I'm tired of anemic guitar amps!

[toobfreak]

I might look it up out of curiosity but I'm too old and busy to do much with the info.
Edit: The Julius Futterman OTL Story

Two modern companies also worth looking into and studying their OTL designs are Atma-Sphere and NYAL (New York Audio Labs).

 

[Chuz Life]

Two modern companies also worth looking into and studying their OTL designs are Atma-Sphere and NYAL (New York Audio Labs).

It's pretty embarrassing I had a whole career in electronics and still seeing tube designs that change everything again.

 

[scruffy]

One interesting thing about the original -

Check this out, those dinky little grey wires supply over 5 amps to the heaters.

I'm not sure, it might be nichrome wire or something weird like that, they're tiny, they look about 20 gauge.

That's the pilot light they're connected to, the green wires come straight out of the PT.

Also you see a bozo no-no in the above pic, someone replaced the original on-off switch with one having screw terminals. Bad idea! When I opened this amp up there were several loose nuts floating around under the chassis, that's what happens after 20 years on the road.

Here's the gray wires after removing the PT:

 

[toobfreak]

It's pretty embarrassing I had a whole career in electronics and still seeing tube designs that change everything again.

One of the best tubes ever made, the Western Electric 300B, was actually made for intercontinental telephone lines. They ran cables from continent to continent along the sea floor to wire telephone service around the world, and every so far, they had to add a repeater to boost and regenerate the signal due to I2R losses.

Once those amps were dropped down on the ocean bottom, they had to work for years never to be seen by the hand of man again.

I once ran into a graduating class that had just graduated electronics; So I asked the class if anyone there could answer me what a Fleming Valve was.

You never saw so many deer in the headlights.

 

[Chuz Life]

One of the best tubes ever made, the Western Electric 300B, was actually made for intercontinental telephone lines. They ran cables from continent to continent along the sea floor to wire telephone service around the world, and every so far, they had to add a repeater to boost and regenerate the signal due to I2R losses.

Once those amps were dropped down on the ocean bottom, they had to work for years never to be seen by the hand of man again.

I once ran into a graduating class that had just graduated electronics; So I asked the class if anyone there could answer me what a Fleming Valve was.

You never saw so many deer in the headlights.

I'm trying to imagine the engineering involved given the harsh conditions, pressures and the voltage drops - for the power needs.

 

[scruffy]

Well hell, you've done this shit before. . . . I'll sit back and learn now. I certainly can't speak from experience on a build like this.

This amp will clean up very nicely. The hardest part will be re-powdercoating the front panel.

I've been doing amps for lots of years. Recently restored a Fender Model 26, definitely one of the highlights of my career.

Worked on half a dozen Twin Reverbs over the years - if you feel like modding yours, I have some ideas for what you can do with that nasty-sounding master volume pull. Twins are great amps to begin with, CBS added too much stuff to them and it didn't always improve the sound, IMO.

They're supposed to be loud and clean, I don't know why anyone would want to add nasty sounding grit to that (I'd just put a pedal in front of it if that's what I wanted). You can easily get more gain out of them if you wish, by changing a couple of resistors, and if you're really serious about it and you don't mind giving up your normal channel you can convert the tube into a cathode follower and add it into your reverb channel. That'll give you a great sounding Bassman type grit, much loved even by Marshall users.

If you check out the 5F8-A Twin on schematicheaven.net you'll see it's much like a Bassman. They had to reduce the gain to keep it clean, by using a 12AY7 at low voltage, but you don't have to do that. You can use an AX at 330 volts and you can get a gain of 70 per stage by changing a couple of resistors.

Figure a single coil guitar input is maybe 5 mV, with a gain of 70 that becomes 350 mV, and if you put the two input stages in series you can drive the cathode follower with over 20 volts when you dime the volumes. (The input to the PI needs about 8 volts to drive the power tubes into breakup, so it's a good excuse for a master volume). Personally I like the 6L6's much better than Marshall's EL34's, a driven 6L6 amp is pretty aggressive and sounds great.

 

[scruffy]

So here's something interesting and educational.

Go here: https://www.ampbooks.com/mobile/amplifier-calculators/vacuum-tubes/12AX7/

You'll notice the defaults are set to 300v supply with a 100k plate resistor, which is "about average" for a Fender amp. Click the green button, and you can see where the tube is operating.

Note the value of the cathode resistor. To get this down to the 1.5k that Fender uses, you have to put plate current = 1 mA and grid bias = -1.5v.

And you'll notice that the tube is still sitting around 200 volts, it's not operating near its midline. To see the operating point more clearly you can change the load to 500 (k) instead of 220, the part values don't change but the green line gets a lot closer to the red DC load line.

Now go back, and use 350 volts and a 220k plate load. When you push the green button, you'll see that the plate voltage is now about 160 volts, which is just about perfect - and notice your cathode resistor has gone up to 2.2k.

Now go here to the preamp gain calculator:

Preamp Gain and Output Impedance Calculator

www.ampbooks.com

Plug in the values 220k for plate and 2.2k for cathode (and we want bypassed=yes because Fender uses a big ol 25uF bypass cap), and push the Recalculate button. Look at the gain. Wow! Almost 80. And your tube is operating about halfway through its swing which means it could go to 100 volts peak to peak and still stay linear.

Soldano and others use these part values along with the slightly higher supply voltage to get a very clean very gainy first stage. To my ear it sounds infinitely better than a lower voltage mockup, and if you need less gain just turn down the volume! (What a concept).

 

[Chuz Life]

So here's something interesting and educational.

Go here: https://www.ampbooks.com/mobile/amplifier-calculators/vacuum-tubes/12AX7/

You'll notice the defaults are set to 300v supply with a 100k plate resistor, which is "about average" for a Fender amp. Click the green button, and you can see where the tube is operating.

Note the value of the cathode resistor. To get this down to the 1.5k that Fender uses, you have to put plate current = 1 mA and grid bias = -1.5v.

And you'll notice that the tube is still sitting around 200 volts, it's not operating near its midline. To see the operating point more clearly you can change the load to 500 (k) instead of 220, the part values don't change but the green line gets a lot closer to the red DC load line.

Now go back, and use 350 volts and a 220k plate load. When you push the green button, you'll see that the plate voltage is now about 160 volts, which is just about perfect - and notice your cathode resistor has gone up to 2.2k.

Now go here to the preamp gain calculator:

Preamp Gain and Output Impedance Calculator

www.ampbooks.com

Plug in the values 220k for plate and 2.2k for cathode (and we want bypassed=yes because Fender uses a big ol 25uF bypass cap), and push the Recalculate button. Look at the gain. Wow! Almost 80. And your tube is operating about halfway through its swing which means it could go to 100 volts peak to peak and still stay linear.

Soldano and others use these part values along with the slightly higher supply voltage to get a very clean very gainy first stage. To my ear it sounds infinitely better than a lower voltage mockup, and if you need less gain just turn down the volume! (What a concept).

All fantastic stuff.

Sorry I didn't see your comments sooner.

Because I have two twins, I would like to get one reworked to as close to a blackface as I can get it. Then, when I'm happy with the sound of that one, try some of the mods and maybe even some ideas of my own on the second one.

 

[Sherlock Holmes]

100 watts is sufficient for now.

200 watts would require a considerably larger OT, and there isn't room.

A couple years back I built a 200 watter in one of Mojo's "tall Marshall" cases, it fit but boy was that sucker heavy. I sold it to a guy who swears by it, says he never has to turn it more than halfway up.

You'd need 1000 watts if you want the sound to be twice as loud as 100 watts.

 

[scruffy]

All fantastic stuff.

Sorry I didn't see your comments sooner.

Because I have two twins, I would like to get one reworked to as close to a blackface as I can get it. Then, when I'm happy with the sound of that one, try some of the mods and maybe even some ideas of my own on the second one.

AB763 is the classic blackface Twin Reverb.

If you compare to the silverface models they're very similar, but there are a couple of very important differences. Starting with the power supply.

Like, if you look at the ultralinear TR, it's running at 520 volts which exceeds the 6L6 spec. You can only do that in ultralinear mode. The AB763 runs around 460 volts which is at the very top of spec.

There is no choke in the ultralinear because it's not needed, the OT provides enough inductance. Also the ultralinear uses a bridge rectifier whereas AB763 is full wave, it's a different power transformer with a separate bias winding instead of a tap.

The ultralinear version was designed at a time when tubes were getting scarce and it was hard to find a matched set of power tubes. That's why they have that horrible "bias balance" adjustment. I vastly prefer the older style AB763 bias control but it's vital to have matched tubes in that case.

So you can't just willy-nilly remove the screen taps and run a silverface as a blackface, your power tubes won't be happy that way. The idea behind ultralinear is as its name implies, linearity at high power, it tends to stay clean all the way up. Which may or may not be what you want. My ear likes the dirt, it likes a little bit of imbalance. The two output sections sound pretty close but not identical, the ultralinear is definitely cleaner at high volume. You "can" keep the ultralinear output section and blackface the rest of the amp, that works and it sounds considerably better but it's not a full blackface. The advantage is it's a lot less work (and less cost).

If you're really a fanatic and insist on a full blackface conversion you have to do metalwork, you'll need a new power transformer, a choke, and some rewiring of the fiberboard. IMO the best bang for the buck is to focus on the area around the phase inverter, you can keep the master volume control but get rid of all the horrible mess around it.

If it were me I'd keep the lower preamp voltages on the ultralinear, the AB puts 50 additional volts on the preamp tubes which is totally not necessary. Also the AB puts 460 volts on the reverb driver which is not necessary either (the silverface reverb actually sounds a lot better to my ear).

They used higher voltages in the AB to try to get more volume out of the amp, by driving the PI a little harder. If it were me I'd keep the silverface ultralinear output (that way you get the extra volume) and just tweak a couple of the power supply resistors to get nicer behavior in the preamp. By the time you get done with a full blackface conversion you could almost buy/build a new one, from a kit or whatever.

One of my favorite tactics is to start with an existing beat up old amp that's "close". You can find amps that use the same voltages as an AB but are otherwise undesirable, and convert them as needed. In this case I'd pay attention to the fundamentals, good strong chassis, rust free transformers, "most" of the chassis holes already present, etc. Amp handles, are very important. Intact cabinet, etc. Cabinets are expensive, Mojo charges a fortune for a chassis and faceplates but their cabinets are even more (they're good cabs, worth the money if you like the "new" look and feel).

For electronic parts I usually go to tubesandmore.com, they stock all the essentials. Also found a place in the LA area called apexjr.com, they have a lot of the oddball stuff like wire, that people don't usually think about up front. Grommets, solder lugs, that kind of thing. Tubes... y'know... subjective. RCA blackplates are the holy grail, but my ear thinks JJ's are just as good and they're a lot cheaper lol.

 

[Chuz Life]

AB763 is the classic blackface Twin Reverb.

If you compare to the silverface models they're very similar, but there are a couple of very important differences. Starting with the power supply.

Like, if you look at the ultralinear TR, it's running at 520 volts which exceeds the 6L6 spec. You can only do that in ultralinear mode. The AB763 runs around 460 volts which is at the very top of spec.

There is no choke in the ultralinear because it's not needed, the OT provides enough inductance. Also the ultralinear uses a bridge rectifier whereas AB763 is full wave, it's a different power transformer with a separate bias winding instead of a tap.

The ultralinear version was designed at a time when tubes were getting scarce and it was hard to find a matched set of power tubes. That's why they have that horrible "bias balance" adjustment. I vastly prefer the older style AB763 bias control but it's vital to have matched tubes in that case.

So you can't just willy-nilly remove the screen taps and run a silverface as a blackface, your power tubes won't be happy that way. The idea behind ultralinear is as its name implies, linearity at high power, it tends to stay clean all the way up. Which may or may not be what you want. My ear likes the dirt, it likes a little bit of imbalance. The two output sections sound pretty close but not identical, the ultralinear is definitely cleaner at high volume. You "can" keep the ultralinear output section and blackface the rest of the amp, that works and it sounds considerably better but it's not a full blackface. The advantage is it's a lot less work (and less cost).

If you're really a fanatic and insist on a full blackface conversion you have to do metalwork, you'll need a new power transformer, a choke, and some rewiring of the fiberboard. IMO the best bang for the buck is to focus on the area around the phase inverter, you can keep the master volume control but get rid of all the horrible mess around it.

If it were me I'd keep the lower preamp voltages on the ultralinear, the AB puts 50 additional volts on the preamp tubes which is totally not necessary. Also the AB puts 460 volts on the reverb driver which is not necessary either (the silverface reverb actually sounds a lot better to my ear).

They used higher voltages in the AB to try to get more volume out of the amp, by driving the PI a little harder. If it were me I'd keep the silverface ultralinear output (that way you get the extra volume) and just tweak a couple of the power supply resistors to get nicer behavior in the preamp. By the time you get done with a full blackface conversion you could almost buy/build a new one, from a kit or whatever.

One of my favorite tactics is to start with an existing beat up old amp that's "close". You can find amps that use the same voltages as an AB but are otherwise undesirable, and convert them as needed. In this case I'd pay attention to the fundamentals, good strong chassis, rust free transformers, "most" of the chassis holes already present, etc. Amp handles, are very important. Intact cabinet, etc. Cabinets are expensive, Mojo charges a fortune for a chassis and faceplates but their cabinets are even more (they're good cabs, worth the money if you like the "new" look and feel).

For electronic parts I usually go to tubesandmore.com, they stock all the essentials. Also found a place in the LA area called apexjr.com, they have a lot of the oddball stuff like wire, that people don't usually think about up front. Grommets, solder lugs, that kind of thing. Tubes... y'know... subjective. RCA blackplates are the holy grail, but my ear thinks JJ's are just as good and they're a lot cheaper lol.

So much to take in. I have watched several youtubes of guys performing and discussing much of what you said. Mods and stuff too.

This is motivating me to get my bench going again. It's becomming a catch all mess.

Thanks for all those insights!

 

[scruffy]

Oh - it probably goes without saying, "stay away from Chinese tubes"! They're horrible, awful. When they break they're likely to take the rest of the amp with them.

 

[Sherlock Holmes]

Oh - it probably goes without saying, "stay away from Chinese tubes"! They're horrible, awful. When they break they're likely to take the rest of the amp with them.

Have you never heard of microphonics?

 

[The Duke]

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.

You need you some toobfreak.
Me? I'd just get a 100W Marshall tube amp, fix it, and make my own speaker cabinet.
Something about them Marshall tube amps.

 

[Chuz Life]

You'd need 1000 watts if you want the sound to be twice as loud as 100 watts.

The ol decible level scaling. I haven't seen that applied to wattage before.
However, the point is made. To double the decibles level, is to multiply it times 10 instead of 2.

The amp would have to be more efficient and deliver more spl per watt - to get around that.

 

[The Duke]

The ol decible level scaling. I haven't seen that applied to wattage before.
However, the point is made. To double the decibles level, is to multiply it times 10 instead of 2.

The amp would have to be more efficient and deliver more spl per watt - to get around that.

You wanna damage your hearing, huh?

 

[The Duke]

I know this was Marshall tube amps:
And tube Wurlitzer, too.

 

[Sherlock Holmes]

The ol decible level scaling. I haven't seen that applied to wattage before.
However, the point is made. To double the decibles level, is to multiply it times 10 instead of 2.

The amp would have to be more efficient and deliver more spl per watt - to get around that.

Yes, the human perception of sound is logarithmic.

 

[The Duke]

Idk how it works now, but back in the day, they'd line the 100w amps into a P.A. type system.

 

[toobfreak]

Yes, the human perception of sound is logarithmic.

Side note: a bit of trivia if anyone doesn't know: The decibel was originally called a Bel, named after Alexander Graham Bell inventor of the telephone, but the Bel proved to be too large a unit to be useful, so they revised it into smaller units (1/10th) and named it the decibel.