Hi everyone,
On request on one client, I installed a London Power Scaling SV1 in a custom amplifier that I built. However, even if it technically works as expected; the owner told me that is not working as he was expecting.
According with some research that he did on the internet, he was expecting that when lowering the power the amplifier will sound with a lower volume but it would keep the same tone. For instance, if the amp is clean with Max power, it will remain same clean at lower power. If the amp is crunch, it will remain also the same crunch. He mentioned some amps from Morgan and Reeves that provide the same feature, behave as he was saying. Of course, he only saw that on the internet, never with a real amplifier in front of him.
Instead, when there is less power, the amp saturates quicker when the gain and master remains at the same level. The only way to keep the tone is to lower also the gain and master.
For me it makes sense that it behaves like this, because by changing the screen and plate voltages you change also the headroom. As soon as you change the operating point of the output valve, the dynamic of the amplifier changes.
Could anyone with experience on the topic can help me a bit on this?
On request on one client, I installed a London Power Scaling SV1 in a custom amplifier that I built. However, even if it technically works as expected; the owner told me that is not working as he was expecting.
According with some research that he did on the internet, he was expecting that when lowering the power the amplifier will sound with a lower volume but it would keep the same tone. For instance, if the amp is clean with Max power, it will remain same clean at lower power. If the amp is crunch, it will remain also the same crunch. He mentioned some amps from Morgan and Reeves that provide the same feature, behave as he was saying. Of course, he only saw that on the internet, never with a real amplifier in front of him.
Instead, when there is less power, the amp saturates quicker when the gain and master remains at the same level. The only way to keep the tone is to lower also the gain and master.
For me it makes sense that it behaves like this, because by changing the screen and plate voltages you change also the headroom. As soon as you change the operating point of the output valve, the dynamic of the amplifier changes.
Could anyone with experience on the topic can help me a bit on this?
How the heck will it still stay clean with less power? Same old story, I don't want more power, I just want more headroom (working the opposite of power scaling).
You have it right, if you think of what changes you are making to the circuit (lower voltage) the effects are as you said. Power scaling can only do so much. To augment the power scaling a master volume helps to get more of a pleasing tone.
You have it right, if you think of what changes you are making to the circuit (lower voltage) the effects are as you said. Power scaling can only do so much. To augment the power scaling a master volume helps to get more of a pleasing tone.
i agree with Printer2
on tube pedals design, the low voltage results in 'plate starvation' and the effects are low headroom, more distortion;
what i 'think' is viable (pardon me if it gets out of the point)
*at least in brazil, its quite expensive to import a power scaling device plus hiring a tech to install it, so:
-its cheaper to purchase a single-ended el84 amp loaded with two 8 inches x 15w speakers, retaining true cranked tone
*its possible to use an distortion/od/fuzz pedal (better if valve driven) and set the amp volume on 10, then set the pedal gain cranked with output near 1
not trying to disappoint, but when i studied attenuation/power scaling, there was always something undesirable as a subproduct (loss of harmonics, decreased headroom, ice pickness)
maybe because power scaling is an attempt to emulate something and synth will never be the real deal
but i am here to help, its not meant to disencourage, so if the advices i gave ain't useable, then let me know what is plausible to reach acceptance so we can work it out or i'll refrain from posting if my knowledge on the subject is mediocre to be worth a difference
best wishes
edit: maybe installing a custom switching system that halves the natural power (power pentodes into triodes, bypassing preamp gain stages, different slope/tonestack settings) would do a better job and grant the tech some good money with little effort, since adding a footswitch that changes a little the path ain't sooo much of an effort, for a professional tech, less than 2 days
and the client would be happier than an attenuator (nothing against power scaling devices or their manufacturers, some peoplevlile it and its alright then... but if your client wasn't happy with the results, maybe we shalt appeal into other alternatives rather than insisting on something that didnt work as his mind wanted... unless he'd get vicious on the money he spent and demands the *&^/$ to magically operate on differentials... i totally undersand it, here in brazil we choose if the income is to buy food or new and necessary equipment... when the gear is not what we expected, then we squeeze the hell out of it, to make it worth the meal we didnt eat!)
on tube pedals design, the low voltage results in 'plate starvation' and the effects are low headroom, more distortion;
what i 'think' is viable (pardon me if it gets out of the point)
*at least in brazil, its quite expensive to import a power scaling device plus hiring a tech to install it, so:
-its cheaper to purchase a single-ended el84 amp loaded with two 8 inches x 15w speakers, retaining true cranked tone
*its possible to use an distortion/od/fuzz pedal (better if valve driven) and set the amp volume on 10, then set the pedal gain cranked with output near 1
not trying to disappoint, but when i studied attenuation/power scaling, there was always something undesirable as a subproduct (loss of harmonics, decreased headroom, ice pickness)
maybe because power scaling is an attempt to emulate something and synth will never be the real deal
but i am here to help, its not meant to disencourage, so if the advices i gave ain't useable, then let me know what is plausible to reach acceptance so we can work it out or i'll refrain from posting if my knowledge on the subject is mediocre to be worth a difference
best wishes
edit: maybe installing a custom switching system that halves the natural power (power pentodes into triodes, bypassing preamp gain stages, different slope/tonestack settings) would do a better job and grant the tech some good money with little effort, since adding a footswitch that changes a little the path ain't sooo much of an effort, for a professional tech, less than 2 days
and the client would be happier than an attenuator (nothing against power scaling devices or their manufacturers, some peoplevlile it and its alright then... but if your client wasn't happy with the results, maybe we shalt appeal into other alternatives rather than insisting on something that didnt work as his mind wanted... unless he'd get vicious on the money he spent and demands the *&^/$ to magically operate on differentials... i totally undersand it, here in brazil we choose if the income is to buy food or new and necessary equipment... when the gear is not what we expected, then we squeeze the hell out of it, to make it worth the meal we didnt eat!)
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A master volume, depending on where in the circuit it is. I kept my answer short as I was running out of the house. To get the most 'fidelity' out of volume reduction I would run the preamp section at the normal voltage, power scale the PI and output, master volume before the PI, Powersoak type of attenuation after the OT. And if you really need it there is the variable flux speakers.
I suggest you drop a line to Mr. London Power himself---Kevin O'Connor: amps@londonpower.com
He used to be a member here, but was dismissed----????
In my experience, he WILL answer you, though it may be a bit caustic.
He used to be a member here, but was dismissed----????
In my experience, he WILL answer you, though it may be a bit caustic.
I send him and email, and he replay quite fast 
He explained me that I needed to add a driver compensation control. Now, by lowering the signal at the Phase Inverter, I keep "exactly" the same tone at a lower level right before the EL84. The kit works properly now.
I have to admit that it was my fault. I didn't noticed that detail in the documentation.
He explained me that I needed to add a driver compensation control. Now, by lowering the signal at the Phase Inverter, I keep "exactly" the same tone at a lower level right before the EL84. The kit works properly now.
I have to admit that it was my fault. I didn't noticed that detail in the documentation.
Hi
The London Power Power Scale kits include the controls needed to maintain exact tone BUT you have to install them THEN you have to use them:
POWER SCALE controls the amount of power the amp will produce
DRIVE COMPENSATION maintains the drive to the output stage at a level appropriate to the POWER SCALE setting. if you make the amp look like it is much smaller - say dialing a 50W amp down to 3W - the "3W" output stage will overdrive much sooner with all other things left alone. So, DRIVE COMPENSATION lets you set the drive into the smaller output stage so you can get the same sound I have this on my amp and you just set the two controls the same for exact tone. You can also set them different for other sounds.
If the amp is cathode-biased that is the control set. If the amp is fixed-biased a bias pot is included to set idle at full power and then is not touched until you change tubes.
From what I've read in Kevin O'Connor's books and on his site, you can easily disable the Power Scale for "instant headroom". With the new SV Power Scaling kits, this is as simple to do as lifting the wiper of the Power Scale pot and bypassing the Drive Compensation control. The kit notes show that.
blackwhaleamp, your customer might want that instant-headroom on a switch for going from clean without Power Scaling to dirty with Power Scaling.
re the dismissal - what I heard is that the forum was doing something illegal, he called them on it and they banned him. The forum's loss and ours too.
The London Power Power Scale kits include the controls needed to maintain exact tone BUT you have to install them THEN you have to use them:
POWER SCALE controls the amount of power the amp will produce
DRIVE COMPENSATION maintains the drive to the output stage at a level appropriate to the POWER SCALE setting. if you make the amp look like it is much smaller - say dialing a 50W amp down to 3W - the "3W" output stage will overdrive much sooner with all other things left alone. So, DRIVE COMPENSATION lets you set the drive into the smaller output stage so you can get the same sound I have this on my amp and you just set the two controls the same for exact tone. You can also set them different for other sounds.
If the amp is cathode-biased that is the control set. If the amp is fixed-biased a bias pot is included to set idle at full power and then is not touched until you change tubes.
From what I've read in Kevin O'Connor's books and on his site, you can easily disable the Power Scale for "instant headroom". With the new SV Power Scaling kits, this is as simple to do as lifting the wiper of the Power Scale pot and bypassing the Drive Compensation control. The kit notes show that.
blackwhaleamp, your customer might want that instant-headroom on a switch for going from clean without Power Scaling to dirty with Power Scaling.
re the dismissal - what I heard is that the forum was doing something illegal, he called them on it and they banned him. The forum's loss and ours too.
Indeed, I talked with Kevin and he explained me that I didn't attached the DRIVER in the circuit. He also explained that I could add another stage to the Power Scaling kit and drive the whole Power with a single POT. At the end, I just add the DRIVER POT in the amplifier and it sounds great.
But now I was wondering, what is the difference between having just the DRIVER and the DRIVER+POWER Scaling. For this 18W amplifier I didn't noticed any large difference between both. So, having a POT just right after the PI is just good enough. Maybe this kit is more useful for a 100W amp
Hi blackwhaleamp:
A lot of players are lazy and don't want extra controls or to have to learn new things, so they probably will accept one control but wine about having two new controls. Tell the guy to stop wining like a school girl and use both controls. kevin explained to me why he provides the tewo controls and why he thinks it is better than using a single control.
With one control for Power Scaling and Drive Compensation, you get Power Scaling. This is great and simple to use with the tone being as you expect.
With the two controls, you can set them about the same and have Power Scaling OR use them separately. this lets the player have a clean preamp and dirty power amp, dirty power amp with clean preamp, both dirty or both clean. Versatility is the bonus of having two controls. If you look at the amps kevin builds he obviously likes versatility - there's a lot of strange controls that other amps don't have.
Scaling extra parts of the amp but not the whole amp gives a bit of automatic drive compensation but limits versatility. I think kevin said he had an "auto drive" kit in the wings for people who want a one-knob solution.
Simple amps are kind of a fake out. Players afraid of learning new things or who are just lazy and don't want to work for their tone say they want no knobs - or a 3 knob amp or some other silly thing. But then they get an amp with no controls and they can't dial in a good sound, or the amp only has one sound. They're lucky if they like that one sound but most players get bored quickly. I put Power Scaling on my amp with the two knobs andadded the bias kit too so I can use any tube I want and now the amp is like ten amps.
A lot of players are lazy and don't want extra controls or to have to learn new things, so they probably will accept one control but wine about having two new controls. Tell the guy to stop wining like a school girl and use both controls. kevin explained to me why he provides the tewo controls and why he thinks it is better than using a single control.
With one control for Power Scaling and Drive Compensation, you get Power Scaling. This is great and simple to use with the tone being as you expect.
With the two controls, you can set them about the same and have Power Scaling OR use them separately. this lets the player have a clean preamp and dirty power amp, dirty power amp with clean preamp, both dirty or both clean. Versatility is the bonus of having two controls. If you look at the amps kevin builds he obviously likes versatility - there's a lot of strange controls that other amps don't have.
Scaling extra parts of the amp but not the whole amp gives a bit of automatic drive compensation but limits versatility. I think kevin said he had an "auto drive" kit in the wings for people who want a one-knob solution.
Simple amps are kind of a fake out. Players afraid of learning new things or who are just lazy and don't want to work for their tone say they want no knobs - or a 3 knob amp or some other silly thing. But then they get an amp with no controls and they can't dial in a good sound, or the amp only has one sound. They're lucky if they like that one sound but most players get bored quickly. I put Power Scaling on my amp with the two knobs andadded the bias kit too so I can use any tube I want and now the amp is like ten amps.
I agree with you. At the end, I left the two controls for the Power Scaling. I had to remove the MID because I didn't have too much space in the front. I could had removed the STANDBY but the holes don't match.
I plan to do a MKII for this amp. I'll put back the MID, and move the POWER to the back. STANDBY is not really needed. I won't put back the FX LOOP as there is no added value on it
I added some pictures
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