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Make The Ampeg V4 Midrange Control Like SVT
By Sid of Stone Marmot
The Ampeg V4 series amplifiers (V4, V4B, and VT22), as well as the V2 and VT40, have the same unique active midrange circuit as the Ampeg SVT. But the SVT has the selectable center frequencies set for about 220 Hz, 800 Hz, and 2500 Hz, whereas with all the other amps listed above have the selectable center frequencies set for about 300 Hz, 1000 Hz, and 3000 Hz.
If you want to change the center frequencies of your midrange to match those of the SVT, you only need to change two capacitors. In the SVT, these capacitors have the reference designations C16 and C19 and are each 0.68 microfarads. In all the other above listed amps, the reference designations for the corresponding caps are C203 and C204, each of which are 0.33 microfarads. Simply change both C203 and C204 to 0.68 microfarads. (See http://www.drtube.com/guitamp.htm for the schematic of your Ampeg amp. The Ampeg schematics have the reference designations.)
0.68 microfarad capacitors with a 400 V minimum breakdown voltage may be hard to find. You could just parallel another 0.33 microfarad, 400 V minimum, capacitor, which should be easier to find, across each of the present C203 and C204 capacitors. The 0.02 microfarad, or 3 %, difference in capacitance is well within the tolerance of the parts and is negligible.
You could also add a switch which parallels the new 0.33 microfarad capacitors across the exiting ones. This way you can change from the stock V4 center frequencies to the SVT center frequencies with the flick of a switch. Be sure you use a double pole, single throw switch that switches both caps together. Also, be careful with your wire routing to the switch so you don't increase the noise in your amp.
Note that all this applies only to the V4s that don't have the distortion control. The late 1970's V4s and VT22s with the distortion control have a much different circuit which I haven't analyzed and have no experience with.
Notice I haven't shown any pictures of where these parts are located. Anyone with lots of electronics experience should be able to quickly find the parts based on the schematic. If you don't have that kind of experience, you probably shouldn't do these modifications yourself as some of the voltages in these amplifiers, which can be stored for a long time on some of the capacitors even after the amp is off and unplugged, are well over 500 V and can be VERY LETHAL! Do yourself a favor and take your amp and these instructions to someone with experience to do the mods for you.
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