Shop Talk—Its Time to Turn Down

December 1, 2009

I HAVE A 1959 TWEED FENDER CHAMP THATSOUNDS great except when I turn the volume control past nine. When the volume goes up that high or higher, the distortion is buzzy and unmusical. Also, I own a 1957 Fender Deluxe Reissue. When it is past halfway up, the low notes seem to compress so hard that it chops off the front of the note. Any suggestions? —John Robinson

Let’s talk about the Champ first, which is a single-ended, Class A amplifier. That means that the signal is being amplified by an output tube that is drawing either more or less current. We have all seen a picture of a sound wave where the top of the wave looks like the hill and the bottom of the wave looks like a valley. On a single-ended amp, the tube amplifies the hill portion of the sound wave by drawing more current, and it amplifies the valley portion by drawing less current. As the volume is turned up, the hills and valleys get larger, so the tube draws even more current and less current. It can come to a point where the hill portion of the tube is drawing as much current as is possible, and the valley portion is drawing so little current that it is driven into cutoff—literally, the bottom of the wave is cut off. This results in buzzy, homogenized, and non-musical distortion. Some Champs have more gain than others depending on whether there is a cathode bypass capacitor on the first gain stage. The cathode bypass cap isn’t shown in the schematic or circuit layout, but many Champs were shipped with one. If your amp has a cathode bypass cap, the preamp signal could be strong enough to drive it into cutoff, which would explain the buzzy sound when you turn it up. The solution? You could remove the cathode bypass cap easily enough, but since it is only there to boost volume, you can get the same effect by simply turning your amp down. Removing the cap would also increase the compression slightly, but Champs are compressed sounding enough as is, so I’d recommend leaving it in place and just running the volume a little lower.

With the 1957 Deluxe reissue, compression can be a real problem. One of the fixes for this is to replace the tube rectifier with a solidstate replacement that fits into the same socket. This will increase the amp’s headroom and give you a better overall range of clean tones. You can also use the interactive volume controls to scoop out the mids and get a better clean sound. Here’s how to set the controls for piano string clean tones:

1) Plug your guitar into the #1 input of the Instrument channel.

2) Set the volume on the Instrument channel half way up.

3) Turn the volume control on the Mic channel all the way up.

4) While playing and listening to your guitar, readjust the Mic channel volume down a bit until the best clean tone is achieved. Set the tone control by ear. It will probably sound best almost all the way up. This should give you a pristine clean tone with minimal compression. —Gerald Weber, Kendrick Amplifiers

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