I'm tired of anemic guitar amps!

[toobfreak]

Concept guitar built for this no-name dude:
View attachment 1155596

That dude looks like an illegal alien. You should deport him.

 

[scruffy]

Okay, all good. Nice and stable 570 volts. Still waiting for a few parts, and I have to drill mounting holes for a can and a tube socket. But the PI and driver are in place, all I have to do is phase them. Started building out the power chain. Here's today's accomplishments:

The reverb circuitry goes here:

This reverb is going to be really, really cool. You won't find anything like it on the market. With this amp, you can get "reverb only", either clean or crunchy, as much of it as you want, and you can choose how hard to drive the springs, and you can run it through a tone stack. When it's mixed with the drive, the reverb has its own tone control. You can run any combination of drive and reverb, so like, clean drive and crunchy verb, or vice versa.

I'm going to take this amp to the NAMM show and turn some heads. Maybe play at the after-show concert. Or maybe just let some of the guitar players experiment with it. Bogner's jaw is going to drop, trust me. He'll probably steal my ideas, but he'll have to spend some months in design and on the bench to make it work.

lol

 

[Chuz Life]

Okay, all good. Nice and stable 570 volts. Still waiting for a few parts, and I have to drill mounting holes for a can and a tube socket. But the PI and driver are in place, all I have to do is phase them. Started building out the power chain. Here's today's accomplishments:

View attachment 1156172View attachment 1156173

The reverb circuitry goes here:

View attachment 1156174

This reverb is going to be really, really cool. You won't find anything like it on the market. With this amp, you can get "reverb only", either clean or crunchy, as much of it as you want, and you can choose how hard to drive the springs, and you can run it through a tone stack. When it's mixed with the drive, the reverb has its own tone control. You can run any combination of drive and reverb, so like, clean drive and crunchy verb, or vice versa.

I'm going to take this amp to the NAMM show and turn some heads. Maybe play at the after-show concert. Or maybe just let some of the guitar players experiment with it. Bogner's jaw is going to drop, trust me. He'll probably steal my ideas, but he'll have to spend some months in design and on the bench to make it work.

lol

I hope you will let us hear it, somehow, when it is finished.

 

[scruffy]

I hope you will let us hear it, somehow, when it is finished.

I'll post on YouTube when it's ready.

I call this design "dial-an-amp". You can literally dial up any amp you want.

Up front, is a very precise clean section. The first thing is, you can dial up the cathode cap in the very first section. This gives you varying degrees of "bass rolloff", at different frequencies. Then your selected signal is fed through a tone stack, which you can switch between TMB (Fender) and Baxandall-with-midrange. Then you boost the 1 volt signal up to about 20 volts, and feed it through the coupling cap of your choice. In other words, "after" the up-front tone stack you can roll off some more bass if you wish, at different frequencies. So like, you can boost the bass and mids in the tone stack, and then roll them back off at a different frequency, this way you get complete control over your mids. If you want a Mesa-style sound with fat mids and skinny edges, you can get that.

After that you run through a switchable gain stage, which is very much like a Soldano but you can take the cold clipper out of the stack. If you take it out, you run straight from a 20 volt drive into a cathode follower, which is the Marshall sound on steroids. If you leave the cold clipper in, you can switch the cathode resistor to get any amount of clipping you want. At 39k it's a Mesa Dual Rectifier, at 10k it's a JCM-800, at 4.7k you get Scruffy's favorite setting which no one else has, or you can just run it at 1.5k like a normal gain stage which puts you in Bogner/Engl/Krank land.

While you're doing all this, the gains are automatically adjusted so the signal that appears at the reverb input is consistent. You can the tap the reverb into any stage, and mix it with the drive from any stage. The phases switch along with your settings, so your reverb is always in phase with the drive.

If all this sounds complicated, it's not. You have two knobs, "amount of drive" and "amount of reverb", and then you have a master volume. There is a second tone stack between the reverb and the master volume, so you can dial up an "overall" contour for whatever comes out of the mixer. This second tone stack is more like a Marshall, it has the presence control and some NFB contouring.

If you want to run completely clean, you can switch out the entire gain section, then you just get a Twin Reverb. If you want to sound like a Marshall instead, you can choose what "type" of Marshall, JMP, JTM, or JCM. If you switch the preamp tone stack to Baxandall you're an Ampeg or an Orange. (And in Orange mode you can switch the coupling caps just like the real thing).

To get to never-never land, you switch everything in and dial back the drive to the cathode follower. There's so much gain you can get an extremely ratty sound out of the reverb, without killing the springs. You can get some delicious reverb tails by dialing in the thump in the drive section, then switching the reverb into crunch mode and boosting the dwell. When you rake your guitar strings it goes thump-thump-thump and then shhhh. It's a wonderful effect for pizzicato. For southern rock you can use a cleanish front end and set the reverb so it's just on the edge of crunch, this way you get varying degrees of bite depending on how hard you pick the strings. If you like to shred, or tap-and-shred, you can boost the drive and turn down the dwell, that way your tapping will be delivered with clarity even when the overall sound is rich with reverb.

It's a tremendously versatile and responsive amp. I've done similar designs three times before, so I kinda know what I'm doing by now. Only last time I used a 12AT7 PI, which can't really drive the KT-88's hard enough. It sounded great about halfway up but lost some oomph at full volume. "Too clean", is what it was lol. This time I'm using two 12AU7's instead of one 12AT7, so instead of being forced to crank the signal to drive the power tubes, there's enough clean volume to push the KT-88's up into blues land, get that really sweet slightly crunchy gain the blues players love, only LOUD, much louder than a tweed Twin. With this amp you don't have to work to keep up with a loud drummer. You can still feed into the board if you want, there's an effects loop and a separate tap for "reverb only", so if you want you can disconnect the power amp and run the entire front end at 3 watts. The recording engineers love this, they can do whatever they want with the sound and you'll have just enough monitor to hear yourself. They won't have to put you in a sound booth when you want Zakk Wylde, you can just sit there on the couch next to the engineer and do the whole thing at Pignose volume. Then step outside, engage the power amp again, and amaze your fans with what you just learned in the studio.

 

[Chuz Life]

I'll post on YouTube when it's ready.

I call this design "dial-an-amp". You can literally dial up any amp you want.

Up front, is a very precise clean section. The first thing is, you can dial up the cathode cap in the very first section. This gives you varying degrees of "bass rolloff", at different frequencies. Then your selected signal is fed through a tone stack, which you can switch between TMB (Fender) and Baxandall-with-midrange. Then you boost the 1 volt signal up to about 20 volts, and feed it through the coupling cap of your choice. In other words, "after" the up-front tone stack you can roll off some more bass if you wish, at different frequencies. So like, you can boost the bass and mids in the tone stack, and then roll them back off at a different frequency, this way you get complete control over your mids. If you want a Mesa-style sound with fat mids and skinny edges, you can get that.

After that you run through a switchable gain stage, which is very much like a Soldano but you can take the cold clipper out of the stack. If you take it out, you run straight from a 20 volt drive into a cathode follower, which is the Marshall sound on steroids. If you leave the cold clipper in, you can switch the cathode resistor to get any amount of clipping you want. At 39k it's a Mesa Dual Rectifier, at 10k it's a JCM-800, at 4.7k you get Scruffy's favorite setting which no one else has, or you can just run it at 1.5k like a normal gain stage which puts you in Bogner/Engl/Krank land.

While you're doing all this, the gains are automatically adjusted so the signal that appears at the reverb input is consistent. You can the tap the reverb into any stage, and mix it with the drive from any stage. The phases switch along with your settings, so your reverb is always in phase with the drive.

If all this sounds complicated, it's not. You have two knobs, "amount of drive" and "amount of reverb", and then you have a master volume. There is a second tone stack between the reverb and the master volume, so you can dial up an "overall" contour for whatever comes out of the mixer. This second tone stack is more like a Marshall, it has the presence control and some NFB contouring.

If you want to run completely clean, you can switch out the entire gain section, then you just get a Twin Reverb. If you want to sound like a Marshall instead, you can choose what "type" of Marshall, JMP, JTM, or JCM. If you switch the preamp tone stack to Baxandall you're an Ampeg or an Orange. (And in Orange mode you can switch the coupling caps just like the real thing).

To get to never-never land, you switch everything in and dial back the drive to the cathode follower. There's so much gain you can get an extremely ratty sound out of the reverb, without killing the springs. You can get some delicious reverb tails by dialing in the thump in the drive section, then switching the reverb into crunch mode and boosting the dwell. When you rake your guitar strings it goes thump-thump-thump and then shhhh. It's a wonderful effect for pizzicato. For southern rock you can use a cleanish front end and set the reverb so it's just on the edge of crunch, this way you get varying degrees of bite depending on how hard you pick the strings. If you like to shred, or tap-and-shred, you can boost the drive and turn down the dwell, that way your tapping will be delivered with clarity even when the overall sound is rich with reverb.

It's a tremendously versatile and responsive amp. I've done similar designs three times before, so I kinda know what I'm doing by now. Only last time I used a 12AT7 PI, which can't really drive the KT-88's hard enough. It sounded great about halfway up but lost some oomph at full volume. "Too clean", is what it was lol. This time I'm using two 12AU7's instead of one 12AT7, so instead of being forced to crank the signal to drive the power tubes, there's enough clean volume to push the KT-88's up into blues land, get that really sweet slightly crunchy gain the blues players love, only LOUD, much louder than a tweed Twin. With this amp you don't have to work to keep up with a loud drummer. You can still feed into the board if you want, there's an effects loop and a separate tap for "reverb only", so if you want you can disconnect the power amp and run the entire front end at 3 watts. The recording engineers love this, they can do whatever they want with the sound and you'll have just enough monitor to hear yourself. They won't have to put you in a sound booth when you want Zakk Wylde, you can just sit there on the couch next to the engineer and do the whole thing at Pignose volume. Then step outside, engage the power amp again, and amaze your fans with what you just learned in the studio.

Patent Worthy!

 

[scruffy]

Today:

That's everything but the input stage (which is the hole for the missing tube socket on top).

So now we're gonna go get some Mogami mini console cable to wire up the front panel. Shielded two conductor cable is perfect for this application. You have to be very careful wiring up long cables carrying DC. For instance the presence control carries DC from the tail of the phase inverter, and the tone stack is directly connected to the plate or cathode of its driver. In both cases the circuit won't behave (or sound right) if you just use a long cable to wire up the pot(s).

For an example, consider the famous Bassman 5F6a circuit, which was the predecessor of the Marshall amps. Note the presence control, which is the 5K pot at the bottom of the phase inverter, where the negative feedback connects.

https://schematicheaven.net/fenderamps/bassman_5f6a_schem.pdf

If that control is too far away from the tube, your amp starts howling. If you need to run a long wire to the front panel, you have to put a 10k resistor right next to the tube, and then run the wire to a 10k pot in parallel. This provides "enough" DC return to mitigate the capacitance from the cable.

Generally volume controls are okay because they don't carry any DC. With those, you can run as long a cable as you want, and all you have to worry about is a little hum, which good shielding will keep out.

 

[Chuz Life]

Today:

View attachment 1156637

That's everything but the input stage (which is the hole for the missing tube socket on top).

So now we're gonna go get some Mogami mini console cable to wire up the front panel. Shielded two conductor cable is perfect for this application. You have to be very careful wiring up long cables carrying DC. For instance the presence control carries DC from the tail of the phase inverter, and the tone stack is directly connected to the plate or cathode of its driver. In both cases the circuit won't behave (or sound right) if you just use a long cable to wire up the pot(s).

For an example, consider the famous Bassman 5F6a circuit, which was the predecessor of the Marshall amps. Note the presence control, which is the 5K pot at the bottom of the phase inverter, where the negative feedback connects.

https://schematicheaven.net/fenderamps/bassman_5f6a_schem.pdf

If that control is too far away from the tube, your amp starts howling. If you need to run a long wire to the front panel, you have to put a 10k resistor right next to the tube, and then run the wire to a 10k pot in parallel. This provides "enough" DC return to mitigate the capacitance from the cable.

Generally volume controls are okay because they don't carry any DC. With those, you can run as long a cable as you want, and all you have to worry about is a little hum, which good shielding will keep out.

Wish I lived close. I could help you clean up that soldering some.

 

[scruffy]

Wish I lived close. I could help you clean up that soldering some.

Every connection will be gone over, all the wires neatly arranged with cable ties, etc.

I just tack everything in first and turn it on and make sure it works. Cosmetics are the very last thing I pay attention to. Because, no one's going to need to service this amp for at least 20 years, and most likely the service person will be me.

I use tricks I learned in the radio world. For instance every ground connection has a fat wire sticking up vertically from it. Because, the front panel cables need to go in, and every single one of them is ground shielded. It doesn't work trying to cram shields into little terminal lugs that already have two big fat #18 wires in them.

I did mention the layers, right? They're vertical. This is old school point to point wiring. Electrically it is perfect. You make the amp work first, then you shape things. Trying to do it the other way around is not only a waste of time but you end up taking things apart for cosmetic reasons, which is a great way to ruin a working circuit.

All voltage grounds and everything that carries current goes "into" the terminal strips. Electrical safety comes first, cosmetics are a distant last place unless you're in production selling amplifiers, in which case the little Filipino ladies cut and strip every wire to length.

Remember, I still have to tweak the rail voltages. All those power resistors are coming out again. I soldered them in such a way that the posts can be easily cleaned with a wick. First I want to make sure audio comes out of the speakers, for that part the particular voltages aren't critical. They only become critical when you want to make the amp sound like it's supposed to, which is still a couple of weeks away.

 

[Chuz Life]

Every connection will be gone over, all the wires neatly arranged with cable ties, etc.

I just tack everything in first and turn it on and make sure it works. Cosmetics are the very last thing I pay attention to. Because, no one's going to need to service this amp for at least 20 years, and most likely the service person will be me.

I use tricks I learned in the radio world. For instance every ground connection has a fat wire sticking up vertically from it. Because, the front panel cables need to go in, and every single one of them is ground shielded. It doesn't work trying to cram shields into little terminal lugs that already have two big fat #18 wires in them.

I did mention the layers, right? They're vertical. This is old school point to point wiring. Electrically it is perfect. You make the amp work first, then you shape things. Trying to do it the other way around is not only a waste of time but you end up taking things apart for cosmetic reasons, which is a great way to ruin a working circuit.

All voltage grounds and everything that carries current goes "into" the terminal strips. Electrical safety comes first, cosmetics are a distant last place unless you're in production selling amplifiers, in which case the little Filipino ladies cut and strip every wire to length.

Remember, I still have to tweak the rail voltages. All those power resistors are coming out again. I soldered them in such a way that the posts can be easily cleaned with a wick. First I want to make sure audio comes out of the speakers, for that part the particular voltages aren't critical. They only become critical when you want to make the amp sound like it's supposed to, which is still a couple of weeks away.

Ok, I get it. I'm just jelly and wanted to play.

 

[ThisIsMe]

I'm going to build an amp. And post the whole thing on YouTube. Show the world how it's done.

My ear has had it with anemic amps. I like thump, I like an authoritative sound. I like the power tubes to break up at full volume, none of that saggy blues stuff.

My other requirement is it has to be bulletproof. It has to be able to fall out the back of a truck and survive. So no PC boards, all old school point to point wiring. The worst thing that happens is a tube blows and then you replace it and you're done.

If you're into guitar amps, check out the schematic of this 200 watt Marshall. Notice the 12AU7 driver, in front of the power tubes. That's there because the KT-88's require 50 V rms to reach full power. They sound great when they do, they're thumpy and they have great dynamics, but they need some beef backing them up.

https://schematicheaven.net/marshallamps/major_1967u_lead_200w.pdf

So I'm going to have a 400 VA toroid that can supply almost an amp at 560 volts (the tubes draw 640 mils at full power), but it weighs less than half as much as a big metal power transformer.

And I'm going to make it a dial-an-amp, so you can get any sound you want just by flipping a few switches. If you want a Fender sound with reverb and the tone stack up front you can get that, and if you want a Marshall sound with the tone stack in back you can get that too. And anything in between, and above and beyond.

By using a 12AU7 as a phase inverter, ahead of the driver, I get a combined gain of about 60 for the power amp, which is just about perfect, that means about 0.8 volts will drive it to full power. With a long tailed pair, it'll have the same sparkle as a Marshall Major about halfway up, and then it'll get really aggressive when it's cranked.

I want to blow some windows out this year. It's one of my New Year's resolutions.

Yep, I see this in your future...

 

[scruffy]

Ok, I get it. I'm just jelly and wanted to play.

I have two lovely transformers and a bunch of working Amperex tubes from the old amp, would you like them? The OT in particular is well made, the only draw back is it's 8 ohms "only".

You could build up a Marshall chassis, they're less than 100 bucks from MojoTone. Another 100 will get you tube sockets and most of the components (you don't need a choke). For a Marshall build the biggest cost is the LCR caps at 20 bucks apiece, you only really need two of them.

 

[scruffy]

Yep, I see this in your future...


View attachment 1156657

I don't have the speakers yet.

 

[Chuz Life]

I don't have the speakers yet.

Not that kind of playing. I see your build and imagine how I might/ would maybe approach it differently.

Not that there's a damn thing wrong with what you've done. Freaking impressive!

 

[scruffy]

I see your build and imagine how I might/ would maybe approach it differently.

Excellent!

That's how we do it.

When you're done with yours, I'll take a look at it and see how I can improve mine.

 

[toobfreak]

When you're done with yours, I'll take a look at it and see how I can improve mine.

Actually Scruff, your amp is looking pretty good, certainly up to the same standards or better than you would find buying most any commercial factory amplifier. I've seen a few things I didn't understand or might have done differently, but attribute that more to style than anything of much electrical significance.

I especially like how you have been stopping and testing each phase of work first before moving on so that you know what you have under you in the many untried parts first and if you run into a problem now, won't be faced with so many places to look to for the issue.

 

[scruffy]

Actually Scruff, your amp is looking pretty good, certainly up to the same standards or better than you would find buying most any commercial factory amplifier. I've seen a few things I didn't understand or might have done differently, but attribute that more to style than anything of much electrical significance.

I especially like how you have been stopping and testing each phase of work first before moving on so that you know what you have under you in the many untried parts first and if you run into a problem now, won't be faced with so many places to look to for the issue.

This is the kind of annoying stuff I run into when raising an amp off the top of my head (what we'd call "prototyping").

If I had my 'druthers I would have moved the cap cans a little farther away from the tube sockets. This would have left more room for the terminal strips and generally reduced clutter.

However, there's a big ol reverb tank in the way. It sits on top of the chassis on the same side as the cans. It's mounted on four "posts" (long #4 screws) and connected by springs that sit between the plate and the posts.

Vertically, there's a stiff wire between each pair of end posts, with three knots in it. Two of them are on either side of the plate, and one is under the chassis. This keeps the tank from moving too far vertically in either direction, when you're carrying the amp around. (Don't blame me, it's what I was given to work with - maybe how AIMS saved money relative to Fender lol. The only other option, other than externalizing the tank, would have been to buy a new tank and mount it upside down on top of the inside of the cabinet, and I've tried that and it works but it's a PITA to get the tank in there and even more of a PITA if you ever have to take it out. Plus I really like the original Accutronics reverb, it sounds better than the new ones).

So I've already built up half the amp, and just now noticed that parts of my power section are occluding the mounting holes for two of the posts on one side of the plate. You can kinda see the problem in this pic:

The little hole just to the left of the bias pot is the "middle" hole where the stiff wire goes. You can see the bottom hole beneath the white 1k power resistor on the bias strip. However the top hole is directly under the two fat black ground wires on the first terminal strip of the power chain, to the right of the white 10w resistor.

I could probably get into the bottom hole to mount the post near the bias strip, but it's not safe. If that post ever comes loose it'll ground the bias voltage, and there go the output tubes. However I can't get into the top post at all, turns out it's too close to the terminal strip. When I was laying out the chassis I figured "okay, I'll leave a little room", but the distance between the terminal strip and its mounting hole is greater than I imagined.

So now I have to move both the power strip and the bias strip.

The bias strip is easy, I'll just move it over a little closer to the edge of the chassis and turn it sideways. The power strip is not so easy, because the PT is sitting right above it on the other side of the chassis. I'll have to move it downwards, which ruins the neat cosmetics for the power chain, but it's vital for electrical safety, we don't want any part of the tank touching the power. "Maybe" I can turn the 10w resistor 90 degrees and mount the strip up and down, we'll see how that plays with the wiring.

All in a day's work. Still waiting for parts so there's time.

 

[toobfreak]

This is the kind of annoying stuff I run into when raising an amp off the top of my head (what we'd call "prototyping").

Building prototypes is hard stuff, you always run into problems and find stuff you'd do different or better if doing again. It really takes a lot of thought building it in your head first, thinking it through, to avoid serious problems.

If I had my 'druthers I would have moved the cap cans a little farther away from the tube sockets. This would have left more room for the terminal strips and generally reduced clutter.

I had considered early on when I saw them up on top whether it might have been better/easier to mount them underneath, but that takes up a lot of real estate.

However, there's a big ol reverb tank in the way. It sits on top of the chassis on the same side as the cans. It's mounted on four "posts" (long #4 screws) and connected by springs that sit between the plate and the posts.

Ah yes, the ol' reverb tank. Remember early on, I urged building that separate from the amp, but you had some reason you liked for keeping it integral with the amp.

So I've already built up half the amp, and just now noticed that parts of my power section are occluding the mounting holes for two of the posts on one side of the plate. You can kinda see the problem in this pic: The little hole just to the left of the bias pot is the "middle" hole where the stiff wire goes. You can see the bottom hole beneath the white 1k power resistor on the bias strip. However the top hole is directly under the two fat black ground wires on the first terminal strip of the power chain, to the right of the white 10w resistor.

My approach would be probably to leave all the existing parts where they were, then mill some custom brackets to mount to the existing holes, go around the other parts, then leave a new mounting hole over those parts above to mount the reverb tank to, or some approximation in-between.

Maybe" I can turn the 10w resistor 90 degrees and mount the strip up and down, we'll see how that plays with the wiring.

Speaking of 10 watt resistors, you might want to check this solder point, it looks like it still needs soldered.

 

[scruffy]

Yeah ... I need two inverse terminal strips, with the mounting on the other side.

Maybe there's some I can cannibalize from another prototype.

 

[scruffy]

What made me notice the Orange wire going to chassis, was that voice in my head of all my supervisors in my past - yelling at me for not knowing better.

Here Chuz Life, just for you.

I don't want you getting mad at me for a little thing like this.

I did like you said, stripped an old AC line cord.

I promise to straighten the wire out before installing the tubes.

 

[toobfreak]

Yeah ... I need two inverse terminal strips, with the mounting on the other side.
Maybe there's some I can cannibalize from another prototype.

One other suggestion, Scruffy, I'm not sure why I didn't see this earlier. If you don't already have one, do yourself a favor and invest in a good deburring tool. You can spend more, but a simple one like this is very effective:

Husky Swivel Blad Deburring Tool with Quick Release Cap and Blade Storage 410-197-0111 - The Home Depot

Remove burrs from metal and plastic pipe or tubing by using this Husky Deburring Tool. Features hardened-steel blade construction.

www.homedepot.com

You can buy better ones from a machinist tool company but frankly, any ol' average one at the hardware store usually works more than good enough. Any time you cut or drill metal, you run this around inside the hole, etc., and it breaks off all of the chips and smooths the roughness.

Highly recommended.

Even that roughness can actually over time saw through insulation with enough rubbing or vibration and cause a short. And chips left after drilling always end up falling off later and getting stuck somewhere you didn't intend a connection.

I say this because I noticed a lot of hanging chip in one of your mounting holes for your reverb tank.

People often think that after they mount the hardware and tighten the fastener, all this stuff will be trapped underneath, squashed flat, but actually sometimes, the act of clamping down hardware on these chips actually facilitates their breaking off. And once they break off and fall out, whatever they were under might loosen up.

Definitely deburr that hole.