Lt Boob amp PART 2 - the other boob!

[Greg_L]

well take as long as needed to get it sorted out ....... I don't want magic smoke over here.
Plus I want that 'bonk' so don't do anything that unbonks it

Ha there won't be any smoke. Since the bias is not adjustable I'm trying to find the balance between great tone and good tube life.

[Lt. Bob]

well great tone comes first ...... I can buy tubes .... but obviously I don't want to buy them every 6 months

[Greg_L]

well great tone comes first ...... I can buy tubes .... obviously I don't want to buy them every 6 months

Exactly. And I really don't think it will be a problem. I just gotta go through the tests and the math. The only thing about it is that there's no pot to turn to adjust the bias. I have to physically remove and install resistors and do a bunch of measurements and calculations to retest every time. I don't mind. It's good work and good practice.

[Lt. Bob]

the boob whisperer ........ wait, that don't sound right

[Greg_L]

the boob whisperer ........ wait, that don't sound right

Sounds awesome!

So I'm leaning towards an old trick I've discovered.

I'm gonna try to knock the secondary voltage coming out of the transformer down a hair before it even gets into the rectifier. I think I need to drop about 5-10% across the board. This will affect all of the voltages within the amp, but not so much the smaller downstream preamp voltages. This will mainly hit the huge power section voltages, which could stand to be tamed a little.

First thing though, I'm gonna try it with the variac. If I get results from using the variac then I'll pursue this option.

[Greg_L]

So the variac results pan out great. If I drop the incoming voltage from the wall, everything falls in line really nicely. That's not really practical though. So now I need to work on something like an internal "variac", or just drop the voltages after the rectifier. The PT just puts out a little too much juice. Even the heater voltages are a little high. More research...

[Greg_L]

Okay @Lt. Bob I think I've got this sorted now. The verdict: we're just gonna leave it alone.

After much much testing and research I've concluded that the internal voltages, even being a tad high, are within acceptable tolerances. The power tube's plate voltages are higher than original spec, but so is our modern wall voltage. That obviously explains the high voltages everywhere. Mojo should probably re-think their power transformer design. Or, maybe they know better and know that things will be fine. Also these voltages are nowhere near as high as similar EL84 amp designs that run forever, so I'm comfortable with them.

One area I did address is the screen voltage. A tube can handle high plate voltage. But high plate voltage combined with high screen voltage will really shorten tube life. So I beefed up the screen resistors in value and wattage. That got the screens down to a more acceptable level relative to the plate voltage. The only potential negative to this arrangement is that at wide open throttle the amp's tone might compress a little more and it won't be quite as loud. I personally think that's a good thing. You're not gonna use it at max loudness anyway. And with the bias now running the tubes nice and smooth and safe, we should be good.

And all of this only relates to tube life. There's nothing about this that's dangerous or gonna cause a mushroom cloud.

And if by chance it still goes through power tubes faster than you'd like, you can use Sovtek EL84M. These are super-duty EL84s.:
https://www.mojotone.com/amp-parts/Outp ... acuum-Tube

[WhiskeyJack]

I am pretty stoked to hear and see this new bewb.

So now i have a question from something up thread. and forgive my novice ignorance here @Greg_L

You mentioned a variac, and my only knowledge of these things it that Johnny 2 bags dude from social D. I think i recall you saying Angoose from AD/HD used one too? But 2 Bags has a variac in his two knob Satellite amp rig. If I understand it is so he can crank the amp to get it cooking but use the variac to dial it down volume wise? Or make it louder i don't know?

Could you ever design an amp that just has that built into it? And i am sure there are amps out there that do this, Like JD's H&K that has multi watt settings. that would essentially be like a built in variac wouldn't it be?

but say a guy wanted a simple circuit like that but need to more or less juice? Would it make sense to design an amp that way?

Forgive my ignorance if I am way out to lunch here but just curious.

Sorry to hijack the thread dudes.

[Lt. Bob]

I am officially naming the new amp BOB ....... short for Bob's Other Boob!

So the deluxe is 'gRamp' and this one is 'BOB'!

There .... now that i've done the important work we'll get back to the amp!

[Greg_L]

I am pretty stoked to hear and see this new bewb.

So now i have a question from something up thread. and forgive my novice ignorance here @Greg_L

You mentioned a variac, and my only knowledge of these things it that Johnny 2 bags dude from social D. I think i recall you saying Angoose from AD/HD used one too? But 2 Bags has a variac in his two knob Satellite amp rig. If I understand it is so he can crank the amp to get it cooking but use the variac to dial it down volume wise? Or make it louder i don't know?

Okay bud, grab a beer and see if this helps.

First off, All a variac is basically is a big giant variable resistor. It plugs into the wall, your device plugs in to the variac. You can alter the voltage going into the device with the variac by turning a great big knob. It really is that simple. I'm not sure if you can over-voltage....I've never tried it...but you can definitely go under wall voltage, and that's useful sometimes. Many of the amps we love the most were designed when wall voltage was much lower than it is now. A variac can get you into that just-right voltage range for the design of a particular amp. The problem with using a variac for tone on a running amp is that it lowers the voltage across the board. Sure, it might lower some high voltages down to more "vintage" levels, but it also lowers the heater voltage which needs to stay where it is. It lowers all the voltages. The heaters suffer, the preamp voltages suffer.

I think the variac thing is way way overblown. I've tested it on numerous amps. It does something for sure, but not enough IMO to even think about. I have one. I'd use it if it did something worthwhile. IMO it doesn't, as far as tone goes. It's a great tool for amp work. It's not a magic tone bullet. It can take a too loud amp down to still too loud. Great. No big win there. Eddie Van Halen is famous for bringing this simple lowly testing device to our attention, but even for him it didn't perform magic. He'd still be EVH without it. I don't know how Johnny two-bags uses one, but I'm certain he would survive just fine without it. He may actually use "power scaling" if it's built into the amp. More on that in a bit. To me this is one of those things that's just self-serving tone masturbation. Sure, if it makes someone happy, then go for it. Lowering voltage does slightly juice up the gain a tiny bit. Slightly. But a variac is totally unnecessary as a part of a guitar rig. And they're never used to go louder. That would be fire and explosions. I think a simple but well designed post phase inverter master volume is a far better option for gain and volume control.

Could you ever design an amp that just has that built into it? And i am sure there are amps out there that do this, Like JD's H&K that has multi watt settings. that would essentially be like a built in variac wouldn't it be?

but say a guy wanted a simple circuit like that but need to more or less juice? Would it make sense to design an amp that way?

Forgive my ignorance if I am way out to lunch here but just curious.

Sorry to hijack the thread dudes.

Okay, for one, you pretty much never want more juice. Amps pretty much already run at their max. Everything we do with tone controls and volume controls takes away signal. For example, turning up the bass knob isn't adding bass. You're just letting all of the available bass through. Same with volume. Turning up volume isn't adding volume. When you crank an amp it gives you all it has. You can't add any more. Everything we do is subtractive. Some amps have active controls that can boost frequencies and stuff, but they're not very common. Most amps are passive. Crank a typical amp with everything at 10 and that amp is giving you all it has. You can't safely and/or reliably squeeze more out of it.

I'm not sure how various amps cut output wattage, but there are different ways. Output power and all the internal voltages are not entirely related. You can have a low power amp that runs big internal voltages. A Deluxe Reverb runs pretty big voltages, but only makes 22 watts. Most amps use what's called a pentode power tube. These tubes have five elements as part of their construction. They're very efficient and provide a lot of amplification. One method of cutting the output power is to disable the extraneous elements of those tubes so they run on the very basics. This is usually how "half power" switches work in amps. You cut a pentode down to triode operation and it greatly reduces the efficiency of the power tubes thereby decreasing output.

There are ways to build internal power reduction into amps. The easiest way IMO is to use a power transformer design that gets you the voltages you want. Simple. Another way is to use an additional low voltage secondary transformer to subtract from the output of the main power transformer. This takes up internal space but it's an option. These transformers are small. You could use something like a 9v wall wart...like a One Spot power supply...wired in parallel with the power transformer and subtract 9 volts from the power supply. You wouldn't actually use a wall wart, but that's the general idea. A small high primary, low secondary step-down transformer can take away voltage. With this option you could preserve the heater voltage but reduce what becomes the rectified DC high voltages. And then there's power scaling, which is like voodoo magic. Power scaling pretty much just focuses on the output tubes. This method supposedly maintains the response of a cranked amp down to whisper levels. Naturally this is really only useful on amps that need to be CRANKED to get the goods...like Johnny Two-Bags's Satellite amps. Power scaling basically preserves the output tube's character by proportionately dialing back voltage and bias strictly at the power tubes. The rest of the amp is left alone. The tubes behave the same at low voltages as they would at massive voltages because everything is dropped in scale. The downside of this is that the system requires some extensive mods and it creates heat. It would be easy to build this system into a fresh-build amp. It's more difficult to fit it into an existing amp. And it's not really necessary on high preamp gain master volume amps.

Any of these options is better than a variac IMO. But a variac certainly wins on simplicity.

[Lt. Bob]

When you crank an amp it gives you all it has. You can't add any more. Everything we do is subtractive. Some amps have active controls that can boost frequencies and stuff, but they're not very common. Most amps are passive. Crank a typical amp with everything at 10 and that amp is giving you all it has.

and actually most of the time they're giving you all they have at much less than cranked ....... with most amps after a certain point you're overdriving the amps input and turning it further simply compresses and distorts the sound ...... which is, of course, part of the sound we desire to achieve.

[Greg_L]

and actually most of the time they're giving you all they have at much less than cranked ....... with most amps after a certain point you're overdriving the amps input and turning it further simply compresses and distorts the sound ...... which is, of course, part of the sound we desire to achieve.

Yeah for sure. Where and when the distortion tone happens just depends on the layout of the preamp and power stage.

Like on your Deluxe Reverb, the input goes straight to the first tube stage. Bam, instant gain. The output of that first stage is hot. You wouldn't know it though because even though it's super amplified, it's not clipped. It's just a huge version of the input signal. Then it passes right to the volume pot. That's where you turn it down. You're just bleeding off all that gain to a level that is pleasing to your ear and/or not driving the output hard. As you turn the volume up you're just letting more of what's already there pass through to the output stage. At some point it kicks the output tubes into clipping and then you get distortion. It doesn't really get louder, it just gets more distorted. Almost every amp has this same phenomenon. And that's where they rate an amps output. Your 22w Deluxe Reverb is 22 watts at max clean output. If you push it into distortion and calculate for clipping, it's probably somewhere closer to 30 watts.

The non-master Marshalls do the same thing. The master volume Marshalls are much different. They clip before the output stage.

[WhiskeyJack]

I am pretty stoked to hear and see this new bewb.

So now i have a question from something up thread. and forgive my novice ignorance here @Greg_L

You mentioned a variac, and my only knowledge of these things it that Johnny 2 bags dude from social D. I think i recall you saying Angoose from AD/HD used one too? But 2 Bags has a variac in his two knob Satellite amp rig. If I understand it is so he can crank the amp to get it cooking but use the variac to dial it down volume wise? Or make it louder i don't know?

Okay bud, grab a beer and see if this helps.

First off, All a variac is basically is a big giant variable resistor. It plugs into the wall, your device plugs in to the variac. You can alter the voltage going into the device with the variac by turning a great big knob. It really is that simple. I'm not sure if you can over-voltage....I've never tried it...but you can definitely go under wall voltage, and that's useful sometimes. Many of the amps we love the most were designed when wall voltage was much lower than it is now. A variac can get you into that just-right voltage range for the design of a particular amp. The problem with using a variac for tone on a running amp is that it lowers the voltage across the board. Sure, it might lower some high voltages down to more "vintage" levels, but it also lowers the heater voltage which needs to stay where it is. It lowers all the voltages. The heaters suffer, the preamp voltages suffer.

I think the variac thing is way way overblown. I've tested it on numerous amps. It does something for sure, but not enough IMO to even think about. I have one. I'd use it if it did something worthwhile. IMO it doesn't, as far as tone goes. It's a great tool for amp work. It's not a magic tone bullet. It can take a too loud amp down to still too loud. Great. No big win there. Eddie Van Halen is famous for bringing this simple lowly testing device to our attention, but even for him it didn't perform magic. He'd still be EVH without it. I don't know how Johnny two-bags uses one, but I'm certain he would survive just fine without it. He may actually use "power scaling" if it's built into the amp. More on that in a bit. To me this is one of those things that's just self-serving tone masturbation. Sure, if it makes someone happy, then go for it. Lowering voltage does slightly juice up the gain a tiny bit. Slightly. But a variac is totally unnecessary as a part of a guitar rig. And they're never used to go louder. That would be fire and explosions. I think a simple but well designed post phase inverter master volume is a far better option for gain and volume control.

Could you ever design an amp that just has that built into it? And i am sure there are amps out there that do this, Like JD's H&K that has multi watt settings. that would essentially be like a built in variac wouldn't it be?

but say a guy wanted a simple circuit like that but need to more or less juice? Would it make sense to design an amp that way?

Forgive my ignorance if I am way out to lunch here but just curious.

Sorry to hijack the thread dudes.

Okay, for one, you pretty much never want more juice. Amps pretty much already run at their max. Everything we do with tone controls and volume controls takes away signal. For example, turning up the bass knob isn't adding bass. You're just letting all of the available bass through. Same with volume. Turning up volume isn't adding volume. When you crank an amp it gives you all it has. You can't add any more. Everything we do is subtractive. Some amps have active controls that can boost frequencies and stuff, but they're not very common. Most amps are passive. Crank a typical amp with everything at 10 and that amp is giving you all it has. You can't safely and/or reliably squeeze more out of it.

I'm not sure how various amps cut output wattage, but there are different ways. Output power and all the internal voltages are not entirely related. You can have a low power amp that runs big internal voltages. A Deluxe Reverb runs pretty big voltages, but only makes 22 watts. Most amps use what's called a pentode power tube. These tubes have five elements as part of their construction. They're very efficient and provide a lot of amplification. One method of cutting the output power is to disable the extraneous elements of those tubes so they run on the very basics. This is usually how "half power" switches work in amps. You cut a pentode down to triode operation and it greatly reduces the efficiency of the power tubes thereby decreasing output.

There are ways to build internal power reduction into amps. The easiest way IMO is to use a power transformer design that gets you the voltages you want. Simple. Another way is to use an additional low voltage secondary transformer to subtract from the output of the main power transformer. This takes up internal space but it's an option. These transformers are small. You could use something like a 9v wall wart...like a One Spot power supply...wired in parallel with the power transformer and subtract 9 volts from the power supply. You wouldn't actually use a wall wart, but that's the general idea. A small high primary, low secondary step-down transformer can take away voltage. With this option you could preserve the heater voltage but reduce what becomes the rectified DC high voltages. And then there's power scaling, which is like voodoo magic. Power scaling pretty much just focuses on the output tubes. This method supposedly maintains the response of a cranked amp down to whisper levels. Naturally this is really only useful on amps that need to be CRANKED to get the goods...like Johnny Two-Bags's Satellite amps. Power scaling basically preserves the output tube's character by proportionately dialing back voltage and bias strictly at the power tubes. The rest of the amp is left alone. The tubes behave the same at low voltages as they would at massive voltages because everything is dropped in scale. The downside of this is that the system requires some extensive mods and it creates heat. It would be easy to build this system into a fresh-build amp. It's more difficult to fit it into an existing amp. And it's not really necessary on high preamp gain master volume amps.

Any of these options is better than a variac IMO. But a variac certainly wins on simplicity.

Shit dude thanks for that. that is awesome. What my head was telling me a variac does was just not good. I don't know that i'd ever have a use for something like that.

Power scaling on the other hand sounds fucking rad and for someone like me i could probably make use of a design that incorporated that. i'm not a high gain guy but i like grunt and overdrive, and decibel options.

thanks for the clarification.

[Greg_L]

Shit dude thanks for that. that is awesome. What my head was telling me a variac does was just not good. I don't know that i'd ever have a use for something like that.

Power scaling on the other hand sounds fucking rad and for someone like me i could probably make use of a design that incorporated that. i'm not a high gain guy but i like grunt and overdrive, and decibel options.

thanks for the clarification.

You bet. Power scaling is probably the best option these days, but it's difficult to implement. And again, you have to have an amp that can really benefit from it. Any amp that needs to actually be cranked or damn near cranked could benefit from power scaling. A Plexi, Twin Reverb, AC30, any old school amp that needs to be very loud and doesn't use a lot of preamp clipping could use power scaling to get cranked tone at low volumes. One issue with power scaling, or anything that lowers volume, IMO is that you take the speaker and cab out of the equation. Sure maybe the amp is sounding righteous at a much lower volume, but now you're not exciting the speaker or cab. Those items are a big part of the overall sound. Sometimes an amp turned way down sounds bad and it's not because the amp sounds bad, but because the speaker is bored and stiff and not doing anything. There's no air moving. This is a problem with any method of cutting output volume.

Another option is an attenuator. These don't change anything within the amp. The amp just blasts away and the attenuator soaks it up. But again, when you cut too much volume, the speakers lose their effect. And ten million posts in the original tone thread prove how much speakers matter.

[Greg_L]

I am officially naming the new amp BOB ....... short for Bob's Other Boob!