Couldn't find schematics, but from the description, it sounds like it's the same idea as Ampmaker's VCB. Both use a MOSFET as a DC-to-DC converter. The VCB is very simple, adding just three resistors, a diode, and a pot. London Power takes it much further, adding circuitry to compensate for shifts in response as you dial it down in an attempt to preserve the same level of distortion at lower volumes. Aiming for the ideal volume control. The VCB just lowers the voltage, which lowers the volume while letting the tubes break up earlier. It's up to the user to compensate with existing gain and volume controls.
One drawback to lowering the voltage to the whole amp is that things like tremelo and reverb will change their behavior and probably stop working altogether at lower volumes. Some folks split the B+, dropping only the output section while leaving the pre (and built-in effects) at standard B+. Taken to the extreme, you could control B+ independently to each section by using multiple VCBs. London Power offers kits that basically do just that.
One drawback to lowering the voltage to the whole amp is that things like tremelo and reverb will change their behavior and probably stop working altogether at lower volumes. Some folks split the B+, dropping only the output section while leaving the pre (and built-in effects) at standard B+. Taken to the extreme, you could control B+ independently to each section by using multiple VCBs. London Power offers kits that basically do just that.
I think you mean O'Conner's power scaling kit. Anyway, if you'll notice, AmpMaker is touting his only for CATHODE-BIASED amplifiers. A traditional Deluxe Reverb is an adjustable fixed-bias amp. I am thinking that the bias voltage may need to track with the plate voltage variation.
Hi Guys
There are no tracking problems with dual-pots as a PPI-MV. Such concerns apply to controlling stereo channel levels for hifi, but are irrelevant in the context of a MV. usually, the PPI-MV is added to amps that have only two preamp stages, which by themselves can only produce mild overdrive, so the later MV placement allows the splitter to be used as a third stage.
In a reverb amp there is a third stage in the preamp but there is also significant attenuation for the dry path so that the reverb mix will be reasonable. Still, the reverb channel has a little more overall gain than the standard Fender 2-stage pre.
The PEC pots are of no better quality than a $1 Alpha pot. The PECs are only useful in high-voltage applications since they have higher insulation ratings than non-mil-spec pots, otherwise save your money.
If you need more "volume" but still wish to use the DR, build two proper speaker cabinets and space them apart. The combo format cripples tone but you can use it as one of the two sound sources. The effective loudness is greatly increased even though there is no increase in power, taking advantage of basic acoustic properties and the way our hearing works. SPKR and TUTs explain why.
Power Scaling is the best method to control amplifier loudness while attaining "cranked" tone. There are no issues of scratchy pots, nor does the tone change, and you can dial power down to zero so the player can decide how quiet "quiet" is. Power Scaling controls only what needs to be controlled to maintain the transfer curve of the output stage. Nothing extraneous is altered so there is no tonal deviation but our approach offers four operating modes for the amp, one of which is Power Scaling and the others are simply flexibility for the player.
There are many problems with vvr,vcb, whatever name you apply. It is a poor interpretation of an old form of Power Scaling, copied by Dana when asked to design a PCB by a licenced Power Scale builder. Among vvr's faults: the "current limiting" does not work; the whole amp is scaled which is always a mistake as it changes tone, there are scratchy pots (the amp's stock pots not the one for the kit); it does not work with fixed-bias - vvr has a variation that does but again it is a bad arrangement that increases the impedance of the power tube grid path. TUT7 expliains this in much more detail, once it is released. TUT4 detailed how to do things properly and has been available since 2006 - so no excuses for doing it wrong.
The one benefit that the imitations provide is a means for players to save their hearing - at least partially. In the example on Ampmaker's site, he suggests a power reduction of nine times for a voltage reduction to one-third, which is true. This power change is a sonic difference of "half as loud". However, the full output of an amp produces ridiculous loudness beyond the Human Scale of loudness perception, and the reduced loudness still falls outside that scale. besides, if you could stand far enough away that it is a safe loudness, would the tone be the same with that circuit? No.
Guitar speakers are very efficient, usually 100dB with 1W input. This is a ridiculous loudness level. Most players use much less power when practising on their own, even it feels rather loud. Were one to measure the output of the amp at those times, you would be surprised at how little power is required. Amps are most often clipped on stage, where the player thinks he can finally let it rip, but this is the worst thing he can do for the on-stage sound and for the house sound.
Have fun
There are no tracking problems with dual-pots as a PPI-MV. Such concerns apply to controlling stereo channel levels for hifi, but are irrelevant in the context of a MV. usually, the PPI-MV is added to amps that have only two preamp stages, which by themselves can only produce mild overdrive, so the later MV placement allows the splitter to be used as a third stage.
In a reverb amp there is a third stage in the preamp but there is also significant attenuation for the dry path so that the reverb mix will be reasonable. Still, the reverb channel has a little more overall gain than the standard Fender 2-stage pre.
The PEC pots are of no better quality than a $1 Alpha pot. The PECs are only useful in high-voltage applications since they have higher insulation ratings than non-mil-spec pots, otherwise save your money.
If you need more "volume" but still wish to use the DR, build two proper speaker cabinets and space them apart. The combo format cripples tone but you can use it as one of the two sound sources. The effective loudness is greatly increased even though there is no increase in power, taking advantage of basic acoustic properties and the way our hearing works. SPKR and TUTs explain why.
Power Scaling is the best method to control amplifier loudness while attaining "cranked" tone. There are no issues of scratchy pots, nor does the tone change, and you can dial power down to zero so the player can decide how quiet "quiet" is. Power Scaling controls only what needs to be controlled to maintain the transfer curve of the output stage. Nothing extraneous is altered so there is no tonal deviation but our approach offers four operating modes for the amp, one of which is Power Scaling and the others are simply flexibility for the player.
There are many problems with vvr,vcb, whatever name you apply. It is a poor interpretation of an old form of Power Scaling, copied by Dana when asked to design a PCB by a licenced Power Scale builder. Among vvr's faults: the "current limiting" does not work; the whole amp is scaled which is always a mistake as it changes tone, there are scratchy pots (the amp's stock pots not the one for the kit); it does not work with fixed-bias - vvr has a variation that does but again it is a bad arrangement that increases the impedance of the power tube grid path. TUT7 expliains this in much more detail, once it is released. TUT4 detailed how to do things properly and has been available since 2006 - so no excuses for doing it wrong.
The one benefit that the imitations provide is a means for players to save their hearing - at least partially. In the example on Ampmaker's site, he suggests a power reduction of nine times for a voltage reduction to one-third, which is true. This power change is a sonic difference of "half as loud". However, the full output of an amp produces ridiculous loudness beyond the Human Scale of loudness perception, and the reduced loudness still falls outside that scale. besides, if you could stand far enough away that it is a safe loudness, would the tone be the same with that circuit? No.
Guitar speakers are very efficient, usually 100dB with 1W input. This is a ridiculous loudness level. Most players use much less power when practising on their own, even it feels rather loud. Were one to measure the output of the amp at those times, you would be surprised at how little power is required. Amps are most often clipped on stage, where the player thinks he can finally let it rip, but this is the worst thing he can do for the on-stage sound and for the house sound.
Have fun
Last edited:
"The PEC pots are of no better quality than a $1 Alpha pot. The PECs are only useful in high-voltage applications since they have higher insulation ratings than non-mil-spec pots, otherwise save your money."
I tend to agree with this statement. They could prove to be better in applications where the amp is subject to fog machines. The PEC pots are sealed. This can keep the fog out but also makes it extremely hard if not impossible to clean. They are also very hard to solder to on the backs. The newer style fog mixes are water based and do create issues for amps in general.
Some clients say they think they sound better....I find this hard to believe. I do think they have a smoother feel, most likely due to the increased diameter of the pot. I don't think they are worth 10 bucks each for general use. I used them in the first channel of the AB763 build and Alphas in the reverb channel for the purpose of testing them over time.
There does not seem to be a lot of cost effective choices when it comes to pots. There exist very well made pots but they cost huge money.
I am conducting experiments with this AB763 to demonstrate to my own satisfaction ideas that get kicked around about several things. Currently I am doing capacitor testing which has demonstrated a larger than expected change in sound than I assumed would be there based on brand and type.
It appears that the use of different dielectric material cause noticeable differences in sound based on other properties being the same at least in this amp design.
Cheers,
Billy
I tend to agree with this statement. They could prove to be better in applications where the amp is subject to fog machines. The PEC pots are sealed. This can keep the fog out but also makes it extremely hard if not impossible to clean. They are also very hard to solder to on the backs. The newer style fog mixes are water based and do create issues for amps in general.
Some clients say they think they sound better....I find this hard to believe. I do think they have a smoother feel, most likely due to the increased diameter of the pot. I don't think they are worth 10 bucks each for general use. I used them in the first channel of the AB763 build and Alphas in the reverb channel for the purpose of testing them over time.
There does not seem to be a lot of cost effective choices when it comes to pots. There exist very well made pots but they cost huge money.
I am conducting experiments with this AB763 to demonstrate to my own satisfaction ideas that get kicked around about several things. Currently I am doing capacitor testing which has demonstrated a larger than expected change in sound than I assumed would be there based on brand and type.
It appears that the use of different dielectric material cause noticeable differences in sound based on other properties being the same at least in this amp design.
Cheers,
Billy
That will quite interesting. I have been under the impression that using any of the film caps (polyester, polypropylene, polystyrene) would sound better than the electrolytics or ceramics; but that there isn't much difference in the film caps' sound.
How about the master volume control? Are you happy with just the dual ganged PPI pot?
The first set of capacitors were Cornell Dubilier 630V Polyester Metallized Axial. These were smaller white looking caps.
The second set I changed to were Cornell Dubilier 715P Series 5% 600V Polypropylene radial which were much larger in diameter. These would be called "orange drop" caps I guess. I am not sure the term "orange drop" really tells you very much. The change in sound was quite a good bit, more than I expected. To me, they sounded less distorted and more "HiFi" sounding.
Cornell Dubilier also has something called PS Series which are 600V 10% Polyester which are the same physical size. I have not tried them yet but my guess is they would not sound very different.
I assume, after some study, the use of Polypropylene as a dielectric produces a cap that is closer to the "ideal capacitor". The "ideal capacitor" would have no resistance or inductance, or leakage but of course no such thing exist. Here is a equivalent circuit of what a real world cap looks like.
The subject is a good bit more complex than I am describing. There is not a unlimited number of caps made to try so experimentation most likely works as well as understand all the complexities.
I am not trying to comment on which sounded "better or worse" as that is purely subjective.
PPIMV issue.
I do not have on hand all the parts I need to experiment with the various circuits that exist nor have I tried yet the power scaling idea. I did install a single 1meg pot post phase inverter MV but I did not like the sound.
Some of the other experiments I tried was changing to 5881 output tubes using the same transformers. Everything ran ok with no noticeable ill effects but I assume this could cause reduced transformer life.
I have just as much interest in what fails as what works well so I am pushing the limits at times.
Cheers,
Billy
The second set I changed to were Cornell Dubilier 715P Series 5% 600V Polypropylene radial which were much larger in diameter. These would be called "orange drop" caps I guess. I am not sure the term "orange drop" really tells you very much. The change in sound was quite a good bit, more than I expected. To me, they sounded less distorted and more "HiFi" sounding.
Cornell Dubilier also has something called PS Series which are 600V 10% Polyester which are the same physical size. I have not tried them yet but my guess is they would not sound very different.
I assume, after some study, the use of Polypropylene as a dielectric produces a cap that is closer to the "ideal capacitor". The "ideal capacitor" would have no resistance or inductance, or leakage but of course no such thing exist. Here is a equivalent circuit of what a real world cap looks like.
The subject is a good bit more complex than I am describing. There is not a unlimited number of caps made to try so experimentation most likely works as well as understand all the complexities.
I am not trying to comment on which sounded "better or worse" as that is purely subjective.
PPIMV issue.
I do not have on hand all the parts I need to experiment with the various circuits that exist nor have I tried yet the power scaling idea. I did install a single 1meg pot post phase inverter MV but I did not like the sound.
Some of the other experiments I tried was changing to 5881 output tubes using the same transformers. Everything ran ok with no noticeable ill effects but I assume this could cause reduced transformer life.
I have just as much interest in what fails as what works well so I am pushing the limits at times.
Cheers,
Billy
Hi Guys
"I tried was changing to 5881 output tubes using the same transformers. Everything ran ok with no noticeable ill effects but I assume this could cause reduced transformer life."
How could an OT possibly be damaged this way? It cannot.
If you have an amp with a PT of known current capacity that can support the heaters of all the pin-compatible octals used in audio, then swapping power tubes has no other effect than to change tone. Output power is limited by the PT and OT. If full output is attainable with 6V6s say, plugging in 6550s does not make it a 150W amp, rather, the 6550s will provide the same output as the Vs do but with a different tone.
Billy, you seem intent on reinventing the wheel with your tests and comments. It has all been done before and it has been documented quite a few times. Caps have different properties based on their construction, so obviously different types have different sounds and the reasons they do are well known. The primary attribute is the dielectric and you can generalise based solely on that facet. Resistors have their own attributes, as do many other components.
There is no voodoo in what makes an amp sound how it does. It is all based on science, but you do not have to know that science to enjoy the amp, to modify it, or even to design it.
Experimentation is the best way for an individual to truly learn something. We experience the world through our hands and our senses, so if we modify an amp, change a cap say, and hear the difference between the first and second, then we will retain this info longer than if we simply read it.
For example, the stage setups described in my books are easy to understand and the improvements in apparent loudness and the feeling of fullness of the sound is described. Intellectually, the player comprehends it and says "ya ya". But then they try it. They are converted forever as if they did not expect such a profound change to the sound simply from moving speakers around. Hearing is believing.
However, all the documented knowledge can save you a lot of time and effort in your own Quest for Tone.
Have fun
"I tried was changing to 5881 output tubes using the same transformers. Everything ran ok with no noticeable ill effects but I assume this could cause reduced transformer life."
How could an OT possibly be damaged this way? It cannot.
If you have an amp with a PT of known current capacity that can support the heaters of all the pin-compatible octals used in audio, then swapping power tubes has no other effect than to change tone. Output power is limited by the PT and OT. If full output is attainable with 6V6s say, plugging in 6550s does not make it a 150W amp, rather, the 6550s will provide the same output as the Vs do but with a different tone.
Billy, you seem intent on reinventing the wheel with your tests and comments. It has all been done before and it has been documented quite a few times. Caps have different properties based on their construction, so obviously different types have different sounds and the reasons they do are well known. The primary attribute is the dielectric and you can generalise based solely on that facet. Resistors have their own attributes, as do many other components.
There is no voodoo in what makes an amp sound how it does. It is all based on science, but you do not have to know that science to enjoy the amp, to modify it, or even to design it.
Experimentation is the best way for an individual to truly learn something. We experience the world through our hands and our senses, so if we modify an amp, change a cap say, and hear the difference between the first and second, then we will retain this info longer than if we simply read it.
For example, the stage setups described in my books are easy to understand and the improvements in apparent loudness and the feeling of fullness of the sound is described. Intellectually, the player comprehends it and says "ya ya". But then they try it. They are converted forever as if they did not expect such a profound change to the sound simply from moving speakers around. Hearing is believing.
However, all the documented knowledge can save you a lot of time and effort in your own Quest for Tone.
Have fun
VOODOO.....
a black religious cult practiced in the Caribbean and the southern US, combining elements of Roman Catholic ritual with traditional African magical and religious rites, and characterized by sorcery and spirit possession.
synonyms: witchcraft, magic, black magic, sorcery, wizardry, dark arts, devilry, hoodoo, necromancy, mojo
lol...Yes, of all the possible things that could cause a amp to sound different "voodoo" is not on my list of possible causes...lol
Given with precision the characteristics of a components, and knowing with equal precision the characteristics of the circuit, it is entirely possible to mathematically predict the effect on sound that components will produce. It is also possible to produce a graphic representation of that change in sound. Relating what one mathematically calculates or what is graphically represented by those calculations to the actual sound is difficult at best. Describing those sounds in words is even harder. We then arrive with words that are typically meaningless. A lot of those words relate to visual references. The "sound" is brown or "sparkling", as an example.
Having a clear idea of what a sound of a given frequency with only the fundamental and no harmonics sounds like is not an easy skill to acquire. Lots of us have a usable ability for relative pitch but few of us have perfect pitch and then only in a very limited range of frequencies.
The only way to truly know what a certain type and value of component sounds like in YOUR amp to YOUR ears is to try them.
I am not really on a "quest for tone". I am simply experimenting to "hear" results for myself. It is actually not very important to me if the "experimenting" produces sounds I like better or not. I have as much interest in what "works" as what does not.
Experimenting is fun, and rest assured my experimenting is very unlikely to produce any earth shattering new information...lol 99% or what is currently known about tube guitar amps was well known and well documented by the early 1950's. Much of that information may not have been implemented by the early designers of guitar amps but it was none the less well known to science. Obviously, today we have access to components that did not exist in 1950 but leaving out solid state devices not a huge amount has changed.
Cheers,
Billy
a black religious cult practiced in the Caribbean and the southern US, combining elements of Roman Catholic ritual with traditional African magical and religious rites, and characterized by sorcery and spirit possession.
synonyms: witchcraft, magic, black magic, sorcery, wizardry, dark arts, devilry, hoodoo, necromancy, mojo
lol...Yes, of all the possible things that could cause a amp to sound different "voodoo" is not on my list of possible causes...lol
Given with precision the characteristics of a components, and knowing with equal precision the characteristics of the circuit, it is entirely possible to mathematically predict the effect on sound that components will produce. It is also possible to produce a graphic representation of that change in sound. Relating what one mathematically calculates or what is graphically represented by those calculations to the actual sound is difficult at best. Describing those sounds in words is even harder. We then arrive with words that are typically meaningless. A lot of those words relate to visual references. The "sound" is brown or "sparkling", as an example.
Having a clear idea of what a sound of a given frequency with only the fundamental and no harmonics sounds like is not an easy skill to acquire. Lots of us have a usable ability for relative pitch but few of us have perfect pitch and then only in a very limited range of frequencies.
The only way to truly know what a certain type and value of component sounds like in YOUR amp to YOUR ears is to try them.
I am not really on a "quest for tone". I am simply experimenting to "hear" results for myself. It is actually not very important to me if the "experimenting" produces sounds I like better or not. I have as much interest in what "works" as what does not.
Experimenting is fun, and rest assured my experimenting is very unlikely to produce any earth shattering new information...lol 99% or what is currently known about tube guitar amps was well known and well documented by the early 1950's. Much of that information may not have been implemented by the early designers of guitar amps but it was none the less well known to science. Obviously, today we have access to components that did not exist in 1950 but leaving out solid state devices not a huge amount has changed.
Cheers,
Billy
Last edited:
A couple of years ago, I fed an old 12" guitar speaker with a signal generator, and clipped in various series resistors until the tone from the speaker was no longer obnoxiously loud on my test-bench.
When I measured the AC voltage across the 8 ohm speaker, and calculated the power being delivered, I was shocked to find it was only about ten microwatts - one hundred thousandth of a watt!
At first I thought I'd made a calculation or measurement mistake, so I repeated both, with no change. Then I realized that a 95 dB @ 1W @1m speaker will still deliver an SPL of 45 dB with only ten microwatts of input power. That's quiet, but still very clearly audible.
-Gnobuddy
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.