Is there a good reason I don't see many guitar amps that use things like mosfet CCS's in the LTP tail, or on the plates of drivers? I'm assuming because of overdrive performance/how they distort, but am not sure.
I'm also wondering about AB2 operation.. I've got plenty of transmitting tubes that like class2 operation, and am wondering if a good guitar amp could be made with them?
It's a topology I haven't seen in guitar amps, and am not sure if it's because it's been tried and doesn't sound good, or if it's because most guitar amp designs are basically old fashioned, and not many people have tried a modern approach w/ a source follower driving calss2 finals..
I guess I'm asking, is there a reason to not start experiments with either mosfet CCS's or class 2 operation in overdriven instrument amps?
I'm also wondering about AB2 operation.. I've got plenty of transmitting tubes that like class2 operation, and am wondering if a good guitar amp could be made with them?
It's a topology I haven't seen in guitar amps, and am not sure if it's because it's been tried and doesn't sound good, or if it's because most guitar amp designs are basically old fashioned, and not many people have tried a modern approach w/ a source follower driving calss2 finals..
I guess I'm asking, is there a reason to not start experiments with either mosfet CCS's or class 2 operation in overdriven instrument amps?
I have never tried a CCS as a plate load for guitar amp or on the cathode of a LTP. My take is that as a plate load, it makes the load line horizontal and linearizes the valve, these are good things in HiFi but for instrumentation amplifiers it's an added cost for "sterile" sound.
For the CCS at the cathode of a LTP I am thinking it would AC balance both output signals in turn reducing distortion. Good for HiFi but not really necessary in IA, resistors are cheaper too.
I can't speak for the transmitting tubes but I believe they have to be operated in x2 operation. I have asked this same question several months ago and got some really good answers. Most of the answers were based on squeezing more power from the valves which I was not interested in the actual power out but the blocking distortion found with large signal transients at the grid of the power tubes. So how does it sound? Awesome of course, I just think that it's not worth it for commercial designs. A good design which would include appropriately chosen coupling capacitors can keep the blocking distortion down to where you don't need a special driver. The only commercial designs I found using these types of drivers were Bass Amps trying to squeeze out as much power as possible.
I have been doing so much experimenting lately I am lost with my ideas. I basically can't make up my mind whether to include A2 drive or not in my latest creation. If I do it will be with sand, so Mosfets to drive some 6L6's operated in PP class A (triode connected, and no global feedback). The front end is a Cascode driving a LTP, no tone stack, just a volume control between the Cascode and the LTP. I get 5w out @ 1% THD and 12w @ 5% THD. I use very efficient drivers (103db@2.83v) so a 12 watt amp is all I need to get into the sweet spot.
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Thanks!
I'm playing w/ some compactrons with not exactly linear triodes, which is why I was asking about the ccs in the ltp, but, I think you're correct, I shouldn't worry about that in a guitar amp.
And, I guess I'll try the a2 finals.
I've had a guitar amp on the workbench (really it's in my dining room, where I play it almost daily 🙂 ) all winter, and I'm wondering about the need for a tone stack. I keep mine set flat, and adjust the treble at the guitar. I've got an eq pedal when necessary..
I'm playing w/ some compactrons with not exactly linear triodes, which is why I was asking about the ccs in the ltp, but, I think you're correct, I shouldn't worry about that in a guitar amp.
And, I guess I'll try the a2 finals.
I've had a guitar amp on the workbench (really it's in my dining room, where I play it almost daily 🙂 ) all winter, and I'm wondering about the need for a tone stack. I keep mine set flat, and adjust the treble at the guitar. I've got an eq pedal when necessary..
Same here. I find myself leaving my tone controls flat and never touching them. I use my guitar volume and tone controls a lot compared to most guitarists. So I said this amp is NOT going to have a tone stack. I am worried I will get a gig where the room is unpleasant and needs to be corrected, in this case an EQ as you have mentioned is a better alternative than a tone stack IMHO.
I do have one major use for "tone controls" in an amp. I like seperate "Tone" controls on the distortion and separate tone controls for the "guitar" sound. I achieve this two ways.
1) I apply treble-boost (well, low-cut) pre-emphasis between the first and second stages, then generate some distortion and compression, then apply treble-cut between the third and fourth stages. With pots between each stage I can control exactly where the clean / dirty threshold is. The guitar sound gets treble boosted then cut and sounds comparatively intact. The distortion, on the other hand, being created between the tone pre-emphasis stage and post / correction stage, only goes thru the final treble-cut and thus has its most irritating and abrasive "fizzy" fuzz removed, leaving more musically rich harmonics. It's like enjoying the creamy Clapton "woman tone" without losing all the treble. I use full tone stacks for each and a bright switch across the first gain pot.
2) I also like to have another simple 2-stage "clean" preamp that completely bypasses the "lead" preamp. This 2-stage preamp (with tone stack) and the other 4-stage preamp (really more like two in series) have outputs that are in-phase so i mix them...usually with just a separate pot on the output of each. The sound is much like the famous "double jacking" of a Plexi, where they drive both the clean and dirty channels simultaneously. It's also a trick a lot of recording studios use. But mixing at the output of the preamp channels works better than tweeking the relative gain of each (though I do that too).
Of course, combining both approaches results in a LOT of knobs instead of "voicing" the amp for one sound. You have to work at minimizing noise. Cost goes up. Reliability goes down, but is still good. My one preamp is really 3 normal preamps, but is so flexible it can emulate anything from a Dumble to a clean Fender to Blackface or Plexi or Mesa or 5150. But unless you duplicate that complexity and add an A/B switch, or walk over to turn a knob, I still generally set it and forget it in typical use and use it as a one-channel amp, admittedly one complicated channel. So it's the opposite of a "champ"...but is very flexible and works well with any guitar. (I also include variable input loading).
To each his own, there are advantages to each direction, which is why I also have a few simple champ-type amps I love equally. The down side is that 'voicing" a champ involves a soldering iron, the "post/correction" EQ is achieved in the output tube, output transformer, and speaker selection, the pre-emphasis is a "bright" cap across the volume...the clean / dirty line is determined by volume...and it has only one knob.
1) I apply treble-boost (well, low-cut) pre-emphasis between the first and second stages, then generate some distortion and compression, then apply treble-cut between the third and fourth stages. With pots between each stage I can control exactly where the clean / dirty threshold is. The guitar sound gets treble boosted then cut and sounds comparatively intact. The distortion, on the other hand, being created between the tone pre-emphasis stage and post / correction stage, only goes thru the final treble-cut and thus has its most irritating and abrasive "fizzy" fuzz removed, leaving more musically rich harmonics. It's like enjoying the creamy Clapton "woman tone" without losing all the treble. I use full tone stacks for each and a bright switch across the first gain pot.
2) I also like to have another simple 2-stage "clean" preamp that completely bypasses the "lead" preamp. This 2-stage preamp (with tone stack) and the other 4-stage preamp (really more like two in series) have outputs that are in-phase so i mix them...usually with just a separate pot on the output of each. The sound is much like the famous "double jacking" of a Plexi, where they drive both the clean and dirty channels simultaneously. It's also a trick a lot of recording studios use. But mixing at the output of the preamp channels works better than tweeking the relative gain of each (though I do that too).
Of course, combining both approaches results in a LOT of knobs instead of "voicing" the amp for one sound. You have to work at minimizing noise. Cost goes up. Reliability goes down, but is still good. My one preamp is really 3 normal preamps, but is so flexible it can emulate anything from a Dumble to a clean Fender to Blackface or Plexi or Mesa or 5150. But unless you duplicate that complexity and add an A/B switch, or walk over to turn a knob, I still generally set it and forget it in typical use and use it as a one-channel amp, admittedly one complicated channel. So it's the opposite of a "champ"...but is very flexible and works well with any guitar. (I also include variable input loading).
To each his own, there are advantages to each direction, which is why I also have a few simple champ-type amps I love equally. The down side is that 'voicing" a champ involves a soldering iron, the "post/correction" EQ is achieved in the output tube, output transformer, and speaker selection, the pre-emphasis is a "bright" cap across the volume...the clean / dirty line is determined by volume...and it has only one knob.
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Hi Guys
Most guitar amps are built to be "functional" as that is good enough for MI. Linearising techniques are good for hifi but make the tube amp less tubey sounding, so even most hobbyists don't bother with them. And production amps are all about cost - so simple traditional circuits are ideal.
For your own amp, there is no reason NOT to try any method, circuit, or device type that you have on hand or are simply curious about using. Transmitting tubes make awesome audio amps. Power-grid tubes like the 811A basically sound like really big 6L6s - very neutral yet with some tube character.
The '2' in class-A2 and class-AB2 means that the grids are driven positive. Traditional circuits do not do this nor do they need to to attain full output. Some receiving tubes are not amenable to positive grid-drive where others will only tolerate it intermittently. In an amp designed to drive the grids positive, transformer coupling or a cathode follower is used. In our modern times, the CF can be replaced by BJTs or mosfets. Note that the solid-state devices will require small heatsinks.
If you build a tube amp, do yourself a favour and build it as a head or rackmount NOT a combo. The combo is the worst environment for a tube. TUT3 discusses mechanical aspects of building an amp, as well as proper wiring for best note articulation and lowest noise.
Have fun
Most guitar amps are built to be "functional" as that is good enough for MI. Linearising techniques are good for hifi but make the tube amp less tubey sounding, so even most hobbyists don't bother with them. And production amps are all about cost - so simple traditional circuits are ideal.
For your own amp, there is no reason NOT to try any method, circuit, or device type that you have on hand or are simply curious about using. Transmitting tubes make awesome audio amps. Power-grid tubes like the 811A basically sound like really big 6L6s - very neutral yet with some tube character.
The '2' in class-A2 and class-AB2 means that the grids are driven positive. Traditional circuits do not do this nor do they need to to attain full output. Some receiving tubes are not amenable to positive grid-drive where others will only tolerate it intermittently. In an amp designed to drive the grids positive, transformer coupling or a cathode follower is used. In our modern times, the CF can be replaced by BJTs or mosfets. Note that the solid-state devices will require small heatsinks.
If you build a tube amp, do yourself a favour and build it as a head or rackmount NOT a combo. The combo is the worst environment for a tube. TUT3 discusses mechanical aspects of building an amp, as well as proper wiring for best note articulation and lowest noise.
Have fun
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