Ampeg V4 EQ in a box

[chuckfalcon]

Hi everyone.  I have an Ampeg V4 with a very interesting tone stack.  It has Bass, Middle, and Treble like normal but it cuts and boosts those frequencies.  Its great.  I am interested mostling in the mid section and would like to be able to put it in a pedal form.  Like 1 knob to boost or cut, and then a 3 way selector to select low-mid/mid-mid/hi-mid.  Just a very simple circuit to dial in a good frequency. 

Here's the schematic for my Ampeg V4:

If anyone has any ideas on how to make a simple EQ or something to get me started or some kind of advice it would be much appreciated.  I have only dove into Fuzz pedals and have built several but not EQ just yet.

Thanks,
CF

[mth5044]

The tonestack there is your typical Baxandall, but they don't show the values. Also, I'm only seeing a bass and treble control in that schematic, no mid control. Are you sure that's the right one?

Edit: ah there, it is. I'll have a look at it a bit more.

Edit 2: looks like the midrange control uses a transformer. Mercury Magnetics sells the transformer, but it's probably $100+ and might be large. Check these threads for some info

http://www.ampegv4.com/forum/viewtopic.php?id=929
http://www.ampegv4.com/forum/viewtopic.php?id=1852

you might want to do some research on simulating these things with gyrators, but I can't help with that, unfortunately. I did do something to simulate the mid range control that the SS Vox amps used, but that was a while ago and I don't have access to that info at the moment. This link and LTSpice was very useful though.

http://www.geofex.com/article_folders/eqs/paramet.htm

[Mark Hammer]

Isn't that one of the Boscorelli projects?  Oops, my bad.  The Tone-O-Matic mimics the tone stack from an SVT.

[PRR]

The Bass/Treb stack is James (not Bax).

The midrange uses a *choke*. These are typically custom for the job. Hand-winding would be tedious, even if you knew all the parameters. You might see if FlipTops has it as a spare part.

The 6K11 mid-tone driver can be replaced with an opamp. (Some thought needed.)

Congratulations on using the mid-control. The users of my VT never understood it.

Suggest you look at other mid controls. Gyrators (opamps) can do just about the same thing, without the choke.

[tonight, we ride]

You may want to dig deeper on the www.AmpegV4.com forum... lots of information there and I'm pretty sure this has been discussed in depth, but I'll try to track down the thread i'm thinking of.

I thought that the tone module was discrete, basically a few resistors, caps, and inductors, but it has been a few years since I was on that forum.

I have to say, this would be a worthy project to pursue; I've got two Ampeg V4s and they are two of the best sounding amps I've ever heard (for when I can play that LOUD!).

[PRR]

> I thought that the tone module was discrete, basically a few resistors, caps, and inductors

It is a feedback amplifier. Steve Dove calls it "swinging output", but the figure in that edition of the book really shows "swinging input". My much older edition has the right drawing, and some critique (partly visible in the g-Books snippet). His main objection is loss of headroom. In the Ampeg there is a gain of 140 to an output no more than 30V, signal level is 0.2V, Mid-EQ amplifier can swing 60V, headroom is way ample.

http://oi66.tinypic.com/30cox15.jpg

The part in the dotty line (and some stray bits) *is* an op-amp. It has two inputs of opposite action, and an output, and high gain (about 500). It is not a universal opamp (low Z on one input, no DC gain) because that is not needed here and would cost much more. With some care in DC biasing, a chip opamp will do the same thing. Actually with a couple fewer parts.

R208 R211 split the output in two legs (balanced bridge). R208 leg runs to R202+R203 for gain of about 8. R211 leg runs to R210 for loss of about 8.

Stoppers R209 R212 hang on these legs. If a stopper is shorted to ground, that leg's gain/loss goes way up. Since there is DC in the amplifier, there have to be blocking caps somewhere, and Ampeg put two here. They picked their values to "be" part of the tuned circuit below (getting lower values with added series caps).

50K Lin pot can put load on R209 leg or R212 leg or not much load on either leg.

Load is a series L-C tank. A series tank goes to "zero" impedance at resonance. (Opposite of a parallel tank which goes to infinite impedance at resonance.) The values picked give high impedance at 100Hz or 10KHz, but at the selected frequency the impedance goes to "zero", limited by coil self-resistance and those stoppers R209 R212 plus any part of the pot still in circuit.

Ampeg used a slide-switch (fronted by a rocker). Unless you can get that part, a 1P3T rotary will do the same job (and less contacts to tarnish).

You "can" get the CUT without the opamp, though you will need a little make-up gain.

The boost is all about selective feedback around a hi-gain (3 stages) amplifier.

[tonight, we ride]

Very cool!

Thanks for the correction and for the info PRR!

You definitely saved me some digging through old bookmarks.

[chuckfalcon]

Hey everyone, thank you so much for the help.  Looks like I am opening up a big can of worms with this one.   I didn't realize that I would have to put a full transformer in the pedal to make it work.  I am new(isn) to making pedals.  I have successfully built some univox super fuzzes on some perf boards, and have breadboarded several fuzz faces. 

Would it be possible to make this run on 9V?  I am down to make this thing work and come up with an idea.

[chuckfalcon]

.

[chuckfalcon]

You may want to dig deeper on the www.AmpegV4.com forum... lots of information there and I'm pretty sure this has been discussed in depth, but I'll try to track down the thread i'm thinking of.

I thought that the tone module was discrete, basically a few resistors, caps, and inductors, but it has been a few years since I was on that forum.

I have to say, this would be a worthy project to pursue; I've got two Ampeg V4s and they are two of the best sounding amps I've ever heard (for when I can play that LOUD!).

How do I get onto that forum?  I have tried but it seems as though you have to be invited by an existing member.

[chuckfalcon]

The Bass/Treb stack is James (not Bax).

The midrange uses a *choke*. These are typically custom for the job. Hand-winding would be tedious, even if you knew all the parameters. You might see if FlipTops has it as a spare part.

The 6K11 mid-tone driver can be replaced with an opamp. (Some thought needed.)

When you say "op-amp" are you talking about a "chip"  like the Texas Instrument one in the Tube Screamers?  I have never built my own circuit so I am kinda in the dark as to get certain voltages/gains to make certain transistors or chips to act a certain way.

Congratulations on using the mid-control. The users of my VT never understood it.

Thanks!  It is one of the coolest things about Ampeg, to me at least.  In a band/studio session, sometimes you can't always hear yourself and the answer isn't always turn up the volume knob.  That Mid knob is magic and transport you to an empty space in the sounds spectrum!

Suggest you look at other mid controls. Gyrators (opamps) can do just about the same thing, without the choke.

Looks like I am gonna dig deep into gyrators.  I looked at the links that everyone has posted but I am still trying to learn what the heck is going on with it.

[J0K3RX]

Hey everyone, thank you so much for the help.  Looks like I am opening up a big can of worms with this one.   I didn't realize that I would have to put a full transformer in the pedal to make it work.  I am new(isn) to making pedals.  I have successfully built some univox super fuzzes on some perf boards, and have breadboarded several fuzz faces. 

Would it be possible to make this run on 9V?  I am down to make this thing work and come up with an idea.

These are inductor coils, not a full transformer... and quite small.
Look at the link below and have a look at the "parts list" at the bottom you will see "L1 - 800 mH / L2 - 300 mH / L3 - 100 mH"... This looks like the same on just about all models including the bass amps. You could use individual coils or one multi-tap. 
http://www.vintageampeg.com/wp-content/uploads/2014/07/VseriesDist.jpg

Here is a hand made coil for a SVT which are the same spec as the V series if I am not mistaken..
http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampInductor.html

Looks like Ampeg tried to incorporate this type of tone stack into their solid state amps as well.
Follow this link, look at the G212/G410/G412 schematic
http://bmamps.com/Tech_sch.html

Discussion on this forum.
http://www.diystompboxes.com/smfforum/index.php?topic=98332.0

[tonight, we ride]

How do I get onto that forum?  I have tried but it seems as though you have to be invited by an existing member.

It looks like they currently want people to use the contact page, located here: http://www.ampegv4.com/contact.php
to request registration

There's a lot of good info on there if you own one of these beasts, so don't get discouraged if it takes some time to get a response!

[PRR]

> are you talking about a "chip"

Yes.

> Here is a hand made coil for

Thanks. Good to know it can be hand-wound with an available core and a few hours of tedium.

> Ampeg ....incorporate this type of tone stack into their solid state amps

Thanks. This is a Linden-approved chip implementation, and even using the same choke-coil as the hollow-state design.

The specified chips are remarkably antique, and not critical. TL072 would work as well, easier, well-known to sound fine in guitar work, and readily available today. Pin numbers will be different.

The first opamp provides input and considerable gain; probably more gain than you want to add to a happy guitar chain. The "18K"(!) pot can be 25K just fine. For near unity gain +/-, R2 could be 1.5K, use 10K for the pot. Or the whole first stage could be a unity gain buffer.

Power supply is bi-polar. Ampeg had tons of voltage available from the power amp so they used +/-15V. +/-9V would work fine. To run on single 9V power would need a lot of re-work.

[chuckfalcon]

How do I get onto that forum?  I have tried but it seems as though you have to be invited by an existing member.

It looks like they currently want people to use the contact page, located here: http://www.ampegv4.com/contact.php
to request registration

There's a lot of good info on there if you own one of these beasts, so don't get discouraged if it takes some time to get a response!

Oh yeah!  I remember that page.  I can fill out everything except for the "security code".  It doesn't show me any code just a small blue box with a white question mark.  I have tried refreshing and restarting.  No dice 

[Bill Mountain]

So...with the voltage divider on the input, is the input impedance 100K or 147K?  I've always wondered this.

> are you talking about a "chip"

Yes.

> Here is a hand made coil for

Thanks. Good to know it can be hand-wound with an available core and a few hours of tedium.

> Ampeg ....incorporate this type of tone stack into their solid state amps

Thanks. This is a Linden-approved chip implementation, and even using the same choke-coil as the hollow-state design.

The specified chips are remarkably antique, and not critical. TL072 would work as well, easier, well-known to sound fine in guitar work, and readily available today. Pin numbers will be different.

The first opamp provides input and considerable gain; probably more gain than you want to add to a happy guitar chain. The "18K"(!) pot can be 25K just fine. For near unity gain +/-, R2 could be 1.5K, use 10K for the pot. Or the whole first stage could be a unity gain buffer.

Power supply is bi-polar. Ampeg had tons of voltage available from the power amp so they used +/-15V. +/-9V would work fine. To run on single 9V power would need a lot of re-work.

[PRR]

> 100K or 147K?

Or 79K?

It's not a megabuck experiment. Get 2 jacks, 3 resistors, ohm meter, and find out.