Harsh overtones
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Harsh overtones
I have an older Fender Deluxe that has a horrible harsh overtone when the volume is above 5. Its especially evident when an A chord or note is played. Sounds kinda like someone is playing an A flat along with me. The volume of the overtone is at least as loud as the original note so its not the usual ghosting that I,ve heard on other amps. Anyone heard this before?
Re: Harsh overtones
How fresh are the tubes?
Tubes are always the first suspect.
How fresh are the filter caps?
Have you checked the bias on the power tubes?
If this is an older Deluxe where the power transformer was designed for 110v or 117v you may have higher than desirable voltages.
Here is how to check the bias.
1. Measure the voltage drop on the cathode resistor to ground. Write the value down.
2. Divide this voltage by the value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.
3. Measure the voltage on the plates of the power tubes to ground. Write this down.
4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.
5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6V6s, if it is over 12 watts, then you need to install a higher value cathode resistor. If it's 10.5 watts or less, you need to install a lower value cathode resistor.
6. After installing the new cathode resistor, do ALL of the steps again to see what you now have. You may have to repeat this process several times to get it dialed in, but it is worth it, and your ears will thank you.
Of course if you are biasing a single power tube amp like a 5F1 Champ, omit the "divide by 2" in step 5, as you will already have the power dissipation of the single 6V6
Tubes are always the first suspect.
How fresh are the filter caps?
Have you checked the bias on the power tubes?
If this is an older Deluxe where the power transformer was designed for 110v or 117v you may have higher than desirable voltages.
Here is how to check the bias.
1. Measure the voltage drop on the cathode resistor to ground. Write the value down.
2. Divide this voltage by the value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.
3. Measure the voltage on the plates of the power tubes to ground. Write this down.
4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.
5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6V6s, if it is over 12 watts, then you need to install a higher value cathode resistor. If it's 10.5 watts or less, you need to install a lower value cathode resistor.
6. After installing the new cathode resistor, do ALL of the steps again to see what you now have. You may have to repeat this process several times to get it dialed in, but it is worth it, and your ears will thank you.
Of course if you are biasing a single power tube amp like a 5F1 Champ, omit the "divide by 2" in step 5, as you will already have the power dissipation of the single 6V6
Tom
Don't let that smoke out!
Don't let that smoke out!
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iknowjohnny
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Re: Harsh overtones
I think Structo nailed it with the mention of filter caps. thats a classic symptom of old electrolytics, and you said it's an older amp so thats where i'd look first.