Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Oct Mon 22, 2018 7:24 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 45 posts ]  Go to page 1, 2, 3  Next
Author Message
 Post subject: How to add overdrive to a guitar amp circuit
PostPosted: Jan Sun 07, 2018 6:50 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2274
Location: Spring Hill, FL
This is one of those times where I'm better at taking an existing circuit and running with it, and not so much at designing my own twist. I want to build a guitar amp, and I'm thinking the Fender circuit below is a good start. I would only have one input instead of four. But as the title says, I want to add an overdrive. On a side note, I"m also wanting to add a regular tone stack in place of the single tone control, which I have have the schematic for the standard issue Fender stack.

So on the overdrive, here's my thinking, and where I need help. As I understand it, the overdrive would essentially be just another pre-amp stage, so I'm thinking another 12AY7 so I don't have as much gain, but I don't know how to figure the values for the plate resistor, or the cathode bias resistor and bypass cap. Rather than having two inputs, one clean and one dirty, I'm thinking a selector switch.

I want to learn from this, so I'm not just looking for someone to just say "Here's what values to use". Obviously that helps, but what I'm looking for is how do I go about figuring out those values? Or if I'm barking up the wrong tree altogether, where am I going wrong.

Image

_________________
Michael


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Mon 08, 2018 2:18 am 
Member

Joined: Nov Mon 13, 2006 2:59 pm
Posts: 243
Location: Friendswood, Texas
I have built a couple of guitar amps using the old Fender schematics and turret boards like the originals used.
I make a 5F1(Champ) clone and a 5E3 (Tweed Deluxe) clone. Although I don't play guitar, I have a friend who "Test Drives"
the amps I built. The 5E3 responded well to an overdrive pedal, although if you used a higher gain twin triode in place of the 12AY7,
the distortion point could be lowered. If I built another amp, I would incorporate a Baxandall tone stack with bass, mid, and treble adjustments.
Most put the tone stack after the pre-amp and ahead of the phase inverter. I'm not a pro at amp design but there are plenty of forums out there
with builders as members. a good one is http://www.ax84.com. They have a forum there where you could ask questions about the design you want to build. also tyrhttp://www.tdpri.com/forums/glowing-bottle-tube-amp-forum.96/.
You might try to find an RCA Receiving Tube Manual and look at the Resistance Coupled Amplifier section for design parameters for specific tubes.


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Mon 08, 2018 2:26 am 
Member

Joined: Aug Sun 23, 2015 6:01 pm
Posts: 791
Location: South Jersey East of Philly
First...I'll pretty much echo Dave1138--If you are adding a tone stack, you will need at least 1/2 of a 12AX7 to recover the gain loss from the tone stack..nothing is for free..Then you can use the other half of the 12AX7 to jump the gain some more.
OK....AFA plate resistors---There are formulas to figure that out...but being a guitar player, I found out theory is one thing, sound is what matters. So basically I am saying is what you didn't want to hear--- Copy Fender's Fender's plate, grid and cathode values and go up or down some in values and see what happens to the sound. Main thing is to keep the plate voltage within a reasonable range comparable to the other pre-amp plate voltages so you don't upset the voltage distribution to the other B+ legs.
As simple as all this sounds, there are a lot of decisions to be made in just adding another stage and tone stack to an amp circuit.


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Mon 08, 2018 3:55 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2274
Location: Spring Hill, FL
Tbirdkid wrote:
So basically I am saying is what you didn't want to hear


Actually, what you've offered up is helpful enough to know where to start. Often times in a question like this, some will just throw out suggestions (with good intentions) along the lines of "You could use an OU812 tube with a 10K plate resistor and 1 uF bypass cap" but no explanation how the came up with that to start with.

_________________
Michael


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Tue 09, 2018 2:14 am 
Member
User avatar

Joined: Jun Wed 08, 2011 2:33 am
Posts: 7250
Location: Ohio 45177
How about rob Fender's two stage 12AX7 input stage from something like a Champ for the extra gain driving the phase inverter and see if you can overdrive that input. Would certainly be alot more gain that a single AY7 stage.


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Tue 09, 2018 4:39 am 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 2882
Location: Vancouver Island, Canada
wazz wrote:
How about rob Fender's two stage 12AX7 input stage from something like a Champ for the extra gain driving the phase inverter and see if you can overdrive that input. Would certainly be a lot more gain that a single AY7 stage.
Right, I agree that you can start with a nominal value, copied, and then adjust.
The theory is good background, tho, so read up on it.

There are plenty of 12AX7 circuits to copy. I think I got some good info from Duncan amps, but it was back in the 90's. The design I copied had way too much gain, using the 12AX7 tubes. I swapped in a 12AT7 I think, or 12AU7, lower gain tubes. 12AY7's were scarce as I recall.
I found a good way to do overdrive is with a volume control after the 1st stage, so you can turn it down to where you like it best. Then a resistor could be subbed in, but I've also left the pot in there for adjustable gain.
My best results were with the 5E3 circuit, as I recall. 6V6 push-pull output, the Tweed Deluxe.
I had some designs where the extra gain stage could be switched in or out. I adjusted the bias by trial and error, usually trying to lower it from way too much gain.
I recall turning plate voltage off, that mutes the tube, but they say that can cause cathode stripping. Not good, but do you play guitar 24/7? No, not since high school, anyway. 8)
Sorry if this sounds like bragging, just remembering what worked and what didn't.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Tue 09, 2018 4:57 am 
Member
User avatar

Joined: Jun Sat 18, 2011 3:36 am
Posts: 2010
Location: Milton, FL 32570
Ok, here's a not so obvious question what kind of guitar do you play?

A gibson with humbuckers will overdrive the amp some. Or if you a fender with single coil pickups you could build a lpb-1 i think Electroharmonix boost which will overdrive the circuit some. But do you really want to have overdrive all the time?

Jason


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Tue 09, 2018 6:06 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2274
Location: Spring Hill, FL
reeves03 wrote:
Ok, here's a not so obvious question what kind of guitar do you play?


For now, an Amazon special cheapie. The only real motivation for building an amp is more related to the antique radio hobby and working with tube equipment, and I probably already have half the parts I'll need.

_________________
Michael


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Tue 09, 2018 1:17 pm 
Member
User avatar

Joined: Jun Sat 18, 2011 3:36 am
Posts: 2010
Location: Milton, FL 32570
TPAairman wrote:
reeves03 wrote:
Ok, here's a not so obvious question what kind of guitar do you play?


For now, an Amazon special cheapie. The only real motivation for building an amp is more related to the antique radio hobby and working with tube equipment, and I probably already have half the parts I'll need.

so is it a single coil or a humbucker? A humbucker puts out more voltage usually and therefore more drive.
I built a fender champ for my first one. That thing would get nasty as hell(in a good way) with a humbucker.
jason


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Tue 09, 2018 5:55 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2274
Location: Spring Hill, FL
reeves03 wrote:
so is it a single coil or a humbucker?


Single

_________________
Michael


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Tue 09, 2018 11:51 pm 
Member
User avatar

Joined: Jun Sat 18, 2011 3:36 am
Posts: 2010
Location: Milton, FL 32570
just build the amp you want and since you have a single coil guitar build a lpb-1 Linear Phase boost, it will push the amp into overdrive .
You get to build 2 things that's a win-win in my book.
I can get you a verostrip layout or you could do it point to point or tag strip.

jason


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Wed 10, 2018 4:13 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2274
Location: Spring Hill, FL
I can't seem to find info on how to build one, but I'm also wanting the overdrive circuit to be part of the amp instead of a pedal.

_________________
Michael


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Wed 10, 2018 4:22 am 
Member
User avatar

Joined: Jun Sat 18, 2011 3:36 am
Posts: 2010
Location: Milton, FL 32570
Here you go http://tagboardeffects.blogspot.com/2013/01/electro-harmonix-lpb1-with-trimmer.html

Jason


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Wed 10, 2018 5:09 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19917
Location: Utica, NY 13502 (USA)
As a hi fi amp guy, not a guitar amp guy, please explain "overdrive". Back in technical college in the '60's, my
guitar-playing classmates built things they called "fuzz-boxes". These were simple, two-transistor amplifiers
biased to produce clipping distortion to make their amps to sound overdriven without actually overdriving and possibly
damaging their equipment. Is this what you are wanting to add to your amp?

Dave


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 12:27 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2274
Location: Spring Hill, FL
That's pretty much it, except some amps have them built in, which I'm looking to do. Generally it's a second input, so it's own pre-amps, or it can be in-line with the rest of the amp, with a switch that bypasses it if you want it turned off, which is what I'm looking to do.

_________________
Michael


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 12:55 am 
Member
User avatar

Joined: Jun Sat 18, 2011 3:36 am
Posts: 2010
Location: Milton, FL 32570
Basically your just adding another gain stage to the circuit I'm sure you'd want it switchable for sure though.

jason


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 1:27 am 
Member

Joined: Jan Thu 24, 2013 11:50 pm
Posts: 2813
Location: Bristol, Rhode Island
One totally different way to test getting that overdrive sound without annoying the neighbors would be to add a master volume control. Turn it down, crank the regular volume to distortion levels and you can get very dirty at low volumes. I have added a few master volumes into recent Fender 6G3 builds, using the across the line control Rob Robinette describes. Super, super easy to add to the circuit (takes less than two minutes) and 100 percent reversible if you don't like it.

Coincidentally, I just finished building a 5D5 circuit not two days ago. Left it 100 percent stock, and love it!


Attachments:
IMG_6441s.jpg
IMG_6441s.jpg [ 83.19 KiB | Viewed 1020 times ]
Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 1:43 am 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 2882
Location: Vancouver Island, Canada
How do you keep people from drooling all over it?
Wow! :P

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 1:55 am 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 2882
Location: Vancouver Island, Canada
Michael, I have an amp in my attic that uses a 12AX7 and a 12Au7 type (5963, salvaged from a ship radio).
I wired two paralleled signal paths and put 2 dual on-off-on switches on the feeders to the plates.
If you can picture that, you can build it. :lol:
There are 4 settings. 12AX7 - 12AX7 - 50L6, 12AU7 - 12AX7 - 50L6, plus the 0ther 2 combos.

Sounds best 12AU7 - 12AX7 - 50L6. Rectifier can be a 35Z5, with an iso trans.
But I would look for a transformer or 2, and use a diode, and could go with a 6V6 output.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


Top
 Profile  
 
 Post subject: Re: How to add overdrive to a guitar amp circuit
PostPosted: Jan Thu 11, 2018 5:24 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2274
Location: Spring Hill, FL
Soooo, how do you go about making the metal that shiny????

As to the original question, I appreciate the suggestions, but I think I'm going to stick to adding gain stages. However just to make sure I'm looking at this the right way, I take it I would want those stages to be higher gain? On one hand that makes sense, but on the other hand, it seems as though that might blow out the sound too much too quickly.

_________________
Michael


Top
 Profile  
 
Post New Topic Post Reply  [ 45 posts ]  Go to page 1, 2, 3  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Tarpawns and 3 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB