that are
> mentioned or get into a power debate, I only need to confirm the
> amplifiers rated output and to check it is performing correctly before
> returning to the client.
>
> Thanks
>
A few things.come to mind:
1) 80W is reasonable for clean output on a SF Twin, especially with
modern 6L6-like objects. Actually, it's pretty darned good.
2) Those 100W are "marketing watts" Notice they don't specify
distortion level where the 100W output occurs. Just for fun, crank the
amp to a full square wave, THEN use your T-RMS meter (or measure peak
watts on the scope and DON'T multiply by 0.7 ...this works if it's a
FULL square wave).
3) I've noticed that NiCr wound resistors can generate a DC voltage
when heated up. This can foul up any meter reading by maybe 5-10%. Try
this sometime: Cook the resistor with full output for 5 minutes, turn
off the signal and measure voltage across the resistor. A couple
hundred mV isn't unreasonable.
--Mike
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Mike Schway | [Picture your favorite quote here]
mschway@xxxxxxx |
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