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#1 2008-03-22 02:56:00

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

So, I was working on my VT-22 today, because once again I had a coupling capacitor go out.   The caps that came in my V-4 were absolute garbage,  and I have now replaced EVERY SINGLE ONE. 
I will post a picture of the little bastards.

anyway... because of this I took off the control circuit board,  and I noticed that all of the parts in the Baxandall tone circuit were all contained in this Integrated component.  I will post a picture.

I decided to replace this one component with nice discrete capacitors and resistors.   because my feelings on the bass and treble controls of my V-4 have always been lukewarm.  and this seemed to present a very probable explanation.

After doing this mod (can I even really call it a mod?) I can safely say that my V-4 sounds Leagues better than it ever has.   it was absolutely breath taking how big the difference was...
highs were less harsh,  lows were more defined,  but almost soft.
it was as if everything I disliked about the high and low controls was removed. 

If you have a V-4 with this tone circuit I highly recommend the change.

I took some pictures with my cell phone and will post them soon.  along with simple directions for those who might need them.
probably will have to be tomorrow.

-Edit note-
I changed the title of this thread from "tone circuit mod for Magnavox era V-4s"  to the current title, because it became clear to me that most if not all V-4s,  and most other late sixties ampegs all used this same tone circuit.  (B-25, non-blue diamond Gemini 1s, V-2, B-15-s)   By taking a look at your schematic (keeping an eye out for PEC#6470000) and scoping out the back side of your control PCB it should be very clear if this mod applies to your amp.

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#2 2008-03-22 17:08:16

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

What year V4 are we talking about?
I've got a 1972 and wondering if this applies to my amp.

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#3 2008-03-23 21:09:21

hangman
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

My VT-22 is same Era,  and the schematic dated 1970 shows the baxandall circuit as one 7 pin unit,  rather than discrete components.  So its safe to assume your V-4 could benefit from this.   

here is a schematic with the 7 pins labeled,  and then I inserted the discrete part values.   I meant to post pictures of what it looks like completed... but I don't have a digital camera, and the pics I took with my cellphone are awful.  so be patient with me there.  I'll post them as soon as I can.

http://homepage.mac.com/lighterthanairflight/images/V4tone.png

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#4 2008-03-23 22:26:01

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

OK, I'm with you now. I'm ready to rip into this one as soon as I see some photos. How did you determine the values for the resistors and caps? I don't see any values listed on my schematic?

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#5 2008-03-24 02:15:33

hangman
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

The circuit is pretty standard,  ampeg used it in a lot of their amps, as did magnatone and others.  the values are almost always the same.
for reference I used a B-25 schematic from the Groove tubes book.  which also used PEC#6470000,  however in that schematic they actually labeled the values of each part. 

the bass and treble controls sound soooo much better.   why use a nice hifi eq circuit but use really crummy ceramic capacitors?

I'll try to get some photos posted tomorrow.  I really need to just go buy a digital camera.

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#6 2008-03-24 21:47:45

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

so here they are. 

http://homepage.mac.com/lighterthanairflight/images/V4tone1.JPG
here you can see that ugly rectangular part (PEC#6470000) which is our tone circuit.   this one is kinda bumpy,  like you can see the ceramic caps inside,  but mine was absolutely flat.
http://homepage.mac.com/lighterthanairflight/images/V4tone2.JPG
View from the back side of the circuit board.. if you have doubts about weather or not your amp could benefit from this mod,  just look for that solder terminals labeled 1-7 right behind VR104.   I haven't checked earlier ampegs,  so I really don't know if they have this or not.
http://homepage.mac.com/lighterthanairflight/images/v4tone3.JPG
This picture just shows the same area as the first photo but with PEC#6470000 removed.
http://homepage.mac.com/lighterthanairflight/images/V4tone4.JPG
This picture shows the board with new parts installed. 
the .00047uf is actually a .0005uf.
Also... the parts are not really arranged as cleanly as they could be.  I didn't actually solder them in for the picture because I used my parts V-4 to do this mockup.   You get the idea though.


the hardest part of this is getting the control board out.  which just involves removing all the rocker switches and removing all the screws from the main circuit board to allow a little more room.

because this board is a hassle to get to, you might replace c105 too while you're at it.   my reverb would feedback past 12o'clock until I replaced this capacitor.   (but like i said all the caps in my amp were crap... so take that advice with a grain of salt.)  but if you are having the reverb/feedback problem... take this opportunity to make your tone circuit sound better AND make your reverb work well again.

So good luck!  You wont regret doing this mod!  if you have questions just post them here or email me.

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#7 2008-03-24 22:08:57

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

Am I viewing all of the photographs? I don't know what the heck I'm looking at. Could you send me the jpegs to:
tinabloom@comcast.net

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#8 2008-03-24 22:31:28

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

I resized all four pictures so they wouldn't take so much time to load. 
I also emailed them to you.

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#9 2008-03-25 01:17:14

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

Thanks Steve, I saved your pictures to my hard disc. Last time I was inside my V4, I replaced quite a few caps - just for the heck of it. My plan is to order the rest of the caps that I don't have and change them all out eventually.

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#10 2008-03-25 01:56:44

hangman
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

yeah... normally I tend to leave tone and coupling caps alone unless they are not performing right/well.   it just so happens that in my ampeg,  none of them were performing right/well.

once you've done the mod let me know what you think!

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#11 2008-03-26 00:41:45

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

Steve:
A small correction for the schematic. Pin 5 is not listed. But from your components on the board, it has to be between the .01mfd cap and the 22K resistor.

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#12 2008-03-27 02:47:30

hangman
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

I did notice that,  but because it was the only point on the schematic of PEC#6470000  that was not labeled, i didn't think to label it. 
but perhaps you are right,  best to make things clear.

I've modified the schematic,  thanks for mentioning it!
-steve

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#13 2008-03-28 22:07:01

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

Steve:
I have the parts on order so I can do this mod. Can you walk me through the procedure to get access to the circuit board for this?

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#14 2008-04-06 20:41:45

hangman
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

sorry I didnt see your post earlier diablo. 

To get access to the circuit board,  you have to first unscrew all the rocker switches. 
(remember the orientation of the switches, mark them if necessary.  it will save you the trouble of putting them in backwards)

2. remove the knobs and nuts from the pots.   the control board should be loose at this point.  be aware that there are lock washers on the pot shafts in the inside. 

3. you need to pull the control board,  I found this to only be possible if the main PCB is unscrewed from the chassis too.   otherwise there wasnt enough clearance.   
because the control board will still have all sorts of wires connected to it going to the main PCB you need to be gentle.  the board won't pull out completely,  but you should have plenty of space to work.

once the main board can be moved,  the control borad is much easier to pull up.   

Now perform the mod (see post #6)

I also replaced caps 101,102,103,and 104 which are the caps in the ultra high circuit.    I just really dislike ceramic capacitors anywhere in the signal path,   there are so many better options these days.   I never used the ultra high switch before because it sounded so shrill.  but,  now I actually leave it on.

so once the mod is done, double check your work! replace the control board,  screw the main pcb back into place,  replace the rocker switches (check orientations!)  but don't totally button your amp up... we gotta test it first, and there is always a possibility that a mistake was made.  you don't want to totally reassemble your amp only to find out that you have to disassemble it again.

So test it,  and if everything was done correctly, you will be hearing your V-4 sounding the better than it ever has!

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#15 2008-04-09 23:11:21

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

Thanks Steve for the info. I've got all the parts sitting here and am just waiting for some free time to get started.

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#16 2008-04-10 23:52:18

hangman
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

let me know what you think once you've finished!

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#17 2008-04-13 03:19:30

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

Hey Steve:
I just finished with the tone mode by replacing the Baxendall circuit with the discrete caps and resistors. I cleaned the pots and switches while it was apart. I also replaced all the caps except for C105, C106, C107, and C205. I ordered caps that were too large for those four. I used mostly orange drops and some tubular polypropylenes. I also found that Joe Piazza made a few mistakes on the V4 schematic. Joe lists C205 as a 1 mfd - it should be a 0.1 mfd. Joe also lists R41, R42, R47, and R48 as a 470 ohm - 1 Watt. Those are 2 Watt resistors in my amp. Anyway, I was happy that everything worked like it should when I powered it up. The amp sounds beautiful. At this point I think I'm finished playing amp technician with the V4.

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#18 2008-04-14 20:09:06

hangman
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From: Seattle Washington
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

glad you're happy.   

thanks for the heads up on the mistakes on joes schematics.   I usually use the schematics Ampeg provided.

-steve

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#19 2008-04-14 22:42:52

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

I'm pretty slow and methodical when working on the amp. I was quadruple checking the parts - ampeg schematic, Joe's schematic, actual old part value printed on component, new part value, and measured values of resistors. I like to make sure everything agrees.

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#20 2008-04-14 23:13:14

hangman
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Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

nothing wrong with being thorough.   
what do you think about the eq?  did you hear a big difference?

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#21 2008-04-15 12:14:55

Diablo
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Registered: 2007-12-01
Posts: 139

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

It's hard for me to tell. The best way to know would be to have a pair of the amps side by side to compare. It's tough for my ears to remember what is sounded like a week ago. Thanks for researching the Baxendall circuit and posting up the suggestion. If the original circuit went on the fritz, I wouldn't have had a clue as to what was inside the ugly brown rectangle with the 7 pins. It's a little strange why Ampeg went with that multi-component since they used normal discrete replaceable components every where else in the amp. It seems to go against their philosophy of "servicability". Perhaps you should send a note to Joe Piazza so he can add your mod  to his V4 schematics.

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#22 2008-05-05 20:05:43

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

So,  I performed this mod to my later model Gemini 12.  Because this amplifier is a 2 channel amp,  I only performed the mod on one channel, so I could at least get an idea of the difference.   

the difference is very much as I described.  The highs are less abrasive,  less shrill.    and the lows seem warmer, and more defined.   overall the sound is more clear,  and smooth.
while these differences are most audible when the treble and bass controls are boosted or cut,   to my ears the channel still sounds better with the Eq flat. 

I will try to get some sound samples recorded so those of you interested can hear the difference.   

-Steve

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#23 2010-09-20 16:49:02

Liquids
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From: CT
Registered: 2010-08-01
Posts: 491

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

I realize this is an old thread, hope it's okay to revive it.

I'm a bit skeptical about this kind of mod, and it's not so easily done given the PCB.  I also don't want to pay for the time and trouble to have my tech friend do this, since it's more a 'fun' mod than a needed one, and may not be of much consequence.

However I can discharge caps, solder, and build my own pedals from scratch, etc.

I got to thinking that it might be easy to just build up a similar simple board using some perfboard and maybe some bus wire, if need be.  I could probably insulate it with electrical tape and secure it similarly to how it is now, but I am curious if it would be easier to attach it via the back side of the board - are there space issues in that regard?   

Also, I could use fairly low voltage components here (that would be smaller) since there's no voltage on these caps, correct?  50v polyester caps are probably as small or smaller than those big ceramics.


Matthew

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#24 2010-09-20 23:12:34

hangman
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From: Seattle Washington
Registered: 2006-09-04
Posts: 1848

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

It really isn't that difficult to do.  if you wanted to put the components on perfboard and mount it that way it would be fine on the front or back or the control board. 
i just felt like it was easier to mount all the components strait to the board.
you could use smaller voltage caps if you wanted... but its hardly necessary,  especially if you use sozos  which are smaller than the ICs that I used in the pictures.    plus I'd put the sound of the sozos up against any 50v polyester cap you can find. 

I think the mod is well worth doing, its so simple and since nothing is really changed,  it barley deserves to be called "mod"  its really just swapping out some lousy caps for some good ones.

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#25 2010-09-21 17:38:56

rob
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Registered: 2008-10-28
Posts: 171

Re: Tone circuit upgrade Mod for V-4s (And other Ampeg amplifiers)

Thanks for the great info Steve. Nice job! I use my v4 primarily for guitar. The reason I picked up the peg was I have been shooting for that great Stones live guitar tone from the late 60's to mid 70's. The v4 really nails it. I know from my own amp building experience that ceramic disk caps (although they are cheap and sometimes microphonic) contribute to the raspy signature of many amps. I really like the idea this upgrade, but am worried I may loose some of the characteristics of the amp. What is your opinion?

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