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How To Make The Ampeg V4 Better For Bass Guitar
By Sid of Stone Marmot
Oct. 6, 2008
Ampeg produced the V4 and the VT22 vacuum tube guitar amps during the 1970s, which were conservatively rated at 100 Wrms. The V4 and VT22 electronics are absolutely identical for a given model year. In fact, though the front may be marked "V4" or "VT22," the back chassis of all these amps is stamped with both V4 and VT22 and have the same schematic. The only difference between the V4 and VT22 is that the V4 is just an amplifier head with no speakers while the VT22 is a combo amp with two 12 inch speakers. The V4 head was typically used with one or two V4 speaker cabinets, with four 12 inch speakers in each cabinet.
Ampeg also produced the V4B vacuum tube bass amp in the 1970s. It was also an amplifier head with no speakers, though Ampeg produced a number of different speaker cabinets that would work with it. It was quite loud for only 100 Wrms and, with appropriate speakers, could fill most nightclubs, high school gymnasiums, and small auditoriums with good, solid bass.
The recent demand for good tube amplifiers has greatly increased the demand, and hence, resale prices, for the V4B. Many consider the 300 Wrms Ampeg SVT the ultimate bass amp for live shows. The V4B has pretty much the same sound, including the unique active midrange control of the SVT, but with a more manageable power level, less than half the weight, and a much lower price.
The V4/VT22 is presently not as popular as the V4B, though in the 1970s it was much more popular and many more were produced than V4Bs. Consequently, you can buy a V4 head or VT22 electronics for a lot less, often half the price or less, than a V4B that is in similar condition. V4s are also much easier to find than V4Bs.
The V4/VT22 and V4B electronics are very similar, with absolutely identical circuits for the power supply, power amp, and tone controls (including the active midrange control). Even the power and output transformers are the same, which is unusual, since a bass amp needs a much larger output transformer than a guitar amp to accurately reproduce the low bass notes. The differences between the V4/VT22 guitar amp electronics and the V4B bass amp electronics are:
1) The guitar amps have reverb.
2) The guitar amps have a sensitivity switch in each preamp to select one of three possible gains.
3) The guitar amps have two bright positions (besides off) in the "Ultra Hi" switch while the bass amp only has one.
4) The bass amp has an "Ultra Lo" switch.
5) The guitar amp has three coupling capacitors in the preamps which are each an order of magnitude smaller that the same caps in the bass amp.
Many people have tried using the V4/VT22 for bass guitar with good success, assuming they were driving bass speaker cabinets. The first three items above can be adjusted on the control panel to be the same as the bass amp. Adding the "Ultra Lo" would be complicated as it requires an extremely rare seven pole, single throw switch to implement it in exactly the same way as the V4B (or four pole, single throw, still rather rare, if you only add it to one channel). But I never liked this switch as it filters out virtually all the highs AND midrange from your sound, leaving a pretty muddy sound.
That leaves Item 5 above. These capacitors, C3, C6, and C7, have the same reference designations in both the V4/VT22 and the V4B and perform the same corresponding functions. (See http://www.drtube.com/guitamp.htm for the schematics for each amp. The Ampeg schematics have the reference designations.) Each is 0.01 microfarads in the V4/VT22 and 0.1 microfarads in the V4B. C3 only affects Channel 1 and C6 only affects Channel 2, whereas C7 affects both channels. Changing each of these capacitors in the V4/VT22 will give this amplifier the same low frequency bandwidth as the V4B. The change isn't dramatic (my quick "back of an envelope" calculations only show about a 10 Hz difference in the low frequency cutoff between the amps), but it does improve the clarity of the lowest four or five notes on a four string, standard tuned bass guitar.
After this change, I haven't noticed any difference in the performance of the V4/VT22 (mine is the VT22) as a guitar amp. If you still plan to play guitar through the amp, you may want to only change one channel. For example, you may want to leave Channel 1 for guitar and optimize Channel 2 for bass. In this case, you would leave C3 alone and change C6 and C7 to 0.1 microfarads. C7 would still affect both channels.
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.
I have changed these caps in my VT22 electronics (which I mounted in a home made cabinet to look like a V4) and am very happy with the results. This 100 Wrms tube amp appears to put out as much usable volume as my 350 Wrms Hartke 3500 solid state amp and is more than loud enough for any place I presently play. The EVM15Ls that I'm running the Ampeg through probably help, which are more efficient than the Hartke speakers. And it still sounds good with guitar, even through (especially through) the EVM15L 15 inch speakers.
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