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Marshall Maniac ([email protected])
8/11/2001 8:30 PM
London Power "Power Scaling"
Have any of you ever used London Power's Power Scaling Kits (http://www.londonpower.com/psmadv.htm)? They appear to let you reduce plate voltage on the output tubes from 100% down to
less than 1% using a panel-mounted pot.
Seems like a good alternative to a Hot Plate. I've been using a Power Brake for some time, and the
idea of having nothing between the OT and the speaker (other than a cord) is very interesting to
me.
Thanks for any thoughts!
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Plexinator ([email protected])
8/11/2001 10:00 PM
Re: London Power "Power Scaling"
I am thinking about trying this also
plexi
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JWK ([email protected])
8/11/2001 11:04 PM
Re: London Power "Power Scaling"
The power scaling kits only became available in the last couple of weeks. There are a few people
around who might give you their insight as to how the actual London Power amps perform with the
integral power scaling. I almost ordered a studio about a year ago, but just can't bring myself to
buy any amp I can't hear for myself after the last major "burn" I experienced two years ago.
In any case, I will probably get the kit for a 50 watt amp I'm dreaming up when I gather a bit more
experience and knowledge.
John
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SeanF ([email protected])
8/12/2001 1:47 AM
all I know is what I've heard...
from two people whose ears I trust completely, and both of them rave about it. I'll probably give it
a try shortly.
http://firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=132880
... 8/20/2001
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Ed ([email protected])
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8/12/2001 6:54 AM
Re: all I know is what I've heard...
Please let me know how it turns out.
thank you
Ed
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Tone Doctor ([email protected])
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8/13/2001 4:37 AM
Re: London Power "Power Scaling"
The Power Scaling kits are pretty new, but I have owned a London Power Session 50 amp for a
year or so.
It rocks, and the Power Scaling is the coolest thing in the world.
As you lower the plate voltage the dynamic range of the output section is lowered. If you adjust
the volume at the same time, you can use it to get the same tone as you would use at a higher
SPL, just quieter.
Conversely, if you leave the input volume up, and turn the Power Scale down, the thing starts to
sustain like a violin.
I think the separate bias controls in the kits allow you to use disparate tubes in the output stage,
as well. That can be an interesting sound.
TD
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Plexinator ([email protected])
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8/13/2001 3:54 PM
Re: London Power "Power Scaling"
TOne DOctor
How do you think this would sound on a 50 watt marshall plexi amp?
plexi
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Tone Doctor ([email protected])
8/14/2001 12:54 PM
Re: London Power "Power Scaling"
http://firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=132880
... 8/20/2001
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Plexi
I am planning to do a 100W JMP Marshall myself.
I think it should sound great.
TD
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Matthew Springer ([email protected])
8/13/2001 4:04 PM
Re: London Power "Power Scaling"
Power scaling bascially is a "volume" control for the AC voltage being fed into the rectifiers. The
heart of the idea is to use a dual ganged pot to reduce the secondary AC voltage into the power
supply. So the answer is YES, anything derived from the main secondary PT AC windings (ie.
everything but heaters pretty much) will be affected.
In his most excellent books, Kevin seems to hint that he also derives the bias voltage from the
secondary AC as well (as oppossed to a separate bias winding), so this voltage will also exactly
scale with power scaling (but obviously, being a negative voltage, it scales "upside down").
-Matthew
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Plexinator ([email protected])
8/13/2001 4:11 PM
Re: London Power "Power Scaling"
Does this effect the preamp too?? or just the power section aka power tube plates and screens?
SOmething to Ponder
Plexi
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Tone Doctor ([email protected])
8/14/2001 12:52 PM
Re: London Power "Power Scaling"
Plexi
Kevin could talk to this better than I, but I believe the voltage scaling is applied to the power tubes
only. It varies the plate, screen and bias voltages in proportion.
By not applying it to the preamp or PI, it allows these sections to stay "fast".
TD
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Dave Rich ([email protected])
8/13/2001 4:20 PM
Re: London Power "Power Scaling"
Thats all there is to it????
At low settings that would create a lot of sag. It would have to be cathode biased too I presume.
Thats interesting. I would like to know more about it.
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Tone Doctor ([email protected])
8/13/2001 6:05 PM
Re: London Power "Power Scaling"
It's not as simple as that.
The circuit adjusts the plate voltage and the bias voltage in proportion.
Power Scaling works well in both fixed and cathode biased amps.
TD
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KB ([email protected])
8/14/2001 4:14 PM
Re: London Power "Power Scaling"
he also derives the bias voltage from the secondary AC as well (as oppossed to a
separate bias winding), so this voltage will also exactly scale with power scaling (but
obviously, being a negative voltage, it scales "upside down")
So where does the DC come in as we all know that bias voltage can't be AC,
KB.
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jaysg ([email protected])
8/14/2001 4:37 PM
Re: London Power "Power Scaling"
The heart of the idea is to use a dual ganged pot to reduce the secondary AC voltage
into the power supply.
http://firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=132880
... 8/20/2001
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Should this be rephrased so no one trys a dual-ganged pot across their PT secondary? The pot
must be controlling MosFet gates?
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Tone Doctor ([email protected])
8/15/2001 12:59 AM
Re: London Power "Power Scaling"
There seem to be a lot of opinions about what Power Scaling is and how it works.
I can tell you reading from this thread, that a lot of the opinions are just that. The reason I know
what the curcuit looks like and does, is because I copied the design from my Session amp, and
have used it in other amps.
I really hope that any owner of a London Power amp would never think to publish schematics.
Kevin is a great guy and we should support him. I can tell you from knowing the components
involved, that he is charging a very reasonable price for his kits.
And look at the bright side, after you buy one you won't have to think, "I wonder how he does
that?"
TD
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Plexinator ([email protected])
8/15/2001 3:19 AM
Re: London Power "Power Scaling"
Well i will probably end up getting one for the plexi 50 watt that i am building...After i get my plexi
done....i will give you guys a run down of how it sounds! I am pretty sure its gonna sound amazing
because...well Kevin O'connor is a very knowledgable person...and i trust him to make a good
product!!
plexi
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Matthew Springer ([email protected])
8/15/2001 4:45 PM
Power Scaling (longish)
Jay,
Should this be rephrased so no one trys a dual-ganged pot across their PT secondary? The pot
must be controlling MosFet gates?
No. I'm _pretty_ sure the whole cricuit is just a big, lowish impedance, high wattage dual ganged
pot right across the secondary windings and you feed the rectifiers (two for the main b+ and one
http://firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=132880
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or two for the bias rail) right off the wipers. No MOSFets are involved as far as I can make out.
Although I'm sure Kevin uses them elsewhere in his amps. My guess is he does something clever
with diodes to ground at each bottom leg of the wiper to reduce the staic power disipated by the
control.
DISCLAIMER: I haven't seen any schematics either, but I have tried to grok my way through the
book.
If you think about it the actual power disipated by say a 10K dual ganged pot across a 300VRMS
voltage source is ~ 9W total DC. They make 25W rated wire wound pots. Let's see if I can still do
this...
P = V * I, V = I * R, so P = I^2*R
A 10K pot (we'll do one leg of the AC winding) therefore draws 30mA DC, so that disipates 30A^-3
* 10k = 9W. Assuming a diode in the bottom leg to reduce this by half (so you don't draw current
when your side of the secondary winding is negative, but you've got two pots in one case so x2 =
~9W.
This is strictly back of proverbial envelope so don't try this at home kids, but you might be able to
get away with it withthe right pot.
-Matthew
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Matthew Springer ([email protected])
Date:
8/15/2001 8:42 PM
Subject:
And another thing...
Anybody even remotely interested in Power Scaling or amp building in general should run out and
immediately buy every single book every written by Kevin O'Connor (I'm dead serious). These
books are some of the finest refernce materials/textbooks on analog circuit design I've ever read
and I've got a masters in EE. I think between him and RG and Randall, I've saved maybe a year of
lab time. Whenever Randall writes his treatise on amp building I'll be the first in line for that one
too. And I certainly don't intend to give away Kevin's secrets in this or any other forum. But I think
everybody agree's that discussing generalized approaches seems to be ok, discussing actual design
specifics would probably be less ok.
Anybody interested in actually building a power scaling amp should certainly run out and buy his
kit. Whatever design problemns we (the community) are going to encounter designing and building
a collapsing B+ rail he's doubtlessly already solved in his most elegant of ways.
-Matthew
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MBSetzer ([email protected])
8/15/2001 9:05 PM
Re: And another thing...
Well, in the research lab it is often said:
*a year at the lab bench can sometimes save you more than an hour of time in the library*
Mike
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... 8/20/2001
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O'Connor ([email protected])
8/18/2001 3:30 AM
Re: And another thing...
Thanks for the kind words and support!
For those of you who have "grogged through the books" and actually understand it, you will know
right off that Power Scaling may in fact be do-able using ganged pots, but is most certainly NOT
elegant or even how I do it in my amps or kits.
TUT2 presented many ways to achieve a Power Scaling result but the term was never mentioned so
very few people ever connected the dots between the book and the Power Scaling Dialogue on our
site. There are about 60 distinct ways to do it and endless variations of those. Every method
cannot be universally applied-- although there are some that can be. The technique used in my
amps has limited applicability as does that for the kits-- which is why there are three kits and likely
more to come.
I'm hoping to get TUT3 finished for Christmas--it will be filled with construction approaches for
standard circuits and their variants, and cover grounding, noise abatement, etc, in detail. But after
that, there will be three books that will shed light on what many deem cutting edge retro: "Power
Scaling Tutorial", "Super Scaling Tutorial" and "Power Management Tutorial". Each covers the
development of the topic area in great detail, with tonnes of schematics and tips.
You can get a taste of this technology in the kits we sell, and learn a whole lot more from the
upcoming books.
Power Scaling does only what is claimed and it works because it only has to do one thing, It just
has lots of beneficial side-effects.
Have fun
Kevin O'Connor
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Matthew Springer ([email protected])
8/18/2001 5:04 AM
Kevin O'Connor on Kevin O'Conner
So, Kevin Now that we have your attention...
As a curious experimenter and engineer but also as a concerned and ethical netizen: I'm curious to
know what level detail and discussion about your designs you would like to see in this and other
forums.
As a person who's been on quite the cutting edge of "Tube Knowledge for the Masses", you are in
some ways on both sides of the intellectual property debate; trying to disseminate information
without giving away the farm.
I was wondering if you'd care to comment on what you consider an appropriate balance is between
all of us DYING to know how you did things and also respecting the hard work you've obviously put
into your designs.
http://firebottle.com/ampage/bbs/fireBB.cgi?forum=ga&thread=132880
... 8/20/2001
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And about the dual pots, I THOUGHT after reading _Secrets_ revealed that was the idea I got, but
when I sat down to write a post supporting this idea yesterday and actually did the power
dissipation calculation I realized this couldn't have been how you did it either. I also looked at your
kits and relaized you'd incorporated current limiting features which means MOSFETS and zeners,
not pots.
(sigh) back to the books for me....
-Matthew
alternate display for printing
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... 8/20/2001