Ampeg V4 vs V4B: The Definitive Guide to the Differences

Discussion in 'Amps and Cabs [BG]' started by StuartV, Nov 29, 2012.

  1. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    What are the differences between a V4 and a V4B?

    I see this question asked pretty often. And it usually does not receive an answer that I think is truly complete and correct.

    After much reading, looking at schematics, etc., I decided to post this with the hope that you fine folks will correct me on anything I get wrong here and future question-askers can just be referred to this thread.

    I am only going to address the differences for models from the 1972 - 1974 era, as that is what I have.

    Here is my '72 - 74 V4:

    Here is Groovy Gravy's V4B from the same era:


    From the outside, the differences are:

    - V4 has a Reverb knob. V4B does not.

    - V4B has an Ultra Lo switch. V4 does not.

    - V4B Ultra Hi switch has 2 settings: 0 and +. V4 has 3 settings: -, 0, and +.

    - V4 has sensitivity switches for Channels 1 and 2. Settings are -9dB, -6dB, and 0. V4B does not have any sensitivity switches.

    Contrary to what I've seen posted on TB before, both amps have the same Midrange switch frequency points: 300Hz, 1000Hz, and 3000Hz.

    As far as I know, the backs are identical. They are both switchable to run at 2 ohms, 4 ohms, or 8 ohms.


    Schematics are available.

    The differences are:

    - Capacitors C3, C6, and C7 are 0.01uF in the V4 and 0.1uF in the V4B.

    These caps affect the low end cutoff frequency in the input section. C3 only affects input Channel 1. C6 only affects input Channel 2. And C7 affects both. I was told by an electrical engineer type that the V4 caps would start a low end rolloff around 40Hz and changing them to V4B specs would lower the rolloff frequency by around 10Hz. 40Hz is approximately a low E, so the general consensus is that the difference between a V4 and V4B due to these caps would be extremely hard to hear. Particularly if you're using a 4-string bass. The low B on a 5-string is 32Hz, so a discerning ear might hear a slight difference down there, between a V4 and V4B.

    - The V4 has capacitors C2 and C5. The V4B does not have these.

    These caps are in the portion of the circuit where the input sensitivity switch lives. C2 affects input Channel 1 and C5 affects input Channel 2. My tech tells me that these caps provide a little bit of extra gain in the input. He also said that, to make the V4 input circuit identical to the V4B, you would need to remove C2 or C5 or both (depending on what channel(s) you want to convert to V4B specs) and set the Sensitivity switch to 0.

    - The V4 has a sensitivity circuit. The V4B does not.

    See above, regarding capacitors C2 and C5.

    - The V4 Ultra Hi has 3 circuit paths. The V4B only has 2.

    The following info is based purely on my reading of the schematics - which I am far from expert at! So I could be wrong.

    For the V4B, it looks like the 0 position is a pass through and does not affect the circuit. For the + position, the signal goes through a 500pF cap.

    For the V4, it looks (to me!) like one position (right or left, but not sure which) is a pass through and does not affect the circuit. In the 0 position, the signal passes through a 120pF cap. And the other side position passes the signal through a 0.001uF cap.

    Hopefully, one of the technical guys on here will provide some clarification on this. In the meantime, I can only conclude that the V4 and V4B are only the same when the Ultra Hi on both are set to pass the signal through without filtering through any caps. I think that's the 0 position on the V4B, but not sure which position it is on the V4. You'd think it would be the 0 position there, but the schematic makes it look like that's not the case.

    - The V4 has a Reverb circuit.

    I believe that with the Reverb turned all the way off, this circuit does not affect the sound.

    - The V4B has an Ultra Lo circuit.

    I believe that with the switch set to 0, this circuit does not affect the sound. With the switch set to +, it is my understanding that the circuit substantially reduces the midrange portion of the sound, thus making the low end relatively louder. Many TB posters have stated that they never user the Ultra Lo because it makes the sound muddy and that the V4B does not have enough power to really push a good sound with that much bass frequency in it (as low frequency sound requires more power to achieve the same volume).

    What does it all mean?

    If you are a bass player that wants to use a V4 (ostensibly, a guitar amp) for bass, the only potential downsides are not having Ultra Lo available and the Ultra Hi being a little different. A lot of people don't use Ultra Lo or Ultra Hi, so they wouldn't care. In which case, the V4 is almost identical to the V4B. And if you want to make it exactly identical (except for no Ultra Lo and different Ultra Hi), you can have 2 or 3 caps changed (C3 and/or C6, plus C7), 1 or 2 caps removed (C2 and/or C5), set the Sensitivity to 0, and set the Ultra Hi to off. Then it will be identical to a V4B with the Ultra Lo set to 0 and Ultra Hi set to 0. Changing and removing caps is quick and easy (for somebody who knows what they're doing) and the caps are cheap. So, the overall cost to do this is very low. Probably, roughly, one hour of a tech's time.

    I have a V4 and I have now had it modded so that Channel 2 is identical to V4B specs. I had C6 and C7 changed to 0.1uF caps and I had C5 removed. Channel 1 remains all original except for the change to C7 which, technically, does affect Channel 1. However, I would say that the difference of just that one cap is so small that I don't think ANYBODY could actually tell a difference in the sound, whether it was a bass or a guitar playing through it.

    Hope this helps somebody...
    BluesOnBass likes this.
  2. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Didn't know that the V4 didn't have an ultra-lo. Guess it wouldn't need to, though. Oh well, I never use it anyway.
  3. Groovy_Gravy


    Apr 26, 2012
    I actually just acquired my V4B. All the possible EQ settings on the amp seem overwhelming. Maybe it would be a good idea to also have a post where people give their EQ settings and also what kind of bass they are using and type of cab (410, 115, 215, etc..)

    Im kind of lost when it comes to the switches and the knobs. is it a parametric type eq? the knobs raise or cut correct? what frequencies are they effecting? say i have the mid switch in the middle its 1000hz but what do the knobs do? Im trying to dial this baby in and get a good grip on how it all works!

    Im trying to get a nice classic bass sound, 60s/70s, (rock n roll!!) any EQ tips? Im using a Pbass through a trace elliot 410...any suggestions?

    Edit.. one more question. Why doesnt my amp have the Ampeg logo on the front? did it fall off? did the V4b not get them?
  4. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Somebody more knowledgeable than me will have to chime in on what all the knobs do. I can say this:

    The Mid switch sets the frequency center for the Mid knob. Yes, all the knobs do boost/cut.

    These Ampegs (like the V4, V4B and SVT) are known for their EQ circuits because the knobs do affect each other. Or such is my understanding. In other words, how you set the Mid switch and knob can affect exactly what the Bass knob does, etc..

    As for the logo, I think that is a difference between specific years within that "black line" era. It may also be a difference between the V4 and V4B. Not sure, though. I really just started this thread to document the differences between the two. Not really document how to use them. Other folks like johnk_10, JimmyM, coreyfyfe and numerous others are much more knowledgeable about that stuff.
  5. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
    The first few years didn't have them.
  6. TussinBot.


    Aug 21, 2010
    Burbank, Ca
    Id ask in the V4 club thread, maybe others would chime in with what settings they use. I havent used my v4b with 10s, but with my 1x15 and formerly with my 2x12 I use two pretty different eq settings. Id set everything to noon and start flipping through the midswitch, seeing where you want those, then add or remove bass and treble to taste.
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Vintage Ampeg amps that have the ultra-hi switch, including the V4, V4B, and SVT, state in their manuals that the amount of treble boost is dependent on the volume setting. They don't explain how it works.

    If you look at the schematic, you'll see that a small capacitor that is connected to the volume control wiper, is switched into the circuit when the ultra-hi is turned on. The transition point between normal frequency response and boosted treble response varies based on the position of the volume knob. The V4 and V4B will react differently because the component values are different.

    The bottom line is, the lower the volume, the more treble boost there is and the lower the frequency at which the treble boost kicks in. As an example, based on an analysis of the circuit of the B42X which has similar ultra-hi circuit topology to that of the V4B, the transition is at 636Hz with the volume at half, and 424Hz with the volume at one quarter. At full volume, there is no treble boost. For more information, check out Guitar Amplifier Preamps by Richard Kuehnel.
    rodl2005 likes this.
  8. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Wow, that's a cool tip I never knew about, David!
  9. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I suppose that in the real world, many people operate their amps within a limited volume range so it might be difficult to hear a shift in the transition frequency as you adjust the volume.
  10. Tuned


    Dec 6, 2007
    The input bias cap (C5) and coupling caps (C6, C7) have a high-pass filter effect to regulate how deep the amp will go, partly for tone's sake and also to keep from frying guitar speakers. That's the most important difference, C6&7 are bigger on the V4B and no input bias cap at all. The V4 EQ values aren't strange for bass purposes really, unless you're accustomed to the V4B's.

    The reason the EQ knobs interact is because the midrange is at a totally different point in the signal chain than the bass and treble. Between the bass/treble knobs and the midrange knob are all three triodes of the 6K11 (at different plate voltages!). More bass will saturate the 6K11 so the midrange will have more grit to it and sound very different than with the low end lower. It's a rather bizarre configuration really, and definitely where a lot of the unique magic happens. I'm a fan of slamming the entire preamp to the point of a little grit and backing off with a master volume if necessary. Works great with my SVC-PL too.
  11. Bearded Wizard

    Bearded Wizard

    Oct 26, 2012
    Thanks for this! I use an old V4 and was wondering the differences between that and the V4B. I do play a 4 string bass tuned standard, the V4 works fine for me.
  12. metalinthenight

    metalinthenight Supporting Member

    May 5, 2008
    Charlotte, NC
    Super nerdy and super neat. Thanks!
  13. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    As both an electronics student, and someone who is in negotiations to trade for a V4, this thread makes me smile.

    So, here's my question. Being that I play mainly 5 string, would you guys suggest that I switch out the caps for the ones that roll off lower? Would I notice a real difference?

    Thanks for the really interesting post Stuart!
  14. StuartV

    StuartV Finally figuring out what I really like Supporting Member

    Jul 27, 2006
    Manassas, VA
    Well, as an electronics student, I expect you could do the mod yourself for about $2 in parts and maybe 2 hours of your time. So, in my mind, the question is why NOT do it?
  15. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Me personally, I'd wait before I heard it before I came to any conclusions about modding it.
  16. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I think what Jimmy says makes sense. Use the amp for a while, get to know it. Then perform whatever mods you want and listen for differences. You can always undo them.

    My gut tells me that I would want to make it into a V4B. This was a dedicated bass amp, the V4 was not. Also keep in mind that most designs are a compromise. They use certain capacitors because that is what was in other amps. By purchasing a lot of one type that is used in many amps, they got a better price. So that is what they used in their designs. They had constraints. The biggest example of this is the cap cans in the SVT. I feel that they skimped at the cost of performance in order to use the same cans that were in other amps. So you can consider going beyond mods of a V4 to a V4B.

    On the down side, players expect certain tone, headroom, and distortion characteristics that are what they have become accustomed to hearing. You feel comfortable with what you know. This becomes the benchmark that they look for. Improve an amp too much and they might not like it. So when listening and evaluating, you have to do it without any preconceptions. Keep in mind that mods can be done to improve the amp without going so far as to change its character.

    These type of mods are fully reversible. You can spend a lot of time clipping in and out components to get the sound just right. Go to town and have a good time doing it. :p
  17. Model the circuit in SPICE and see!

    Personally, I'd leave it. I've used my V4 with my 5 string and there's plenty of low end. There's numerous threads floating around here concerning the misconception of 1st harmonics through cabs and amps
    rodl2005 likes this.
  18. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    So far we have only used Multisim (NI). Are the modeling capabilities similar? (Hope this is not a derail. I will be glad to share what I find if I can model it out.) If I know Multisim, is SPICE pretty easy to get around?

    I agree that less is more when doing mods. I tend to err on the side of don't do it unless there's a problem. But the point that I can do, and undo, with little or no time/effort is valid.
  19. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Modeling is good and gets you into the ballpark but swapping parts and listening is the ultimate test. Today a designer would do both. Some of these mods are so simple, quick to perform, and fully reversible that, if you have the tools, they are well worth giving them a try.

    These amps were designed with data sheets, load line charts and rules of thumb. Their calculators were slide rules. They built prototypes, listened, and refined the designs. They did a pretty good job.

    When I'm voicing an amp, I sit there with a bag for resistors and clip them in and listen to get things to sound the best. Experimenting can sometimes provide some surprising results.
  20. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA
    Good advice beans.