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V4 - V4B Conversion

Old Fart

Old Fart

Mar 11, 2011
2,518
4,371
#1
I've been using a VT-22 I want to put back in the combo cab since it is such a pristine example. So...I landed a V4 that is going to my tech. He is a good tech and familiar with these amps, but apparently not familiar with the capacitor swap. Can anybody tell me what caps and values go into the input stage to change the mid frequencies?

By the way, the V4 already sounds better than the VT-22 because the former has been recapped and the latter has not. When I got my SVT recapped, it sounded remarkably better. I'm no expert, but I can hear. Which is why I love these Ampegs. Would you disagree, JimmyM?
 
#3
You definitely have to keep up with stuff like that to get the most out of them, yes. Don't have to be obsessive about it, but cap swaps do need to be done from time to time. Can't help you with values, but from what I've heard, both the V4B and V4 were the same.
 
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#4
This is true, RoadRanger.

The frequency switch cutoffs are different Jimmy. From the top of my head, the highest of the three cutoffs on a V4 is 3.0K Hz, while the V4B is 2.5K. Simple capacitor change. Plus the V4 has the input sensitivity switches and reverb tank while the V4 B, I believe, has an Ultra Low switch. There are differences. That said, I believe they sound the same with the tone knobs at 12:00. The V4, without the conversion, is already a good bass amp. Very good. The old ones are, anyway.

My point on the capacitor thing is that recapping an amp of this age changes its performance. More articulate. Less flabby. According to my ear.
 
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#5
They were labeled that way, but Bill Hughes, the guy who invented them, said they were the same. Didn't say which one they went with, but I'm pretty sure it was the one with the 3k. I have heard of folks modding them and getting really good results with that mod, though, so don't let me stop you.
 
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#6
On the V4 the caps are c3, c6, and c7, the coupling caps between the first few gain s. Change them from 0.01uf to 0.1uf.

Both the V4 and V4B were labeled for the same mid range points and I believe used the same PEC tone module for the tone controls. You can see them in the portaflex wiki pages.

imag0087-jpg.jpg
 
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#7
The V4 and the VT-22 are the same amp. As was mentioned, there are slight differences in the V4B. The V4 also differs in that it has a sensitivity switch at the input. If you are never going to be using the reverb, the tube and reverb pan can be removed and stored. It saves wearing out a tube. The circuit boards and power supply are the same in the V4B, the boards just aren't populated as they are in the V4/VT-22.

The PEC modules used in the tone circuit are the same in all the Ampeg amps.

It is important to find the right schematic for your V4. As noted on the schematics, some parts at the input may or may not be installed in the amp. So just because it's on the schematic, it doesn't mean that the part should be there. This is due to revisions. In the case of the early V4B schematics, what is shown on the schematic should be in the amp.
 
#9
coreyfyfe said:
On the V4 the caps are c3, c6, and c7, the coupling caps between the first few gain s. Change them from 0.01uf to 0.1uf.

Both the V4 and V4B were labeled for the same mid range points and I believe used the same PEC tone module for the tone controls. You can see them in the portaflex wiki pages.
Click to expand...
Oops....you're right. I forgot they were labeled the same, too.
 
#11
This is not one of those "do a search" lectures. There is a really good thread on here about it. The guy goes very deep into detail about it. I used to own one and it was a great read at the time.

As stated earlier, the schematics are online everywhere. If your tech can't find them and doesn't get the differences, perhaps you may want to find a different tech.

The conclusion of most is that the differences are very subtle, even if you can hear them at all.

Mine was a very early V4 (not B). It sounded amazing. I just have gear ADD so I like to swap out gear often. I can't think of one bad thing to say about a vintage V4 though. Pretty tough to beat.
 
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#12
Here's the aforementioned thread:

Ampeg V4 vs V4B: The Definitive Guide to the Differences | TalkBass.com

Here's what was done to my V4's pre-amp to put it at V4B spec's:

1) CH.1 changed to V4B specs; C2 and C7 replaced with .1uF Orange Drops.

2) Preamp tube bias R5 changed to 3.3K. R3, R4, C2 removed from board.

3) Sensitivity switch CH.1 disconnected. Circuit removed from board.

4) Bass bright caps changed to 500pF & 250pF

I left Channel 2 alone and did not remove the reverb. Of course the conversion doesn't net you the ultra-low switch which sucks anyway.
 
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#14
You guys are the BEST. I will do some reading.

Some folks seem to be saying there is no difference in the tone circuits, and others say there is. Right? That is what I am hearing? Irrespective of who is right or wrong.
 
#15
Old Fart said:
You guys are the BEST. I will do some reading.

Some folks seem to be saying there is no difference in the tone circuits, and others say there is. Right? That is what I am hearing? Irrespective of who is right or wrong.
Click to expand...

Not quite, everybody is right. There' s a little encased brown rectangular module with 7 pins that is part of the amp's tone circuit. This is called the James (some people call it the Baxandall) tone circuit. The same part is used in all Ampeg amps of that era. There are other external components in the V4/VT-22 that are not part of or differ from that in the V4B.


Below is a reference V4B schematic that you can use. Also attached is a VT-22/V4 schematic and service manual. It may not be exactly the same as yours, depending on the year your amp was made.
 

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#17
Awesome, BOT. I wish I had your knowledge. Then again, I guess I do. Because you share it with us.

Another quick question (I am taking the amp to my tech today): This amp was 'serviced' by someone I don't actually trust. He swapped out all the vintage tubes in favor of modern units. <frown> The amp has a terrible buzz/hum. It also gets VERY HOT. The power tubes are EL-34s. Is this the reason why it gets so hot? Would the (untrusted) tech have rewired it in order to accommodate the EL-34s?
 
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#18
Modifications have to be made to the amp to accommodate EL34s, resistors in the bias section and the screen resistors, so if you want to convert back to 7027/6L6 you'll need to correct that. Also I think you have to make a change on the tube socket, check that too.

The heat may be excess heater current draw of EL34s vs 7027As.
 
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#19
Pin 1, suppressor grid, of the EL34 needs to be grounded or better yet returned to the raw negative bias supply. The 7027 does not have such a pin being a beam tetrode. The heaters require 5-600mA more for each tube putting an additional strain on the PT. Bias needs to be lowered by about ten volts to keep an EL34 happy. All in all it's not a "hotrod" modification and is one I would not recommend be made.
 
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#20
I would carefully check the amp wiring against the schematic. You can never assume that it is wired correctly. As Corey and Paul said there is a larger heater current draw by the EL34, 1.5A per tube vs 0.9 for the 7027A. The extra 2.4A for four tubes does put a strain on the power supply and would make the amp run hotter than normal.

There are many reasons for the hum. Could be bad power supply caps, especially if the hum is loud. One thing to check is the speaker out and ext. speaker jacks. They both need to be isolated from the chassis or there will be hum caused by a ground loop. This is done with Switchcraft S1029 and S1028 shoulder and flat washers. If you look at the speaker output jacks, you normally see a non metallic washer behind the nut. Same on the inside.
 
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